CNC milling is one of the most common processes when looking to produce complex parts. Why complex? Whenever other fabrication methods like laser or plasma cutting can get the same results, it is cheaper to go with them. But these two do not provide anything similar to the capabilities of CNC milling.
So, we are going to take a deep dive into milling, looking at the various aspects of the process itself as well as the machinery. This will help you understand if you require CNC milling services to produce your parts or is there a more cost-effective alternative available.
We are going to look at the process, machinery, etc. in later paragraphs. But lets first make clear what CNC milling means and bring clarity to some of the more confusing points about the term itself.
First, people often ask for CNC machining when looking for milling. Machining entails both milling and turning but these two have distinct differences. Machining refers to a mechanical cutting technology that uses physical contact to remove material, using a wide range of tools.
Secondly, all CNC machining uses CNC machines but not all CNC machines are for machining. Computer numerical control is what lies behind these three letters. Any machine using CNC utilises computerised systems for automating the cutting process.
So CNC machining is a mix of these two terms, bringing us the answer to the question posed in the heading. CNC milling is a substractive fabrication method that uses computer numerical controls systems for automating the process.
G code can also be written manually, as was done in the past. This, however, prolongs the whole process considerably. Therefore, we would suggest making full use of the possibilities modern engineering software offers.
Although CNC machines do the cutting work automatically, many other aspects of the process need a machine operators hand. For example, fixing the workpiece to the worktable as well as attaching the milling tools to the spindle of the machine.
Manual milling depends heavily on the operators while newer models have more advanced automation systems. Modern milling centres may also have live tooling possibilities. This means they can change the tools on the go during the manufacturing process. So there are fewer stops but someone still has to set them up beforehand.
The milling process usually comprises of a few different operations but this depends on the shape of the final product and the state of the raw piece. Often, milling is necessary for giving a precise finish and adding a few features like slots or threaded holes.
But it is also suitable for creating a finished part from a block of material. The first operations use larger tools to quickly carve away the material to fasten the process until getting to an approximate shape of the final piece.
A tool change is necessary to create the highly accurate machined parts. The great precision milling is known for is achieved in the last stage, taking engineering tolerances and surface roughness to levels hard to match with any other fabrication process.
Now, lets see what makes up a milling machine. While new milling centres have the capabilities to perform all kinds of operations, they are also more complicated. So we are sticking to the more traditional benches here to give an overview of the machine components.
Horizontal milling machines derive their name from the positioning of the tool their axis lies horizontally. The images above show one way of using them which is plain milling. Of course, horizontal mills are also suitable for end milling.
Of course, newer machines look a little different, making them suitable for automation. Horizontal milling centres may have several spindles with a variety of tools on them for quicker turnaround times. Also, the table as well as the tools can move in more directions, including rotational axes.
There are a few differences between horizontal and vertical milling machines. But the main components are still similar. The machine head is at the end of a ram. The spindle for cutting tools is attached to the head.
The modern 5-axis vertical mills offer possibilities to rotate the part for more access and faster turnaround times. Automating all the movements results in better accuracy, quicker lead times and close to identical batches of parts.
There is a lot of variety available on the market today. Numerous ways of classification also exist. The basics remain pretty much the same everywhere, with a few modifications bringing about more possibilities and hence another type of milling machine.
The design of bed-type milling machines includes a stable machine bed. While large and heavy parts can result in instability with knee-type machines, bed-type ones can hold their ground. The long bed means that multiple parts can be attached onto the bed at once, diminishing idle times and increasing the efficiency on the workfloor.
The worktable attaches directly to the bed of the machine and can move in 2 directions. The spindle head, of course, can move axially to determine the cut depth. The position of the axis depends on the machine, as there are both horizontal and vertical bed mills, as well as universal machines. All of them can also be automated by using CNC.
Another way to increase productivity is using a two-machine stand. This helps to either mount a number of parts onto the table for simultaneous processing or one large part. This loses the necessity for re-clamping it to process the other end. It is important to note that this setup opens the possibility to tool collision which can be prevented by a correct CNC program.
These machines are suitable for producing parts ranging from small to medium size. The limitation comes from the fact that knee-type mills provide less stability than, for example, bed-type milling machines. Also, the frame sets its own limits for part dimensions.
A traditional knee-type mill is a great option for producing one-off parts, maintenance work, preparatory tasks, etc. The unidirectional movement of the cutting head limits the possibility of accidents. Using them for preparing the workpiece for later refining on a CNC station is common.
These machines require a manual change of tools after every operation, making the whole process a little slower. Still, modern CNC machining centres include the capabilities of knee-type milling machines.
The ram-type mill has its cutting head mounted on a ram that can slide back-and-forth. This increases the tool movement to 2 axes X and Y. Both horizontal and vertical options of the ram mill are available on the market. Many of such mills also include the ability for swivelling the cutting head.
Planer-type mills are very similar to bed-type milling machines. Both have large worktables and spindles that can move in 3 directions. The main difference comes from the planer-type milling machines ability to accommodate more milling tools at once. The number of different tools usually goes up to 4.
A 3-axis vertical mill means that the table can move in 2 directions X and Y. This enables positioning the workpiece relative to the cutting tool while the distance remains the same. So the third, Z-axis, is added by allowing to lower the cutting tool.
A 4-axis mill has all the 3 axes as previously described. But another one comes in the A-axis. Now the table can rotate around the X-axis, allowing face milling the sides without repositioning of the workpiece.
5-axis CNC machining centres cost a lot more than the other options but make it possible to produce very complex parts in one go. No extra setups are necessary while the tool life increases through making the suitable part positioning possible.
6-axis CNC milling centres are not too common because of the hefty price-tag. They can be up to 75% quicker than 5-axis machines but the necessity of such capabilities is rare enough to justify the expenses. The video above also shows a comparison of a 5-axis and 6-axis mill.
Milling is suitable for many different features, including threading, chamfering, slotting, etc. This allows for producing complex designs on a single CNC milling centre with enviable accuracy. The tolerances for CNC machining are around +/- 0.1 mm.
Surface milling can use different cutters, wide or narrow, depending on the necessary outcome. Using a wide cutter can result in fast material removal when coupled with slow cutting speed, fast feed rate and coarse teeth of the cutter. Of course, the surface finish of such cutting may not meet the requirements.
Therefore, a second step can include a change of tools to use finer teeth. This also requires faster cutting speeds and slower feed rates, so the amount of material removal per time unit is slower. At the same time, the final finish is more accurate. Thus, the combination of the two makes for a good choice from an economic standpoint.
Face milling often comes after surface milling, as it can produce more intricate contours and leaves a nice finish. The teeth on the sides do most of the cutting work while the teeth on the tip take care of the surface finish.
In case of a regular 3-axis mill, the use of different cutters makes the most sense. These can be dovetail cutters to produce angled grooves or just a mill with a conical cutting head for chamfering. Note that these two are basically the opposites of each other.
Form milling helps to create these surface contours in a single cut. The tools can help create round recesses, round edges, etc. The tools must have the right parameters to achieve the desired outcome.
