Lazy "Z"

[vpl]

I have been running this router for a while now.



I bought it in pieces and with the help of several cnc zone members its up and running. Its come a long way in a short period of time. Thanks!

I have done lots of 2d and 3d projects and all is well. I have made plenty of mistakes along the way and have learned quite a bit. I have come up with a weird issue as of late. I use Bobcad-cam v24 & Mach 3. I have been roughing out a skateboard press mold with a standard 3d rough. No issues. When I finish it with a 3d finish it doesn't look great. So I have been trying to do a planar finish. Every once in a while it losses "z" zero. It seems to lose it at the end of an "x" pass. See the pic below.


Groove #1 was towards the end of the planar slice finish- As soon as it cut that deep I stopped the run. Otherwise it would likely have just kept cutting as deep. I checked where "z" zero and it was relative to the new depth of cut.

Groove #2 was a start over. It went fine until the end and it cut like .075" too deep.

Groove #3 was the last time around. It looks like "z" never came up after the "x" pass. It dislodged the work piece and made a funny cut at the end of the slot.

This "z" issues only occurs (or seemingly so) on "planar slice". I have hours and hours of cut time without any crazy zeroing or wandering issues.

My first guess was the coupler. So I over tightened it-of course. Not convinced I went a head and marked it and then ran it.



The resulting issues was groove #3 and the marked area did not slip. The coupler is fine. In addition you can manually run "z" to the top or bottom stop with the thumb wheel on the stepper motor. You can crank on it real hard and nothing slips in either direction.

All the cables with the exception of an extension cord for the router are shielded and have a ground wire. The ground wires are run into the control box and grounded. The control box is grounded to the earth.


I pulled the vacuum tube from the dust collection system on 2 of the 3 grooves, feeling it may be discharging static electricity.

I have looked the tool path over and it looks good.

In short once zero is lost on "z" it appears to stay there. If you ask the machine to "go to zero" after it has its issue "x" and "y" are fine. "Z" will be how ever much lower the groove is below where its supposed to be.

The only other input I can mention is the little nema 23 gets fairly hot. Hotter than the other steppers. Hotter than any other operation that I have done so far.


Any thought?? Thanks for taking the time to read this post-


VPL


PS I'm running a PorterCable 3.25 router-
I just purchased a 3 KW Chinese spindle and VFD......Waiting on some aluminum and VFD Cable to finish up the job. The spindle and VFD came quickly from China. I also purchased a ball screw directly from China. Its arrived quickly and I have been using it ever since....

[harryn]

It would be good to post some more pictures detailing the construction of the Z, the stepper motors, and the stepper driver to help with the diagnostics.

I will go out on a limb here and say that the problem is that somehow, the torque on the Z motor is not sufficient to hold its position.

One way this can happen is that sometimes, the stepper motor drivers will reduce the standby current in the motor if it isn't being called on to run for a while. When this happens, then in theory, the Z motor might not have enough torque to hold its position and the screw could start sliding down.

The other thing that could be happening, is that if the motor is in fact getting full current, it could be overheating. Similar things could be going on in the driver board.

Since the problem is intermittent, then hot electronics might make sense as a possible culprit. In that case, put some fans on the driver and motor and see if that helps.

Of course this is a complete guess, but fans are cheap.

[adprinter]

I experienced a similar problem some time ago. Happened at seemingly random times. After going through the construction of an elaborate (-)Z Limit switch assembly, I later discovered the cause of the problem- the mounting bolts for the Nut assembly on the Z axis had vibrated loose! I retorqued them, and have not had any problems since. You might want to check yours.

[Treischl]

check that the motor shaft is not slipping inside the coupling. I caught mine doing that the other day, giving me the exact same problem you are seeing.

[louieatienza]

Have you checked if the ballscrew is moving in the bearing block?

[ger21]

Try cutting your Z axis acceleration in half.