There are many different factors that go into the objects you can create with a CNC mill, but the biggest contributor is going to be your end mills. At the most basic level, end mills are like drill bits but instead of only drilling vertically, they can also cut horizontally and have several form factors to get the cleanest cut for specific materials. Some are used only for plastics to keep from heating too much, some are designed to keep wood from chipping or tearing out, and some are designed for ultra fine detail work. With all the different specifications you can find with end mills, explaining each part individually would better serve your understanding, so my goal today is for you to have the considerations necessary to lead you down the path to success. Lets get started!
From one end mill to the next, the most obvious difference you will find is that end mills come in many shapes and sizes. Some are thin and pointy, and others are wide and rounded. Some of the most common shapes you will find are fishtail (or flat), ball-nosed, and bullnose, and each of these can be a straight cut or a tapered cut.
Size is the biggest determination of what you can do with any given end mill. Large ones excel at grinding through a lot of material at once, but you dont get a lot of detail out of your parts. With CNC milling, the radius of your end mill is the radius of any internal corner, so you will almost never have a perfectly square corner on the inside of a milled object. Smaller and smaller end mills can be used for each pass to clean up an edge and get the part to the final dimension and shape. However, the smaller your end mill, the more fragile it gets, so if you try to cut through material too fast with a 1/16 end mill and youre snapping it off into the workpiece. In most cases the most efficient use of a tool is to cut at around half of the diameter of the tool; with a tool, have a maximum depth of .
There are generally two forms of end mills: straight and tapered. This is a choice based on the geometry of your finished part because a tapered end mill wont be able to do all the same things as a straight end mill, and a straight end mill may not be the most efficient choice. By using a tapered end mill, the cross sectional area is larger than a straight end mill of the same tip diameter, creating a much stronger end mill that is less likely to bend while milling. For perfectly vertical walls you will need to use a straight end mill as the taper just wont reach. However for angled walls, using a straight end mill is not the most efficient, ideal choice.
Fishtail end mills are generally used to cut simple profiles out of a medium, like big letters out of a piece of wood. They cut best using the side of the mill, so most cutting software will slowly ramp the end mill down into the material, rather than a simple plunge. With fishtail end mills, you will have nice square corners at the bottom of any inset section of geometry and a smooth, flat surface anywhere it passes over the top of.
Ball-nose end mills have a dome-shaped tip. These excel at high-detail contours like relief artwork or mold and die making, but have what is known as scalloping. Since the tip of the end mill is round, having a perfectly flat surface is a challenging feat and will take many more passes than a simple fishtail to smooth out.
Bull-nose end mills are often called corner radius end mills, and are a combination of fishtail and ball-nose. These have a flat bottom, but rounded corners, so you can have a filleted inner corner while also avoiding the problem of scalloping. These are commonly used to mill molds as you dont need to use nearly as many tool changes to get flat bottom pockets and rounded contours.
The tip profile isnt the only thing that differentiates end mills. The spiral channels on an end mill - called flutes - determine which materials you can cut. Generally, less flutes equals better chip clearing at the expense of surface finish. More flutes gives you a nicer surface finish, but worse chip clearing. The softer and gummier the material, the quicker you need to remove the chips away from your part. Using a 6 flute end mill on plastic is going to melt the material more than it cuts it, and if you use it on aluminum you run the risk of generating enough heat to friction weld the aluminum to your end mill, completely ruining both pieces. The guideline for soft metals, plastic, and woods is to use one or two flutes; for high-detail milling use three or four flutes, and for carbon fiber, six or more flutes.
If you're a newcomer to the CNC milling, try starting with a two-flute, up-cut end mill and see how that works for you and your material. Considering that the material options for desktop CNC milling aren't too crazy, you can mill most of the different materials these machines are capable of using a two-flute end mill, but you will need to adjust feedrate and spindle speed.
End mills are made of a few different materials, but high-speed steel (HSS) and tungsten carbide are two of the most common. The HSS tools are more forgiving than carbide, as carbide is brittle and can chatter and shatter. HSS is also cheaper than carbide, but it tends to dull faster than carbide. In order to improve tool performance, manufacturers apply different coatings to extend the life of the tool and keep it sharp longer.
Finding the right end mill for the job is all about finding the balance between the different factors that make up the tool. Dont forget that standard procedure for CNC milling is swapping out your tools depending on which step of the process you are working on. Its perfectly normal to have dozens of tools - end mills, drill bits, engraving bits, and others - that you rotate through to get a progressively closer shape and finish to your final product. I hope that Ive either gotten you interested in using more end mills or at least given you a better understanding of how once differs from the next.
When the mill is rotated without feed or with very fine feed, the rods are in parallel alignment and in contact with one another for their full length. New feed entering at one end of the mill causes the rod charge to spread at that end. This produces a series of wedge shaped slots tapering toward the discharge end.
The tumbling and rolling rods expend most of their crushing force on the coarse fractions of the feed material and only to a lesser degree on the finer material filling the interstices in the rod charge. The horizontal progression of material through the mill is not rapid compared to the movement of the rods and material resulting from rotation of the mill. The average particle is subjected to an action similar to many sets of rolls in series, before it is discharged. Because of this, the rod mill can effectively reduce 1 feed size to 10 mesh or finer in open circuit.
The voids (or interstitial space) within a rod load are approximately half those in a ball mill grinding load. Rods in place weigh approximately 400 pounds per cu. ft. and balls in place approximately 300 pounds per cu. ft.. Thus, quantitatively, less material can progress through the voids in the rod mill grinding media than in the ball mill, and the path of the material is more confined. This grinding action restricts the volume of feed which passes through the mill, without causing an overload condition.
The conical or convex head of our Rod Mill forms a receiving pocket at the feed end which facilitatesentrance of the feed to the grinding charge uniformly. This permits maximum grinding efficiency at the maximum rate possible before an overload occurs. In addition, this type of head construction permits the use of rods the full mill shell length, and reduces wear on the end liners.
The discharge end pocket receives and readily discharges broken rod pieces which otherwise may remain in the rod charge and reduce grinding effectiveness.Vertical feed or discharge end liners may be substituted for the conical liners, when and if desired.
The old and common terms impact and attrition are not satisfactory for designating types of grind. The reason will be obvious when it is seen that high speed and low speed gave about the same type of grind. Furthermore, the term attrition is not as specific as it was formerly regarded when it was used to signify the undesirable work of excessively small media. The reason why it is not specific is shown in this report; in batch tests, when the amounts of subsieve material were the same, the excessively big media left too many coarse particles of ore and in that respect failed as would excessively small balls. Surely attrition does not apply to the failure of the large balls; hence, attrition does not suit. Nonselective is a better term because it covers both extremes of poor work, and selective is descriptive of good work on the coarse material.
The term overgrinding is much used in conversation with mill men, but search of the literature indicates that a good definition does not exist. This is due probably to the absence of a satisfactory antonym. Selective and non-selective grinding are used here to compare products that have the same amount of the subsieve size. Then the product with the least amount of coarse sizes shows good selective grinding and the others are ground nonselectively. Stage grinding which is by repeated passes followed by removal of the finished material, is the best means of obtaining selective grinding. These terms must not be confused with differential grinding, which has to do with the relative grinding rates of two or more minerals in an ore.
Is it better to use a grinding mill with large balls or will small rods? How do you decide between using a ball mill or a rod mill? Many investigators have attributed the selective grinding of rods to line contact. Other things should be considered. In the two pairs of tests shown in table 12 the relative deportment of large balls and small rods in batch wet grinding is shown. The two loads had the same volume. The rods required about 12 percent more power and their better selective grinding is obvious.
In considering the selective grinding of the rods, it must be remembered that the rods were heavier than the heaviest balls; they weighed 7 pounds each, whereas the largest balls weighed only 5 pounds each. On the basis of weight, the rods were larger than the balls although their diameters were much smaller. The rods, being only 35 inches long, may be regarded as much more rigid than rods regularly used.
These observations should be compared with table 4, which shows that the heavier stuffed pipes did more selective grinding than the light pipes. There the diameters were the same, and unquestionably selective grinding was due to the greater weight. Hence, weight as well as diameter of the medium has to be considered in appraising selective grinding and ball milling generally.
