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When I refer to accuracies, I mean your machine’s performance. For example, if you program a cut to a 2.0000" diameter on a lathe, will it stay at 2.0000" from beginning to end? It should. Accuracy checks are carried out in a variety of ways using an indicator and magnetic base, a ball bar or a laser. First, check the axes for “backlash:” that is, a mechanical loss of motion, which can be caused by anything in the feed system. The most common culprits include the servomotor, positional feedback device (pulse coder, encoder, DC-type tachometer, etc), gear belt, couplings, bearings, ball screw, hydraulic actuator or even the axis itself. Testing With an Indicator When using an indicator to test for backlash, load the axis bearings on both sides of the balls. Make sure the displayed position is the same actual position whether you are moving in a positive or negative direction on the axis. Most manufacturers prefer that you not use the manual pulse generator located on your control or on the hand pendant. The preferred method is to use either MDI or a program that feeds into your indicator in both directions. Be careful: many indicators have “lost their life” when doing this check. Mount the indicator in mid-stroke of the axis. Using a pulse handle, drive the axis into the indicator until it reads zero. (I usually only go into the indicator no more than half a revolution.) You will be feeding back into the indicator from the opposite direction. Here is an example program for checking the X-axis: G0X3.0Z3.0 (indicator set at zero) Try to slow the axis down because you are feeding back into your indicator. In the program, when the X-axis makes its last move, it is heading in the opposite direction from where we started. If there is no backlash in the X-axis, your indicator will return to zero. If not, there is a problem. This program is an example of what I use on an absolute-style axis on a machine with rapid backlash compensation. If your machine has compensations for rapid and feed, you will have to write a program using both feed-type commands. Refer to your machine manuals to find out where and how to change your backlash compensation. Most are entered in a metric format and you may have to convert your findings if you use a standard indicator. Contact your machine tool builder for the allowable amount. The Drive Mechanism To find out where the backlash is coming from, you will have to inspect the entire drive mechanism. I advise caution in going further because any alterations can, and probably will, change your axis position. It is best to consult with an experienced service person to become comfortable with how the axis works. Usually the fault will lie in either the ball nut or the associated bearings. To check this, mount a ball (with a dab of grease) to the end of the ball screw (use the center hole on the end of the shaft). Mount your indicator and zero it out. Now move the axis in either direction while checking the indicator after every change of direction. The indicator should never move off zero. If it does, there is something wrong with your thrust bearings. Mounting your indicator on the axis in question and placing the tip of the indicator on the threads of the ball screw itself checks another common fault. If the indicator moves with the motion of the axis, you have backlash in the ball screw. Again, do not to try to fix this unless you are fully confident in this area of repair. If you know how to check the feed system, even if you do not make the repair, you can save your company a day’s worth of service charges. The checks are more accurate using a ball bar or laser, but you still have to know what to do to the axis once the faults have been given. Most will give you a printed readout of what the percentages of fault are, and you will see that you are limited in what adjustments can be made. Also, if your machine has scales on the axis, you should never see any backlash at all when checking with the sample program that I gave you. Next time we will further investigate accuracy checks on lathes. Talk to you next month. Mike Breen is a Field Service Engineer with Ellison Machinery Company of Wisconsin, holding this position for more than six years. For comments and inquiries about these articles, his e-mail address is mbreen@ellisonwi.com. - November 2000 |
This month we are going to start addressing what I consider the second level of PM: checking accuracies.