Newbie- Discovery 300 sx-15 Z axis won't stay put

[nederjack]

Hope all you tech folks won't find this question to stupid but my machine started to lose its ability to maintain its Z axis positioning accuracy. I slowed down the global max feed rates and was able to get away with 290ipm, and finish up my last couple of gadgets. But now the same problem is back and getting worse. Checked to make sure the compression clamp drive pulleys assemblies were snug and O.K. so now I'm out of my league. My next best guess is that I have an encoder problem and hope that I can send it to a wizard who knows how to fix these things. If any of you folks have run into the same problem would sure like to hear back as I am located a good distance from any metro center and repair techs. Have looked through the forum and haven't found anyone who has had the same problem recently. Hoping someone can point me in the right direction.
Thanks
nederjack

[machintek]

Does it move because of rapid? Or can it lose position due to cutting force?
Have you marked the pulleys and shafts?
I recently had a lathe making bad parts because the X axis belt had broken fibers in it, thus it could stretch with load to the point that it would jump a few teeth. Just another item to check - the Z belt.
But yes it could be a loose encoder coupling in the motor or a bad encoder, or a BMDC that is not counting correctly.
If it is the SX it has the bridgeport drives and DC motors. Thus brushes to check. Newer machines had Siemens motors and drives.

George

[nederjack]

Hey George,
Thanks for the response. I have read several of your responses to other folks , and it seems that you really know what you're talking about. I want you to know that I really appreciate you taking the time to help those of us who possess fewer technical skills. I did as you suggested and marked the pulleys to the shafts , and all is as it should be and the belt is in good shape. The positioning error directly proportional to the velosity of the Z axis motion, i.e. the faster the feed the greater the error. Cutting pressure does not appear to be a factor. Planning on pulling the motor today after I get some turn work going. Must admit that I'm on foreign ground here and have read through the service manual, specifically the section on the drive system. I am assuming that the encoder contains both the DCS and HBD boards and when you speak of the BMDC, you are referring to what the manual calls the output drive module. Sorry that I am unfamiliar with the basic terminology as I am a designer/cutter and am now learning the inner workings of the tools I use. Would appreciate any other advice as far as checking for “a loose encoder coupling” and brush replacement (i.e. special tools involved) or do I just pull the motor, and send it to someone that can check and rebuild? And, yes they are DC servo motors with the encoder/tach module attached to the back of the motor.
Thanks again,
Nederjack

[machintek]

I must apologize. If you have a SX machine, you have a FMDC. It has 8 LED across the top of the board. It controls positioning. All the encoders go to it.
I would swap the offending motor with another axis to see if it follows the motor. If not the problem may be the FMDC board or possibly a cable.

George