Ball mill and rod mill are the common grinding equipment applied in the grinding process. They are similar in appearance and both of them are horizontal cylindrical structures. Their cylinders are equipped with grinding medium, feeder, gears, and transmission device.
The working principle of ball mill and rod mill machine is similar, too. That is, the cylinder drives the movement of the grinding medium (lifting the grinding medium to a certain height then dropping). Under the action of centrifugal force and friction, the material is impacted and ground to required size, so as to realize the operation of mineral grinding.
Grate discharge ball mill can discharge material through sieve plate, with the advantage of the low height of the discharge port which can make the material pass quickly so tha t to avoid over-grinding of material. Under the same condition, it has a higher capacity and can save more energy than other types of mills;
It is better to choose a grate discharge ball mill when the required discharge size is in the range of 0.2 to 0.3 mm. Grate discharge ball mill is usually applied in the first grinding system because it can discharge the qualified product immediately.
Overflow discharge ball mill can grind ores into the size under 0.2 mm, so it is very suitable for the second grinding system. The capacity of it is about 15% lower than grate discharge ball mill in the same specification, and the loaded grinding medium is also less than that one.
It can be divided into three types of rod mills according to the discharge methods, center and side discharge rod mill, end and side discharge rod mill and shaft neck overflow discharge rod mill.
It is fed through the shaft necks in the two ends of rod mill, and discharges ore pulp through the port in the center of the cylinder. Center and side discharge rod mill can grind ores coarsely because of its structure.
This kind of rod mill can be used for wet grinding and dry grinding. "A rod mill is recommended if we want to properly grind large grains, because the ball mill will not attack them as well as rod mills will."
It is fed through one end of the shaft neck, and with the help of several circular holes, the ore pulp is discharged to the next ring groove. The rod mill is mainly used for dry and wet grinding processes that require the production of medium-sized products.
The diameter of the shaft neck is larger than the diameter of the feeding port about 10 to 20 centimeters, so that the height difference can form a gradient for ore pulp flow. There is equipped with a spiral screen in the discharge shaft neck to remove the impurities.
It has high toughness, good manufacturability and low price. The surface layer of high manganese steel will harden rapidly under the action of great impact or contact. The harder index is five to seven times higher than other materials, and the wear resistance is greatly improved.
It has high toughness, good manufacturability and low price. The surface layer of high manganese steel will harden rapidly under the action of great impact or contact. The harder index is five to seven times higher than other materials, and the wear resistance is greatly improved.
It is made of several elements such as chromium and molybdenum, which has high hardness and good toughness. Under the same work condition, the service of this kind of ball is one time longer than the high manganese steel ball.
After the professional technology straightening and quenching processing process, a high carbon steel rod has high hardness, excellent performance, good wear resistance and outstanding quality.
The steel ball of ball mill and the mineral material are in point contact, so the finished product has a high degree of fineness, but it is also prone to over-grinding. Therefore, it is suitable for the production with high material fineness and is not suitable for the gravity beneficiation of metal ores.
The steel rod and the material are in line or surface contact, and most of the coarse particles are first crushed and then ground. Therefore, the finished product is uniform in quality, excellent in particle size, and high in qualification rate.
The cylinder shape of the rod mill and the ball mill is different: the cylinder of the rod mill is a long type, and the floor area is large. The ratio of the length to the diameter of the cylinder is generally 1.5 to 2.0;
The cylinder of the ball mill is a barrel or a cone. And the ratio of the length to the diameter of the cylinder is small, and in most cases the ratio is only slightly larger than 1, and the floor area is small, too.
The above is the main content of this article. The ball mill and the rod mill are the same type of machine on the appearance, but there are still great differences in the interior. It is very necessary to select a suitable machine for the production to optimize the product effect and maximize its efficiency.
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The materials of the beads (grinding media) are generally divided into glass, ceramic and steel. Except for the ordinary 2-3/3-4mm glass beads used by the vertical sand mill, other equipment uses zirconia beads with a size of 0.8-1.2mm.
The working principle of the bead mill is that the beads (grinding medium) and the material are placed together in the container, and the container is stirred by the high-speed rotating dispersing disc, so that the solid particles and beads (grinding medium) in the material produce strong shearing, scouring, Collision, to achieve the purpose of crushing, grinding and dispersion.
Many researchers have done a lot of experiments on the grinding efficiency of ball mills and bead mills, and the conclusion is that bead mills are much better than ball mills in terms of grinding efficiency and preparation cost. Moreover, the experiment also proved that the bead mill can produce powder with an average particle size of less than 0.5m, which is much smaller than the particle size of the product ground by the ball mill.
Researchers put 3 kg of piezoelectric ceramic composite material in the ball mill container, vibratory grinding hopper and bead mill barrel respectively for pulverization. All of them are processed by wet processing, and samples are taken at regular intervals to measure the particle size and distribution. Compare the processing time of the three types of equipment when the powder particle size D50 reaches about 0.5m and the particle size distribution of the processed powder.
It can be seen from the experimental results that from the perspective of crushing efficiency and particle size distribution, the vibratory milling process is better than the ball milling process, and the bead milling process is significantly better than the ball milling and vibrating milling processes. This is related to the size of the grinding ball used for crushing and the speed and energy of the grinding ball. Generally, the smaller the grinding ball, the greater the grinding effect, the finer the pulverization, and the narrower the particle size distribution.
In the experiment, the diameter of the zirconia ball used in the bead mill is 1.5mm (1.5 Kg), and the diameter of the zirconia ball used in the ball mill and vibratory mill is 20mm, 15mm and 8mm (7.5 Kg). The grinding surface area of the sand mill is much larger. For ball milling and vibration milling. The running speed of the grinding ball during sanding is 1800 rpm, the vibration speed of the grinding ball during vibratory grinding is 600 times/min, and the running speed of the grinding ball during ball milling is 65 rpm. Therefore, the bead mill has the highest crushing efficiency, the finest powder particles, the narrowest particle size distribution, and the best crushing quality.
Based on the above pulverization test results, the researchers concluded that the piezoelectric ceramic composite material is pulverized by a bead mill, which is more conducive to improving the microstructure of piezoelectric ceramics, and improving the mechanical properties and piezoelectric dielectric properties of piezoelectric ceramic materials. For the ultrasonic atomization transducer element, it can reduce the performance attenuation rate by 50%, and obviously extend its service life from 5000h to 8000h.
Compared with other grinding equipment (jet mill), the bead mill has the advantages of high fineness, strong continuity, and high efficiency. The fineness requirements can be adjusted by adding or subtracting grinding media or grinding with different media. The grinding medium of the bead mill runs at high speed, and the grinding action is mainly impact and shear force. The zirconia bead grinding media used in the experiment achieves zero pollution to the raw materials, and has a small particle size and many contact points, which can be ultra-fine grinding and dispersion.
The raw materials are grinded at a high speed in a grinder, the liquid phase is uniformly mixed and fully reacted to obtain a slurry, and then through a spray dryer, the slurry is atomized, and in contact with hot air, the moisture quickly vaporizes. In this process, the slurry The material is granulated and dried to obtain a lithium battery electrode material.
Although the bead mill has some advantages unmatched by other types of equipment in terms of dispersion and grinding, the bead mill also has its own application limitations. For example, the bead mill has requirements for the fineness of the raw materials to be ground, and the general requirement is less than 80 Mesh (<0.18 mm). In addition, the bead mills currently used in the coatings industry have relatively small specifications, and the output of a single device is not large, making it difficult to meet the output requirements of the ceramic industry.
In the processing of many materials, it is common to see the combined use of ball mills, jet mills, and bead mills. Some researchers have used a combination of ball mills and agitated bead mills to significantly shorten the grinding time. Improve the grinding efficiency, and also save the grinding power consumption. Some processes are first handed over to a ball mill to stir and mix, and then grind with a bead mill. In some processes, a bead mill is used to grind, and then the sintered material is pulverized with a jet mill. The three cooperate with each other to achieve the optimal processing and best performance of the material.
