Drill bit tolerances

mattathayde had written this in response to

:

------------------------------------- Michael Koblic wrote:

what are you measuring with, 12-15 range .18-.189, also do you have runout on your press, how fresh are the bits, etc. your dealing with such small differences that it will be very hard to get a good read unless you have very high quality measuring devices.

-matt

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Any bit not sharpened properly--especially one with the point not centered will drill oversize..

Have you measured the diameter of the drill bits? I could not find tolerances on drill sizes, but drill blanks are the nominal size plus nothing minus .0003 inches for blanks up to half inch. I would expect drills to have about the same tolerances.

Dan

Note that it matters where you measure, too. Drill bits taper very slightly from the cutting end to the shank (to avoid binding in the hole, I guess). Certainly don't trust a measurement of the shank to give the size of the hole it will produce.

As has already been mentioned -- it all depends on how well it has been sharpened. If the angles or lengths of the flutes are different, the drill will run off-center, and cut with only one flute, while pivoting on the "center" as defined by the sharpening, not the center-line of the bit itself.

Enjoy, DoN.

...

Also, are you using a mill-drill, a drill-press, or a hand-held drill motor? And what is the material being drilled, ie, wood, aluminum, steel? Clamped or hand-held?

If using a drill press or better and drilling into steel or aluminum, I would regard it as an unusual problem -- with the press, the bit, the material, or the work-holding -- if I drilled a hole with a #13 bit (0.1850") and was then able to push a #12 bit (0.1890") through the hole without cutting at least a little more metal.

I've used the shank of drill bits as gages; they measure within .001" of the marked size.

As for the holes, the #12 makes a hole bigger than #13 and smaller than #11 (i.e. look at the gap between sizes), unless it's dirty/bent/dull. Fractional drills, some of 'em, have been off by .005" recently, but they might have been made-for-WallyWorld.

People who need real precision don't use twist drills, of course, for the final dimensions.