axis movement stalls

we have one hitachi seiki make (cooper) horizontal machining center(HB-800) with NUM-750 controler, the machine is 13 years old. we are facing one peculiar problem recently, the machine after being programmed, stops moving in "feed" motion in all the four axes, randomly but this problem frequents x and z axes.
any solutions or diagnosis will be welcomed.
best regards
iniyavan

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Not familiar with that machine, but sounds like an "in position" type error. If it is, do a little testing and you should find one axis is causing the problem(x or z). If an axis does not come to the programmed position(usually about .002,) the next line of the program will not execute. The control is waiting for the programmed move to complete. The servo drive may need adjustment to bring axis into position. If the control will display axis lag, quick way to check it. Not an uncommon problem as brushed DC motor age and dynamics change.
AL

I cant help you with your problem but I will say that I was impressed by your website. That glassware machine was quite impressive looking. I dont know what it makes but it looks interesting anyway. Good luck with your problem.


Charles

the machine is an a.c servo machine with NUM 750 control, do u think a faulty proximity sensor or a limit switch or even the encoder itself is a problem here?
thanks for the help!
iniyavan home page

Virtually every time I have seen this type of problem,(was CNC tech for Dynapath) the problem could be attributed to one axis. Can be hard to spot when it happens using various programs during machining. Does the control dispay an "in position" indicator (usually an * after the axis position data. Check for one axis not being "in position" when the machine stops in a feed move. This usually happens at the end of a move.(control will not execute next move until axis is in position). If running a program, check the program for the move just executed to see which axis was moving. It is usually easier to find by testing the machine axis independantly(Ex. g1 5. f5 g1 xo. g1 x5. g1 x0 etc). A slower feed rate will usually make the error more predominant as faster moves have the advantage of inertia to help get into position. If the stop occurs during a move(not at end of move)check the feed rate over-ride potentiometer. A bad spot can cause an open, which is feed rate O, and will stop movement also. I do not believe it is a faulty sensor or switch as USUALLY this would cause a fault condition on the control(travel limit etc.)
AL

The point I'm trying to make is to test and find out which axis is actually causing the problem.
I don't believe it is an encoder, as they tend to be go, no go, either they work or they don't.
for unusual encoder probems, scratch on disc,or loose encoder coupling,or intermittant wire contact you would get loss of pulses and and accuracy/repeatability problems. Most Ac drives will fault if total loss of encoder feedback, so don't think you problem is there.
If the control will display axis lag or following error, (option in some set up pages).
Manually jog each axis at a slow feed rate and see if each axis comes back to o.oooo when you stop jogging.(following error will be greater the faster you move).if one axis does not come back to o.oooo, probably is the problem axis. It may be right at the edge of the controls allowable "in position" range. Why the problem is intermittant, most times it is in position, every once in a while it isn't.
If you find one axis as described, check the manual for the servo drives, for adjustment procedures. Of course I am assuming that mechanically the machine is okay. A sticking gib or other mechanical load can cause in position problems at lower feed rates.
Al

finally sorted out the problem, it was no fancy stuff here simple over temperature of the drive module and the 5 volts power transformer.
i fitted a blower on the x'former and directed the cold air duct onto the drive module directly, the ambient temperature was 68 degrees and lo it came down to 40. now it works fine 24/7.
thanks a lot all and best regards
iniyavan