Progressing on our cnc projects we decided it was time to try out a compression bit, with the promise of our supplier that we would be able to contour in a 15mm ply in a single pass with good finish on both sides. Sounds good. Well we have a stock shapwoko 3 xxl with makita router, the bit is the one in the link below:
We got the 8mm and we were advised by the supplier to use 18k @4000mm/min feed and 16mm depth per pass for hardwood.
Honestly it sounded unlikely from the beginning to do such a cut but we gave it a go on birch ply overriding the feedrate at 50% initially. Even at 2000min/min slotting at full depth resulted in horrible sounds, huge vibrations and very poor finish.
We tested other values but weren’t able to find a good setting to cut in a single pass, so we resolved cutting with a 5mm doc which resulted in a good finish but much more cutting time.
Has anyone got any advice about how to achieve a good cut with a doc of 15mm with this kind of bit?
My belief and experience is that the shapeokos are not stiff enough (due to belts in my opinion ) to run at production speeds at that deep of cuts.
I can cut 3/4” Baltic birch full depth with a 1/4” compression bit. 18k rpm and 60 ipm but it burns the bits up because it’s not meeting chipload recommendations. I need the bits to last more than a couple weeks so I have two alternatives.
First is if I am pocketing already I run a 1/4” down cut down to 1/4” above stock bottom leaving 20k stock to leave. Then come in with the compression bit to finish the job.
If I am not pocketing I use a 1/4” compression bit at 18k rpm 150 ipm 1/4” doc and do it in multiple passes.
As Ryan says the Shapeoko just isn’t big, heavy and rigid enough to make that cut.
I use my compression bits as finishing cutters after roughing out with a regular low helix upcut leaving a couple of mm to come back and take off in full depth finishing passes with the compression cutter.
The figures for the endmill are for machines where the endmill will give way and snap before the machine is even aware of any issue.
So in industrial environments these ratings are more like limitations of the endmill, where as in the Shapeoko world they are aspirations for the machine.
Compression bits are meant to run at full depth cutting both the top and bottom of plywood at the same time. That would work with a heavy duty 3 HP Router or in an industrial CNC machine. A Shapeoko is not a 3HP Router or an industrial CNC machine. The idea of a compression bit is the bottom of the bit is an up cut bit and the top of the bit is a down cut bit. This is perfect for cutting full depth but a Shapeoko just cannot do that.
A good rule of thumb of any bit is DOC is half of diameter maximum. So if you have a 1/4 inch bit you should only cut 1/8 of an inch deep per pass. This rule of thumb keeps stress off the bit and keeps it from overheating and losing its temper. Even Carbide is sensitive to excessive heat. If you look at the DOC of a #201 upcut bit in softwood is 0.060 inch. Now compare that to 0.060" cut to .71875" depth of cut with a compression bit that is almost 12 times more doc of cut then what Carbide3d recommends for a 1/4 inch #201 bit in softwood.
A compression bit is capable of cutting full depth in 18MM plywood, just not with a Shapeoko. You need much more power to cut successfully at full depth. What your supplier is telling you is true for the bit but not for a Shapeoko. Do you think your supplier knows what kind of machine you are using?
@gdon_2003 hi guy, thank you very much for this very clear explanation.
What Disappoints me now is that I indeed asked for advise my supplier specifying machine, router spec, material and type of cut I was willing to achieve…maybe they don’t know the Shapeoko but what kind of machine could run a 700w makita? Its clearly not going to be an industrial grade cnc…
Industrial grade CNC’s are incredibly powerful. Incredibly. Forces equivalent to those that forged the universe. Well, perhaps a little less than that.
So, thinking of the Shapeoko as being equivalent to a trim router being automatically moved around by an average strength human is probably a good idea.
I think recommendation for trim routers is to never to cut more than 5mm (3/16") in one go, so I’m not sure you’d try pushing the trim router by hand through wood at a 16mm depth of cut.
I treat the endmill specs as headroom - upper limits of the endmill (ie: don’t push the endmill more than this) as opposed to minimum speeds (ie: you must run it this speed to make it cut properly).
I suggest you give the compression bit a try as a finishing cutter at full depth but only 0.5mm to 1mm sideways stepover, mine does an excellent job on birch ply that way. As Guy says, the Shapeoko can’t push that cut at the full width of the cutter, but it can shave the sides down.
You’ll need a roughing toolpath first to hog out the slot with an upcut bit going down at depths the Shapeoko can manage.
I agree it’s unlikely your tool supplier knew what a Shapeoko was or how limited the machine is compared to a $50,000 industrial machine.
It really all depends on what you bought the cutter to do, if you just want to get to a clean finish on the wood then use it as a finishing cutter full depth but small sideways, if you wanted a cutter that can slot full depth, full speed, the cutter can but you need a much more expensive machine to run it.
@LiamN hi Liam thanks for your suggestion, I could use the compression bit to shave off the last mm but the upcut bit I use for roughing tears off long splinters that I would need to shave off 3 or 4 mm to get a good finish…is that because I am being too aggressive? We are cutting with a doc of 3mm with 6,35 carbide #201…or maybe the Bit is not Sharp enough?
I’ll soon post a picture to get you an idea of what I mean…
Try roughing with a downcut bit not quite through the bottom, but deep enough to engage both directions in the compression bit then finish roughing and run the finish cut with the compression bit.
It could be feeds & speeds, it could be the wood. I found in high quality baltic birch I could get very good finishes with very little tearout, just running a marking knife over the top (when finished) took away the stringy bits. However, in lower quality ply I had some really bad tearout such as that you describe.
In the lower quality ply I ended up using a cheap Yonico low helix downcut (from Amazon) in a shallow first pass to cut through the top layer, which is very similar to David’s suggestion.