While trying to identify the root cause of failure of my overshooting on the depth when I was doing a perimeter cutout, I had assumed that I had slippage in the collet (a problem I had when I first started out a couple of years ago), but when I went back and looked at it the bit did not seem to be pulled out any.
So my question is: If I run an upcut bit too aggressively can the forces in the z-axis overcome the holding power of the stepper motor?
Details
Shapeoko Pro XL (the unnumbered one) with 65 mm spindle (ER-16 version)
Bit Cadence MFG 1/4 inch upcut extended reach
Feed rate 100 inch/min (I noticed I did the specs for the 1/4 uptown jenny rather than the extended reach so this is a little more aggressive)
Plunge rate: 50 inch/min
Spindle speed: 12500 rpm
Pass depth: 0.125 inch
Max cut depth 38.1 mm
The bit getting pulled out was my original thought (hence the request to have the option to recheck length on pause), but when I looked the bit it seem to be where I put it in the collet. This extended reach bit has a relief between the flutes and the part of the shaft that clamps in the collet. I always place that relief just below the collet (so there is full diameter in the collet). It seemed to be in the same position. That’s why I was wondering about pulling on the stepper motor losing steps.
My initial thoughts on solutions, before I noticed the bit seemed to be where it was supposed to be, were to look into torque specs on the collet nut, measure the stick out and record for comparison (hence the request to have the machine be able to do a check)
Since this was a 1.5 inch thick piece of material (38.1 mm) I tried to think of other scenarios where there might be lost steps. I don’t think it could be from prior bits since there is a measure at tool change. This was third item cut out in the sequence and the first two were fine.
So, I can’t seem to figure out the why.
Strategy going forward
back off on the feeds&speeds on cutting out
Measure bit stick out and note down, so I can check on pause if bit is pulling out
Possibly do some clear cutting outside the part perimeter using a roughing bit to give a bit more space ( pocket a band from 3 mm to 16 mm outside the perimeter down to about 6 mm thickness to give plenty of clearance) I already need to do this in some areas for some shorter bits where I am cutting a wedge shape to clear the collet nut
And if I go all OCD, look into getting a torque wrench that will do 18 ft-lbs on a 22 mm nut with clearance for the bit.
Not sure if it’s been mentioned elsewhere but your workpiece may be distorting under the cut.
If it’s a larger thin workpiece that is only secured on the perimeter, the up cut could be pulling the workpiece up off of the table a tiny bit towards the center. (Or towards the edge If the tabs aren’t beefy enough)
That could look like a deeper than expected cut when the workpiece settles.
Yes, but it wouldn’t cut 5 mm into the spoilboard, I can see the ramp down in the spoilboard (and the remains of the stock). I used bottom of stock as the reference point so it should stop there.
One thing that occurred to me, which I didn’t check (but don’t think is really plausible) is did it pull the spindle down in the mount.
Yikes, 5mm is rough. The spindle mount is an interesting idea but I think you’re right, doesn’t seem particularly plausible.
That large of a gouge rules out quite a few of the usual suspects
It’s not the bit slipping.
It’s not the spoiler bending up.
It’s not the gantry bending down.
It’s probably not lost steps…with it being so deep, if you carefully jog to origin after you cut you would be able to see if it’s that far off. Same test would work of it’s the spindle slipping in the mount.
Sounds like a tricky problem you got on your hands. I’d probably blue sharpie every metal thing I could think of in the off chance that it happens again. Make it very visible to see what might have slipped.
