Know The Compensation Types. Key Concept

Transcription

1 Key Concept 3 Know The Compensation Types CNC machining centers provide three kinds of compensation to help you deal with tooling related issues. In essence, each compensation type allows a CNC programmer to create complete program without having to know every detail about the tooling needed for the job. The setup person will enter certain tooling-related information into the machine separate from the program. You should now have a pretty good understanding of the physical attributes of a CNC machining center. You know the machine s components, axes, and have been introduced to the most important buttons and switches. And you know that many of the things you will be doing on the machine require that you follow step-by-step procedures. In Key Concept number three, we will be studying some functions that help you deal with the tooling related to a job fixtures and cutting tools. For example, you know many of the machining operations that can be performed on a CNC machining center (from Key Concept number one), as well as the cutting tools that are used to perform these operations. Some of the setup tasks that are related to cutting tools may be pretty obvious. Cutting tools must, for instance, be assembled and placed into the machine s automatic tool changer magazine. Other things you must do with cutting tools may not be so obvious at least not to a CNC newcomer. For example, the CNC machine must be told the length of every cutting tool meaning the length of every cutting tool must first be measured. For some cutting tools, like certain milling cutters, the machine must also be told how big the cutting tool is (the radius of the cutter). We will also be taking a close look at workholding devices. As the name implies, a workholding device holds the workpiece during the CNC cycle. The most common example is a table vise. You have probably seen or possibly used a bench vise. As you turn the vise handle, a moveable jaw moves in and out and can be clamped on the object you want to hold in the vise. In similar fashion, a table vise mounted on a machining center s table can clamp onto and securely hold the workpiece to be machined in the cycle. As with cutting tools, there are some pretty obvious tasks that must be performed with workholding devices. They must, for instance, be mounted on the machine s table. Other tasks are not so obvious. The machine must be told the position of the workholding device s location surfaces in each axis (surfaces that contact the workpiece being held). With each of these lesser obvious tasks, there is a related form of compensation. And the focus of Key Concept number three will be to help you understand the compensation types. Copyright 2011, CNC Concepts, Inc. Machining Center Setup and Operation Page1

2 Key Concept Number Three is made up of four lessons: 7: Introduction to compensation 8: Fixture offsets 9: Tool length compensation 10: Cutter radius compensation Two of the compensation types are related to cutting tools: tool length compensation and cutter radius compensation. We ll discuss them in Lessons Nine and Ten. The third compensation type, fixture offsets, is related to work holding devices and we ll discuss fixture offsets first, in Lesson Eight. Copyright 2011, CNC Concepts, Inc. Machining Center Setup and Operation Page2

3 Lesson 8 Fixture Offsets And Assigning Program Zero The programmer will choose an origin for the program which is called the program zero point. The use of a program zero point simplifies the task of programming; it is usually the setup person who must determine where the program zero point is located in the setup and who must enter this position in fixture offsets. You know that a CNC machining center has three linear axes: X, Y, and Z. It may additionally have one or two rotary axes. The CNC program tells the machine how to move these axes in a way that causes a cutting tool to machine the workpiece. To this end, the programmer specifies a series of coordinates through which the tool will move. Coordinates are simply positions specified within the program. Think of the global positioning satellite (GPS) system in an automobile. It works by monitoring the car s current position on the earth. And this position is specified with coordinates using longitude and latitude. With CNC machines, coordinates are specified in each axis to cause a positioning movement. Consider these X and Y coordinates: X0.5 Y0.5 These coordinates may be specifying the position at which a hole must be drilled and they send the machine to a position of 0.5 in each axis (X and Y). When this command is completed, the center of the spindle will be positioned directly above the hole-location. But in order to cause such a motion, the machine must know the origin for the coordinates that is, the location from which the coordinates are taken. In CNC terms, we call this origin location the program zero point. How is the program zero point determined? It is the programmer who determines the position of the program zero point and most programmers use a logical method for deciding where it is placed. The method is based upon how the blueprint is dimensioned. They will pick the location in each direction (axis) from which the dimensions begin. Consider the drawing shown in Figure 8.1 that shows an example of how program zero is determined in the X and Y axes. Program zero placement example With this drawing, program zero selection is pretty obvious. Since the lower left corner is the datum surface for each direction, it makes the perfect program zero point. Program zero X Y Figure 8.1 An example of program zero point placement in the X and Y axes Copyright 2011, CNC Concepts, Inc. Machining Center Setup and Operation Page3