First comes gear milling. The material softness enables creating the part with more ease while achieving great tolerances. The gears then go through a heat treatment process to harden the surface. After that, CNC turning will be responsible for the final outcome.
Milling is the most common form of machining, a material removal process, which can create a variety of features on a part by cutting away the unwanted material. The milling process requires a milling machine, workpiece, fixture, and cutter. The workpiece is a piece of pre-shaped material that is secured to the fixture, which itself is attached to a platform inside the milling machine. The cutter is a cutting tool with sharp teeth that is also secured in the milling machine and rotates at high speeds. By feeding the workpiece into the rotating cutter, material is cut away from this workpiece in the form of small chips to create the desired shape. Milling is typically used to produce parts that are not axially symmetric and have many features, such as holes, slots, pockets, and even three dimensional surface contours. Parts that are fabricated completely through milling often include components that are used in limited quantities, perhaps for prototypes, such as custom designed fasteners or brackets. Another application of milling is the fabrication of tooling for other processes. For example, three-dimensional molds are typically milled. Milling is also commonly used as a secondary process to add or refine features on parts that were manufactured using a different process. Due to the high tolerances and surface finishes that milling can offer, it is ideal for adding precision features to a part whose basic shape has already been formed.
The time required to produce a given quantity of parts includes the initial setup time and the cycle time for each part. The setup time is composed of the time to setup the milling machine, plan the tool movements (whether performed manually or by machine), and install the fixture device into the milling machine. The cycle time can be divided into the following four times:
Following the milling process cycle, there is no post processing that is required. However, secondary processes may be used to improve the surface finish of the part if it is required. The scrap material, in the form of small material chips cut from the workpiece, is propelled away from the workpiece by the motion of the cutter and the spraying of lubricant. Therefore, no process cycle step is required to remove the scrap material, which can be collected and discarded after the production. Cutting parameters In milling, the speed and motion of the cutting tool is specified through several parameters. These parameters are selected for each operation based upon the workpiece material, tool material, tool size, and more.
During the process cycle, a variety of operations may be performed to the workpiece to yield the desired part shape. The following operations are each defined by the type of cutter used and the path of that cutter to remove material from the workpiece.
Milling machines can be found in a variety of sizes and designs, yet they still possess the same main components that enable the workpiece to be moved in three directions relative to the tool. These components include the following:
The above components of the milling machine can be oriented either vertically or horizontally, creating two very distinct forms of milling machine. A horizontal milling machine uses a cutter that is mounted on a horizontal shaft, called an arbor, above the workpiece. For this reason, horizontal milling is sometimes referred to as arbor milling. The arbor is supported on one side by an overarm, which is connected to the column, and on the other side by the spindle. The spindle is driven by a motor and therefore rotates the arbor. During milling, the cutter rotates along a horizontal axis and the side of the cutter removes material from the workpiece. A vertical milling machine, on the other hand, orients the cutter vertically. The cutter is secured inside a piece called a collet, which is then attached to the vertically oriented spindle. The spindle is located inside the milling head, which is attached to the column. The milling operations performed on a vertical milling machine remove material by using both the bottom and sides of the cutter. Milling machines can also be classified by the type of control that is used. A manual milling machine requires the operator to control the motion of the cutter during the milling operation. The operator adjusts the position of the cutter by using hand cranks that move the table, saddle, and knee. Milling machines are also able to be computer controlled, in which case they are referred to as a computer numerical control (CNC) milling machine. CNC milling machines move the workpiece and cutter based on commands that are preprogrammed and offer very high precision. The programs that are written are often called G-codes or NC-codes. Many CNC milling machines also contain another axis of motion besides the standard X-Y-Z motion. The angle of the spindle and cutter can be changed, allowing for even more complex shapes to be milled.
The tooling that is required for milling is a sharp cutter that will be rotated by the spindle. The cutter is a cylindrical tool with sharp teeth spaced around the exterior. The spaces between the teeth are called flutes and allow the material chips to move away from the workpiece. The teeth may be straight along the side of the cutter, but are more commonly arranged in a helix. The helix angle reduces the load on the teeth by distributing the forces. Also, the number of teeth on a cutter varies. A larger number of teeth will provide a better surface finish. The cutters that can be used for milling operations are highly diverse, thus allowing for the formation of a variety of features. While these cutters differ greatly in diameter, length, and by the shape of the cut they will form, they also differ based upon their orientation, whether they will be used horizontally or vertically. A cutter that will be used in a horizontal milling machine will have the teeth extend along the entire length of the tool. The interior of the tool will be hollow so that it can be mounted onto the arbor. With this basic form, there are still many different types of cutters that can be used in horizontal milling, including those listed below.
Another operation known as a straddle milling is also possible with a horizontal milling machine. This form of milling refers to the use of multiple cutters attached to the arbor and used simultaneously. Straddle milling can be used to form a complex feature with a single cut. For vertical milling machines, the cutters take a very different form. The cutter teeth cover only a portion of the tool, while the remaining length is a smooth surface, called the shank. The shank is the section of the cutter that is secured inside the collet, for attachment to the spindle. Also, many vertical cutters are designed to cut using both the sides and the bottom of the cutter. Listed below are several common vertical cutters.
All cutters that are used in milling can be found in a variety of materials, which will determine the cutter's properties and the workpiece materials for which it is best suited. These properties include the cutter's hardness, toughness, and resistance to wear. The most common cutter materials that are used include the following:
The material of the cutter is chosen based upon a number of factors, including the material of the workpiece, cost, and tool life. Tool life is an important characteristic that is considered when selecting a cutter, as it greatly affects the manufacturing costs. A short tool life will not only require additional tools to be purchased, but will also require time to change the tool each time it becomes too worn. The cutters listed above often have the teeth coated with a different material to provide additional wear resistance, thus extending the life of the tool. Tool wear can also be reduced by spraying a lubricant and/or coolant on the cutter and workpiece during milling. This fluid is used to reduce the temperature of the cutter, which can get quite hot during milling, and reduce the friction at the interface between the cutter and the workpiece, thus increasing the tool life. Also, by spraying a fluid during milling, higher feed rates can be used, the surface finish can be improved, and the material chips can be pushed away. Typical cutting fluids include mineral, synthetic, and water soluble oils.
In milling, the raw form of the material is a piece of stock from which the workpieces are cut. This stock is available in a variety of shapes such as flat sheets, solid bars (rectangular, cylindrical, hexagonal, etc.), hollow tubes (rectangular, cylindrical, etc.), and shaped beams (I-beams, L-beams, T-beams, etc.). Custom extrusions or existing parts such as castings or forgings are also sometimes used.
When selecting a material, several factors must be considered, including the cost, strength, resistance to wear, and machinability. The machinability of a material is difficult to quantify, but can be said to posses the following characteristics:
The material cost is determined by the quantity of material stock that is required and the unit price of that stock. The amount of stock is determined by the workpiece size, stock size, method of cutting the stock, and the production quantity. The unit price of the material stock is affected by the material and the workpiece shape. Also, any cost attributed to cutting the workpieces from the stock also contributes to the total material cost.
