Z Axis dropping

[GM Barton]

We purchased a used 3 axis CNC Router (home built) running WinCNC with Anaheim Automation stepper motors and drivers.

All was well for a year or so, then we starting having problems with the Z axis dropping down and gouging the part. The PC display (WinCNC) shows the Z axis coordinates that should be cutting, but it keeps dropping lower and lower into the part.

It seems to be temperature related. It always starts out fine, but then as the transformers and driver boards start to heat up, the Z axis suddenly starts dropping down into the part. We thought that the Stepper Driver Boards may be overheating (Anaheim BLD 75) and sometimes a red light would turn on the Z axis board. We replaced the board and attempted to avoid overheating, but the problem is getting progressively worse. It runs shorter and shorter times before the Z axis starts dropping. We've also replaced some of the stepper motors.

Most recently, we cut with no load (moving but not cutting any material) fine for several minutes, but as soon as we start cutting and experience a load, the Z axis drops. In this test program, we're cutting a flat plate, so there the Z axis is not moving via the program at all. Most of the movement is on the Y axis with small stepover on the X axis after each pass. So the Y axis is doing most of the work, but the Z axis is what fails and drops down.

We're at a loss to find the problem. I've just ordered a replacement PCI 7200 board from WinCNC to see if that could be the problem.

Any suggestions of what might be causing this?

Thanks,
Greg

[jalessi]

Greg,

Why not swap the drive with the X or Y and see if the error moves to the new axis?

You can swap motors also, however only move one part at a time so you can isolate the problem.

Jeff...

[LeeWay]

Swapping parts may help to pinpoint the issues. It may also be sub par components. IE, the power supply may be too small, motors too small, drive too small, etc.
This may not be the case and heating may be affecting it.
It may be a holding torque issue when heated.
Another thing you may try that would be inexpensive is a bungee cord or small spring to give the Z extra holding power. In other words, take some of the weight off that axis. It may not take much to do it, especially if it is a torque thing.

[GM Barton]

Yes, I've tried switching the stepper drives, replacing the Z drive with a new one and also replacing the motor on the Z drive, but the problem only exists on the Z drive no matter what we do.

The spring or bungee cord sounds like a good idea. I see if we can work something with that.

- Greg

[CarveOne]

This sounds to me more like a mechanical problem at whatever is being used to couple the z axis motor shaft to the lead screw instead of an electrical problem. My z axis Delrin coupler can do that, and needs to be re-checked each time I use the machine. I have no means of support at the bottom of my z axis lead screw and the shaft will slowly slide outward from the coupler.

If yours has lower end support, check if anything else can move vertically and cause this problem.

CarveOne

[YZF]

I agree this sounds like a mechanical problem. Loose coupling. A loose coupling will behave this way because it will tend to slip more under the higher load of lifting the axis.

[GM Barton]

I put marks on the couplings and shafts. Then ran for 23 minutes. The Z axis held constant, then dropped 5mm during the last 2 seconds (it was continuing to drop when I hit the emergency stop). I then checked the shafts/couplings and none of them had moved from the marks I made.

[dertsap]

is it possible your slides are sticking and causing the motor to loose steps , this happens to me every so often because i cut so much mdf and the dust tends to get into the linear slides bearings , i just spray the crap out of the slide bearings with some wd40 and it flushes the dust out and its good again

[YZF]

Were the marks were on the motor shaft, the screw and the couplings? That's a good test and it seems to pretty much rule that out. As you've swapped motors and drives perhaps the problem is with the PC or the cabling?

[CarveOne]

Even with the all surface area of the lead screw threads I have had that side of the coupling move also. It happened during the EMC2 configuration testing when the z axis was going up and down repeatedly. I think the Delrin coupler had relaxed (cold flow) under tightened screws over a period of time and the reversals made it slowly unscrew a little each time it started going upwards. probably had the acceleration setting too high also.

Just for grins, try lowering your acceleration setting for the z axis and see if it has any effect on your problem. I doubt that it will, but it's something else to try.

CarveOne

[GM Barton]

I replaced the WinCNC computer control board. Also made a solid aluminum connection between the stepper motor drivers mounts and a large aluminum plate - to draw excess heat out of the boards. Neither of these changes has helped.

Now I'm going to replace the Z-axis stepper motor and double check the connection. It's a keyed connection so should not be moving, but will confirm.

[LeeWay]

If it's not moving until things heat up, then I would suspect the drive. Possibly the motor. Surely electrical though. I personally have been through the same things and found each time that it was mechanical. I have now locked all my connections with lock tighted set screws and I drilled seats for them as well. Basically now like a keyed connection. Keyed connections can still slip, but it sounds like the motor is simply loosing power or torque.
The spring thing may help. You should be able to rig up something temporary just to see.
If that helps, I would look into the drives, motors or PS.