Tapping for 10-32

I will go with #21 as others have said. Always worked for me.
 
Oskar
Are you hand tapping, or drillpress or using a tapping device that clutches foreword and reverse as you push or pull the tap. ???
It really sounds to me that you are having a problem with the tap itself and need to get a good quality one (get a few if you really are tapping a lot of holes). I think that I might have broken 4-5 taps in my whole life and I typically tap for 4-40, 8-32 and 10-32 fasteners. A cheap tap is just that, cheap. A good quality tap should make a bug difference, but get at least a pair just in case.
 
Tap Charts are not a necessity The Henry Ford Trade school taught us how to use a calculator!
The Formula is divide 1 by the pitch minus the major thread diameter and use the answer to find the hole size
Always go up to the next nearest size drill
3/8" minus 1/16 = 5/16 5/16 is the drill size 3/8 16 tap
this will work for all size taps
 
@oskar,

Little Machine Shop has a nice thread chart here:
https://littlemachineshop.com/Reference/tapdrill.php

What I like about that one is that it has columns for 75% thread and 50% threads, so you can choose what works best for your project and materials.
(also note the link on that page for a "printable version" of the table. I laminated it and hung it in the shop.)

-brino
 
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I hand tap guys and its the most boring job but it has to be done

I buy taps in the middle of the price range and in the last 10 years I only broke 2 taps for M5 bolts.
 
If you haven't yet try tap magic it works great
 
use a x-press tap that was patented by Besley, also call a expander tap. It was first used on soft metal, now they use on everything, it doesn't cut it expands the metal
I can see this post went over everyones head more than likely no one herd of x-press taps, no metal removal it expands and makes a stronger hole.
 
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As mentioned a #21 drill will give you slightly more clearance. I would recommend good quality HSS gun taps when threading through holes. The gun tap pushes the chips forward and through the hole. When doing blind holes I would start with a plug tap and finish with a bottoming tap. Be careful to remove the tap and clean out chips regularly when doing blind holes. Also when using hand taps make sure to reverse direction every 1/4 turn or so. This breaks the chips and allows them fall into the relieved area between cutting edges. If you attempt to turn more than 1/2 a turn without reversing in gummy metals the cutting edges will get clogged with chips.

The only time I would consider anything other than a high quality HSS tap is for a one off job. Carbon steel taps don't last long and break easily when they get dull. I did buy a carbon steel 9/16-18 left hand tap for what I thought would be a one off job. Low and behold I've done the "one off job" a dozen more times. I think it's time to invest in some HSS ones before I hear the dreaded "crack" which, if it happens, I'm sure would be followed by a string of colorful 4 letter words
 
See I don't even bother with the chart too often as my drill index has this handy dandy chart for tap drill size ranging from 2-56 tpi thru 1-8 tpi
It also gives recommended body drill or clearance hole sizes. After you have done this for 30+ years, some of those numbers just stick in yer head, like #7 for 1/4-20 or in a pinch you can use 13/64 .... boggles my mind just how much of this stuff I personally take for granted because I have done it and used the info for so long...I have always wondered about the % thread stuff... class of fit has a purpose but for most home shops, I cant see that ever being something to worry about.
 
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