The production cost is a result of the total production time and the hourly rate. The production time includes the setup time, load time, cut time, idle time, and tool replacement time. Decreasing any of these time components will reduce cost. The setup time and load time are dependent upon the skill of the operator. The cut time, however, is dependent upon many factors that affect the cut length and feed rate. The cut length can be shortened by optimizing the number of operations that are required and reducing the feature size if possible. The feed rate is affected by the operation type, workpiece material, tool material, tool size, and various cutting parameters such as the axial depth of cut. Lastly, the tool replacement time is a direct result of the number of tool replacements which is discussed regarding the tooling cost.
The tooling cost for machining is determined by the total number of cutting tools required and the unit price for each tool. The quantity of tools depends upon the number of unique tools required by the various operations to be performed and the amount of wear that each of those tools experience. If the tool wear exceeds the lifetime of a tool, then a replacement tool must be purchased. The lifetime of a tool is dependant upon the tool material, cutting parameters such as cutting speed, and the total cut time. The unit price of a tool is affected by the tool type, size, and material.
The milling process industry is made up of all manufacturers that are involved in process technology that is used for breaking down, separating, sizing or classifying bulk materials. Milling processes can also be used to remove contamination or water/moisture from bulk materials to produce dry particles prior to transport.
Milling allows you to control the particle size of your product in the manufacturing process. Particle size consistency enhances product flow rate and reduces the size, which can assist in the downstream process, depending on the product goals.
A milling process refers to the complete process of cleaning, grading, breaking down, sizing, separating or classifying dry bulk materials. The actual milling process involves the mill that is used to break solid bulk materials into smaller pieces by grinding, cutting or crushing.
Dry grinding creates a lot of dust that results in the need for proper filtering systems. This can be of specific importance during grinding of substances whose dust, when mixed with aerial oxygen, can result in explosive mixtures. Elaborate technical solutions such as systems for dust removal and inert gas overlay are often necessary to achieve the required level of safety. The cost for these filtration machines often significantly exceeds that of the actual mill.
Wet milling, also called wet media milling, is a process in which particles are dispersed in a liquid by shearing, by impact or crushing, or by attrition. Wet milling can be a more intensive process, but it can reduce a bulk product into more components, and into finer particles in the micron & submicron (or nanometer) particle size range.
The milling process industry is made up of all manufacturers that are involved in process technology that is used for breaking down, separating, sizing or classifying bulk materials. Milling processes can also be used to remove contamination or water/moisture from bulk materials to produce dry particles prior to transport.
Milling allows you to control the particle size of your product in the manufacturing process. Particle size consistency enhances product flow rate and reduces the size, which can assist in the downstream process, depending on the product goals.
Corn wet milling and dry milling are the most important methods of processing corn and each method produces distinct co-products. Thecorn wet-milling processis designed to extract the highest use and value from each component of the corn kernel.Wet milling involves soaking or steeping whole corn to soften the kernels, after which further processing separates components used in a variety of products. In general, ethanol is not the primary focus for wet milling processors.
The corn dry-milling process is a less versatile, less capital-intensive process that focuses primarily on the production of grain ethanol. In this process, the corn kernels are hammer milled into a medium-to-fine grind meal for adding to the ethanol production process.
It is very important to understand which performance can be achieved by a particular type of mill, and for which type of bulk solids. It will allow the process engineer to properly select a mill type for new process, or perform troubleshooting on an existing process.
There are many milling process innovations and new technologies available in global processing industries. Some of them present small developments in available machine models; some present agricultural innovations and some present revolutionary technologies.
Milling is a process performed with a machine in which the cutters rotate to remove the material from the work piece present in the direction of the angle with the tool axis. With the help of the milling machines one can perform many operations and functions starting from small objects to large ones.
Milling machine is one of the important machining operations. In this operation the workpiece is fed against a rotating cylindrical tool. The rotating tool consists of multiple cutting edges (multipoint cutting tool). Normally axis of rotation of feed given to the workpiece. Milling operation is distinguished from other machining operations on the basis of orientation between the tool axis and the feed direction, however, in other operations like drilling, turning, etc. the tool is fed in the direction parallel to axis of rotation.
The cutting tool used in milling operation is called milling cutter, which consists of multiple edges called teeth. The machine tool that performs the milling operations by producing required relative motion between workpiece and tool is called milling machine. It provides the required relative motion under very controlled conditions. These conditions will be discussed later in this unit as milling speed, feed rate and depth of cut.
Normally, the milling operation creates plane surfaces. Other geometries can also be created by milling machine. Milling operation is considered an interrupted cutting operation teeth of milling cutter enter and exit the work during each revolution. This interrupted cutting action subjects the teeth to a cycle of impact force and thermal shock on every rotation. The tool material and cutter geometry must be designed to bear the above stated conditions. Depending upon the positioning of the tool and workpiece the milling operation can be classified into different types.
Milling Cutters There are a lot of cutting tools used in the milling process. The milling cutters named end mills have special cutting surfaces on their end surfaces so that they can be placed onto the work piece by drilling. These also have extended cutting surfaces on each side for the purpose of peripheral milling. The milling cutters have small cutters at the end corners. The cutters are made from highly resistant materials that are durable and produce less friction.
Surface Finish Any material put through the cutting area of the milling machine gets regular intervals. The side cutters have got regular ridges on them. The distance between the ridges depends on the feed rate, the diameter of the cutter and the quantity of cutting surfaces. These can be the significant variations in the height of the surfaces.
Gang Milling This means that more than two milling cutters are involved in a setup like the horizontal milling. All the cutters perform a uniform operation or it may also be possible that the cutter may perform distinct operations. This is an important operation for producing duplicate parts.
Milling is a metal removal process by means of using a rotating cutter having one or more cutting teeth as illustrated in figure Cutting action is carried out by feeding the workpiece against the rotating cutter. Thus, the spindle speed, the table feed, the depth of cut, and the rotating direction of the cutter become the main parameters of the process. Good results can only be achieved with a well balanced settings of these parameters.
Feed Rate It is the rate with which the workpiece under process advances under the revolving milling cutter. It is known that revolving cutter remains stationary and feed is given to the workpiece through worktable. Generally feed is expressed in three ways.
Depth of Cut Depth of cut in milling operation is the measure of penetration of cutter into the workpiece. It is thickness of the material removed in one pairs of cutter under process. One pairs of cutter means when cutter completes the milling operation from one end of the workpiece to another end. In other words, it is the perpendicular distance measured between the original and final surface of workpiece. It is measured in mm.
Owing to the variety of shapes possible and its high production rates, milling is one of the most versatile and widely used machining operations. The geometric form created by milling fall into three major groups:
Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.
