A laser tracker can be an ideal tool to check and calibrate a CNC machine. Typically, CNC machine tool calibration has been done by using a laser interferometer upon machine installation. This is usually a linear calibration that is performed separately on each axis. Typically, it is a repeatability calibration. In other words, if I send the machine to the same location over and over again, how accurately can it go to the same location in a single axis? A laser tracker can be used to volumetrically check not just the repeatability, but also whether or not the machine is traveling to the locations selected. With some newer CNC controllers, the error data can then be fed into the controller and self correct the machine motion.
Older NC/ CNC machine tools typically had a very rigid structure and did not have a controller that could correct for errors. Many newer machines are constructed in more of a modular way to reduce costs and have CNC Controllers; which do have both linear and volumetric compensation. Depending on the configuration of a CNC machine, verifying volumetric accuracy can be a single process or multiple processes. The single process would be a configuration where an SMR could be located in a position to allow the capture of all axes of motion. If it cannot, then multiple measurement processes would need to be done. The data would then require merging. For example, I will illustrate a single process with a typical “travelling bridge” style machine. This is common for large format CNC 3 axes profilers, routers, plasma, and water jet cutters. The plasma and water jet are typically X, Y- 2 axes programmable with automated or manual 3rd axis. However, the procedure is similar and the controller can still correct for flatness and squareness issues, along with positioning accuracies.
In order to quickly perform a volumetric test, you must first prepare a program for the CNC machine. The program should consist of a representative set of point locations throughout the work volume of the machine. The program should approach the locations in both directions of the axes of travel in order to compensate for backlash. For example, if you are locating and measuring a position at X5Y0Z0, you would approach it from X0 and from X10, taking the measurement each time. This will capture positional error in both directions of the X axis. A point out in the middle of the work volume would be approached 6 times in order to thoroughly compensate for +/-X, +/-Y, and +/-Z.
In order to ensure a stable target and that the machine is not shortcutting any of the positions, the programmer must set a dwell at each position and use G1 (federate mode) instead of G0 (rapid mode) to ensure the machine successfully moves to each location. Once this program is written, it can be used each time a test or calibration is desired.
For the laser tracker, very little prep work is needed. If using the FARO® Laser Tracker Vantage with FARO® CAM2® Measure 10, setup the Vantage and SMR so all measurements will be in the line of site of the tracker. Set the software to take a measurement whenever there is a stable point for slightly less than the dwell time programmed for the CNC machine. With the CNC at its start point, go to Measure > Inspect Surface. Press the measure button and the Vantage will take a point. Then, start the CNC program. The Vantage will automatically take points at each dwell location. After the final point, hit “end measure” and close the measure surface window. Go to Report, and you can save the data as an Excel spread sheet.Now, you have a spread sheet of actual measurements and a G-code CNC program; which is essentially your nominal’s. The G-code program can be imported into the spread sheet, stripping out the Alpha characters and non relevant data upon import. The 2 sets of data might be vertically misaligned but this can be adjusted within Excel. From this data, the errors can be extrapolated and the correct signs (+/-) can be added to satisfy the requirements of the CNC control or the data could just be used for reference. If you are not familiar with Excel, this would be a good time to find someone who knows it well. The familiarity with Excel can develop formulas and automate this entire process. Once this is done, the Excel template can be used whenever you wish to perform a calibration in the future. Volumetric calibration will compensate for the 21+ sources of significant volumetric error in a 3 axis motion control system. These include squareness, flatness, and straightness of each axis, roll, pitch, yaw, and backlash/ linear positioning of each axis. After loading the compensation data into the controller, run through the measurement process again. You should see significant improvement. If the results are worse, it is likely that +/- signs on the calibration data have been reversed.A common objection to using the tracker is, “well, my CNC machine is accurate to within .0002 inches and the tracker is not that accurate.” This comes from the fact that most machine tool builders do not report volumetric accuracy but instead state repeatability per axis. Repeatability per axis is not volumetric accuracy. It is simply the ability of the machine to go to the same position (the same incorrect position) over and over again. Once this is explained, most machine operators understand the differences.
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