Odd problem with reliable motion

Mostly Printed CNC - MPCNC Troubleshooting
#1

Hi everyone,

So I have a larger MPCNC (600x600x80mm build volume.) When I was pocketing with a block machining operation from Estlcam, I noticed that parts of the pocketing of the gap was not cutting out all of the material, there were lines of wood left in the gap area.

Realizing this might be a misconfiguration of my steps/mm, I checked my GRBL config. I have my steps/mm on my 96oz steppers at 100 steps per rev, as that’s what the calculators pop out for the right value when I account for my belt and pulleys, etc. I don’t use a RAMPS board for controlling; I have external stepper controllers for each high-power motor.

I made a new test object, a 50x50x12mm box, and I configured Estlcam to cut that box and have a 30mm gap around it so that I’d have plenty of space to measure and see the problem in action. Motion in the direction where the gantry moves on the lower crossbar is dialed in perfectly. 50mm on that axis (my Y axis) resulted in a box side of 50mm through the entire length of that axis on the test cut.

Motion on the upper crossbar, however, produced a wiggly, variable cut length between 43mm and 45mm. At no point was the side of that box ever 50mm. I also noticed the same banding in the gap area. I used the caliper to measure the width of those bands of uncut wood and they were between 1.3 and 1.8mm thick.

At this point, it seems like something in my motion system is loose. I think the z-axis could very well be loose, but I’m wondering if there’s any other place in the build that might contribute to this wobbling.

Has anyone had to diagnose similar issues, and if so, what was your fix?

Thanks in advance,
Pete

#2

Do you have a picture of this cut?

Cut speed?

With the grain, across the grain, what kind of wood?

Spindle?

Bit?

#3

Sorry, I didn’t include pictures because I was asking more as a “does this kind of problem ring a bell with anyone” – I’ll get some pics all the same.

#4

I understand, but it really could be anything.

I naturally assume CAM issues. It is by far the most easily messed up.

Since you are using grbl and some very non standard steppers I have to assume you got that right. Most steppers are 1.8 or .9 degree (200, 400 steps per rev.)

#5

I setup an album on imgur with my photos.

The material I was cutting, I thought it was Pine, but it sure smelled like Cedar during the cut, so it could be either pine or Cedar.

Spindle is a Kress 1050 FME-P

Bit is a HQMaster 1/8" end mill

DOC is 4mm, Spindle is set to 18krpm, I was moving at 30-60IPM (I changed it mid-run to try to ascertain whether my “whining” bit was due to too fast/slow.)

[Album] Cut issues

#6

Try shallower cuts. With that much height you’re limited to depth of cut. Pretty sure what’s happening is the endmill is pulling the spindle into the cut, that’s what it looks like on that lower end of the square anyway. I’ve had that happen and it actually overcame the holding torque of the steppers and started cutting loop to loops in some plywood. That was exciting! :lol:

#7

Sounds like it was exciting indeed! I did a similar test at 1mm DOC and the bit sang like it was annoyed. The 4mm DOC the bit was quieter, but still sang a bit. My understanding is that hearing the high-pitched whine from the bit indicates the tool is not being fed properly (either DOC, feeds/speed are wrong). The bit I used, the suggested feeds/speed was 4mm DOC at 30ipm and 21k rpm.

What depth of cut do you recommend I try? I could do 500 microns instead of 1mm next I suppose.

Pete

#8

Something else you can try is put a thicker piece of wood under whatever you’re cutting. That’s a pretty tall machine for cnc. Most of my cuts in plywood, which is harder (to cut) than solid is between half a millimeter and sometimes 1 or 2 millimeters. It’s hard to give good feed rates for these machines since everybody’s is slightly, or greatly different.

#9

if you have no plans to use it as a 3d printer I would knock those legs down for sure. and how tight are those belts?

#10

Your CNC is far too high.
Reduce its height, that’s the only good way.