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Milling is one of the processes by chip removal machining. The milling process requires a basic equipment. Basic equipment consists of milling machine, work-piece, fixture and cutter. The most important element in the process of milling is necessary machinery. This machine is called milling. There are different types of milling. The routers according to tool orientation can be horizontal, vertical or universal. The special milling can be circular, copiers, gantry or mobile bridge. The routers according to the number of axes can be three, four and five axis.
In Gestin de Compras we have the necessary means to produce a wide range of parts made by milling, always with the best quality and the most competitive prices. It is possible because we work with companies that have all necessary equipment, from manually milling machines to 5-axis CNC milling machines, to adapt efficiently production means to get the desired part.
Milling is an important process of manufacturing technology and basically it refers to the removal of metal from the work piece using a tool which has several cutting points and is rotating about its axis. Thus each cutting point removes a little bit of the metal but since there are multiple such points and the tool is rotating at a fast speed, the overall removal is quite brisk.
The main advantage of the milling machine is that it can be used to perform literally any operation with a great degree of accuracy and hence it is an indispensable machine for any workshop worth its salt.Milling machines come in a wide variety of flavours such as universal, horizontal, vertical, drum type to name a few. Each of these have their own unique features and used for various operations. We wont go in these details in this article but will try to see what sort of milling methods can be deployed using these machines as follows.Methods of MillingGiven below are few of the most commonly used milling methods in the manufacturing industry. This list is certainly not exhaustive but only indicative of the wide variety of uses that a milling machine can be put to in the industry.Single piece milling this method is used for milling a single job work which is held on the milling machine. It is important to note that the piece has to be worked in a single machine cycle for it to be classified under this category of milling methods.String milling is quite similar to single piece milling but the only difference being that instead of a single piece there are several parts which is simultaneously fixed and are worked upon.Index milling refers to a special kind of milling operation wherein the machine is set to perform identical operations on a work piece. Each of these identical operations is performed one after the other by indexing the work piece into a new position. A very good example of such a process is the gear cutting operation wherein the gear grooves are cut in succession one after the other.Copy milling technique refers to where a design or cut is difficult to make by itself, hence it is first made in the form of a master template which is turn is used to guide the tool path. Hence basically the template acts as an original from which the other parts are copied just like you would photocopy a document from an original paper in the Xerox machine.Gang milling refers to the situation when a gang or group of cutters is used to simultaneously work on the work piece in order to produce the desired shapeReciprocal milling refers to the process wherein the time taken to load and upload the job work from the fixture on the milling table is minimized by utilizing two fixtures which are ready at the same time for milling one after the other.There are several other types of milling methods apart from these and each of these methods have their own unique features. The reader is advised to refer to any good textbook of Production Technology in case he/she is interested to gain advanced knowledge about this area of manufacturing technologyImage CreditsThe Hong Kong Polytechnic University: Machining and Metrology Unit Website
Milling machines come in a wide variety of flavours such as universal, horizontal, vertical, drum type to name a few. Each of these have their own unique features and used for various operations. We wont go in these details in this article but will try to see what sort of milling methods can be deployed using these machines as follows.
Given below are few of the most commonly used milling methods in the manufacturing industry. This list is certainly not exhaustive but only indicative of the wide variety of uses that a milling machine can be put to in the industry.
Single piece milling this method is used for milling a single job work which is held on the milling machine. It is important to note that the piece has to be worked in a single machine cycle for it to be classified under this category of milling methods.String milling is quite similar to single piece milling but the only difference being that instead of a single piece there are several parts which is simultaneously fixed and are worked upon.Index milling refers to a special kind of milling operation wherein the machine is set to perform identical operations on a work piece. Each of these identical operations is performed one after the other by indexing the work piece into a new position. A very good example of such a process is the gear cutting operation wherein the gear grooves are cut in succession one after the other.Copy milling technique refers to where a design or cut is difficult to make by itself, hence it is first made in the form of a master template which is turn is used to guide the tool path. Hence basically the template acts as an original from which the other parts are copied just like you would photocopy a document from an original paper in the Xerox machine.Gang milling refers to the situation when a gang or group of cutters is used to simultaneously work on the work piece in order to produce the desired shapeReciprocal milling refers to the process wherein the time taken to load and upload the job work from the fixture on the milling table is minimized by utilizing two fixtures which are ready at the same time for milling one after the other.There are several other types of milling methods apart from these and each of these methods have their own unique features. The reader is advised to refer to any good textbook of Production Technology in case he/she is interested to gain advanced knowledge about this area of manufacturing technologyImage CreditsThe Hong Kong Polytechnic University: Machining and Metrology Unit Website
String milling is quite similar to single piece milling but the only difference being that instead of a single piece there are several parts which is simultaneously fixed and are worked upon.Index milling refers to a special kind of milling operation wherein the machine is set to perform identical operations on a work piece. Each of these identical operations is performed one after the other by indexing the work piece into a new position. A very good example of such a process is the gear cutting operation wherein the gear grooves are cut in succession one after the other.Copy milling technique refers to where a design or cut is difficult to make by itself, hence it is first made in the form of a master template which is turn is used to guide the tool path. Hence basically the template acts as an original from which the other parts are copied just like you would photocopy a document from an original paper in the Xerox machine.Gang milling refers to the situation when a gang or group of cutters is used to simultaneously work on the work piece in order to produce the desired shapeReciprocal milling refers to the process wherein the time taken to load and upload the job work from the fixture on the milling table is minimized by utilizing two fixtures which are ready at the same time for milling one after the other.There are several other types of milling methods apart from these and each of these methods have their own unique features. The reader is advised to refer to any good textbook of Production Technology in case he/she is interested to gain advanced knowledge about this area of manufacturing technologyImage CreditsThe Hong Kong Polytechnic University: Machining and Metrology Unit Website
Index milling refers to a special kind of milling operation wherein the machine is set to perform identical operations on a work piece. Each of these identical operations is performed one after the other by indexing the work piece into a new position. A very good example of such a process is the gear cutting operation wherein the gear grooves are cut in succession one after the other.Copy milling technique refers to where a design or cut is difficult to make by itself, hence it is first made in the form of a master template which is turn is used to guide the tool path. Hence basically the template acts as an original from which the other parts are copied just like you would photocopy a document from an original paper in the Xerox machine.Gang milling refers to the situation when a gang or group of cutters is used to simultaneously work on the work piece in order to produce the desired shapeReciprocal milling refers to the process wherein the time taken to load and upload the job work from the fixture on the milling table is minimized by utilizing two fixtures which are ready at the same time for milling one after the other.There are several other types of milling methods apart from these and each of these methods have their own unique features. The reader is advised to refer to any good textbook of Production Technology in case he/she is interested to gain advanced knowledge about this area of manufacturing technologyImage CreditsThe Hong Kong Polytechnic University: Machining and Metrology Unit Website
Copy milling technique refers to where a design or cut is difficult to make by itself, hence it is first made in the form of a master template which is turn is used to guide the tool path. Hence basically the template acts as an original from which the other parts are copied just like you would photocopy a document from an original paper in the Xerox machine.