4 As you can see, all dimensions in Figure 8.1 begin from the left side in X and the lower side in Y. For this reason, the programmer will make the program zero point the lower-left corner in X and Y. If you have machine shop experience, and if you are assigned to machine this workpiece on a conventional machine, these two surfaces (left and lower sides) would be the location surfaces in your setup. That is, you would locate the workpiece from the left and front side. But maybe you don t have shop experience. When a machinist locates a surface, it means they will use some kind of stationary stops in the setup and hold the location surfaces of the workpiece against these stop while clamping the workpiece. In this manner, each workpiece to be machined can be loaded into the setup in precisely the same position. Figure 8.2 shows another example. It is the same workpiece, but it has been dimensioned differently. This time, the program zero point location is the upper-left corner in the X and Y axes. Another program zero placement example With this drawing, program zero point selection is not so obvious. For X, notice that dimensions begin from the left surface. For Y, notice that dimensions begin from the upper surface. X Y Program zero Notice that all Y coordinates are negative Figure 8.2 Another example of program zero point selection For the workpieces shown in Figures 8.1 and 8.2, the workholding device will probably be a table vise. For the workpiece shown in Figure 8.2, the workpiece will be oriented in such a way that the upper surface is placed against the fixed jaw of the vise. This locates the workpiece in the Y axis. For the X axis, an end-stop will be mounted on the vise in such a way that the workpiece can be held against the stop while the vise handle is tightened. This locates the workpiece in the X axis. Figure 8.3 shows a workpiece (though not one in Figures 8.1 or 8.2) held in this fashion. Figure 8.3 A workpiece located in a table vise Copyright 2011, CNC Concepts, Inc. Machining Center Setup and Operation Page4

5 So one way to describe how the program zero point is determined is to say the programmer chooses the position on the drawing from with all dimensions begin. Another is to say it is the position a machinist will use to locate the workpiece in a setup. Either method will render the same location. What about program zero in the Z axis? We ve described how programmers determine the location of the program zero point in the X and Y axes. And as with X and Y, a program zero point must be selected for the Z axis. While most workpieces are placed in the workholding device in such a way that they are located against the bottom surface (as is the workpiece in Figure 8.3) most programmers select the top surface of the workpiece and use it as the Z axis program zero point. This is especially true for vertical machining center programs. Figure 8.4 shows an example. Program zero in the Z axis is often the top of the workpiece Program zero Z0.1 Z Figure 8.4 Typical program zero point location in the Z axis Frankly speaking, it doesn t matter where the programmer places the program zero point as long as the setup person knows where it is. If the setup person is responsible for assigning program zero, of course, they must know its position. Setup documentation must include this information. Most setup sheets include a drawing or sketch that shows where the workholding device is mounted to the table. This drawing should also show a workpiece, as well as the location of program zero on the workpiece. Assigning program zero You must understand that just because the programmer wants the program zero point to be in a particular location doesn t mean the CNC machining center is automatically going to know where it is. A conscious effort must be made to assign program zero. Program zero assignment is the task of telling the CNC machining center where the program zero point is located in the setup. You can think of this task as marrying the CNC program to the workholding setup. Program zero assignment values Program zero assignment involves determining the distance in each axis between the program zero point and a special reference position. For the X and Y axes, the special reference position is the spindle center while the machine is at its X and Y axis zero return position (the zero return position is described in Lesson Five). The program zero assignment values in X and Y are the distances between the program zero point and the centerline of the spindle while the machine is at its zero return position in the X and Y axes. Figure 8.5 shows the program zero assignment values for the X and Y axes. Copyright 2011, CNC Concepts, Inc. Machining Center Setup and Operation Page5

