richard harris wrote: > Hey, > > I am running emc 3.2.0 off the BDI with Jon Elson's pwm servo boards. The > machine is a knee mill, of similar size and mass to a hurco. > > This machine gets little use as its bigger brother does all the heavy lifting > so the problem i describe below might have existed for some time and not been > noticed based upon part configuration and tolerance. > I notice during the last few days, while machining a thin wall box that one > sidewall was thicker than the other. At first i assumed that i had a chip in > the vise when i loaded the part, however the next part had a similar > condition. I re-zeroed the machine and noted that it showed i was .010" off. > Ran the next part and it was fine, the second part showed some issues, and > the third clearly had a problem. Part is taking 17 minutes, and the > properties pull down shows 400" of motion. > I checked the machine for a loose belt, loose ball screw to thrust washer, > and loose ball nut, all of these looked fine. > I continued to run the code, cutting air and double checking the machine > position to the zero point on the vise. I am loosing .002-.005" to the in > the minus x direction. > I have checked for backlash within the working envelope with a dial indicator > and have not found anything that would alarm me. > Any ideas on what would causing a loss of position in one direction only it > is repeatably losing in the minus direction only. Mechanical, electrical, > anything at this point. > OH oh! Well, what encoders are you using? Are you sure it never did this before, or is this the first part where you might have noticed such an error? Are the encoders on the motor? Is is possible the shaft couplings or pulleys are slipping, so the leadscrew is shifting relative to motor position? You can mark things like this with a marker pen to detect shifting of the hubs on the shafts.
If these are old encoders, the light source may be wearing out, or possibly the glass disc is starting to crash into the analyzer gratings. If the latter is so, you will rapidly see the problem get MUCH worse and develop servo faults. Is it only one axis that is having trouble? You might re-seat all connections to the encoder for that axis. You can also mark the motor shaft, make a number of back and forth movements and then return to the same readout position. If the motor shaft is dead on but the machine position is off, then it eliminates electrical problems. If the motor position doesn't reproduce, then it is either the encoder or the encoder counter. You may have to break out instruments like an oscilloscope to delve into encoder signals looking for a disturbance there. It could be electrical interference from a spindle drive VFD or similar noise source, weak signals from the encoder, maybe even a cable that is developing a broken wire. If you have differential encoders and a differential receiver, you should check the differental signals too for any anomaly. Jon ------------------------------------------------------------------------------ The Planet: dedicated and managed hosting, cloud storage, colocation Stay online with enterprise data centers and the best network in the business Choose flexible plans and management services without long-term contracts Personal 24x7 support from experience hosting pros just a phone call away. http://p.sf.net/sfu/theplanet-com _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