Gang milling refers to the situation when a gang or group of cutters is used to simultaneously work on the work piece in order to produce the desired shapeReciprocal milling refers to the process wherein the time taken to load and upload the job work from the fixture on the milling table is minimized by utilizing two fixtures which are ready at the same time for milling one after the other.There are several other types of milling methods apart from these and each of these methods have their own unique features. The reader is advised to refer to any good textbook of Production Technology in case he/she is interested to gain advanced knowledge about this area of manufacturing technologyImage CreditsThe Hong Kong Polytechnic University: Machining and Metrology Unit Website
Reciprocal milling refers to the process wherein the time taken to load and upload the job work from the fixture on the milling table is minimized by utilizing two fixtures which are ready at the same time for milling one after the other.There are several other types of milling methods apart from these and each of these methods have their own unique features. The reader is advised to refer to any good textbook of Production Technology in case he/she is interested to gain advanced knowledge about this area of manufacturing technologyImage CreditsThe Hong Kong Polytechnic University: Machining and Metrology Unit Website
There are several other types of milling methods apart from these and each of these methods have their own unique features. The reader is advised to refer to any good textbook of Production Technology in case he/she is interested to gain advanced knowledge about this area of manufacturing technologyImage CreditsThe Hong Kong Polytechnic University: Machining and Metrology Unit Website
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Po Hu, Zhenyu Han, Yunzhong Fu, Hongya Fu, "Implementation of Real-Time Machining Process Control Based on Fuzzy Logic in a New STEP-NC Compatible System", Mathematical Problems in Engineering, vol. 2016, Article ID 9814973, 14 pages, 2016. https://doi.org/10.1155/2016/9814973
Implementing real-time machining process control at shop floor has great significance on raising the efficiency and quality of product manufacturing. A framework and implementation methods of real-time machining process control based on STEP-NC are presented in this paper. Data model compatible with ISO 14649 standard is built to transfer high-level real-time machining process control information between CAPP systems and CNC systems, in which EXPRESS language is used to define new STEP-NC entities. Methods for implementing real-time machining process control at shop floor are studied and realized on an open STEP-NC controller, which is developed using object-oriented, multithread, and shared memory technologies conjunctively. Cutting force at specific direction of machining feature in side mill is chosen to be controlled object, and a fuzzy control algorithm with self-adjusting factor is designed and embedded in the software CNC kernel of STEP-NC controller. Experiments are carried out to verify the proposed framework, STEP-NC data model, and implementation methods for real-time machining process control. The results of experiments prove that real-time machining process control tasks can be interpreted and executed correctly by the STEP-NC controller at shop floor, in which actual cutting force is kept around ideal value, whether axial cutting depth changes suddenly or continuously.
Machining efficiency and quality of finished parts can be improved by monitoring, analyzing, and diagnosing the process of product manufacturing. It is hard to model the machining process accurately due to its complexity and variability. Therefore, artificial intelligent algorithms are usually used to build the relational model of machining parameters, cutting tool wear, and quality of finished part, with the purpose of developing machining process controllers that shorten machining time, prevent damage of tools, and improve quality of finished part. Li et al. used back propagation neural network for multiobjective cutting parameters optimization in sculpture parts machining to increase surface quality . Huang et al. proposed a fuzzy control strategy based on constraint of spindle power in end milling process for reducing machining time of complex shape machining . Zuperl et al. proposed an adaptive control strategy based on neural network to maximize the feed rate subject to allowable cutting force on the milling tool . Off-line optimization and on-line adaptive adjustment based on neural control scheme (NCS) are combined to control the cutting force . Research works have also been carried out to analyze and improve the performance of artificial intelligence algorithms. Osaba et al. presented a Standstill & Parade strategy for subpopulations communication in parallel genetic algorithms . Precup et al. introduced an approach for analyzing the stability of nonlinear processes controlled by Takagi-Sugeno fuzzy logic controllers . Qian proposed and studied the global attractivity of periodic solutions for nonlinear difference equation . Guerra and Vermeiren analyzed the stabilization of nonlinear systems that can be modeled by Takagi and Sugeno (TS) discrete fuzzy models by using nonquadratic Lyapunov functions in . From above publications, it could be concluded that the result of optimization or adjustment is greatly affected by the rapidity of machining process control, which is expected to be implemented as soon as possible, which means in real-time at shop floor in this case. However, two bottlenecks limit the rapidity of machining process control. One is to transfer high-level product manufacturing data between CAPP systems and CNC systems. The other is to embed real-time adaptive control algorithms or methods in the kernel of implementation platform.
The standard of STEP-NC supports bidirectional data transmission between CAD/CAM systems and CNC systems, which provides a possible way for solving the first bottleneck. Kumar et al. presented a STEP-NC compliant process control framework for discrete components and relevant self-learning algorithms in order to compensate errors and improve the surface quality of finished part . Laguionie et al. proposed a STEP-NC-compliant manufacturing scenario to optimize the process routes in multiprocess manufacturing environment . Campos and Hardwick proposed a feature-based traceability approach based on STEP-NC in order to feed back the manufacturing data associated with the monitored processes . Then the traceability interface for STEP-NC is defined and associated with other standards such as ISA-95 and MTConnect to support traceability activities in collaborative manufacturing scenario . Wosnik et al. presented a STEP-NC-compliant data model and process chain architecture for the optimization of machining process based on feedback process data . Kumar et al. established the information models and implementation mechanism for machine tool process control based on STEP-NC . Ridwan et al. proposed the STEP-NC data models and relevant optimization algorithm for real-time process control and monitoring, so that the high-level machine condition monitoring can be used for optimizing machining process . Machine condition monitoring (MCM) based on STEP-NC standard is proposed, which enables optimization during machining in order to shorten machining time and increase product quality. The MCM system consists of three modules, namely, optiSTEP-NC, AECopt, and knowledge-based evaluation (KBE) . However, most of the machining process control methods based on STEP-NC, implemented at machining process stage, are still off-line. It is difficult for a few on-line adaptive control algorithms, which are implemented on external machining process controllers at shop floor, to realize real-time machining process control. The root of this problem is the above publications did not find suitable platform for solving the second bottleneck.
In this paper, a framework and implementation methods of real-time machining process control based on STEP-NC are studied. STEP-NC data model for real-time machining process control is defined in order to transfer high-level product information between CAD/CAM systems and CNC systems. A software-based STEP-NC controller, which interprets and executes high-level information in STEP-NC files, such as manufacturing features, machining operations, and real-time machining process control functions, is designed and developed. An adaptive control algorithm based on fuzzy logic is also embedded within the software kernel of STEP-NC controller in order to improve the real-time performance of machining process control. The STEP-NC data model, software STEP-NC controller, and implementation methods proposed in this paper can be used to realize real-time machining process control at shop floor. The rest of the paper is organized as follows. In Section 2, the architecture of different types of machining process controllers is analyzed and compared. In Section 3, the STEP-NC data model and implementation methods for real-time machining process control are proposed, along with the design of STEP-NC controller that is able to interpret the newly defined entities. In Section 4, the fuzzy control algorithm for real-time machining process control is introduced and validated. Finally, experiments are carried out to verify the proposed framework, STEP-NC data model, and implementation methods for real-time machining process control.