6 The machine is currently at the zero return position in the X and Y axes Fixture Offsets And Assigning Program Zero Program zero assignment values for X and Y Program zero X value FANUC Machine Position Y value 10M X Y Z Top view of vertical machining center Figure 8.5 You must be able to determine the program zero assignment values for the X and Y axes For the Z axis, the point of reference will be based upon how your company chooses to use a feature called tool length compensation. Based upon our recommended method, the special reference position for the Z axis is the spindle face while the machine is at its Z axis zero return position. The Z axis program zero assignment value is the distance between program zero in Z and the spindle face while the machine is at its Z axis zero return position. Figure 8.6 shows the program zero assignment value needed for the Z axis. The machine is currently resting at the zero return position in the Z axis Program zero assignment value for Z Spindle face Program zero Z value FANUC 10M X Y Z Machine Position + The Z axis program zero assignment value depends upon how your company uses tool length compensation. Front view Figure 8.6 You must determine the program zero assignment value for the Z axis How program zero assignment values are determined By one means or another, someone must determine the distance in each axis between the program zero point and the zero return position. And again, Figures 8.5 and 8.6 show the needed values. There are two basic ways to determine program zero assignment values by physically measuring them during the setup (setup person does this) and by calculating them prior to making the setup (programmer does this). Which method is used is determined by the programmer and is based upon whether or not your company makes qualified workholding setups and whether or not you use predictable workholding tools (fixtures). Copyright 2011, CNC Concepts, Inc. Machining Center Setup and Operation Page6

7 How predictable are your workholding tools? When an engineer designs a fixture, they specify certain dimensions from workpiece location surfaces on the fixture to the components that locate the fixture to the machine table (commonly keys or pins). If the fixture is made accurately meaning these dimensions are correct it will be possible for the programmer to calculate the program zero assignment values before the setup is made. Frankly speaking, not many programmers know all of the related dimensions or make the effort to find or measure them so very few programmers calculate program zero assignment values. What is a qualified workholding setup? By qualified workholding setup, we mean one that can be placed on the machine table over and over again in exactly the same location. Product producing companies can often justify the higher fixture costs related to making qualified workholding setups. If a workholding setup is truly qualified, program zero assignment values will be exactly the same every time the setup is made. While it may not be possible to predict (calculate) program zero assignment values for the very first time the setup is made, it will not be necessary to repeatedly measure them every time the job is run. Many companies run the same jobs over and over (lots of repeat business), and for them, qualifying workholding setups can save a lot of setup time. With known program zero assignment values, the programmer can include commands in the program that automatically assign program zero. This keeps the setup person from having to do anything relative to program zero assignment, and again, will save time. Unqualified setups and few repeated jobs If your company doesn t have qualified workholding tooling maybe you seldom see the same job a second time the program zero assignment values will have to be measured on the machine after the workholding device is placed on the machine s table. This, of course, will be the responsibility of the setup person. Manually measuring the program zero point location on the machine Again, when an unqualified setup is made, or when a qualified but not predictable setup is made for the first time, the setup person must measure program zero assignment values right on the machine. There are two popular methods for doing so. Which one you will use is determined by whether or not your machine has a spindle probe and if so, whether your company uses it. With a spindle probe, the program zero values will be automatically measured. Without a spindle probe, you must manually measure the program zero values. Let s begin by showing how to manually measure them. You will be using the position display screen to help with taking these measurements. With the relative position displays, you are allowed to reset (set to zero) or preset (set to a specific value) the axis display registers at any time. This allows you to specify a point-of-reference for the measurement. In essence, you ll be using the machine as a measuring tool to help you determine the program zero assignment values. If you have had experience running a knee mill that has digital displays, you will find the related techniques to be familiar. But we will not assume you have had this experience and will describe the procedure in detail. Measuring program zero assignment values in X and Y for a square or rectangular workpiece This procedure involves using an edge finder and techniques similar to those a machinist will use on a manual milling machine when picking up an edge. Figure 8.7 describes two types of edge finders the wiggler type and the optical type. As with many setup related tasks, this requires some basic machining practice skill. While you may be able to easily understand this presentation, if you do have questions, ask an experienced person in your company or school to demonstrate how an edge finder is used and to let you practice picking up edges. We will be using a wiggler-type edge finder having a inch diameter (0.100 inch radius). When this edge finder is flush with the edge of the workpiece, the centerline of the spindle will be precisely inch away from the surface being touched by the edge finder. Copyright 2011, CNC Concepts, Inc. Machining Center Setup and Operation Page7

