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Inconsistent cut depth (not levelling-related)

Hi! I noticed on some pocket cuts that the actual cut depth on neighbouring paths can vary by as much as 0.6mm - my only theory is that this is caused by the head flexing when cutting towards one path in one direction and the neighbouring one in the opposing one, but the difference seems extremely large, which is worrying me a bit, and is forcing me to do a lot of cleanup

Some details

  • 1500x1500 machine, mafell router head
  • these pockets are along the Y axis, but the same issue occurs when cutting in Y direction
  • These particular ones were cut with a 6mm 2 flute downcut endmill, at about 12k rpm, 2000 mm/min, 3mm depth of cut, in birch ply
  • stepover was quite conservative (about 3mm)

I’m also having problems with chatter and vibration in the router head - I’ll make a separate post about that, but mentioning it here as I wouldn’t be surprised if the problems are related

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Hi Rob,

Thanks for your question.

For your speeds and feeds, I would say the RPM is a little low. I would increase it to 18k rpm and see if that helps.

Also, 3mm pass depth could be a little deep for birch ply. Have you thought about doing 3mm pass depths, but on the last pass, reduce it to 1mm so you can get the bottom as good as possible?

Thanks!

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Hi Rob,

Ok does the endmill you are using give a recommended RPM or just a chip load? I just based my value on Amana Tool datasheets and found an endmill that sounded similar to the one you were using.

I think 3mm could be fine for hogging, but I would reduce it for a finishing path.

Thanks

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It is an amana one, I actually haven’t been able to find exact numbers for their metric endmills, so I’ve been going off the 1/4 inch one and just using the formulas.

Their datasheets indeed all assume 18k rpm, but that’s for 4500mm/min at up to 6mm DOC (and obviously assuming a very beefy industrial machine). I’m finding it quite tricky to toe the line between taking chips that are too thin and overloading the tool, but I do think my assumptions are skewed towards the latter because of how aggressive all the values in the datasheets are.

I’ll try out higher RPM combined with a gentler finishing pass later this week. I did actually try this with a similar upcut endmill before and still got a lot of vibration and chatter (to the point where i had to stop the cut), but I think I need to go back and test a bit more systematically.

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@kirberich Hi Rob, I see, we always stick to their stated RPM regardless of chip load. The datasheets are very aggressive, so just tone down the pass depth, and try to keep the feedrate higher so you do not rub.

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I did see that most posted numbers for RPM on the workbee are much higher than expected - the cutter I’m using has a recommended chip load of .12mm, which results in a theoretical rpm of ~8300 at 2000mm/min - I already reduced the chip load to 0.08mm to account for the fact that the workbee isn’t the most rigid machine, but 18k would mean a chip load of 0.05mm, which to me sounds like getting close to the tool just rubbing - but maybe my expectations are off here, I’m still quite new to this!

If you have some guidelines on how far you generally derate endmills for the workbee, it’d be quite useful to know!

The cut depth is an interesting one though - i didn’t think 3mm with a 6mm endmill would be pushing it, but I’ll try with a gentler finishing pass and see if that makes a difference!

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Author avatar Rob Kirberich will be eternally grateful.
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