TEST METHODS FOR COORDINATED MOTION OF FIVE-AXIS MACHINING CENTERS

Transcription

1 TEST METHODS FOR COORDINATED MOTION OF FIVE-AXIS MACHINING CENTERS Shawn P. Moylan, Ronnie R. Fesperman, and M. Alkan Donmez Manufacturing Engineering Laboratories National Institute of Standards and Technology 1 Gaithersburg, Maryland, USA INTRODUCTION Despite the increase in demand and usage of five-axis machine tools and considerable research in the area (e.g., [1-3]) users and builders are not getting optimum performance from their machine tools because a robust set of measurements assessing the accuracy of coordinated complex motion involving multiple linear and rotary axes does not yet exist. Currently, a standard specifying certain kinematic tests for machining centers, concerning the accuracy of paths described by the simultaneous movement of three or more numerically controlled linear and/or rotary axes is in draft status [4]. This draft standard proposes four tests. Two tests are intended to check the accuracy of the circular trajectories when the motion of two linear axes is synchronized with the rotation of one rotary axis. The other two tests check the accuracy of the trajectories when three linear axes and two rotary axes are controlled simultaneously. Of these four tests, three offer alternative methods of performing the tests. This paper presents the results of the tests we carried out to compare these alternative methods applied on a five-axis machine tool in our shops. DESCRIPTION OF THE MACHINE The machine used for this study is configured with a tilting rotary table. The machine has a non-orthogonal tilting B'-axis, inclined from the Y-axis by 45 (see Fig. 1). THE EXPERIMENTS The draft standard proposes tests for various structural configurations of five-axis machines. These tests involve creating a tool path to keep a constant positional relationship between the tool point and the workpiece, either at the same C -axis B -axis FIGURE 1. Schematic of the machine used in the current study with rotary axes directions. nominal position or at a constant radius from a nominal workpiece position. Therefore a fixed set of linear displacement sensors or alternatively a telescoping can measure the deviations from an ideal tool path in three orthogonal directions in the workpiece coordinate system (WCS). To maintain the constant positional relationship, the rotary axis motion is commanded and the motions of the linear axes are automatically coordinated using the machine s tool center point (TCP) control function. The difference between the maximum and minimum deviations recorded for each measurement is reported to represent the accuracy of the coordinated motion. Descriptions of the measurements Test 1 checks coordinated motion of the tilting (B ) axis with the three linear axes (45 orientation of B'-axis requires the coordination of three linear axes). A test sphere mounted in the spindle should be positioned a distance R away from the rotary axis average line. The rotary axis is rotated at 360 degrees/minute and the full travel of the axis is tested (or the maximum stroke allowed by possible interferences). Path deviations resulting from the coordinated motion are measured in three orthogonal directions in the WCS. 1 Official contribution of the National Institute of Standards and Technology (NIST); not subject to copyright in the United States. The full descriptions of the procedures used in the paper require the identification of certain commercial products. The inclusion of such information should in no way be construed as indicating that such products are endorsed by NIST or are recommended by NIST or that they are necessarily the best materials, instruments, software or suppliers for the purposes described.

2 Instead of measuring in the WCS X-, Y-, and Z- directions as prescribed by the standard, we measured in the directions tangential, radial, and parallel to the motion generated by the rotary axis. This change allows a more direct comparison between alternatives and eases diagnosis of machine errors. For the current study, the three measurements were performed simultaneously using an device [1] consisting of a three-sensor nest calibrated against a reference sphere. An alternative method for this test is to generate a tool path that keeps a constant distance between the tool point and the workpiece point allowing measurement with a telescoping. With this alternative, the three measurements are carried out with the oriented in: a) the direction tangential to the rotation of the rotary axis under test, b) the direction radial to the rotary axis under test, and c) the direction parallel to the rotary axis under test. The table mounted sphere should be located a distance R from the rotary axis under test. The radius, r, of the path of the spindle 2 2 mounted sphere should be a) r = LB + R, b) r = L B + R, and c) r = R where L B is the nominal length of the telescoping (100 mm in the current case). All other test parameters are the same as those previously mentioned. The other tests are designed similarly to those already described above. Test 2 checks coordinated motion with the rotary table (C - axis). This test also has an alternative using the telescoping. The only notable difference for this test is that because our rotary table has infinite travel, the test was performed over 360 with an additional 10 at the beginning and end of each move for lead in and lead out. Test 3 checks coordinated motion of all five axes. Again there are three measurements. They are taken as a) tangential, b) radial, and c) parallel to the rotation of the rotary table (C -axis). For this test the rotary table is directed to move from 0 to 360 while the tilting axis is directed to move from 0 to 90 and from 90 to 0. This five-axis test also has a alternative. Analyzing and reporting data It is important to note that a best-fit circle is not removed from the recorded data. This differs from circular contouring tests (tests that examine coordinated motion of two linear axes) where a best-fit circle is removed in order to remove effects of setup error. When coordinated motion involves at least one rotary axis removing a best-fit circle is inappropriate because, while effects of setup error will be removed, it will also mask errors associated with the rotary axes (location errors of the axis average line with respect to the linear axes in the machine coordinate system). Setup Since a best-fit circle will not be removed from the data, the operator must take special care to minimize any error in the measurement setups. Specifically, the operator should ensure the test sphere is properly mounted in the spindle. Firstly, the sphere center must be aligned to the spindle axis average line. Secondly, the distance from the spindle gage line to the center of the sphere must be accurately determined (likely with a tool setting station) and input into the controller as the tool height. These two aspects of the setup are of primary importance because the TCP function of the controller positions the center of the tool of a known length. A simple example can illustrate the sensitivity of these tests to an improper setup. Measurement a) of test 2 is the most easily visualized. For the sake of example, the test is setup with the test sphere misaligned to the spindle average line by 10 μm in the Y-direction. As the axes move throughout the test the WCS rotates with the rotary axis, but the tool coordinate system does not. The effect of this is a perceived deviation in motion that is double the initial setup error (see Fig. 2). This sensitivity can be compounded in the case of the test if the common procedure of using the tool mount to set the table-mounted sphere is followed. table mounted ball spindle mounted ball (with setup error) spindle axis position (A) start of test 2 L + 2 B μm L B -10 μm 10 μm (B) after 90 rotation of axis FIGURE 2. Schematic demonstrating the effect of setup error on Test 2 measurement a). RESULTS Results of all of the tests and measurements performed are shown in Table 1. Plots of the data for several of the measurements are shown

