Official Luthiers Forum!

When cutting a deep cut, like the outer perimeter of a solidbody body, is it necessary that the bit's length of cut be >= the depth of the cut? Assuming multiple passes are being made to get to depth, and the shank diameter is not larger than the cutting diameter, can you safely cut deeper than the bit's length of cut?

-Mike

Mike, it depends (good answer huh). It depends on the rigidity of the machine, fixturing / clamping, it also has a lot to do with the cutter diameter. Long skinny cutters will whip around leaving chatter marks, but if you had something like a 1/2 shank, 1/2 dia. cutter you could take a pretty hefty cut, assuming you have the horse power.
If you're using a 1/4" shank cutter and 1/4" diameter, I would take several light passes. If you rough cut the body on a band saw so the full diameter of the cutter isn't buried in the wood, the you could more than likely clean it up in one pass. I'd do two passes however, leaving about .02-.03 on the first pass and come back for a second pass to finish up.
The length of flutes on a cutter is not a good indicator of allowable depth of cut.
So, I would use the largest diameter cutter I could and start light and work up to what your machine can do.
Hope that's of some help anyway.

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Jim Watts
http://jameswattsguitars.com

Thanks for the reply Jim. Just as a rule I always make several passes (or have in the past....I'm asking these questions in anticipation of delivery of a new machine)...but just so I'm clear I'm wondering about the ability (multiple passes being a given) of being able to cut the full 1.75" depth with a bit that has an overall length deep enough to do it, but the cutting portion of the bit is only (for example) 1.25" long. So on the final pass, the shank of the bit would be buried .5" deep relative to the top of the material. I guess your "it depends" re: rigidity, etc. would still be very relevant. I guess I just wanted to make sure it wasn't expressly a no-no even on the most rigid machine.

To put the question another way - is it safe for the shank of the bit to contact the material?

In the past I've used long (2" cutting length) 1/4" diamater bits in a CNC Shark, since it was limited to 1/4" bits. So I never had to flirt with the shank contacting the material. But I also got lots of chatter and deflection whipping that long, skinny bit around in a not-very-rigid machine.

Mike, I've had a number of setups over the years where I've had to bury the bit beyond the flutes.
Seems to work ok even though the shank may tend to burnish the surface just a bit.
I think that the unfluted portion of the shank may actually help support the rest of the cutter.
This has usually been with smaller endmills such as 1/8" and 1/4".
Should add that I'm normally cutting ebony in these conditions.
Nelson

Depends on the cutter and material how much the rubbing will effect things, but in general it's fine. If you have an accurate machine, you can offset the cuts enough that it won't rub but the difference won't matter after sanding (0.001-0.002")

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

I do it all the time with metal but it requires caution to blow the chips out of the way with coolant.

With wood you don't want to use a slow feed rate if you can avoid it. The shank will burn the wood if it dwells too long in one place and the rpm is too high. Use a compressed air stream to make sure you aren't getting chips between the shank and the wood. Don't press your luck too much with regards to depth. If the tool has an inch of cutting length and you try to bury it in a two inch groove you might get away with it but it's not a good idea to rely on this method.

One thing you might try is to cut an initial groove that is offset by .2" or so from the contour line and down to the tool's cutting length. Gives the chips somewhere to go when you finally bury the flutes.

Mind you....don't use a tool that has been sharpened to do this as the diameter of a sharpened tool is going to be less than the original shank. That'll be burnish city.

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I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

Thanks guys...you've set my mind at ease about some things.