Z-axis Skip during 3D cut

[Chazz]

I am having a problem that is new to me -- my Z-axis stepper is sometimes skipping during a cut. It has happen three times now -- twice in the last cut. The first two skips (or what appeared to be a skip) were very small. The last one... changed my Z by about 3/8"!!!

It has only happened so far, while doing a 3D finishing cut with a 1/4" ballnose bit. The 3D pattern has some shear Z-Drop surfaces; but the tool seemed to run those fine.

Also, I admit that I was not there watching. So some of this is supposition.

Lastly, I have recently finished the 5.5" Z upgrade. However, it seems to travel just fine. The spring is in place.

Thoughts?
Thanks,
Chazz

[scottp55]

Chazz,
Sound like you might be losing steps?
What Diameter was Roughing bit,material,and 3D finish Move Speeds?

[MogulTx]

Also check to make sure the bit is not slipping in the collet.

[Chazz]

Chazz,
Sound like you might be losing steps?
What Diameter was Roughing bit,material,and 3D finish Move Speeds?

Using Router
1/4 ballnose (Amana Tool #46294)
harder wood (like white oak) sorry, not up on my wood IDs
Roughpath's machining allowance: 0.1"
Finish path's feed speed: 2.0"/s
Finish path's plunge speed: 2.0"/s

First skip on this last cut was about 0.03" and the second one was about 0.13".

I can't really tell if the bit slipped but as a sort of test, I tried to use the motor to drive the ballnose into the spoiler board (router off) and it didn't seem to move in the collet.

[MogulTx]

Chazz.

The combination of the pulling forces of the router being driven into the material and pulled against it in a rotational manner, may move the bit. You should be able to measure the bit extension out of the collet and tell if it moves after XXX minutes of run time. ( The collet might heat up and expand and allow the bit to slip.) The fixes for that are (1) replace a loose or damaged collet (2) tighten the collet nut more. (3) replace the bit if it is loose in the collet from being milled too small. (4) replace the collet with the next size smaller collet so that it grips more aggressively ( assumes a metric collet and a loose fit and that a 1 mm reduction in collet size would force it to grip more aggressively). A dirty collet can also be a problem. If it is clogged with sawdust, in the "fingers" of the collet, it may not grip an appropriately sized bit with enough strength to entrap it. Solution if to blow it out and or scrub it with a brass or plastic bristle brush to remove debris.

Dull bits can contribute to bit pulling out. Bad feed /speed rates can contribute. ( I have never run my systems at 2.0 in the Z while doing a 3D roughing cut. Is it wrong? I dunno. But I would back it down to 50% of that and see what the performance is. Maybe even to .7 and see what the performance looks like. I am guessing that you may have pulled a mostly well secured bit out of your collet... but it is merely a guess at this time....)

Work on it. Let us know what progress you make.

[scottp55]

Well, So much for my theory...with using the same bit, those speeds, and that allowance, it shouldn't be a toolpath thing.
Does it do it running regular 2D operations as well, or just 3D with the 2IPS plunge?
When you did your Z-upgrade, did everything look good with no wear marks and moved freely on rails?
Can you inspect back there to make sure no new wear marks, all bolts still tight,and nothing protruding where it shouldn't.
On a Keypad Move/Jog (multiple times to limits) does it return to the zero you set (maybe use a softwood test block and feeler gauges)?
Did you install your upper Z prox, and is it working?
Did you redo set-up and check the box (I think third page?) "I have Z prox" and does it now run the Z routine with a double tap/pause at VC set safe-Z briefly, then continue to upper Z prox and retract an 1/8"?
Does it do it on an aircut of that model?
I might make a VCP file with material set-up safe Z set to approx what your 3D Z is and, run an aircut using your common 2D settings just to try to isolate it.
Good Coms and packet size test?
Out of ideas then...So I'd call Tech and see if they had run across a similar problem.
Imagine you've checked most of these already.

[Chazz]

Chazz.

The combination of the pulling forces of the router being driven into the material and pulled against it in a rotational manner, may move the bit. You should be able to measure the bit extension out of the collet and tell if it moves after XXX minutes of run time. ( The collet might heat up and expand and allow the bit to slip.) The fixes for that are (1) replace a loose or damaged collet (2) tighten the collet nut more. (3) replace the bit if it is loose in the collet from being milled too small. (4) replace the collet with the next size smaller collet so that it grips more aggressively ( assumes a metric collet and a loose fit and that a 1 mm reduction in collet size would force it to grip more aggressively). A dirty collet can also be a problem. If it is clogged with sawdust, in the "fingers" of the collet, it may not grip an appropriately sized bit with enough strength to entrap it. Solution if to blow it out and or scrub it with a brass or plastic bristle brush to remove debris.