Machining process control methods are usually designed based on automatic control theories such as classical control theory, modern control theory, information theory, system theory, and artificial intelligence theory. Machining process control systems become more integrated, interoperable, and intelligent with the development of computer and network technologies in recent years. The procedure of machining process control consists of three stages, namely, information collection, information processing, and control output. According to real-time property, machining process control can be classified into three categories.
(1) Off-Line Machining Process Control. Machining process information is acquired and saved during or at a certain stage of machining process. Then the acquired data is saved and analyzed by external machining process controllers, which adjust machining parameters of later machining process plans.
(2) On-Line Machining Process Control. Machining process information is acquired in real-time or at a certain stage of machining process. Then the machining process is paused to analyze the data and evaluate the process condition. Cutting parameters of subsequent working steps are adjusted to fix or optimize the machining process.
(3) Real-Time Machining Process Control. Machining system executes tasks of motion control, position control, data acquisition, data analyzing, and parameter adjusting in every interpolation period in order to keep the machining condition at desired and ideal state, which means all tasks of part machining, machining condition monitoring, and adaptive control are executed in real-time.
Machining process controllers (MPC), which analyze the process condition and adjust machining parameters, can be implemented at process planning stage or shop floor stage as shown in Figure 1. For MPC at process planning stage, CNC systems execute motion control command without adjustment even if the machining process is unstable. Optimization can only be done when the next part is being machined. For external MPC at shop floor, machining process data is firstly used for adjusting machining parameters on-line and then transferred to CAPP systems for further analysis in order to optimize process plan. MPC at shop floor is usually implemented on an external computer, which sends adjusted machining parameters to CNC systems. For integrated MPC at shop floor, CNC systems execute machining tasks by interpreting high-level integrated machining process data directly and include dimension and position of manufacturing features, machining methods, and process parameters. Machining process data is acquired and analyzed by real-time machining process control algorithms simultaneously with interpolation computing. The data acquired during machining process is integrated with input data and sent to CAPP system in order to preserve manufacturing knowledge for later machining process planning. Separation of MPC and CNC systems will lead to the delay of machining process control, which means integrated MPC at shop floor is a possible solution for implementing real-time machining process control.
Fluctuation of cutting force has great influence on machining system stability, cutting tool life, dimensional accuracy, and surface quality of finished part. It is necessary to select proper machining parameters with constraint of maximum or optimal cutting force. In this paper, the control of cutting force at specific direction is chosen as controlled object to implement real-time machining process control. As shown in Figure 2, side milling of planar face with end milling tool is taken as an example. The cutting force normal to the machined surface, which will cause the deformation of cutting tool, workpiece, and fixture, is selected as the controlled object. Optimal cutting force is calculated by CAD/CAM system and sent to CNC system at shop floor along with other process planning data. The prediction algorithm of differential tangential (), radial (), and axial () cutting forces is given by Engin and Altintas in  as
Edge cutting coefficients , , and and shear force coefficients , , and are various in different cutting conditions, which can only be identified from plenty of cutting tests. It is hard to build an accurate model for cutting process as the interrelation of machining parameters and cutting force is too complicated. As a result, the control algorithms based on empirical equation and off-line optimization usually become inappropriate at real cutting process, because of tool wear, varying cutting conditions, and complex shape of part. In this paper, real-time optimization algorithm is used to calculate the ideal machining parameters for current cutting condition by analyzing actual cutting force signal, which is acquired by a dynamometer. There are four machining parameters that are mainly related to cutting force, namely, axial cutting depth (), radial cutting depth (), feed rate (), and spindle speed (). Axial cutting depth and radial cutting depth are determined by machining allowance and shape of part, which are usually hard to be adjusted in real-time during the cutting process. Feed rate , which has more influence on cutting force than spindle speed , is adjusted in real-time in order to keep actual cutting force equal to optimal value. Acquired cutting force data should also be preserved and sent to machining knowledge management system for further analysis. Therefore, a data model for describing and transferring information of real-time machining process control tasks and machining process condition parameters is needed.
STEP-NC data transfer standard describes the machining process plan with entities such as project, workplan, workpiece, machining_workingstep, machining_feature, machining_operation, machining_function, and technology, which are defined by using EXPRESS language, a basic descriptive method of STEP-NC standards . Machining system gathers information of machining condition and associates it with entities of machining process plan. However, there is no data model for real-time machining process control and machining process condition information in STEP-NC data transfer standard at present. To pursue the aim of this research, which controls the cutting force of side mill perpendicular to machined surface by adjusting feed rate in real-time, new STEP-NC entities for describing constant force milling function and cutting force data are defined by using EXPRESS method in this paper. Figure 3 is the EXPRESS-G diagram of the developed data model for real-time machining process control. The upper half gives a brief description of existing STEP-NC data model, which is referenced from relevant ISO 14649 Part 10, Part 11, and Part 111 . The lower half expresses the newly defined STEP-NC data model, which is compatible with existing STEP-NC standards of ISO 14649.
Entity const_force_milling is a subtype of adaptive_control, which is defined in ISO 14649 Part 11. Under this entity there are six attributes, _axis_const, _axis_const, _axis_const, _force, _force, and _force. The first three attributes represent the direction of milling force that is supposed to be constant. Then the last three attributes represent the ideal milling force in Newtons. The direction and magnitude of milling force, which is applied to machined surface by milling cutting tool, are defined as three components at , , or direction in feature coordinate system. The attributes of milling force are optional if the value of corresponding Boolean attribute is False. When the adaptive_control attribute of entity machining_technology in STEP-NC file points to an instance of const_force_milling, the STEP-NC controller will execute the corresponding working step with variable machining parameters in order to keep the milling force at optimal value. Meanwhile, real-time intelligent control algorithm is used to realize machining process control.
Entity milling_force_save is defined to preserve the cutting force data. There are six attributes, its_workingstep, start_point, end_point, sample_rate, local_save, and online_save. The cutting force data acquired during cutting process is used for optimizing machining parameters in real-time. Then the acquired data should be associated with corresponding working step and sent to other subsystems for further analysis. Attribute its_workingstep represents the corresponding machining_workingstep that is being executed while the cutting force data is being acquired. Attributes start_point and end_point represent the positions of milling cutting tool in feature coordinate system at the beginning and the end of data acquisition. Attribute sample_rate represents the sampling period of cutting force acquisition in microseconds. Total integration of product manufacturing information can be realized if the real-time machining process condition data is saved in STEP-NC file. However, the quantity of cutting force data is usually too huge for STEP-NC file, especially when the sampling frequency is very high. In this case, the cutting force data is saved in an individual file that is stored in local storage of web server, while the file path or URL is represented as character strings by attribute local_save or online_save. At least one out of the last two optional attributes must exist.
