Very strange question about twist drills

**Schtoo** · 31st January 2007, 12:48 AM

Not sure where I read it, but it wasn't me who invented the theory.

Problem is, it's right.

Sheetmetal will flex, so it's difficult to alter what you are doing to avoid the wobble. If you back it up with something solid and/or change speed, then chances are the wobble will be gone. Narrower angled drills also seem to help, as do pilot holes if possible. Pilot holes can also make it much, much worse since the bit will bite harder and tear, rather than cut. Sticky metals like ally and brass are also more difficult to drill cleanly, since the bit will bite much, much harder.

To a point, grandpa told me that if I am going to drill lots of brass, take the sharp edge off the drill bit. Either make the angle of attack less acute, or reduce the clearance angle, so the bit can't take such a big bite. I have done it once, and it made a world of difference.

I have also re-sharpened a 3 flute countersink for a friend here, and he said that it went from good to excellent. Not because it was sharp, but because the cutting edges were changed slightly and were not all balanced like they were originally. Made much cleaner countersinks, and much more controllable.

Similar to chatter with a handplane or electric router. Firm things up, change the speed or change the rate of bite and the chatter will often (but not always) go away.

**Randy Korr** · 31st January 2007, 04:18 AM

First of all, let me apoligize in case somebody has the true right answer and I’ve just missed it because I’m math impaired. Skew’s answer makes the most sense to me (a combination being plausible and digestible at my reading level). I don’t think the phenomenon ought to be greatly sensitive to the speed of the drill, since spirographs have engaged teeth, but that’s testable. What ought to matter a lot, and it’s testable too, is the size of the hole that’s being countersunk and the size of the bit that’s being used to countersink it.

If the flutes on a 1/2” bit, where they engage the shoulder of a 7/32” hole, are spaced apart (in a straight line or along some arc of sweep) approximately the same distance as the length of one side of a regular pentagon that just fits in a 7/32” hole; then countersinking a progressively larger hole might give a hexagon at some point (where it’s 6 steps around the circle instead of 5). Maybe it goes 5, 6, 7, 8,... ...smooth.

Now it might not be really obvious to you all, but I think I’ve talked myself out of that prediction. It’s a longer way around the hole, but the diameter of the 1/2” bit, where it engages the shoulder of a larger hole, is larger too. It’s farther around the hole, but the bit takes bigger steps. Time to grab a protractor and dust off the trigonometry book. That, or drill a few holes and walk the bit around it.

If I manage to get something other than a pentagon, I will attempt to rationalize that and post pictures.

R. K.

**Bob Willson** · 31st January 2007, 10:57 AM

I agree entirely with Skew, he is just more eloquent than I am.

**BobL** · 31st January 2007, 07:15 PM

Originally Posted by Randy Korr

. . . . .I don’t think the phenomenon ought to be greatly sensitive to the speed of the drill, since spirographs have engaged teeth, but that’s testable. What ought to matter a lot, and it’s testable too, is the size of the hole that’s being countersunk and the size of the bit that’s being used to countersink it.

If the flutes on a 1/2” bit, where they engage the shoulder of a 7/32” hole, are spaced apart (in a straight line or along some arc of sweep) approximately the same distance as the length of one side of a regular pentagon that just fits in a 7/32” hole; then countersinking a progressively larger hole might give a hexagon at some point (where it’s 6 steps around the circle instead of 5). Maybe it goes 5, 6, 7, 8,... ...smooth.

Now it might not be really obvious to you all, but I think I’ve talked myself out of that prediction. It’s a longer way around the hole, but the diameter of the 1/2” bit, where it engages the shoulder of a larger hole, is larger too. It’s farther around the hole, but the bit takes bigger steps. Time to grab a protractor and dust off the trigonometry book. That, or drill a few holes and walk the bit around it.
If I manage to get something other than a pentagon, I will attempt to rationalize that and post pictures.

I'd be interested in that.

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