Chattering with low chipload

I’ve had the Shapeoko XL for a little over a month and have been able to make a lot with it, but I’ve been having problems with chattering from the minute I started working with oak instead of pine. I can adjust cut depth and/or speed to get the oak to mill, but I’m running the bits at 30% of the recommended chipload to do so. Something doesn’t seem right and I can’t figure it out. I’ve tightened up the V-wheels and belts a bit, but not real change.

All the details: I’m using a 1/4in 2 flute spiral upcut from Amana, carbide compact router, 10k RPM, and depth of cut of 6mm, 50% engagement. To not get significant chatter I have to run at 700mm/min when suggested chipload would be almost triple that. It wouldn’t be much of an issue, but I’ve noticed the bit gets moderately warm, which wasn’t an issue when running much faster feeds on softwood. I’ve tried running at faster RPM and shallower depth of cut as well, but it just gives me a proportionate increase in speed and still leaves a warm bit (which isn’t surprising with the low chiploads).

Any ideas would be appreciated.

Hmmm, that does seem odd. For your milling parameters, I’d recommend you try a shallower DOC. 6mm is pretty deep for a 1/4" endmill. I typically go around 3mm DOC for mine, depending on the operation. Try just going shallower and see if it goes away.

Are you pocketing, contouring, adaptive with this toolpath? What are you doing?

Also, is your material held down well? You are taking an aggressive cut and will put a lot of force into the material as you mill it.

@wmoy in his videos more or less suggests 0.05" doc (1.25mm) at 50% engagement at 50 to 60 ipm (without calculator, that is something like 1300 mmpm) for a 1/4"
that’s pretty conservative and you can go a bit deeper… but I see the rule-of-thumb of “no more than half of diameter” for non-adaptive paths

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I concur with the 50%D max DOC as a safe comfort zone for non-adaptive toolpaths. Deeper is possible with a sharp endmill AND maxing out RPMs, but it’s probably better to stay in the safe zone while learning.

But it seems you did try shallower DOC:

what RPM/DOC/feedrate combination did you try ?

Note that manufacturer recommendations are often WAY above what can be achieved easily on a stock Shapeoko. Amana has no clue what CNC will be used, and professional CNCs are much more rigid than a Shapeoko, and they can pull off heavier chiploads.

For a 1/4" bit on a Shapeoko in oak, I would aim for 0.001" to 0.002" chipload. So your setup of using a 2-flute 1/4" endmill at 10K RPM and 27ipm (700mm/min) corresponds to a chipload of 0.0013", so I’m not surprised (that it works, and that chatter may start when you double or triple that chipload value)

Bottomline: cut shallower passes, or alternatively go to adaptive clearing toolpaths (you can then easily go up to 200% DOC, but of course with a much, much lower stepover)

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Most of what I described is already adaptive paths with less than 50% engagement and I already run 3mm or less for doing contour cuts that have full engagement. I’ve ran up to 18k RPM when working with hardwood, and with a 1mm cut depth I was able to push the speed to 2k mm/min (chipload .001"), but with that kind of chipload I’m not surprised that I’m getting a bit hot enough to be uncomfortable to touch.

On pine I can run a .005" chipload without issues and the bit is barely warm. I expect it is something with the rigidity of the machine, but I can’t figure out how to improve it. V wheels seem tight, as do the belts. Is this something the HDZ / Z-Plus is made to improve?

Well I am (surprised).
0.001" is not that low, and should not result in an endmill that is too hot to touch. Are you positive that your tool is still sharp enough ? It sounds to me like you may be (borderline-)rubbing, which you shouldn’t at 0.001" chipload with a reasonable sharp bit. I mean sharp as in:


not

You could get away with a less-than-sharp bit in pine with 0.005" chipload, but the same it would cause issues when reducing chipload to 0.001".

The HDZ/Z-plus make for a sturdier machine, but at this point I’m not sure that’s your issue.

If you want, post a sample project that gives you chatter in oak, and I could try and run it on my machine (I have plenty of oak scraps) to tell you what I see/hear.

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The bit is still razor sharp, and things are similar between a flat endmill and ball end I use so it’s not just one bit. My suspicion is that something is still allowing small amounts of tool deflection despite everything seeming tight. I just the gator tooth clamps from the store to see if it’s the work piece itself isn’t secured tightly enough.