STEP-NC file contains high-level information of product manufacturing without low-level tool path, in which entity project is the root node of tree structure. CNC systems should interpret all instances in an input file in order to get necessary information for on-line tool path generation and execution. The first step of interpreting a STEP-NC file is to map the information of input file to internal data model. C++ classes are defined according to the EXPRESS definition of newly developed STEP-NC entities and added to the ISO 14649 class library, which is originally designed by NIST for off-line STEP-NC interpreting . It is modified and embedded to the software CNC kernel of STEP-NC controller developed in this paper to interpret STEP-NC files on-line. Figure 4 presents the C++ classes for STEP-NC real-time machining process control information. Class iso14649CppBase is the base class for all other classes in the class library. Class instance is the base class for C++ classes that maps STEP-NC instances in STEP-NC files. Member variable iId of instance represents the number of instances. Member variables of C++ classes are pointers to instances of other classes, which represent STEP-NC types or entities. By this way, nodes in tree structure of STEP-NC file are linked to each other.
CNC systems should be able to interpret the STEP-NC files that contain machining process control operations directly, acquire machining process condition data in real-time, and compute interpolation points and optimized machining parameters simultaneously. Most commercial CNC systems have limited interfaces for STEP-NC interpretation and real-time adaptive control. In this paper, A STEP-NC controller based on open architecture software CNC kernel is proposed and developed in order to implement real-time machining process control at shop floor. An integrated data model based on STEP-NC is used to describe the information of geometry, technology, process planning, and machining process condition, which makes all stages of product manufacturing process traceable. The STEP-NC controller interprets STEP-NC files directly while communicating with sensors without external data acquisition and analysis system. Adaptive control algorithms can be embedded into interpolation calculation procedure in order to optimize machining parameters. The architecture of machining process control system that consists of three subsystems is shown in Figure 5, in which ISO 14649 is used as data transfer standard for real-time machining process control. The first subsystem is CAD/CAM system, which is responsible for making the machining process plan that contains real-time machining process control functions. Then the machining process plan is sent to other subsystems in form of STEP-NC files. Machining knowledge got from former product manufacturing process, machine tool capability, and cutting tool condition are considered by CAD/CAM system to optimize the process routes and parameters off-line in order to get better quality of finished part or higher production efficiency. The second subsystem is machining knowledge management system, which is responsible for gathering, fusing, and managing product manufacturing data. The third subsystem is intelligent machining system, which is responsible for interpreting and executing STEP-NC file and optimizing machining process in real-time. At the core of this subsystem is an open STEP-NC controller developed by Research Division of Numeric Control Technology in Harbin Institute of Technology based on Open Modular Architecture Controller (OMAC). The function of STEP-NC controller is realized by a software CNC kernel that consists of four software modules, namely, STEP-NC interpretation module, task generation module, system coordination module, and interpolation calculation module. This paper focuses on the implementation of real-time machining process control at shop floor, and the realization of CAD/CAM system and machining knowledge management system goes beyond the scope of this study.
STEP-NC interpretation module is responsible for interpreting and executing STEP-NC files that contains real-time machining process control functions directly. Most of the manufacturing features derived from entity machining_feature in ISO 14649 Part 10, process data for milling in ISO 14649 Part 11, and milling cutting tool data in ISO 14649 Part 111 are supported by this module . Tool paths are generated on-line by STEP-NC interpretation module and compiled by task generation module to generate task segments such as line feed, arc feed, and rapid move, which are transmitted to system coordination module via temporary storage. System coordination module creates several real-time threads for automatic operation, jog operation, and machining process condition monitoring, which are managed and synchronized by the host process according to scheduled clock period. System coordination module monitors the task segment queue in temporary storage in real-time, extracts one task segment each time when the task segment queue is not empty, and executes it by calling the functions of interpolation calculation module. Interpolation calculation module is responsible for optimizing machining process parameters and calculating the position of interpolation points, which consists of algorithms for speed control, position calculation, and MPC control. Under the interoperation of the four modules in software CNC kernel, the STEP-NC controller can parse and map a STEP-NC project into tree structure of C++ class instances, execute the main work plan, generate and execute tool paths for every working step, and control the machining process in real-time. Besides the software CNC kernel, a software Human Machine Interface (HMI) is developed for receiving instructions from user or communicating with other subsystems, and the hardware interface is used to communicate with servo system and sensors.
Realization methods of real-time machining process condition monitor along with algorithms for real-time machining process control are studied and developed based on the open STEP-NC controller proposed and built in Section 3.3. A real-time thread is created by system coordination module to acquire machining process data from sensors via hardware interface, and the real-time adaptive control algorithm is embedded into interpolation calculation module to synchronize the procedure of machining parameters optimization and interpolation calculation. Procedure of real-time machining process control based on STEP-NC is shown in Figure 6. There are two host processes in the software CNC kernel of STEP-NC controller. The first process is responsible for interpreting STEP-NC file and generating tool paths. Tool paths are encapsulated as task segments, which contain information of real-time machining process control such as start point, end point, feed rate, spindle speed, and ideal cutting force. Task segments are pushed into a first in, first out (FIFO) task segment queue in shared memory. The second process is responsible for executing the task segment in real-time. When the task segment queue is not empty, which means the process of STEP-NC interpreting has started, the real-time process will get the first task segment from the queue and execute it until the queue is empty. Real-time machining process control algorithm is called to analyze cutting force signal, which is acquired from dynamometer, and to adjust the feed rate. Interpolation algorithm is called to calculate the position of interpolation point according to the new feed rate in real-time. Servo system, which drives the feed shafts and spindle, is controlled via hardware interface to implement the adaptive control algorithm. The acquired cutting force data is saved in a data file and associated with corresponding working step by modifying the entities of input STEP-NC file and saved as output STEP-NC file.
The complexity of cutting process makes it hard to be modeled accurately with state-space equations. Artificial intelligence algorithms, which are able to handle unpredictable, nonlinear, multivariable, and incertitude controlled objects, can be a feasible solution. However, most artificial intelligence algorithms are used for off-line machining process optimization due to the limitation of computational complexity. Fuzzy logical algorithm, which has high computational efficiency, is suitable for real-time machining process control. In this paper, a fuzzy control algorithm with self-adjusting factor is proposed to adjust the feed rate in real-time in order to get constant cutting force. The principle of proposed control algorithm, which is designed by modifying the conventional fuzzy control algorithm, is illustrated in Figure 7. The input of control system is ideal cutting force while the controlled object is actual cutting force. The input lingual variables of fuzzy controller are the error between ideal cutting force and actual cutting force and the variety rate of error. The output lingual variable of fuzzy controller is the variety rate of feed rate. CNC system adjusts the feed rate according to the output quantity of fuzzy controller.
The error and error variety rate of ideal cutting force and actual cutting force can be calculated aswhere is ideal cutting force (N), is actual cutting force (N), is error (N), and is error variety rate (N/s).