8 Edge Finders Wiggler type When this edge finder is rotated (by the spindle), it is easy to tell when it is aligned with the surface being picked up. As it is driven into the surface by an axis movement, the wiggler becomes more and more concentric with its shank. Driven further, it eventually it runs perfectly true with the shank. Driven just a bit further, it runs out wildly. At this instant, the edge finder is perfectly aligned with the surface being touched. Optical type (also called conductivity type) This kind of edge finder is easier to use. As soon as its stylus touches the (metallic) workpiece, a light comes on. At this instant, the edge finder is perfectly aligned with the surface being touched. Wiggler type edge finder in machine s spindle When an edge finder is perfectly aligned with a surface, you know the spindle center is precisely the edge finder s radius from the surface. If the edge finder has a 0.2 in diameter, the spindle center is precisely 0.1 from the surface. Figure 8.7 Edge finders used to pick up program zero surfaces in X and Y Procedure to use a wiggler-type edge finder to measure X and Y axis program zero assignment values 1) Load workpiece and edge finder Begin by making the workholding setup and load a workpiece. Place the edge finder in the spindle and manually start the spindle at about 500 rpm. With your finger, push on the edge finder stylus to cause it to run out (it is now wiggling and ready to pick up a surface in X or Y). 2) Touch off the X program zero surface and reset the X axis display Using the joystick and handwheel, move the table so that the edge finder comes flush with the X program zero surface (the left side of the workpiece shown in Figure 8.2) as shown in the drawing below. With the edge finder perfectly aligned with the surface, reset the X axis relative position display (set it to zero). Copyright 2011, CNC Concepts, Inc. Machining Center Setup and Operation Page8

9 Edge finder is now flush with program zero X surface FANUC X Y Z M Relative Position + 3) Back off in the Z axis, move over inches in X, and reset the X axis display again Using the handwheel, move the edge finder up (plus) in the Z axis to clear the top of the workpiece. Then move the X axis precisely in the plus direction (monitor the X axis position display to confirm the amount of motion). The centerline of the spindle will now be right over the program zero surface in the X axis, as shown in the drawing below. Now reset the X axis display again. Spindle center is now directly above the program zero surface in X FANUC X Y Z M Relative Position + 4) Touch off the Y program zero surface and reset the Y axis display Now repeat the process for the Y axis. Using the joystick and handwheel, move the table so that the edge finder comes flush with the Y program zero surface column side of our example workpiece as shown in the drawing below. With the edge finder in this position, reset the Y axis display (set it to zero). Copyright 2011, CNC Concepts, Inc. Machining Center Setup and Operation Page9

10 The edge finder is now flush with the Y axis program zero surface FANUC 10M X Y Z Relative Position + 5) Back off in Z, move over inch in Y, and reset the Y axis display again Using the handwheel, move the edge finder up (plus) in the Z axis to clear the top of the workpiece. Then move the Y axis precisely in the minus direction (again, monitor the Y axis position display to confirm the amount of motion). The centerline of the spindle will now be right over the program zero surface in the Y axis, as shown in the drawing below. Now reset the Y axis display again. The spindle center is now directly above the program zero surface in Y FANUC 10M X Y Z Relative Position 6) Send the X and Y axes to the zero return position When you send the X and Y axes to the zero return position, the axis displays will follow along. When the X and Y axes get to the zero return position, the X and Y axis displays will show the program zero assignment values (distances from program zero to spindle center in X and Y while the machine is at the zero return position). Measuring program zero assignment values in X and Y for a round workpiece This procedure involves using a dial indicator mounted in or to the spindle. You ll be using it to precisely align the spindle center with the center of the round workpiece. Again, this involves the need for some basic machining practice skill, so ask an experienced person in your company to demonstrate. Figure 8.7 introduces a dial indicator. Copyright 2011, CNC Concepts, Inc. Machining Center Setup and Operation Page10

