Drill Bit Size for Taps?

[Ben]

I haven't run a tap in decades. I was just looking to see what I need to run a 1/4" tap and am getting both 7/32" and #7. Is there any significant difference between fractional and wire size drill bits for running taps? If I can just use bits I have now versus buying a whole set of wire size bits, that would be nice. I could swear that the last time I did this I just used fractional bits that I already had.

[zxcvbob]

Do you have a 13/64" bit?  That is just a couple of thousandths smaller than the minor diameter of a 1/4x20 thread, so it'll give you 100% threads but you risk breaking the tap so go slow and careful and clear the chips a lot.  (or hog out the hole just a little when you drill it by wobbling the bit)  If it's too tight you can try the 7/32 but it's hard to go the other way if that's too loose.

[dogmush]

IThe book answer is it really depends on what you're tapping, but for any non SpaceX projects, no you can just go up to the nearest 1/64" fractional bit.

Be very careful with 100% threads, as Bob underplayed how easy it is break threads when running that tight.  Metal actually flows a bit as you cut the threads, that's what the extra space in the minor diameter is for, letting the metal flow a bit.That said, most drill bits don't drill their nominal size unless you put it in a mill, so a 13/64 homeowner grade bit is fine for a 1/4-20 hole.

[Ben]

So both you guys are saying 13/64. Does that mean the internet 7/32 is wrong, or just a little too loosey goosey?

This is no technical project. I'm just tapping into 1"x2" 1/8" steel tubing on target stands I'm welding together just to thread a bolt through to hold the wood 1x2s in place - basically copying a commercial stand I already have.

That said, I'd like to make a note of the right bit sizes for common taps in general, if I have to tap holes somewhere on the farm.

[dogmush]

So both you guys are saying 13/64. Does that mean the internet 7/32 is wrong, or just a little too loosey goosey?

Yes.

#7 bit is .201" 13/64 is .2031, and 7/32 is .2187.  Too big, especially if the hole wallows at all.

[Ben]

Got it, thanks.

[tokugawa]

IThe book answer is it really depends on what you're tapping, but for any non SpaceX projects, no you can just go up to the nearest 1/64" fractional bit.

Be very careful with 100% threads,

  ^ "very" as in, just don't.  :'( busted taps are no fun.

 1/4- 20 use a 13/64".  That is .002 bigger than a #7.
 7/32" might work for a 1/4-28, I would not use it on a 1/4-20.

   75% thread gives about 95% of the power of a 95% thread.

[French G.]

When I make target stands I drill a 1/2 hole in the tube then tack a 3/8 nut over that. Helps to have a steady supply of scrap stainless tube, no rust maintenance for my lazy ass.

[zxcvbob]

1/4- 20 use a 13/64".  That is .002 bigger than a #7.

You're right.  For some reason I thought it was .002 smaller than a #7.

[zahc]

There's no magic here. My tap chart shows 2 different drill sizes, one 75% thread for Aluminum and one 50% thread for steel.

Don't tell anyone but sometimes I just hold the drill bit up to the tap and eyeball the right size. You'll know pretty quick when tapping if you guessed wrong.

[Ben]

There's no magic here. My tap chart shows 2 different drill sizes, one 75% thread for Aluminum and one 50% thread for steel.

Don't tell anyone but sometimes I just hold the drill bit up to the tap and eyeball the right size. You'll know pretty quick when tapping if you guessed wrong.

The tap chart almost confuses me more. 

https://www.littlemachineshop.com/Reference/tapdrill.php

[Declaration Day]

I'm a machinist.  I use 13/64 for tapping 1/4-20.

[tokugawa]

The tap chart almost confuses me more. 

https://www.littlemachineshop.com/Reference/tapdrill.php

 We have not even gotten to sizing holes for reamers yet......

[Ben]

We have not even gotten to sizing holes for reamers yet......

 

Anyways, just drilled and tapped. The 13/64ths worked fine. If anything, on the slightly loose side.

[dogmush]

Anyways, just drilled and tapped. The 13/64ths worked fine. If anything, on the slightly loose side.

Drill bit's prolly dull.

[Ben]

Drill bit's prolly dull.

Likely. I have an embarrassing amount of half full drill bit sets in various states of disrepair. I need to really relegate all of them to the Polaris toolbox for field stuff and buy once cry once on a nice, large set for the shop that stays in the shop. Then try to remember how to properly sharpen bits. My dad taught me when I was a kid, and I did it through my 20s, but then I got lazy.

[Cliffh]

Got one of the Drill Doctor bit sharpeners years ago, can't remember the model number.  Anyway, it does a pretty good job.