Losing Z Height in Mid Operation

I bought my CNC router with Mach 3 configured with a USB connection instead of a Serial. Recently I started dropping Z height in Mid Operation. Checking bearings, slop in the screw, connections, and anything else I could think of is good. I'm running in a metric configuration, and when I calibrate it sets at 355 "steps per". Operation starts out good, but after several minutes Z height will drop 5 or 6 mm. The manufacture suggested changing the Steps Per to 160. At that configuration, things seem to be working better, but when I do a profile cut, the depth only reaches 55% of where it should go. Anybody think that having it with a USB setup might be my problem or is there any other advice?

Changing the steps/mm will result in an incorrect distance traveled, and is not the answer.

Try reducing the acceleration by 50%.

One of the several things I tried was deducing the Velocity In's or mm's to 50% and the Acceleration up to 75%. I also tired changing the Step Pulse and Dir from 1 to 5, but same.

Could be your Z motor is not powerful enough, or there's some binding in the Z somewhere... likely it's losing steps going up, so it starts lower and lower.

Is your Z motor getting to hot? I'm having the same problem with mine. After I let the machine cool down for about a half an hour it 'll be OK...but trying to get 2 hours of continuous production and my Z gets shallower and shallower. Motor gets too hot to touch. I'm going to try a old computer fan to cool it and I'll post my results under the "Misstress" build log.

Greg

What program do you use to generate the g-code? I had a similar problem when I used the BobCad-cam.

I'm using Aspire, but wrote a code using NotePad to raise the Spindle up and down twelve times, and checked it and run another one to go up and down five times, dropping 5 mm at the longer session, 1.9mm at the shorter session. So I'm discounted the problem being the software. I've watched the screw when it's working and it seems tight and no bad sounds, so that's my next thought is the motors is too weak or getting hot.

We can give all the advice we can, but more info on the machine would be helpful. ACME or ballscrew? Pitch or lead of said screw? Stepper size, rated stall torque? PSU? Drive? Microstep setting? If your controller is USB based then most likely the pulse generator is on a separate board and not done on the computer, so I think we can rule out the USB connection. So that leaves:

Power issue - Inadequate stepper torque, PSU too small, motor miswired
Mechanical issue - Leadscrew too fine, causing stepper to spin too fast, resulting in lost torque at higher RPMs
Setting issue - Rapids and accel set too high

louieatienza, knowing the information you suggested is a little out of my expertise, however, I took the pulley off the shaft to inspect if the bearings were a problem and noticed when I turn the motor by hand, it is stiff and clicks. However, if I power it from the computer it seems to work okay. Is that normal for a stepper motor to be fairly hard to turn by hand? Gary

You'll feel the motor "cog" as you turn it by hand. It is tough to diagnose what the problems may be since there are too many unknowns; we're just guessing here. The only thing I can say would be to run your machine slower; set your feedrates and rapids slower, and slower, and see if the problem still exists. I would also read up here and elsewhere and get some familiarity with your machine and its parts so that you'll be able to work on your machine.

A couple of things to consider:

Mechanical issues: Physically mark all connections between the leadscrew and the motor! You want to do this to verify that nothing is slipping on a shaft.

Electrical issues: This actually sounds like an electrical noise issue to me. You need to observe what is happening during machine cycling, paying special attention to anything turning on or off. Especially off. If you have a spindle drive driven by a relay or a solenoid valve turning an air last on or off that could cause a problem. Generally the problem crops up when a device is turned off.

Years ago I had a problem with a series of lathes that would loose a couple of microns every five cycles or so. We eventually traced it to a relay in a packaged spindle drive. You could cycle the axises back and forth all day and never loose position with the spindle off but with it on you would gradually loose position in a very predictable manner. This was a tough one at the time due to the crude CNC system and the fact that it took a while to build up a significant error.

There are other possibilities such as the motor stalling but if that was happening with stepper you should hear some nasty noise. It would probably be a good idea to make sure the firmware in your uSB pulse generator is up to date.

Quote:

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Originally Posted by garylmast

One of the several things I tried was deducing the Velocity In's or mm's to 50% and the Acceleration up to 75%. I also tired changing the Step Pulse and Dir from 1 to 5, but same.

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i have the same problem here, can it be that the ltp port is not stable enough on the pc ?
i lose 9 mm depth on 12 cm distance........strainge.
i use mach 3 on 1 ghz pc xp .
i have nema 34 1250 oz steppers, en cooled stepperdrivers on 60v. one per driver.
to make g-code i use deskproto 7.0 full.
i have used espire 8.5 same results.
test with zero the z axis end move up and x and y move and pusch go to zerro then i lose 2mm hight.
all is in order the coupler, the bearings, the bolts, the spindel, all oke.

what wil be the problem.?????? thx in advanced

I had to install a larger, double torque stepper motor to control the Z-Axis which solved the problem. The original one was just too small. I also added, just for good measure, a 1" diameter spring on the outside to help hold the spindle. Since the modification, absolutely no problems. On a newer machine I bought, I had it equipped with the larger stepper motor, but with air cylinders to help for the weight of the spindle. Again, no problems.