tapping and drill bit sizes

OK, maybe this might be something really simple for some of you, but what size bit do you choose for which tap? Is there a chart someplace out there? I need to drill and tap 2 10mm holes and I'm wondering what size I need to go up to: drill 10mm and tap 11mm? 12mm? or drill 11mm and tap 12? 13?

And for future reference is there a rule of thumb for both metric and SAE? so if I have a hole that's 5/16" that has the threads stripped, what should I drill it out with and what should I tap it with?

Thanks

ever heard of Google? first hit when searching for "drill tap sizes" : http://www.engineersedge.com/tap_drill_chart.htm

BrettM said:

ever heard of Google? first hit when searching for "drill tap sizes" : http://www.engineersedge.com/tap_drill_chart.htm

Click to expand...

:worship:
when I ran my search I didn't come nowhere near close to what you got. Thanks

If you are tapping in metric, it couldn't be easier. You take the diameter and subtract the pitch. I think those are the proper terms. So for example a 10x1.5mm tap, you would use an 8.5mm drill. Now an 8.5mm drill may not be easy to come up with so you would want to use one you got or buy a cheaper standard drill. so take the metric size that you came up with and devide that by 25.4 so you would have 8.5 / 25.4 and you would get about .355 so you need to get a drill that diameter. Thats not always possible to get the exact size so you would have to decide which is best and that just comes with experience i think. You really dont want to get more then .002 or .003 way from the drill you need. There are exception depending on what the function is and the overall size of the tapped hole.
I'm trying to get back on here more often and probably will have the bug for the next couple of days, so if you need to know an exact size or a tip, Just say what size hole you are needin to tap and i'll go look at my chart and let ya know the drill size.

well stupid me typed all that out then realized you guys allready gave him a link

Gene MJ

Have you got an Orchard Supply Hardware handy? Check out the tool corral - you should find a nice 6" stainless ruler near the taps and dies. It goes for about $6.50.

Flip it over, and there's drill sizes for UNC (#1 to 4",) UNF (#0 to 1-1/2",) NPT (1/8" to 4",) and the formulae for metric drilling as well - and figuring out drills for any other size of thread with 75% engagement (pretty standard.)

On the front it's 0-6" by 64ths, and a chart of decimal values for each 1/64".

Well worth the money - I have several (home, school, shop, ...)

5-90

If you have to order it, it's General Tools P/N 715. General Tool Company is online, and they're worth checking out anyhow...

5-90