Dull bits can contribute to bit pulling out. Bad feed /speed rates can contribute. ( I have never run my systems at 2.0 in the Z while doing a 3D roughing cut. Is it wrong? I dunno. But I would back it down to 50% of that and see what the performance is. Maybe even to .7 and see what the performance looks like. I am guessing that you may have pulled a mostly well secured bit out of your collet... but it is merely a guess at this time....)

Work on it. Let us know what progress you make.

I don't recall ever having this problem in 2D... ever.

I doubt the collet has any issue. I have never hit metal in operation and while there is alway dust on it when I pull it to do my bit change, it always comes off by just blowing on it. It seems brand new.

The bit has a line between the silver and gold portions which would make an easy reference mark for checking for slippage in the future. However, I didn't note where it was when I put it in this time.

The bit is pretty new as in this week was the first time I used it. That said, I suppose there could be a defect that I am not able to see.

I don't know what my roughing plunge speed was off hand. The 2.0 (and the problem so far) is only in 3D finishing cuts.

I have a theory which I will post shortly...

[Chazz]

Well, So much for my theory...with using the same bit, those speeds, and that allowance, it shouldn't be a toolpath thing.
Does it do it running regular 2D operations as well, or just 3D with the 2IPS plunge?
When you did your Z-upgrade, did everything look good with no wear marks and moved freely on rails?
Can you inspect back there to make sure no new wear marks, all bolts still tight,and nothing protruding where it shouldn't.
On a Keypad Move/Jog (multiple times to limits) does it return to the zero you set (maybe use a softwood test block and feeler gauges)?
Did you install your upper Z prox, and is it working?
Did you redo set-up and check the box (I think third page?) "I have Z prox" and does it now run the Z routine with a double tap/pause at VC set safe-Z briefly, then continue to upper Z prox and retract an 1/8"?
Does it do it on an aircut of that model?
I might make a VCP file with material set-up safe Z set to approx what your 3D Z is and, run an aircut using your common 2D settings just to try to isolate it.
Good Coms and packet size test?
Out of ideas then...So I'd call Tech and see if they had run across a similar problem.
Imagine you've checked most of these already.

It just happens in 3D finishing cuts.... I don't KNOW that it is a stepper slip in a plunge, it could be bit slippage upon the upward move.
In the upgrade, I did not find any worn parts. Nothing required a hammer to get back together. I would say that the long spiral gear coming out of the Z stepper didn't seem as well lubricated as the others. I gave it a shot of lub; but it just doesn't have the same free feeling. However, this cut shouldn't have taken the Z travel to the top end of the screw gear...

As for Z testing, I have always wanted to modify the Z-zeroing script to report what the difference was from the old Z setting to the new.... hummm
I have not attached the Z-prox switch nor changed any settings to let it know.

I have done some pretty severe fluting passes in 2D mode which would be kind of like this 3D stuff... no problems noted. However, it was a medium dense wood rather than hard.

[Chazz]

0630151936_resized.jpgI think that there are some contributing factors and I have a way forward for some testing....

1) The pattern had a steep drop-off -- as in vertical.

2) In the more severe case, the bit could have been pinched between the 7/8" pattern drop-off and the 7/8" 3D cut boundary.

3) By leaving 0.1" machining allowance on the roughing pass, there was a lot to pull out on those vertical surfaces.... all at once... like, for a moment, doing a 7/8" deep pass through the hardwood material. (opinions on this please.... how much of a machining allowance when using 1/4 upcut for rough and 1/4 ballnose for finish?)

I still don't know if it was a stepper skip or bit slip; but I can note when I put the bit in and report if it happens again)

I will take a picture and post shortly... (should have done this at first)

[Chazz]

So, I had left the desktop and all on, with the bit in and decided to redo the cut as a test.

1) I zero'd the Z to the spoiler board.
2) I moved it to Z 0.
3) I measured from the spoiler board to the bottom of the plexiglass under the router.
4) I ran the 3D finish cut again. It did jump again -- in a different spot -- BEFORE it got to the new material that had not been cut yet....
5) I moved it over to the spoiler board and eye-balled it down to just touching. It was at +0.04" Z.
6) I remeasured from the spoiler board and it was the same measurement. From this, I would claim that the bit did not slide in the collet.

7) I removed the bit and ran the router down to the lowest I wanted to get it to.
8) I zero'd Z to that point.
9) I cautiously ran the router to the highest I could comfortably get it. This turned out to be about 5.4
10) I made a long script of "jz 0" followed by "jz 5.4" over and over again.
11) I ran the script.

While the script was running, I noticed odd movement in the servo screw gear down from the Z servo. From the router moving from top to bottom, the bottom of that screw gear was moving side-to-side by 0.15" I think I can adjust something to get rid of that movement; but I don't see this as being enough to cause my original problem. Especially since the 0 - 5.4 test ran for 20 minutes or so without skipping a beat.

Thoughts?