View Full Version : Drill cutting oversize hole?

01-20-2011, 03:56 PM
I was tapping holes for some 8 Ė 32 set screws when I noticed that the threads didnít look right. Turned out that the hole drilled with a #29 drill was oversize. I tried a #30 drill which is 0.1285 and it produced a 0.136 hole . These holes were drilled using a drill press. I would normally drill them on the Mill and tap in the mill with the slow speed. Any insight into why these holes were that much oversized?

01-20-2011, 04:00 PM
It could be the runout in your drill chuck or an uneven lip height on the point.

01-20-2011, 04:15 PM
Did you measure the drill bits to see if they were the right size?

A bad grind can make them drill bigger also.

01-20-2011, 04:27 PM
+1 on bad grind/uneven lip. If the chisel point is off center then the drill will cut big. Take a close look at the tip of the bits. If they're damaged or off center then you have your culprit.

Al Messer
01-20-2011, 04:50 PM
I was taught in school that drill bits usually DO drill oversize and to start off a couple of sizes under size and work up to the final size and that if I REALLY wanted the hole the exact size, then ream it!

01-20-2011, 05:02 PM
Drill a center hole first.

01-20-2011, 05:33 PM

Yeah, any of the things already mentioned can cause it. To add to the list, with drills that small, they can flex a bit as you drill. Rubbing the side of the hole enlarging it. Often seen when drilling at a too slow of speed. And not clearing the chips often enough on deep holes can also cause over sized holes.


01-20-2011, 06:14 PM
I had the same experience with an 'F' drill from the cheap Grizzly gold colored set I got as a gift when I bought my X-3. It's the funky grind they put on them and I couldn't believe how much oversize they'd cut. I thought I was reading the mike wrong!

I use them only to chain drill or for hogging crappy metal. One of these days when I meet someone I really don't like, I'll give 'em away.;)

01-20-2011, 07:05 PM
A machinist friend of mine, tool and die maker actually, once told me that for really accurate holes a twist drill won't do the job. His theory was that as you drilled the bit "untwisted" some and drilled a slightly larger hole than advertised. True or not I can't say but they make reamers for something.

In your case I would wager your drill isn't sharpened correctly.

01-20-2011, 08:19 PM
Where did you but the drill bit? Did you use cutting fluid and continually clear chips? Cheap bits can lead to problems. If you do not clear the chips they will rub and bind creating heat and causing general problems with the hole.

01-20-2011, 08:50 PM
incorrectly sharpened drill, to much down pressure while drilling the hole and drilling without lube.

01-20-2011, 10:56 PM
Yes, very very common. If the center point of the bit is EXACTLY on center it will drill oversize, to the degree that the point is off center. If it has to be accurate, drill undersized then ream.