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I have an issue with this when CNC carving archtop plates with a 3/4" ball nose endmill particularly in spruce. The center of the cutter, even though it is "center-cutting", drags and tends to rip the material rather than cut. The ripping is somewhat intermittent based on the slope of the surface being cut.
I have considered tilting the router spindle slightly, say 15 degrees, so that the cutter presents some SFPM at it's tangency with the wood. Anyone tried this?
I've always used a cutpath parallel to the grain and wondering if I should try cutting at an angle or even perpendicular to the grain. I see a bit of an analogy here to feeding a board at an angle thru a drum sander perhaps? Someone suggested using a ball nose burr for finishing which I tried cut with little success.
My goal here is to minimize sanding after machining. Any help or suggestions would be appreciated as always.
Thanks
Nelson

If you visualize it, the center does relatively little of the cutting if your feed rate is more or less correct, it just sort of scratches across the surface and burnishes the one or two thou left over from the outside of the cutter passing through. The side of the cutter does always leave a better finish, but I've never had something that wasn't 220-able out of the machine using a sharp ballnose.

Cutting across the grain might work better, but be careful on the ends since that's where across-grain=splits. If you're noticing more fuzzing and whatnot at angle changes, there could be some motion smoothness issues, as well.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

I need to post a closeup photo of the cutter I'm using. It's two brazed carbide flutes are flat and overlap each other slightly. There is a small gap between the two faces of the flutes and perhaps shavings are hanging up here rather than clearing out causing the intermittent tearout. I should perhaps try a true center cutting ball nose. Another thought might be to use HSS for the finish cut since it will take a sharper edge than carbide?
I appreciate the "sounding board", guys.
Nelson

spruce always wants to tear out and you probably will not be able to eliminate it 100%. Also...carving on an incline typically just relocates the problem areas...it probably will not get rid of them.

Make sure you are using a good quality helical ball mill (onsrud makes nice tools) and do some experimentation with feed & speed to see what works best.

It also sounds like you are using a parrallel toolpath...this could potentially aggravate things since it probably lines up with the grain direction (the reaction loads from the tool are pointing in the same direction the wood wants to tear out). I bet you could get better results by angling the toolpath (20 degrees is probably adequate) just as you might angle a piece of wood going thru a planer. Alternatively, you could try different toolpath forms.

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http://www.PeakeGuitars.com

The brazed-type cutter you're using could have a lot to do with it. If you get a four-flute carbide ball, then you can be pretty sure that everything is lined up really accurately and that you're getting the smoothest cut you can with your setup. With improper tooling, your setup could be fine and you'd still get poor results.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

As an aside to what Bob mentioned...4 flutes are great as long as you have the power to run them. If you are using a machine with low HP (i.e a router based machine) then you might want to stick with a 2 flute. Otherwise, you'll have to slow the feed rate down...and if you have to slow it down too much, then you run the risk of getting some burning and maybe even a few glowing embers.

If you have a big fancy Fadal like Bob, then run the heck out of that 4 flute...!

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http://www.PeakeGuitars.com

You might try relieving the center a little where the cutting lips meet. Just a few thou would give you from a chordal height a reasonable clearance but not enough to be greatly visable. It would be better to have a little flat than tear out too!

Due to greatly varying SFPM you might jack the speed up and do a few light passes right at the end too.

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Billy Dean Thomas
Covina, CA

"Multi famam, conscientiam, pauci verentur."
(Many fear their reputation, few their conscience)

If your machine is using any sort of full-size router for a spindle then the problems you're describing aren't a symptom of using four-flute cutters. If the machine is too slow or the spindle is too fast, then you will get more burning with more flutes, but that's an improper feed rate problem. At the same feed rate, a four-flute should require the same amount of torque or less than a two-flute.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

It's interesting in that at least one of the 3/4" ballnose that I use has an offset of approx. 1/32" between the faces of the two brazed straight flutes. This gives the effect that I believe you're talking about and the flat is hardly noticeable. The problem is that the spruce shavings get trapped in this small gap and cause tearout for a short distance before it clears out.
If I understand correctly what you're suggesting, it would be like a non-center-cutting endmill is ground? As an alternative it looks like a person could almost take a center cutting ballnose and grind a small flat on the "tip" with relief in opposite directions to serve the two flutes. The problem might again be that the center of the cutter is not rotating. (subject to argument of course )
For what it's worth, I'm running a Bosch 1617EVS router at 25K up to 200 IPM. Would love to retrofit a variable frequency spindle. That router will just about drive me out of the house depending on how heavy a cut I'm taking.