Losing steps in Z - need help

[zeeway]

I am trying to run the famous VCarvePro Bullshead pub sign. My router starts off cutting the little squiggly design by the B, but by the time it gets to line 5000, the router tip is cutting air - it is .094 inches above the material.

Here's my setup Mach 3, Gecko 540, cncrouterparts.com motors 380oz-in, extension cables and 48v-7.3A power supply. Typical router (41.5" by 24" overall) running on skate bearings and pipe. Feed screws for x and y are 1/2-10 acme 5 start with Dumpster ABN. Feed screw for Z is 1/2-10 acme with delrin nut - no antibacklash...I figure the weight of the router motor provides some backlash compensation.


1. Slowed Z speed and accel way down 50 ipm and 10 i/s/s.
2. Lubed all feed screws.
3. Set all tool offsets to zero to make sure these were not sneaking in.
4. Tested with router on and off (above material).
5. Changed pulse width to 5 - but Mach3 kicks it back to 4.
6. Ran a trial program of about 50 z moves which ultimately came back to zero...and this worked like a charm.
7. Opened control cab to make sure gecko was not overheating - also checked by hand - I could hold my hand on there, so I am guessing it is 140 degrees F or less. Power supply is cooler than that.
8. I also chopped the Bullshead pub program in pieces - and it appears that all the z axis is out of position by line 5000.
9. I also used the Mach3 function to calibrate the X, Y and Z axes to match actual movement, and this looks good.
10. Marked couplings and feed screw on Z - could not detect any slip. Also cannot detect slip from coupling to motor shaft.
11. Disassembled Z axis and realigned tubes for the skate bearing carriages.
12. Remounted X, Y, and Z motors to make sure they is not hanging on anything.

I did have a slight problem in x and y axes not returning to correct zero, but the above seems to have fixed this problem.

Every so often when I try to jog the z axis, it does not respond. If I go into the motor tuning screen and change the smallest thing, it wakes up again. Is this normal?

So the only thing I have thought of now is to change out the Z axis feed screw to go to 1/2 - 5 start with a dumpster ABN...guessing it may be some kind of backlash problem.

Any other ideas. I would appreciate any help.

[ger21]

Try the Z axis accel at 5. I had mine at 10 and it was fine 99% of the time, but I did a carving the other day, and it started rising up. Set it to 5 and it cut fine.

[zeeway]

Thanks, Gerry. I'll give that a shot...by the way, why would a higher accel number cause the axis to rise?

[jsheerin]

It tries to accelerate down at the higher acceleration. The force of plunging into the wood is too high, and the motor misses steps. The controller thinks the z position is lower than it is. When it raises the z axis up, it moves the full distance due to no cutting load, but now it is higher than it should be. If it does this multiple times, then the z axis gradually creeps upwards. I've had mine not retract due to stalling the motor because of dust jamming the linear bearings. Then on the next vertical move, it plunged the cutter through the table.

Another possibility is that you have a parallel port that can't source enough current to drive the G540's opto isolators. I had this problem. I showed some scope traces in the Gecko forum from when I was trouble shooting it. I ended up putting in a pci dual parallel port and having to make a custom cable to use pins from both ports to get it working. The way I eventually found to reliably diagnose this was to run all axes simultaneously with a program at very low speed. Then watch the motor couplers to make sure they were all turning simultaneously. Also check in both directions. At higher speeds it would typically work which is why it was so hard to track down.

[zeeway]

Thanks jsheerin - that makes sense to me. I am using an older router and maybe it can't handle the chip load on a down feed. Maybe you have just justified a new router for me..

[ger21]

The force of plunging into the wood is too high, and the motor misses steps

When it happened to me, I was running a finishing pass with a 1/16" bit and .005 stepover. Basically zero cutting forces. I think I was just gtrying to go faster than the machine was capable. There were some pretty violent direction changes going on.

[mlabruyere]

Originally Posted by jsheerin I've had mine not retract due to stalling the motor because of dust jamming the linear bearings. Then on the next vertical move, it plunged the cutter through the table.

I had this as well....had to turn over the table due to the huge rip in it.... I also lost a really good 1/4 bit in the process...dang.

I set my Accel on the Z to 5 as well. Don't need more than that since it doesn't go far anyway. No problems now.

[jsheerin]

Actually I didn't mean the router motor - I meant the stepper motor on the Z axis.

[cyclestart]

Originally Posted by zeeway Every so often when I try to jog the z axis, it does not respond. If I go into the motor tuning screen and change the smallest thing, it wakes up again. Is this normal?

That doesn't sound normal at all. I'm not a Mach user but if that was normal Mach operation the complaint would be heard here loud and often.

re: losing steps due to cutting pressure. Run the program without a tool or at some distance above the work. It will prove or disprove that possibility.

I had similar problems with the g540 as have a few others on these forums. In my case it didn't become obvious until attempting 3d cutting. I 'solved' it by lengthening the step timing, solved is in ' ' because it may be a kludge masking another problem with my setup/hardware. jsheerin's response makes a lot of sense. I'm not pointing a finger at the drives, just saying don't rule out the possibility.

[ger21]

Every so often when I try to jog the z axis, it does not respond

When it doesn't respond, do the DRO's change? If they do, then it's an electrical issue. If they don't, then you have either another application interfering with Mach3, or possibly a bad Mach3 install. Try un-installing completely and trying the latest version.