11 Indicating A Round Workpiece Hole Spindle Mounting attachment Dial indicator The goal when indicating a round workpiece attribute (like a hole) is to bring the spindle center in X and Y precisely in line with the center of the round attribute. To begin, keep the indicator s stylus above the workpiece in the Z axis and bring the spindle center close to the center of the round attribute. Rotate the spindle and watch the stylus as it moves around the round attribute to see if it is roughly centered. When satisfied, bring the Z axis down until the stylus contacts the round attribute. Work with one axis (like X) first. Rotate the spindle to bring the stylus to the X plus side of the attribute and zero the indicator. Now rotate the spindle to the X minus side. The amount of indicator movement during rotation will tell you how far you must move the X axis to center the spindle over the attribute (actually you must move the X axis by half the indicator movement). Repeat the process for the Y axis. Figure 8.7 Spindle-mounted dial indicator 1) Load the workpiece and dial indicator Begin by making the workholding setup and load a workpiece. Place the dial indicator in or on the spindle. 2) Using the joystick and handwheel, align the spindle center with the workpiece center When finished, the center of the spindle will be directly over the center of the workpiece as shown in the drawing below. Now reset both the X and Y axes relative position displays. The spindle center is now directly above program zero in X and Y FANUC 10M X Y Z Relative Position 3) Send the X and Y axes to the zero return position When you send the X and Y axes to the zero return position, the axis displays will follow along. When the X and Y axes get to the zero return position, the X and Y axis displays will show the program zero assignment values in X and Y (distance between program zero and the spindle center in X and Y while the machine is at the X and Y zero return position). Copyright 2011, CNC Concepts, Inc. Machining Center Setup and Operation Page11

12 Measuring program zero assignment values in the Z axis Again, we are looking for the distance between the Z axis program zero surface (commonly the top of the workpiece) and the spindle face while the machine is resting at the Z axis zero return position. We recommend using a gauge block (a block of known height) between the workpiece and spindle face to keep from scuffing the spindle face (we re using the three inch side of a block). 1) Make the workholding setup and load a workpiece For this procedure, the top of the workpiece is program zero in Z. Place a gauge block on top of the workpiece. The gauge block is, of course, resting on the Z axis program zero surface. 2) Touch the spindle nose to the gauge block and set the Z axis display Using the joystick and handwheel, move the Z axis until the spindle nose touches the gauge block as shown in the drawing below. Now set the Z axis relative position display to the thickness of the gauge block (3.000 in our case). FANUC 10M X Y Z block Relative Position 3) Send the Z axis to the zero return position When you send the Z axis to the zero return position, the axis display will follow along. When the Z axis is resting at the zero return position, the Z axis display will be showing the program zero assignment value (distance from program zero to spindle nose in Z while the machine is at the Z axis zero return position). Measuring the program zero point location using a spindle probe If your company does not make qualified workholding setups, you will have to measure the program zero assignment values for every setup you make. Workpiece- and tooling-producing companies, for example, tend not to make qualified workholding setups. They cannot justify the additional tooling costs like productproducing companies can (because product-producing companies tend to make the same setups over and over again). As you have seen from the previous discussions, manually measuring program zero using an edge finder or dial indicator can be quite tedious, error prone, and time consuming. A spindle probe will dramatically simplify the task. It will automatically measure program zero assignment values in less than a minute, and it will actually assign program zero (assigning program zero will be presented a little later). Figure 8.8 shows a spindle probe. Copyright 2011, CNC Concepts, Inc. Machining Center Setup and Operation Page12

