how much bigger does a drill bit drill?

[Robert LaLonde]

Everybody says "drill bits drill oversized" like its an absolute.

Last week I had to setup a bit of a finicky job. I milled a mold cavity in aluminum, and then had to setup to drill an 1/8" angled hole out of the cavity for a core pin. I made a guide plate, set the mold in an angle vise and took it over to the mill drill to fdrill it manually. Because I was not drilling into a flat surface I started the hole with an extended 1/8" end mill. Then I swapped out to an 1/8" x 6" aircraft bit to drill the hole. I figured it would be oversized, but the media that goes in the mold doesn't tend to flow into very small gaps. That's handy for venting, but its also handy when fitting hardware into castings I grabbed a piece of 1/8" teflon rod and it wouldn't got into the hole. In fact a .125" chucking reamer wouldn't go into the hole without removing little material either. I wound up using a .126" (.001" oversize reamer) in order to feed the teflon core pin into the hole.

Afterwards I measured the bit, and across the flutes it came in at .124. It sure didn't drill much larger than that. In this case I was actually counting on it to drill oversized and it didn't. I have to say I was surprised. The little mill drill actually is quite good for an import, (as a drill press) but I would have though some flex and runout of a bit that long and that thin would have wallowed out the hole some.

As an aside note. I was surprised the drill bit was that close in size. If you have ever measured the flutes on small drill bits in a cheap index you will find the actual size is all over the place. Usually they are off by couple thousandsths sometimes more. This one was from a middle price set of import 6" aircraft bits.

 

[jlsmithseven]

Speaking from experience, I've had the most drill bit wander about .012. This was because the flute was super long and my piece was a little uneven. Most of the time though if I had a flat enough piece and a regular drill bit in, it won't go over .004" bigger. I usually just use a drill size down.

-However, in the manual lathe, if you're drilling to start a boring hole, make sure you're setup is rigid. I once had to use a 7/8" drill bit that was super long and drill about 2" deep. I went down 2 or 3 drill sizes for the recommended boring drill, and it actually took my measurement right on what I needed for internal threads and I didn't even have to bore! So keep that in mind, the more rigid the setup, the better and more accurate.