As grinding equipments, ball mills and rod mills are both high efficiency grinding equipments. Their common points are: 1. They have the same function, they are all used to grind materials, so that the materials can reach the state of powder; 2. The shape of ball mills and rod mills are similar, they are all horizontal and rotatable cylinder; 3. The overall design of ball mills and rod mills are compact, with simple operation and easy maintenance. But the both are different in performance and applications. In this article, the similarities and differences between ball mills and rod mills are analyzed from 9 aspects, and suggest how to choose ball mill or rod mill.Table of Contents Ball Mills vs Rod Mills1. Shape and Structure2. Ore Discharge Types3. Grinding Medium4. Medium Filling Rate5. Fineness of Product6. Running Cost7. Performance Aspects8. Stability Aspects9. Application AspectsHow To Choose Ball Mill or Rod Mill1. According to Ore Properties2. According to Granularity Properties3. According To Mill Properties
The ratio of cylinder length to diameter of rod mill is generally 1.5-2.0, and the inner surface of liner plate on end cover is vertical plane. In most cases, the ratio of the length to the diameter of the ball mill is only slightly greater than 1.In addition, with same specificationthe, the cylinder rotating speed of the rod mill is lower than that of the ball mill.
The rod mill usually uses 50-100mm diameter steel rod as grinding medium, while the ball mill uses steel ball as grinding medium. The length of steel rod is 25-50mm shorter than the cylinder, and it is usually made of high carbon steel with carbon content of 0.8% 1%; the loading capacity of rod is about 35% 45% of the effective volume of rod mill.
In the ball mill, there are lattice ball mill and overflow ball mill in common use (named by different ore discharge structure), while there are only overflow type and open type rod mill. Generally, with the same specification, the diameter of the hollow shaft at the ore discharge end is larger than that of the ball mill.
Medium filling rate refers to the grinding medium percentage in the mill volume. For different grinding methods, mill structures, medium shapes and operating conditions, the medium filling rate has a available range.Generally, the filling rate of ball mill is 40% 50%, and that of rod mill is 35% 45%.
The power consumption of the ball mill is slightly higher than that of the rod mill, and the wear rate is also higher than that of the rod mill. The daily maintenance requires constant addition of a certain amount of steel balls, while the rod mill only needs to be inspected and replaced in a period of time.
To a great extent, the hardness and granularity of ore will affect the selection of grinding equipment. Some ores with dense structure, small crystal and high hardness are difficult to grind. Therefore, a longer grinding time, a harder grinding medium and a larger grinding capacity are required in the grinding process.
Rod mill adopts high carbon steel rod with carbon content of 0.8% 1% and diameter of 50-100 mm as grinding medium. In the grinding operation, most of the coarse particles are ground first, and a small part of the fine particles are ground. Therefore, the products of rod mill are rough, uniform in texture, excellent in particle shape and high qualified rate of products.
Ball mill uses steel ball as grinding medium. Because the steel ball is in point contact with the ore, the grinding products are of high fineness, and the particle size of the products is relatively fine, which is prone to over grinding, so ball mill is not suitable for the re-selection production line in ore dressing.
It is necessary to design the layout plan of the grinding workshop for concentrator in advance, so it should be considered whether it can meet the installation conditions of the mill. The cylinder of the rod mill is long cylinder type, and the cylinder of the ball mill is cylinder type or cone type.
The simplest grinding circuit consists of a ball or rod mill in closed circuit with a classifier; the flow sheet is shown in Fig. 25 and the actual layout in Fig. 9. This single-stage circuit is chiefly employed for coarse grinding when a product finer than 65 mesh is not required, but it can be adapted for fine grinding by substituting a bowl classifier for one of the straight type so as to enable the W/S ratio of the overflow to be kept below the 4/1 limit usually necessary for flotation. On account of the greater efficiency of the bowl classifier the trend of practice is towards its installation in plants grinding as coarse as 65 mesh.
Single-stage grinding is generally to be recommended for small plants on account of its simplicity. Variations in the size and character of the ore are unavoidable in most plants, but they are, as a rule, very much more noticeable when operations are on a small than when they are on a large scale. Multi-stage grinding as practised in large installations may, therefore, prove impossible to control on a very small scale, and for this reason the simplicity of single-stage grinding is likely to result in a greater overall efficiency than would be obtained with a multi-stage arrangement. When, however, the capacity of a plant approaches or exceeds 1,000 tons per day, two-stage grinding becomes preferable because the effect of normal variations of the ore is less marked and control becomes correspondingly easier.
The usual type of two-stage circuit is shown in Fig. 26, and is one that can be employed for any degree of grinding, although a straight must be substituted for a bowl classifier in the second stage when a 48-mesh product is required. It used to be the practice at one time to omit the first classifier and to pass the feed straight through the primary mill to the secondary circuit, but it was not a good method because either the secondary mill received pieces of ore that were too big or else the primary mill overground a large proportion of the feed. Much better results are obtainable by keeping the coarse ore circulating round the primary circuit, which is set for the efficient grinding of such material, until it is fine enough to be sent to the secondary circuit where the machines are set to grind fine ore more efficiently than coarse. It should be noted that the overflow of the primary classifier is sent to the secondary classifier, not direct to the mill, in order that all material which has been ground fine enough in the primary circuit can be discharged immediately without any chance of its entering and being overground in the secondary ball mill.
So important is it from the point of view of efficiency to get the undersize out of the circuit at the earliest possible moment, whether it is produced in the primary or in the secondary mill, that a special intermediate bowl classifier is often installed between the two stages. Such an arrangement has been found very useful in plants in which improvements in dry crushing practice have resulted in a reduction in the size of the feed to the grinding mills with the result that they have been able to take larger tonnages; the classifiers have then becomeoverloaded, especially in the case of older installations in which both stages were equipped with straight classifiers.
The method of installing a bowl classifier to overcome the difficulty is shown in Fig. 27. This circuit is usually adopted in modern practice, but with a bowl instead of a straight classifier, if necessary, in the closed circuit of the secondary ball mill.
Any increase in the efficiency of classification gives greater economy of power by reducing the amount of ore that is overground, so making a larger proportion of the power required to turn the mill available forgrinding the particles that are still too large. From a theoretical point of view, the ideal method of grinding would consist of a series of ball mills, each in closed circuit with a classifier and each so short that the ore in its passage through the mill would be struck only two or three times by the balls, any undersize produced being removed at once by the classifier ; in this way the chance of a particle being struck again after it had reached the required size would be reduced to a minimum. Practice circuit design approaches the ideal:
Open circuit grinding consists of one or more grinding mills, either parallel or in series, that discharges a finalground product without classification equipment and no return of coarse discharge back to the mill. Some very simplistic examples of open circuit grinding are see below and are made of aRod mill,Ball Mill or aRod mill, ball mill combination.
Not all ores can be ground in an open circuit type ofarrangement. Some conditions which do favor open circuit grinding such assmall reduction ratios,reduction of particles to a coarse, natural grainsize,recirculation of cleaner flotation middlings forregrinding anda non-critical size distribution of the final groundproduct.
Closed circuit grinding consists of one or more mills discharging ground product to classifiers which in turn return the coarse product from the size separation back to the mill for further grinding. In this circuit, grinding efficiency is very dependent upon the size separation effected so care should be exercised in selecting the type and size of classifier used to close the system.
This type of grinding is the most common circuit found in mineral processing facilities, mainly because a lot of ores and product requirements are not suitable for open circuit grinding. Some advantages presented by grinding in closed circuit are that this arrangement usually results in higher mill capacity and lower power consumption per ton of product, it eliminates overgrinding by removing fines early and it avoids coarse material in the final ground product by returning this material to the mill.
Although closed circuit grinding offers many choices for arrangement of the equipment as well as combinations of equipment, some of the more common circuits arerod mill/classifier,Ball mill/Classifier,Rod mill/Ball mill/Classifier and Rod mill/Classifier/Ball mill/Classifier.