13 Using a spindle probe to measure the program zero point 1) The spindle probe is manually positioned to within 0.5 or so of the program zero point in each axis. 2) A special program is run that causes the probe to automatically measure the program zero assignment values. 3) This program assigns program zero. Probe at zero return position Program zero Figure 8.8 A spindle probe used to measure program zero assignment values Though the specific procedure varies from one manufacturer to another, a spindle probe is usually pretty easy to use. You simply make the workholding setup, load a workpiece and place the spindle probe into the spindle. Then you first manually position it to within about 0.5 inch of the program zero point in each axis. Finally, you execute a special probing program. This program will cause the probe to touch each program zero surface (X, Y, and Z) and determine the related program zero assignment values. The special program will additionally assign program zero (the topic of our next discussion). When finished, most machines will leave the spindle probe back where it started. Machine tool builders commonly supply probing programs for a variety of program zero possibilities, including any corner of the workpiece (lower-left, lower-right, upper-left, and upper-right), the center of a hole, the center of a plug, and the center of a slot. Any company that seldom or never makes qualified workholding setups should be able to easily justify the cost of a spindle probe. Fixture offsets Now that you know how to determine program zero assignment values, let s discuss how program zero is actually assigned. We are going to assume that you do not have a spindle probe to measure and assign program zero so you must assign program zero manually. If you do have a spindle probe, it will assign program zero as part of its function meaning it will automatically do what we are about to discuss. As stated, offsets are storage registers within the control. They are used to store tooling-related values and will not be used until they are invoked by the program. In general, offsets are used to separate certain tooling related values from the CNC program keeping the programmer from having to know them when programs are written. Fixture offsets are the offset registers used to assign the program zero point/s. They allow the separation of program zero assignment from the CNC program. In fixture offsets, you will be placing the program zero assignment values (as measured in the last discussion). Most machines come with at least six fixture offsets, meaning up to at least six different program zero points can be assigned and used by a program. With many vertical machining center applications, only one program zero point is required. If more than one workpiece is being machined by a program, it is likely that more than one fixture offset will be required. Figure 8.9 shows the fixture offset display screen page of a popular CNC machining center control. Copyright 2011, CNC Concepts, Inc. Machining Center Setup and Operation Page13

14 The first page of the fixture offsets display FANUC WORK COORDINATE SYSTEM SETTING 16M #0: COMMON X Y Z #2: G55 X Y Z #1: G54 X Y Z #3: G56 X Y Z Figure 8.9 First page of fixture offsets display screen Notice that fixture offsets are organized by number. Figure 8.9 shows the common offset (#0 on the fixture offset display screen page) as well as fixture offsets numbered one through three. The second page of fixture offsets (not shown) displays fixture offsets numbered four through six. Again, many machining center programs use but one program zero point and when this is the case, it is common practice for programmers to use fixture offset number one to assign it. When more than one program zero point is required, each one must be assigned meaning the tasks required to assign one program zero point must be repeated for each of the other program zero points. Setup documentation will specify how many program zero points are used by the program as well as which fixture offsets are used to assign them. For now, let s concentrate on assigning but one program zero point per program. And again, most programmers will use fixture offset number one to invoke it. When looking at Figure 8.9, notice that fixture offset number one is shown in the lower left corner of the fixture offset page (specified with #1:). Also note the G code next to it is G54. G54 happens to be the G code used in the program to invoke fixture offset number one. When the control executes a G54 in the program, it finds the values stored in fixture offset number one s set of registers and uses them to assign program zero. Remember, the values stored in the X and Y fixture offset register are the distances between the program zero point and the spindle center at the machine s X and Y zero return position. In the Z fixture offset register, it is the distance between the Z axis program zero point and the spindle face at machine s Z axis zero return position. Fixture offset values have a polarity. They are the distances from the zero return position to the program zero point in each axis. When the zero return position is very close to the plus limit for each axis (as it is for almost all machining centers), the polarity of fixture offset values will be negative, as is also shown in Figure 8.9. Once program zero assignment values have been determined, actually assigning program zero is easy. The setup person simply enters them into the appropriate fixture offset registers with the required polarity (again, as negative values for most machines). As stated earlier, if a spindle probe is used to measure program zero assignment values, it will automatically enter the program zero assignment values into fixture offsets. The actual values placed in fixture offsets will be the same regardless of whether they are calculated, whether they are manually measured by the setup person, or whether they are measured and entered by a spindle probe. Copyright 2011, CNC Concepts, Inc. Machining Center Setup and Operation Page14