Then the error and error variety rate are quantified according to the domain of fuzzy rule input aswhere is fuzzy variable for error, is fuzzy variable for error variety rate, is quantitative factor of error (N1), is quantitative factor of error variety rate (s/N), is adjusting factor of , and is adjusting factor of (s1).
The quantitative factors and are associated with ideal cutting force in order to improve the adaptability of fuzzy controller. As shown in Figure 8, the domain of fuzzy rule input is and triangle function, which has been widely used and has low computational complexity, is adopted as membership function of fuzzy rule. The fuzzy language value and membership value of input are expressed as follows: and are fuzzy language values of error. and are fuzzy language values of error variety rate. and are membership value of error. and are membership value of error variety rate.
The self-adjusting factor is used to adjust the weight of error and error variety rate adaptively when calculating the output fuzzy language value. The upper and lower limits of self-adjusting factor are set to 0.85 and 0.45. The fuzzy control output and variety rate of feed rate are calculated aswhere is fuzzy control output, is variety rate of feed rate (mm/s2), is scaling factor of fuzzy control output (mm/s2), and is adjusting factor of (mm/s2).
Factors , , and can be used to adjust the rapidity, accuracy, and stability of fuzzy control system. Lower and can improve the rapidity but reduce the accuracy and stability. Lower can improve the stability but the rapidity is reduced. The proposed fuzzy control algorithm is validated and adjusted by actual cutting tests as shown in Figure 9. The test part is made of aluminum alloy and the cutting tool used here is a high speed steel end mill tool that is 10mm in diameter and has 3 teeth. Considering the inertance of feed shaft, the feed rate is adjusted every 10 interpolation periods. The cutting force data is firstly processed by using mean filter algorithm aswhere is average cutting force at the th acquisition cycle (N), is actual cutting force at the th acquisition cycle (N), is spindle speed (r/min), and is sampling time of cutting force (s).
The test results are shown in Figure 10, in which the subtitle is in format of . The feed rate fluctuates obviously when = 8.0mm/s2 and more steadily when = 4.0mm/s2. The change of cutting force signal is more smooth when = 7.2s1. So in this paper, adjusting factors , , and are set to 0.9, 7.2s1, and 4.0mm/s2. The results indicate the fuzzy control algorithms proposed in this paper can work properly without building the model of metal cutting process with state-space equations.
Experiments are designed and carried out to verify the function of proposed STEP-NC controller for real-time machining process control. The experiment platform consists of Qier XKV715 vertical milling machine, Kistler 9257B dynamometer, and industrial computer is shown in Figure 11. The proposed software CNC kernel of STEP-NC controller along with real-time machining process control algorithm runs on an industrial computer, in which a Rexroth PCM-S11.2 SERCOS master card is used for communicating with servo drivers and IO ports, and an Advantech PCI-1741U data acquisition card is used for collecting cutting force data from dynamometer. Both expansion cards are plugged in the PCI slots of industrial computer.
Two test parts made of aluminum alloy are machined on the experiment platform as shown in Figure 12. Planer face is chosen as manufacturing feature and side milling is chosen as machining operation. High speed steel end mill tool that is 10mm in diameter is used to machine the parts with side milling method. The planar face of test part 1 has six sections that are different in width, which will cause irregularly variation of axial cutting depth in machining process. The shape of planar face in part 2 is a right-angled trapezoid, which will cause linear variation of axial cutting depth in machining process. Taking test part 1 as an example, instances of key entities related to real-time machining process control are extracted from Part 21 STEP-NC file, which is interpreted, modified, and outputted by STEP-NC controller. As shown in Box 1, instance in line #23 connects with planar face in line #29 and side finish milling operation in line #45 is the machining working step to be executed in adaptive mode. Milling technology in line #50 represents the cutting parameters, and in its attribute list, a constant force milling adaptive control strategy is assigned. As represented in line #53, the ideal cutting force is 50N in the minus direction of feature coordinate system. The STEP-NC controller interprets the STEP-NC file, generates tool path for each working step, and executes it in real-time. The acquired cutting force signal is saved in a data file that is stored at local hard disk after a working step is finished. Lines #54 to #56 are added in the input file by STEP-NC controller to preserve the information of real-time machining process condition. Line #56 is the newly added instance of entity milling_force_save, and in its attribute list, there are the path of cutting force data file and corresponding working step represented in line #23, which can be linked with each other. Lines #54 and #55 are instances of cartesian_point, which represent the position of milling cutter in feature coordinate system when the data acquisition is at start and end.
Results of real-time machining process control are presented in Figure 13. The average cutting force per revolution and feed rate are represented as absolute value in the data graph, because all values of milling force and feed rate are negative according to the definition of feature coordination system. Approach feed rate is set to 40mm/min for both conventional and constant force milling, and the adjusting range of feed rate in constant force milling algorithm is from 20mm/min to 180mm/min. For test part 1, cutting force signal is step-like while the feed rate is constant in conventional machining process as presented in Figure 13(a). In constant force milling process, cutting force signal changes suddenly at the start of every section while the proposed constant force milling algorithm decelerating or accelerating until the actual cutting force is equal to ideal cutting force or the feed rate reaches upper or lower limit as presented in Figure 13(b). For test part 2, cutting force signal changes gradually while the feed rate is constant in conventional machining process as presented in Figure 13(c). In constant force milling process, the feed rate is adjusted by the proposed fuzzy control algorithm and actual cutting force is kept close to ideal value as presented in Figure 13(d). Experiment results show that the STEP-NC interpreter can identify the ideal cutting force from STEP-NC file and keeps the actual cutting force around it by adjusting feed rate in real-time. The cutting force signals are recorded and linked with corresponding machining working step correctly. As a result, high-level information of real-time machining process control, instead of low-level information in most other published methods, can be interpreted at shop floor directly by using the proposed STEP-NC data model. Real-time machining process control algorithm is integrated with the software CNC kernel rather than external MPC in other research works.
This study has focused on the information exchanging mechanism, implementation method, and system scenario of real-time machining process control. An advanced solution method for optimization problems in manufacturing is proposed for implementing real-time machining process control at shop floor. Two key issues for implementing real-time machining process control are studied and solved. The issue of exchanging high-level product manufacturing data between CAPP systems and CNC systems is solved by extending STEP-NC standard and building an open STEP-NC controller that interpret STEP-NC data directly at shop floor. The issue of implementing real-time machining process control at shop floor is solved by integrating adaptive control algorithm with interpolation algorithm in software CNC kernel. Cutting force at specific direction is chosen to be the controlled object of real-time machining process control to demonstrate the newly proposed STEP-NC data model and implementation methods. A fuzzy control algorithm with self-adjusting factor is proposed to keep the cutting force constant by adjusting feed rate in real-time. The verification of proposed STEP-NC data model and implementation method is carried out on an open CNC platform. The experiment results indicate that the STEP-NC controller is able to interpret STEP-NC file with real-time machining process control functions correctly and keep the cutting force at specific direction close to ideal value. More research works on performance of real-time machining process control algorithms will be carried out based on the proposed implementation method in the future.
Copyright 2016 Po Hu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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