The importance of the grinding circuit to overall production in any facility should be obvious by now. Because of the responsibilities assigned to grinding it becomes essential that a grinding mill accepts a certain required tonnage of ore per day while yielding a product that is of a known and controllable particle size. This leads to the conclusion that close control over the grinding circuit is extremely important.
There are many factors which can contribute to fluctuationsin performance of a mill, but some of the most common found inindustrial practice are thechanges in ore taken from different parts of the mine,changes in crusher settings,wear in the crushers,screen damage in the crusher circuit.
These are a few things that operators should look for when changes in mill performance are noticed. Stockpiling of ore ahead of the mill can aid in smoothing out some of the fluctuations although it must be stored in such a manner that no segregation occurs.
The reduction ratio in the grinding section is so much greater than in the crushing plant that labour becomes a relatively small item and the power and steel consumption the largest items of cost. Table 18 gives the average total consumption of power that may be expected in modern ball mill installations of various capacities up to 4,000 tons per day, the figures being based on an average medium-hard ore.
The cost of grinding is more difficult to predict than that of crushing because variations in the hardness and toughness of the ore produce proportionately wider variations in the consumption of power and steel. An approximate guide to grinding costs; they are direct costs and include no overhead charges. Power is assumed to cost 0.075 per kilowatt-hour in the case of the smallest plant and to decrease to a minimum per kilowatt-hour for the largest.
As the tonnage rises up to 1,000 tons per day the costs fall rapidly. In plants of greater capacity, however, they do not decrease in the same proportion with increase of tonnage, because the extra capacity is not obtained by increasing the size of the individual machines but by installing two or more similar units side by side, each of equal efficiency.Reduction of costs then becomes more a matter of organization than of plant design.
As already stated, the power and steel costs are the two largest items, those of labour and supplies being small by comparison ; it is on this account that recent progress has been mainly directed towards reducing the consumption of power and steel by means of greater efficiency in classification and by the use of mills of larger diameter.
The way in which the efficiency of classification has been increased has been described in the paragraph headed Grinding Circuits. An increase in the diameter of a mill gives greater economy in two ways : In the first place, the balls do more effective work in a large than in asmall mill, because, falling from a greater height, they shatter the pieces of ore with greater force ; in the second place, the ratio of the deadweight of the mill to the weight of the ball charge decreases as the diameter increases and thus in a large mill the useless weight to be moved is distributed over a greater weight of useful ball load than in a small mill, with the result that a larger proportion of the total power consumption is available to give the balls the cascading and rolling action necessary to break up the ore.
It is essential for the grinding and flotation sections of a plant to be run continuously. It takes nearly half an hour to clear the circuit of even a small grinding unit preparatory to stopping it, and often an hour is necessary to get the circuit fully loaded after restarting ; most of the power required to keep the machines running during the stopping and starting periods is wasted. Moreover, the operation of the flotation machines is poor during these periods so that much of the power required to keep them running is also wasted. In addition, modern practice aims at the elimination of everything likely to cause fluctuating conditions. For these reasons it is the universal custom to run the grinding and flotation plants for 24 hours per day.
Highly automated milling is a versatile machining process that is capable of producing components in almost any shape. There are a wide variety of CNC milling operations adopted for different manufacturing purposes. End milling mainly differs from other processes due to the type of tooling it is used for cutting materials. In this article, well outline the types of end mills, what is end milling, and also figure out the difference between end mill and drill bit.
End milling is a type of milling process that can be used to produce slots, shoulders, die cavities, contours, profiles, and other milling parts. End milling uses the end mill which is a cylindrical cutter with multiple cutting edges on both its periphery and its tip, permitting end cutting and peripheral cutting.
The end mill is a type of milling cutter designed to be able to cut axially and applicable in end milling, profile milling, tracer milling, face milling, and plunging. End mills and other cutting tools can be made from a host of materials, such as the carbide inserts (suitable for high production milling), high-speed steel (when a special tool shape needed), ceramics inserts (for high-speed machining with high volume), and diamond inserts (offer tight tolerances). High-speed steel (HSS) and tungsten carbide are two of the most common materials for making the end mill. End mills allow precision cutting to manufacture milled parts for broad applications, including jewelry, sign making, mold making, circuit boards, wood engravings, machine parts, and more. Endmills are available in varying lengths, diameters, flutes, and types.
1. End mills in different shapes Ball nose end mill: with a radius at the bottom which makes for a greater surface finish, produces a rounded pass, ideal for 3D contour work, shallow slotting, pocketing, and contouring applications. V-bits: with small angles and tips, produce a V-shaped pass, used for engraving, narrow cuts, and small, delicate engraving of lettering and lines, particularly for making signs, its also available for exceptionally sharp edges. The v-bit end mill comes in two forms- 60or 90V-bit. Fishtail end mill: with cutting edges on one end, which may be much thinner than the other end, fishtail end mills can plunge directly into your material and produce a flat surface, prevent any splintering or breakout, create clean edges on thin material and make pockets with flat bottoms, suitable for plunge routing and producing precise contours, like signs. With fishtail end mills, you can also have nice square corners at the bottom of any inset section of geometry. Square end mill: also known as flat end mills, are general-purpose mills that generate flat-surfaced cuts with perfect 90 corners in the workpiece, involves milling operations like side milling, face milling, and more. Square endmills can be used in the roughing or finishing stage. Bull-nose end mills: also called corner radius end mills, this type of cutter is a combination of fishtail and ball-nose, also a flat bottom but with rounded corners. Bull-nose end mills are often used to mill molds (plastic injection molds, die cast molds, etc.)
2. End mills in different number of flutesThe spiral-shaped cutting teeth on the end mill are flutes, which offer an empty path for cutting chip removal during the machining process. End mills available in 2, 3, or 4 flutes, 2 and 4 flutes end mill are more common. More flutes create a smoother surface finish, while fewer flutes are best at chip clearing and keep heat from building up. 2 Flute end Mill: suitable to work with wood and aluminum because they produce large chips 4 Flute end Mill: used to machine most other materials, cut harder materials than 2 flutes
3. End mills in different materials High-speed steel (HSS): cheaper than carbide tools but dull faster as well Carbide: is brittle and can shatter, provide better wear resistance and toughness. Carbide end mills are extremely heat-resistant and used for high-speed applications on some of the hardest materials. Solid Carbide end mills: considerably harder, rigid, and more wear-resistant than other types.
1. Rotary end mills cut side to side or in the horizontal direction, lots of mills can cut both axially and laterally, while drill bits that plunged directly into the material only move up and down and cut in vertically.
4. The tip of drill bits is often ground in cone-shape, the exception is diamond drill bits which have a flat end, end mills are available in various shapes and specifications, a regular end mill has a flat tip, and a ball nose end mill with a profile at the cutting end. You can choose a proper end mill according to the materials to be cut and surface finish required.
FRITSCH Ball Mills FRITSCH BALL MILLS: THE MOST EFFECTIVE MILLS FOR SMALL AND VERY SMALL QUANTITIES For fast batchwise grinding of medium-hard to hard samples For achieving the nest particle sizes Dry or wet grinding For mixing For homogenisation FRITSCH is an internationally respected manufacturer of application-oriented laborator y instruments. For more than 80 years, laboratories worldwide have relied on our FRITSCH. ONE STEP AHEAD. experience, quality, ser vice and innovation - for fast industrial applications as well as for especially accurate results in industr y- and...
are the most effective laboratory mills for rapid batchwise comminution of medium-hard to hard samples down to the finest particle size. The grinding can take place dry or wet. Grinding sets of many different materials are available. FRITSCH Ball Mills are also the ideal and reliable lab assistants for mixing and homogenising. The smallest instrument for small quantities Vibratory Micro Mill Fine comminution and sieving in one unit Operating principle Grinding in a Ball Mill takes place through impact and friction of the sample between the grinding balls and the inside wall of the grinding...
FRITSCH Mini-Mill T H E U LT R A - E F F E C T I V E F R I T S C H M I N I - M I L L For smallest sample quantities up to 5 ml Max. feed size 6 mm, nal neness 5 m Dry, wet and cryogenic grinding in a single unit Extremely compelling in price and in performance Extremely effective grinding due to spherical grinding bowl with plug-style closure and practical quick clamping system Precisely adjustable congurable and reproducible grinding time Very simple operation, cleaning and maintenance PTFE bowl for use in biotechnology The ultra-compact FRITSCH Mini-Mill is the ideal...
