Mark J Posted December 14, 2022 Report Share Posted December 14, 2022 I was surprised to discover that most of my 3/8" drill bits are not 3/8". Long story short, I had occasion to put calipers on them and found that while the Forstner bit was .375", 3 twist bits and a brad point measured .370". 5 thousandths sounds trivial (1/200"), but it's enough to make the other bits fit sloppily in the Forstner's hole. I always thought drill bits were accurately sized, something you could use as a reference standard. Guess not. 1 Quote Link to comment Share on other sites More sharing options...
Mark J Posted December 14, 2022 Author Report Share Posted December 14, 2022 This discussion was interesting. Apparently only the cutting edge of a twist drill bit is the full size. The flutes and the shank are a few thousandths smaller for clearance. I did not know. https://www.practicalmachinist.com/forum/threads/why-are-all-my-drills-undersize.145676/ 2 Quote Link to comment Share on other sites More sharing options...
BonPacific Posted December 14, 2022 Report Share Posted December 14, 2022 My better Forstners are similar, very slightly tapering inward from the scoring edges. 1 Quote Link to comment Share on other sites More sharing options...
Tpt life Posted December 14, 2022 Report Share Posted December 14, 2022 I think some depend on the standard runout on a drill shaft. Most guys handholding for 3/8” will also introduce some error. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.