3 in Figs. 3 through 5. These plots demonstrate that the data from the two alternatives do resemble each other. The standard uncertainty in the measurements a function of the linear displacement sensor and error in the setup is approximately 0.9 μm for the measurements and 0.8 μm for the measurements. As such, the expanded uncertainty for the results shown in Table 1 is 3.7 μm for the measurements and 3.2 μm for the measurements. In addition to the experimental results, the tests were evaluated using a computer model. The model incorporates the nominal kinematics of the machine tool as well as simulated kinematic errors. The trajectories of the spindle-mounted sphere and the table-mounted component were input into the model and their actual paths were output. The model was run with only one error present at a time, allowing the measurements sensitivities to individual errors to be evaluated. Magnitudes of 50 μrad for axis orientation errors (squareness of Y rotated about the Z-axis, COY; squareness of Z rotated about the X- and Y- axes, AOZ and BOZ; mis-orientation of the B around the X-axis, AOB; mis-orientation of the B around the modified Z-axis, COB; and squareness of the C around the X- and Y-axes, AOC and BOC) and 10 μm for axis location errors of the rotary axes (offset of the B in the X- and modified Z-directions, XOB and ZOB; and offset of the C from the B -axis in the X- and Y-directions, XOC and YOC) were used. The results are shown in Table 2. TABLE 1. Range in deviations (in μm) observed in each measurement. Int. indicates that the measurement could not be completed because of interference problems. Measurement Ballbar a) Tangential Test 1 b) Radial 42.2 Int. c) Parallel a) Tangential Test 2 b) Radial c) Parallel a) Tangential 46.2 Int. Test 3 b) Radial 40.0 Int. c) Parallel (A) (B) FIGURE 3. Deviations recorded during Test 1 using (A) the and (B) the. measurements were recorded simultaneously while measurements in series. (A) (B) FIGURE 4. Deviations recorded during Test 2 using (A) the and (B) the. measurements were recorded simultaneously while measurements in series.

4 TABLE 2. Results of computer model evaluations of tests. Ranges in deviation are reported in μm. Error magnitudes are in μrad for the orientation errors and in μm for the location errors. R = 125 mm Error Magnitude Test 1 Test 2 Test 3 Meas. a) Meas. b) Meas. c) Meas. a) Meas. B) Meas. c) Meas. a) Meas. b) Meas. c) COY AOZ BOZ AOB COB XOB ZOB AOC BOC XOC YOC FIGURE 5. Deviations recorded during Test 3 using the. Measurements were recorded simultaneously. CONCLUSIONS The data presented in the previous section lead to two conclusions. The majority of the deviation observed while testing our machine (see Table 1 and Figs. 3 5) is the result of location errors of the axes of rotation (i.e. XOB, ZOB, XOC, and YOC) with a small amount of deviation resulting from mis-orientation of the B-axis (AOB) and possibly squareness of the Y-axis (COY). More importantly, the s and the tests, as currently described in the draft standard and as carried out in this study, will lead to differing results when there are orientation errors present in the linear axes (i.e. COY, AOZ, and BOZ). In order for the measurements to be a true alternative to the measurements and for the tests to better accommodate inclined tilting axes, we recommend users follow certain practices. In the measurements, the three orthogonal directions in which the deviations are measured/reported should be specified with respect to the rotary axis under test (radial, tangential, and axial). This allows misorientations of the rotary axis to be better observed and diagnosed. With the five-axis test, the three orthogonal directions should be with respect to the rotary table. For the alternative to provide similar results to the regarding errors in the linear axes, those axes must travel the same distances in both alternatives. To accomplish this, the spindle mounted sphere of the should be positioned a distance R from the rotary axis. This will result in an offset distance r between the axis average line of the rotary axis and the center of the table-mounted sphere of r = R L B when the is radial to the rotary axis, r = R when the is parallel to the rotary axis, 2 2 and r = R + L B when the is tangential to the motion created by the rotary axis. REFERENCES [1] Weikert S, R-Test, a New Device for Accuracy Measurements on Five Axis Machine Tools. CIRP Annals. 2004; 53: [2] Tsutsumi M, Yumiza D, Utsumi K, and Sato R, Evaluation of Synchronous Motion in Five-Axis Machining Centers With a Tilting Rotary Table. Journal of Advanced Mechanical Design Systems and Manufacturing. 2007, 1,

5 [3] Zargarbashi S, and Mayer J, Assessment of Machine Tool Trunnion Axis Motion Error, Using Magnetic Double Ball Bar. International Journal of Machine Tools and Manufacture. 2006, 46, [4] ISO/CD ;2009, Machine Tools Test Conditions For Machining Centres Part 6: Accuracy of Speeds and Interpolations. DRAFT SUBJECT TO INTERNAL REVIEW