TECHNICAL DATA Electrical details 100-240 V/1~, 50-60 Hz, 90 watt Dimensions w x d x h Bench top instrument 20 x 30 x 30 cm Emissions value of workplace according to DIN EN ISO 3746:2005 Approx. 75 dB(A) (depending on the material to be ground and grinding bowl/balls used) Order no. 23.1000.00 Practical and effective: Spherical grinding bowls for easy assembly Operating panel with integrated, easy-to-clean glass keyboard IDEAL FOR Chemical analysis Comminution and homogenisation of grinding samples for creation of compacts for x-ray fluorescence and infrared spectroscopy (e.g. potassium...
FRITSCH Vibratory Micro Mill T H E F R I T S C H V I B R AT O R Y B A L L M I L L Max. feed size 5 mm Max. sample quantity 10 ml Final neness 10 m Effective comminution in a narrow, homogeneous particle size range Loss-free grinding in a closed vessel dry or in suspension Cryogenic grinding and simple embrittling in the cryo-box Modular system for simple conversion to dry or wet sieving Adjustable oscillation amplitude for easy adaption of the vibration energy to the grinding sample FRITSCH cryo-box for fast, simple embrittlement The FRITSCH PULVERISETTE 0 is the ideal...
TECHNICAL DATA ROHS The FRITSCH Vibratory Micro Mill PULVERISETTE 0 is recommended for sample preparation of RoHS tests (Restriction of Hazardous Substances). Electrical details 100-240 V/1~, 50-60 Hz, 50 watt Weight Net 21 kg Gross 22 kg Dimensions w x d x h Bench top instrument 37 x 40 x 20 cm Packaging w x d x h Cardboard box 50 x 43 x 30 cm Emissions value of workplace according to DIN EN ISO 3746:2005 Approx. 68 dB(A), with sound absorption hood approx. 53 dB(A) (depending on the material to be ground and mortar/grinding balls used) Grinding and sieving in one unit: the PULVERISETTE 0...
GRINDING BOWLS AND GRINDING BALLS For your FRITSCH Mini-Mill PULVERISETTE 23, you require one grinding bowl and the corresponding number of grinding balls. To avoid undesired contamination of the sample through abrasion, we offer a selection of 4 different material types. Normally, grinding bowls and balls of the same material are used. In principle, the grinding bowl material must be harder than the material to be ground. Important: Pay attention to the specified useful capacity as this is not identical to the bowl volume! Material Main component of the material* Density...
MORTAR AND GRINDING BALLS For the FRITSCH Vibratory Micro Mill PULVERISETTE 0, you require a mortar, which must be equipped with a grinding ball. All FRITSCH mortars are rimmed, regardless of the material, in a robust shell of shock-resistant aluminium, which protects the actual mortar. To optimally adapt the grinding to any sample type, you can choose between 6 different materials, whereby mortars and grinding balls of the same material are generally used. Important: The mortar material must always be harder than the material to be ground. For cryogenic grinding, use mortars and grinding...
FRITSCH Ball Mills ENVIRONMENTAL ROHS MOBILE PHONES ARE REDUCED TO DUST For comminution of individual electronic components, such as mobile phones for RoHS analysis, the FRITSCH Vibratory Micro Mill PULVERISETTE 0 delivers very good results depending on the sample characteristics at room temperature or with cryogenic grinding after embrittlement with liquid nitrogen in the practical FRITSCH cryo-box. Mobile phone keypad: Grinding results after embrittlement under freezing conditions PHARMACEUTICALS BONE PREPARATION FOR RESEARCH For comminution of bones, e.g. for XRF analysis for...
ORDERING DATA Order no. Instrument without grinding bowl and bails 23.1000.00 for 100-240 V/1~, 50-60 Hz Grinding bowl 15 ml volume Zirconium oxide Stainless steel Tempered steel Grinding bowl 10 ml volume Zirconium oxide Stainless steel Tempered steel Grinding bowl 5 ml volume 23.1600.00 PTFE Grinding balls 15 mm diameter 55.0150.27 Zirconium oxide Grinding balls 10 mm diameter 55.0100.27 Zirconium oxide Grinding balls 5 mm diameter 55.0050.27 Zirconium oxide Smaller grinding balls (0.1 - 3 mm 0) are also available! VIBRATORY MICRO MILL PULVERISETTE 0 Instrument incl. grinding...
Ball Mills are the most effective laboratory mills for rapid batchwise comminution of medium-hard, soft, brittle, fibrous, temperature-sensitive and moist samples down to the finest particle size. The comminution of the material to be ground takes place through impact and friction between the grinding balls and the inside wall of the grinding bowl respectivelythe mortar.The grinding can be performed dry or wet. In addition to comminution Ball Mills are also the ideal and reliable lab assistants for mixing and homogenising. Grinding sets of many different materials are available to prevent undesired abrasion.
A ball mill is a type of grinder used to grind and blend bulk material into QDs/nanosize using different sized balls. The working principle is simple; impact and attrition size reduction take place as the ball drops from near the top of a rotating hollow cylindrical shell. The nanostructure size can be varied by varying the number and size of balls, the material used for the balls, the material used for the surface of the cylinder, the rotation speed, and the choice of material to be milled. Ball mills are commonly used for crushing and grinding the materials into an extremely fine form. The ball mill contains a hollow cylindrical shell that rotates about its axis. This cylinder is filled with balls that are made of stainless steel or rubber to the material contained in it. Ball mills are classified as attritor, horizontal, planetary, high energy, or shaker.
Grinding elements in ball mills travel at different velocities. Therefore, collision force, direction and kinetic energy between two or more elements vary greatly within the ball charge. Frictional wear or rubbing forces act on the particles, as well as collision energy. These forces are derived from the rotational motion of the balls and movement of particles within the mill and contact zones of colliding balls.
By rotation of the mill body, due to friction between mill wall and balls, the latter rise in the direction of rotation till a helix angle does not exceed the angle of repose, whereupon, the balls roll down. Increasing of rotation rate leads to growth of the centrifugal force and the helix angle increases, correspondingly, till the component of weight strength of balls become larger than the centrifugal force. From this moment the balls are beginning to fall down, describing during falling certain parabolic curves (Figure 2.7). With the further increase of rotation rate, the centrifugal force may become so large that balls will turn together with the mill body without falling down. The critical speed n (rpm) when the balls are attached to the wall due to centrifugation:
where Dm is the mill diameter in meters. The optimum rotational speed is usually set at 6580% of the critical speed. These data are approximate and may not be valid for metal particles that tend to agglomerate by welding.
The degree of filling the mill with balls also influences productivity of the mill and milling efficiency. With excessive filling, the rising balls collide with falling ones. Generally, filling the mill by balls must not exceed 3035% of its volume.
The mill productivity also depends on many other factors: physical-chemical properties of feed material, filling of the mill by balls and their sizes, armor surface shape, speed of rotation, milling fineness and timely moving off of ground product.
where b.ap is the apparent density of the balls; l is the degree of filling of the mill by balls; n is revolutions per minute; 1, and 2 are coefficients of efficiency of electric engine and drive, respectively.
A feature of ball mills is their high specific energy consumption; a mill filled with balls, working idle, consumes approximately as much energy as at full-scale capacity, i.e. during grinding of material. Therefore, it is most disadvantageous to use a ball mill at less than full capacity.
Grinding elements in ball mills travel at different velocities. Therefore, collision force, direction, and kinetic energy between two or more elements vary greatly within the ball charge. Frictional wear or rubbing forces act on the particles as well as collision energy. These forces are derived from the rotational motion of the balls and the movement of particles within the mill and contact zones of colliding balls.
By the rotation of the mill body, due to friction between the mill wall and balls, the latter rise in the direction of rotation until a helix angle does not exceed the angle of repose, whereupon the balls roll down. Increasing the rotation rate leads to the growth of the centrifugal force and the helix angle increases, correspondingly, until the component of the weight strength of balls becomes larger than the centrifugal force. From this moment, the balls are beginning to fall down, describing certain parabolic curves during the fall (Fig. 2.10).