15 An example of program zero assignment Figure 8.10 shows the simple drawing that we ll use for an example. Drawing Vise Figure 8.10 Drawing to illustrate program zero assignment The program Since the workpiece is dimensioned from the lower-left corner, we ll use this corner as the program zero point for the program. Program zero in Z is the top surface of this workpiece. Here is the program to machine the workpiece. O0001 (Program number) N005 G54 G90 S600 M03 (Assign program zero, select absolute mode, turn spindle on CW at 600 RPM) N010 G00 X1.0 Y1.0 (Move the tool into position in X and Y) N015 G43 H01 Z0.1 M08 (Rapid up to workpiece, instating tool length compensation and turn on coolant) N020 G01 Z F3.5 (Drill hole at 3.5 IPM) N025 G00 Z0.1 M08 (Rapid out of the hole, turn off coolant) N030 G91 G28 X0 Y0 Z0 (Rapid to the machine s zero return position) N035 M30 (End of program, this command also turns off spindle) While some of the words in this may look familiar from previous presentations, it is not our intention to explain this program in detain (again, programming is presented in another manual that is sold separately). We want to make but one point. In the line starting with N005, the G54 word tells the machine to look in fixture offset number one to find the program zero assignment values. This tells the machine where the program zero point is located in the workholding setup. While it may not seem like much, consider what the G54 in this program is doing. It is allowing the programmer to write the program without concern for where the vise is placed on the machine s table. Again it allows the programmer to separate program zero assignment from the program. The setup for this example job Before this program can be run, of course, certain setup related tasks must be done. First of all, the workholding setup must be made. For this setup, the vise must be mounted on the table. It must also be aligned so that it is square with the table. (If you have never seen this done, ask an experienced person in your company or school to demonstrate how to square a vise.) Though not shown in the drawing, an end stop must be added to the vise to locate the left side of the workpiece in the X axis. After loading a workpiece in the vise, the program zero assignment values can be determined. For our example, this means actually measuring the program zero assignment values. If we have a spindle probe, this will be as easy as positioning it 0.5 inch away from the program zero corner and activating the appropriate Copyright 2011, CNC Concepts, Inc. Machining Center Setup and Operation Page15

16 program. If we don t have a spindle probe, program zero assignment values must be manually measured as shown earlier in this lesson. Program zero must then be assigned. Again, if we have a spindle probe, has been done automatically. But if we don t, we must manually enter the program zero assignment values just measured into fixture offset number one (as negative values). There are also several other setup-related tasks that must be done before a workpiece can be run (among other things, loading the program, assembling cutting tools, and loading cutting tools into the automatic tool changer magazine). But the task of program zero assignment is complete. You should now see how program zero assignment marries the workholding setup to the CNC program. It tells the machine where program zero is located so that the machine can move the cutting tool/s properly during the program s execution. What is the common fixture offset? In Figure 8.9, the first fixture offset (upper-left on the display) is labeled as the common offset (#0). You may have been wondering what it is for. Our intension in this lesson is simply to show you the most common way that program zero is assigned. But there are other ways. The common offset provides a way to shift the point of reference for fixture offset entry. When set to zero (as it is in Figure 8.9 and as it probably is on your machine/s the special point of reference for program zero assignment values is the zero return position. So program zero assignment values in fixture offsets one through six are specified from zero return to program zero in each axis just as we have said to this point. But the zero return position doesn t always make the best point of reference for determining program zero assignment values. There are applications when shifting the point of reference for fixture offset entries to a more logical position can make it easier to assign program zero. This is especially with predictable qualified setups. What if my machining center doesn t have fixture offsets? Fixture offsets have been available with machining center controls since about So if your machine is under twenty-five years old, you can rest assured that it has fixture offsets. But if you have a machine that is older, it may not have fixture offsets. For machining centers that do not have fixture offsets, program zero is assigned in the program with a G92 command. Though newer controls allow the use of G92 to maintain compatibility with older machines, you should only consider using G92 to assign program zero if your machine does not have fixture offsets. Advantages of assigning program zero with fixture offsets instead of G92 Again, fixture offsets provide a much better way of assigning program zero than G92. Here are some reasons why. Safety A G92 command within the program simply tells the control how far it is from the program zero point to the spindle at the present time. When G92 is used to assign program zero, each axis of the machine must be in a previously planned position before the cycle can be activated. If the machine is out of position, it will not move to the correct location. Instead, it will likely crash the cutting tool into the fixture or workpiece. This is the single largest cause of crashes on (older) machining centers. Ease of use As stated, the operator must be certain that the machine is in the proper location prior to activating a cycle if program zero is assigned with G92. This will require tedious manual positioning movements. With fixture offsets, the operator need not worry about sending the machine to a specific location (other than possibly the machine s tool change position) prior to activating the cycle. Copyright 2011, CNC Concepts, Inc. Machining Center Setup and Operation Page16

