Drill bit tolerances

What is the expected tolerance of drill bits? I have a set of fractional, letter and number drills. No.12 to 15 (including 3/16") seem to produce the same size hole.

Reply to
Michael Koblic
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mattathayde had written this in response to

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------------------------------------- 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.


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

Reply to
Jerry Wass

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.


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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.

Reply to
DoN. Nichols


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.

Reply to
James Waldby

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.

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