With the further increase of rotation rate, the centrifugal force may become so large that balls will turn together with the mill body without falling down. The critical speed n (rpm) when the balls remain attached to the wall with the aid of centrifugal force is:
where Dm is the mill diameter in meters. The optimum rotational speed is usually set at 65%80% of the critical speed. These data are approximate and may not be valid for metal particles that tend to agglomerate by welding.
where db.max is the maximum size of the feed (mm), is the compression strength (MPa), E is the modulus of elasticity (MPa), b is the density of material of balls (kg/m3), and D is the inner diameter of the mill body (m).
The degree of filling the mill with balls also influences the productivity of the mill and milling efficiency. With excessive filling, the rising balls collide with falling ones. Generally, filling the mill by balls must not exceed 30%35% of its volume.
The productivity of ball mills depends on the drum diameter and the relation of drum diameter and length. The optimum ratio between length L and diameter D, L:D, is usually accepted in the range 1.561.64. The mill productivity also depends on many other factors, including the physical-chemical properties of the feed material, the filling of the mill by balls and their sizes, the armor surface shape, the speed of rotation, the milling fineness, and the timely moving off of the ground product.
where D is the drum diameter, L is the drum length, b.ap is the apparent density of the balls, is the degree of filling of the mill by balls, n is the revolutions per minute, and 1, and 2 are coefficients of efficiency of electric engine and drive, respectively.
A feature of ball mills is their high specific energy consumption. A mill filled with balls, working idle, consumes approximately as much energy as at full-scale capacity, that is, during the grinding of material. Therefore, it is most disadvantageous to use a ball mill at less than full capacity.
Milling time in tumbler mills is longer to accomplish the same level of blending achieved in the attrition or vibratory mill, but the overall productivity is substantially greater. Tumbler mills usually are used to pulverize or flake metals, using a grinding aid or lubricant to prevent cold welding agglomeration and to minimize oxidation .
Cylindrical Ball Mills differ usually in steel drum design (Fig. 2.11), which is lined inside by armor slabs that have dissimilar sizes and form a rough inside surface. Due to such juts, the impact force of falling balls is strengthened. The initial material is fed into the mill by a screw feeder located in a hollow trunnion; the ground product is discharged through the opposite hollow trunnion.
Cylindrical screen ball mills have a drum with spiral curved plates with longitudinal slits between them. The ground product passes into these slits and then through a cylindrical sieve and is discharged via the unloading funnel of the mill body.
Conical Ball Mills differ in mill body construction, which is composed of two cones and a short cylindrical part located between them (Fig. 2.12). Such a ball mill body is expedient because efficiency is appreciably increased. Peripheral velocity along the conical drum scales down in the direction from the cylindrical part to the discharge outlet; the helix angle of balls is decreased and, consequently, so is their kinetic energy. The size of the disintegrated particles also decreases as the discharge outlet is approached and the energy used decreases. In a conical mill, most big balls take up a position in the deeper, cylindrical part of the body; thus, the size of the balls scales down in the direction of the discharge outlet.
For emptying, the conical mill is installed with a slope from bearing to one. In wet grinding, emptying is realized by the decantation principle, that is, by means of unloading through one of two trunnions.
With dry grinding, these mills often work in a closed cycle. A scheme of the conical ball mill supplied with an air separator is shown in Fig. 2.13. Air is fed to the mill by means of a fan. Carried off by air currents, the product arrives at the air separator, from which the coarse particles are returned by gravity via a tube into the mill. The finished product is trapped in a cyclone while the air is returned in the fan.
The ball mill is a tumbling mill that uses steel balls as the grinding media. The length of the cylindrical shell is usually 11.5 times the shell diameter (Figure 8.11). The feed can be dry, with less than 3% moisture to minimize ball coating, or slurry containing 2040% water by weight. Ball mills are employed in either primary or secondary grinding applications. In primary applications, they receive their feed from crushers, and in secondary applications, they receive their feed from rod mills, AG mills, or SAG mills.
Ball mills are filled up to 40% with steel balls (with 3080mm diameter), which effectively grind the ore. The material that is to be ground fills the voids between the balls. The tumbling balls capture the particles in ball/ball or ball/liner events and load them to the point of fracture.
When hard pebbles rather than steel balls are used for the grinding media, the mills are known as pebble mills. As mentioned earlier, pebble mills are widely used in the North American taconite iron ore operations. Since the weight of pebbles per unit volume is 3555% of that of steel balls, and as the power input is directly proportional to the volume weight of the grinding medium, the power input and capacity of pebble mills are correspondingly lower. Thus, in a given grinding circuit, for a certain feed rate, a pebble mill would be much larger than a ball mill, with correspondingly a higher capital cost. However, the increase in capital cost is justified economically by a reduction in operating cost attributed to the elimination of steel grinding media.
In general, ball mills can be operated either wet or dry and are capable of producing products in the order of 100m. This represents reduction ratios of as great as 100. Very large tonnages can be ground with these ball mills because they are very effective material handling devices. Ball mills are rated by power rather than capacity. Today, the largest ball mill in operation is 8.53m diameter and 13.41m long with a corresponding motor power of 22MW (Toromocho, private communications).
Modern ball mills consist of two chambers separated by a diaphragm. In the first chamber the steel-alloy balls (also described as charge balls or media) are about 90mm diameter. The mill liners are designed to lift the media as the mill rotates, so the comminution process in the first chamber is dominated by crushing. In the second chamber the ball diameters are of smaller diameter, between 60 and 15mm. In this chamber the lining is typically a classifying lining which sorts the media so that ball size reduces towards the discharge end of the mill. Here, comminution takes place in the rolling point-contact zone between each charge ball. An example of a two chamber ball mill is illustrated in Fig. 2.22.15
Much of the energy consumed by a ball mill generates heat. Water is injected into the second chamber of the mill to provide evaporative cooling. Air flow through the mill is one medium for cement transport but also removes water vapour and makes some contribution to cooling.
Grinding is an energy intensive process and grinding more finely than necessary wastes energy. Cement consists of clinker, gypsum and other components mostly more easily ground than clinker. To minimise over-grinding modern ball mills are fitted with dynamic separators (otherwise described as classifiers or more simply as separators). The working principle is that cement is removed from the mill before over-grinding has taken place. The cement is then separated into a fine fraction, which meets finished product requirements, and a coarse fraction which is returned to mill inlet. Recirculation factor, that is, the ratio of mill throughput to fresh feed is up to three. Beyond this, efficiency gains are minimal.
For more than 50years vertical mills have been the mill of choice for grinding raw materials into raw meal. More recently they have become widely used for cement production. They have lower specific energy consumption than ball mills and the separator, as in raw mills, is integral with the mill body.
In the Loesche mill, Fig. 2.23,16 two pairs of rollers are used. In each pair the first, smaller diameter, roller stabilises the bed prior to grinding which takes place under the larger roller. Manufacturers use different technologies for bed stabilisation.
Comminution in ball mills and vertical mills differs fundamentally. In a ball mill, size reduction takes place by impact and attrition. In a vertical mill the bed of material is subject to such a high pressure that individual particles within the bed are fractured, even though the particles are very much smaller than the bed thickness.
Early issues with vertical mills, such as narrower PSD and modified cement hydration characteristics compared with ball mills, have been resolved. One modification has been to install a hot gas generator so the gas temperature is high enough to partially dehydrate the gypsum.
For many decades the two-compartment ball mill in closed circuit with a high-efficiency separator has been the mill of choice. In the last decade vertical mills have taken an increasing share of the cement milling market, not least because the specific power consumption of vertical mills is about 30% less than that of ball mills and for finely ground cement less still. The vertical mill has a proven track record in grinding blastfurnace slag, where it has the additional advantage of being a much more effective drier of wet feedstock than a ball mill.
The vertical mill is more complex but its installation is more compact. The relative installed capital costs tend to be site specific. Historically the installed cost has tended to be slightly higher for the vertical mill.