17 Efficiency Since the machine must be in a precise location prior to activating a cycle if program zero is assigned with G92, wasted motions must be programmed at the beginning of the program, possibly during each tool change, and at the end of the program. These motions are required just to get the machine to its proper starting position, and are time consuming. Rerunning tools The operator will often need to re-run tools in the program. Re-running tools when program zero is assigned with G92 is more difficult and error prone than when using fixture offsets. Assigning program zero in the program Again, only use G92 if your machine does not have fixture offsets. With this method, program zero assignment is done very close to the beginning of the program. You will simply include the program zero assignment values (the same ones used with fixture offsets) in the G92 command. However, the polarity of the program zero assignment values is reversed as compared to fixture offsets. They are taken from the program zero point to the zero return position meaning they will almost always be positive values. Here is the example program again, but this time G92 is being used to assign program zero (not fixture offsets). O0001 (Program number) N002 G91 G28 X0 Y0 Z0 (Ensure that the machine is truly at its starting point the zero return position) N003 G92 X Y Z (Assign program zero) N005 G90 S600 M03 (Select absolute mode, turn spindle on CW at 600 RPM) N010 G00 X1.0 Y1.0 (Move the tool into position in X and Y) N015 G43 H01 Z0.1 M08 (Rapid up to workpiece, instating tool length compensation and turn on coolant) N020 G01 Z F3.5 (Drill hole at 3.5 IPM) N025 G00 Z0.1 M08 (Rapid out of the hole, turn off coolant) N030 G91 G28 X0 Y0 Z0 (Rapid to the machine s zero return position) N035 M30 (End of program, this command also turns off spindle) In the line beginning with N003, the X and Y words specify the current distances between the program zero point and the spindle center in X and Y (at the zero return position). The Z word in this command tells the control the current distance from the program zero point in Z to the spindle nose (again, at the zero return position). And again, these are the program zero assignment values. As long as the machine is at the zero return position when the G92 command is executed, these values will be correct. The balance of this program is identical to the fixture offset example program. Key points for Lesson Eight: Program zero assignment values must be determined before program zero can be assigned. Program zero assignment values are the distances between the program zero point and the machine s zero return position. There are only two ways to determine program zero assignment values calculate them or measure them. Program zero assignment values can only be calculated if you make qualified workholding setups, if you use predictable workholding tools, and if you know some important dimensions on the machining center. If you make qualified workholding setups but your workholding tooling is not predictable, you will have to measure program zero assignment values the very first time a setup is made. If you document these values, you will not have to perform these measurements again. Copyright 2011, CNC Concepts, Inc. Machining Center Setup and Operation Page17

18 If you don t make qualified workholding setups and program zero assignment values must be measured for every setup, you should use a spindle probe to measure program zero assignment values and assign program zero. If you must manually measure program zero using an edge finder or dial indicator, you re using the machine as a very expensive measuring tool. You must consider the ease of setup as the top priority when it comes to deciding where the program zero point is placed. If your machine has fixture offsets, use them to assign program zero. To assign program zero, the program zero assignment values shown in Lesson Five are placed into fixture offsets. You ve seen a full example of how a program zero is assigned using fixture offsets. Again, fixture offsets offer numerous advantages over G92 but if your machine does not have fixture offsets, you must use G92 to assign program zero. You must be very careful, since the G92 method of assigning program zero is not nearly as failsafe as when using fixture offsets. Copyright 2011, CNC Concepts, Inc. Machining Center Setup and Operation Page18