Special graph paper is used with lglg(1/R(x)) on the abscissa and lg(x) on the ordinate axes. The higher the value of n, the narrower the particle size distribution. The position parameter is the particle size with the highest mass density distribution, the peak of the mass density distribution curve.
Vertical mills tend to produce cement with a higher value of n. Values of n normally lie between 0.8 and 1.2, dependent particularly on cement fineness. The position parameter is, of course, lower for more finely ground cements.
Separator efficiency is defined as specific power consumption reduction of the mill open-to-closed-circuit with the actual separator, compared with specific power consumption reduction of the mill open-to-closed-circuit with an ideal separator.
As shown in Fig. 2.24, circulating factor is defined as mill mass flow, that is, fresh feed plus separator returns. The maximum power reduction arising from use of an ideal separator increases non-linearly with circulation factor and is dependent on Rf, normally based on residues in the interval 3245m. The value of the comminution index, W, is also a function of Rf. The finer the cement, the lower Rf and the greater the maximum power reduction. At C = 2 most of maximum power reduction is achieved, but beyond C = 3 there is very little further reduction.
Separator particle separation performance is assessed using the Tromp curve, a graph of percentage separator feed to rejects against particle size range. An example is shown in Fig. 2.25. Data required is the PSD of separator feed material and of rejects and finished product streams. The bypass and slope provide a measure of separator performance.
The particle size is plotted on a logarithmic scale on the ordinate axis. The percentage is plotted on the abscissa either on a linear (as shown here) or on a Gaussian scale. The advantage of using the Gaussian scale is that the two parts of the graph can be approximated by two straight lines.
The measurement of PSD of a sample of cement is carried out using laser-based methodologies. It requires a skilled operator to achieve consistent results. Agglomeration will vary dependent on whether grinding aid is used. Different laser analysis methods may not give the same results, so for comparative purposes the same method must be used.
The ball mill is a cylindrical drum (or cylindrical conical) turning around its horizontal axis. It is partially filled with grinding bodies: cast iron or steel balls, or even flint (silica) or porcelain bearings. Spaces between balls or bearings are occupied by the load to be milled.
Following drum rotation, balls or bearings rise by rolling along the cylindrical wall and descending again in a cascade or cataract from a certain height. The output is then milled between two grinding bodies.
Ball mills could operate dry or even process a water suspension (almost always for ores). Dry, it is fed through a chute or a screw through the units opening. In a wet path, a system of scoops that turn with the mill is used and it plunges into a stationary tank.
Mechanochemical synthesis involves high-energy milling techniques and is generally carried out under controlled atmospheres. Nanocomposite powders of oxide, nonoxide, and mixed oxide/nonoxide materials can be prepared using this method. The major drawbacks of this synthesis method are: (1) discrete nanoparticles in the finest size range cannot be prepared; and (2) contamination of the product by the milling media.
More or less any ceramic composite powder can be synthesized by mechanical mixing of the constituent phases. The main factors that determine the properties of the resultant nanocomposite products are the type of raw materials, purity, the particle size, size distribution, and degree of agglomeration. Maintaining purity of the powders is essential for avoiding the formation of a secondary phase during sintering. Wet ball or attrition milling techniques can be used for the synthesis of homogeneous powder mixture. Al2O3/SiC composites are widely prepared by this conventional powder mixing route by using ball milling . However, the disadvantage in the milling step is that it may induce certain pollution derived from the milling media.
In this mechanical method of production of nanomaterials, which works on the principle of impact, the size reduction is achieved through the impact caused when the balls drop from the top of the chamber containing the source material.
A ball mill consists of a hollow cylindrical chamber (Fig. 6.2) which rotates about a horizontal axis, and the chamber is partially filled with small balls made of steel, tungsten carbide, zirconia, agate, alumina, or silicon nitride having diameter generally 10mm. The inner surface area of the chamber is lined with an abrasion-resistant material like manganese, steel, or rubber. The magnet, placed outside the chamber, provides the pulling force to the grinding material, and by changing the magnetic force, the milling energy can be varied as desired. The ball milling process is carried out for approximately 100150h to obtain uniform-sized fine powder. In high-energy ball milling, vacuum or a specific gaseous atmosphere is maintained inside the chamber. High-energy mills are classified into attrition ball mills, planetary ball mills, vibrating ball mills, and low-energy tumbling mills. In high-energy ball milling, formation of ceramic nano-reinforcement by in situ reaction is possible.
It is an inexpensive and easy process which enables industrial scale productivity. As grinding is done in a closed chamber, dust, or contamination from the surroundings is avoided. This technique can be used to prepare dry as well as wet nanopowders. Composition of the grinding material can be varied as desired. Even though this method has several advantages, there are some disadvantages. The major disadvantage is that the shape of the produced nanoparticles is not regular. Moreover, energy consumption is relatively high, which reduces the production efficiency. This technique is suitable for the fabrication of several nanocomposites, which include Co- and Cu-based nanomaterials, Ni-NiO nanocomposites, and nanocomposites of Ti,C .
Planetary ball mill was used to synthesize iron nanoparticles. The synthesized nanoparticles were subjected to the characterization studies by X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques using a SIEMENS-D5000 diffractometer and Hitachi S-4800. For the synthesis of iron nanoparticles, commercial iron powder having particles size of 10m was used. The iron powder was subjected to planetary ball milling for various period of time. The optimum time period for the synthesis of nanoparticles was observed to be 10h because after that time period, chances of contamination inclined and the particles size became almost constant so the powder was ball milled for 10h to synthesize nanoparticles . Fig. 12 shows the SEM image of the iron nanoparticles.
The vibratory ball mill is another kind of high-energy ball mill that is used mainly for preparing amorphous alloys. The vials capacities in the vibratory mills are smaller (about 10 ml in volume) compared to the previous types of mills. In this mill, the charge of the powder and milling tools are agitated in three perpendicular directions (Fig. 1.6) at very high speed, as high as 1200 rpm.
Another type of the vibratory ball mill, which is used at the van der Waals-Zeeman Laboratory, consists of a stainless steel vial with a hardened steel bottom, and a single hardened steel ball of 6 cm in diameter (Fig. 1.7).
The mill is evacuated during milling to a pressure of 106 Torr, in order to avoid reactions with a gas atmosphere. Subsequently, this mill is suitable for mechanical alloying of some special systems that are highly reactive with the surrounding atmosphere, such as rare earth elements.
In spite of the traditional approaches used for gas-solid reaction at relatively high temperature, Calka etal. and El-Eskandarany etal. proposed a solid-state approach, the so-called reactive ball milling (RBM), used for preparations different families of meal nitrides and hydrides at ambient temperature. This mechanically induced gas-solid reaction can be successfully achieved, using either high- or low-energy ball-milling methods, as shown in Fig.9.5. However, high-energy ball mill is an efficient process for synthesizing nanocrystalline MgH2 powders using RBM technique, it may be difficult to scale up for matching the mass production required by industrial sector. Therefore, from a practical point of view, high-capacity low-energy milling, which can be easily scaled-up to produce large amount of MgH2 fine powders, may be more suitable for industrial mass production.
In both approaches but with different scale of time and milling efficiency, the starting Mg metal powders milled under hydrogen gas atmosphere are practicing to dramatic lattice imperfections such as twinning and dislocations. These defects are caused by plastics deformation coupled with shear and impact forces generated by the ball-milling media. The powders are, therefore, disintegrated into smaller particles with large surface area, where very clean or fresh oxygen-free active surfaces of the powders are created. Moreover, these defects, which are intensively located at the grain boundaries, lead to separate micro-scaled Mg grains into finer grains capable to getter hydrogen by the first atomically clean surfaces to form MgH2 nanopowders.
Fig.9.5 illustrates common lab scale procedure for preparing MgH2 powders, starting from pure Mg powders, using RBM via (1) high-energy and (2) low-energy ball milling. The starting material can be Mg-rods, in which they are processed via sever plastic deformation, using for example cold-rolling approach, as illustrated in Fig.9.5. The heavily deformed Mg-rods obtained after certain cold rolling passes can be snipped into small chips and then ball-milled under hydrogen gas to produce MgH2 powders.
Planetary ball mills are the most popular mills used in scientific research for synthesizing MgH2 nanopowders. In this type of mill, the ball-milling media have considerably high energy, because milling stock and balls come off the inner wall of the vial and the effective centrifugal force reaches up to 20 times gravitational acceleration. The centrifugal forces caused by the rotation of the supporting disc and autonomous turning of the vial act on the milling charge (balls and powders). Since the turning directions of the supporting disc and the vial are opposite, the centrifugal forces alternately are synchronized and opposite. Therefore, the milling media and the charged powders alternatively roll on the inner wall of the vial, and are lifted and thrown off across the bowl at high speed.
In the typical experimental procedure, a certain amount of the Mg (usually in the range between 3 and 10g based on the vials volume) is balanced inside an inert gas atmosphere (argon or helium) in a glove box and sealed together with certain number of balls (e.g., 2050 hardened steel balls) into a hardened steel vial (Fig.9.5A and B), using, for example, a gas-temperature-monitoring system (GST). With the GST system, it becomes possible to monitor the progress of the gas-solid reaction taking place during the RBM process, as shown in Fig.9.5C and D. The temperature and pressure changes in the system during milling can be also used to realize the completion of the reaction and the expected end product during the different stages of milling (Fig.9.5D). The ball-to-powder weight ratio is usually selected to be in the range between 10:1 and 50:1. The vial is then evacuated to the level of 103bar before introducing H2 gas to fill the vial with a pressure of 550bar (Fig.9.5B). The milling process is started by mounting the vial on a high-energy ball mill operated at ambient temperature (Fig.9.5C).
Tumbling mill is cylindrical shell (Fig.9.6AC) that rotates about a horizontal axis (Fig.9.6D). Hydrogen gas is pressurized into the vial (Fig.9.6C) together with Mg powders and ball-milling media, using ball-to-powder weight ratio in the range between 30:1 and 100:1. Mg powder particles meet the abrasive and impacting force (Fig.9.6E), which reduce the particle size and create fresh-powder surfaces (Fig.9.6F) ready to react with hydrogen milling atmosphere.
Figure 9.6. Photographs taken from KISR-EBRC/NAM Lab, Kuwait, show (A) the vial and milling media (balls) and (B) the setup performed to charge the vial with 50bar of hydrogen gas. The photograph in (C) presents the complete setup of GST (supplied by Evico-magnetic, Germany) system prior to start the RBM experiment for preparing of MgH2 powders, using Planetary Ball Mill P400 (provided by Retsch, Germany). GST system allows us to monitor the progress of RBM process, as indexed by temperature and pressure versus milling time (D).
The useful kinetic energy in tumbling mill can be applied to the Mg powder particles (Fig.9.7E) by the following means: (1) collision between the balls and the powders; (2) pressure loading of powders pinned between milling media or between the milling media and the liner; (3) impact of the falling milling media; (4) shear and abrasion caused by dragging of particles between moving milling media; and (5) shock-wave transmitted through crop load by falling milling media. One advantage of this type of mill is that large amount of the powders (100500g or more based on the mill capacity) can be fabricated for each milling run. Thus, it is suitable for pilot and/or industrial scale of MgH2 production. In addition, low-energy ball mill produces homogeneous and uniform powders when compared with the high-energy ball mill. Furthermore, such tumbling mills are cheaper than high-energy mills and operated simply with low-maintenance requirements. However, this kind of low-energy mill requires long-term milling time (more than 300h) to complete the gas-solid reaction and to obtain nanocrystalline MgH2 powders.
Figure 9.7. Photos taken from KISR-EBRC/NAM Lab, Kuwait, display setup of a lab-scale roller mill (1000m in volume) showing (A) the milling tools including the balls (milling media and vial), (B) charging Mg powders in the vial inside inert gas atmosphere glove box, (C) evacuation setup and pressurizing hydrogen gas in the vial, and (D) ball milling processed, using a roller mill. Schematic presentations show the ball positions and movement inside the vial of a tumbler mall mill at a dynamic mode is shown in (E), where a typical ball-powder-ball collusion for a low energy tumbling ball mill is presented in (F).
In discussions on high energy ball milling, the more generic term ball mills is often used in place of the terms stirred ball mills or Attritors, but the differences between the types of mills are quite distinct.
In addition, Attritors offer the advantage of being able to take samples at any time and making formulation adjustments during high energy ball milling. Alternatively, ball mills are closed systems and do not offer these advantages. Nevertheless, ball mills can be a more appropriate choice for grinding larger sized material starting at 1/4 or larger. It should be noted that ball mills are much larger than Attritors and therefore require a special foundation.
The Attritors design accounts for much of the difference: conventional ball mills turn the entire drum or tank containing the media and the material, while Attritors stir the media in a stationary tank with a shaft and attached arms or discs resulting in a more efficient use of energy for the milling process. Further, Attritor tanks are all jacketed which allows for more precise temperature control during high energy ball milling.
Ball milling is a grinding technique that uses media to effectively break down pigment agglomerates and aggregates to their primary particles. Using a rotor or disc impeller to create collisions of the grinding media, the impact and force created by the bead mills collisions break down the pigment agglomerates. The media can consist of either stainless steel, glass, or ceramic materials. The higher the bead hardness or density, the greater the collision force. The ball-milling process uses a higher concentration of grinding media to mill base in which the chambers are designed to maximize the energy transfer.
When a particle size has to be reduced below 10 microns, bead milling is the technique to use. However, if the material has a very low viscosity, ball milling is a better dispersing process than using a high shear mixing (vertical) system.
Currently, the VMA-Getzmann company offers three product lines for bead milling. They can be dedicated stand-alone systems or accessories that can be added to the high-speed vertical disperser models. Depending upon the model, sample quantities can be as low as 20 ml or up to 20,000 ml.
Our Dispermat SL model line is the current horizontal bead mill system. Milling chamber sizes can start at 50 ml to save on raw material costs. The beads are separated from the mill base by a dynamic gap system. The standard gap uses 1.0 mm diameter grinding media; an optional gap is available to use beads down to 0.3 mm diameter. The Dispermat SL can be selected to run as a single pass or as a recirculation configuration.
One of the unique features is an independent pumping system to feed the mill base into the milling chamber. Instead of the speed of the milling rotor controlling the sample volume the operator can control the volume, through the mixing system pump that fits on top of the milling chamber. Separating the rotor speed from the sample feed system provides more control over the milling process.
Basket bead milling is a relatively new design for ball milling applications. The grinding media is contained in a cylinder (basket), and the mill base is circulated through the basket. The VMA-Getzmann basket mill consists of a stainless-steel cylinder with an opening at the top and a sieve filter on the bottom. The standard diameter size of the grinding media is 1.0 mm. however, it can be ordered to use 0.3 mm bead size.
Since the Getzmann basket mill is attached to aHigh-Speed Dispersermodel, those with an adapter allow the user to switch between the basket mill system and a motor shaft for high-shear dispersing easily.
Attached to the bottom of the basket is a cowles blade that rotates at high speed. The purpose of the cowles blade is to circulate the mill base to ensure all materials enter the basket mill. When you have created the desired particle size, the basket mill is then raised out of the sample container, while the grinding media stays in the basket.
The third system for ball milling applications is the APS (air pressure system). The APS is attached to a high shear disperser. It consists of a sample containerwith a sieve filter at the bottom, a stand to elevate the sample container, along with a sealing system around the motor shaft, and a container lid. The mill base and grinding media is mixed 50/50 in the container. Adisk impeller or pearl mill impelleris immersed into the mixture and rotated from 500 to 5000 RPMs depending on the desired particle size. After the dispersion is completed the stop cock that covers the sieve filter is removed, the lid is clamped tight over the vessel. The lid has an air connection; the air is applied to force the sample through the sieve filter separating the mill base from the grinding media. Aside from the ability to produce small quantities of less than 25 milliliters, another advantage of the APS system is their ease of cleaning.Get in Touch with Mechanic