Drill Bit Size: 1/4 x 20NC Tap?

[Steve in VA]

I need to tap some metal and have this 1/4 x 20NC tap. Every source I see online says to use a 13/64 or #7 drill bit. But the tap has 17/64 printed on it. Is there an advantage of using one size bit vs. the other? Will any bit between the two work?

I realize there is a difference depending upon what material you’re drilling into, but the difference between the two seems a bit much to me.

Calling all machinists for input!

 

[Eric Rorabaugh]

@woodtickgreg

 

[Nubsnstubs]

Steve, that 17/64 is .2656". It's probably for the OD of the bolt, which will or should be .250 +- a few thousandths.

13/64 is for the ID for the threads .2031".

A #7 drill bit is .2010". It's 2 thousandths smaller than the 13/64 bit. ............... Jerry (in Tucson)

 

[woodtickgreg]

Steel I would go with the 17/64
Aluminum I would go with the 13/64
I usually go with what's marked on the tap and that can vary by the manufacturer. But I also refer to a Starrett drill and tap chart that gives decimal sizes and recommendations. When in doubt I go one size smaller and use lots of tap magic cutting oil.

 

[Nubsnstubs]

Steel I would go with the 17/64
Aluminum I would go with the 13/64
I usually go with what's marked on the tap and that can vary by the manufacturer. But I also refer to a Starrett drill and tap chart that gives decimal sizes and recommendations. When in doubt I go one size smaller and use lots of tap magic cutting oil.

Greg, I am going to say up front, I am NOT a machinist, but have been doing my own stuff since '79 when a machine shop moved in next door to my cabinet shop and told me if I needed anything, they would let me use any unused machine that was available. I don't want to start a fight on this, but your info is not correct. Steel or aluminum does not change the sizes needed for a 1/4-20 thread, male or female.

I am also using a Starret, a Dormer and an OSG chart for any size required to thread anything. The hole needed for a proper thread in Steve's case is a #7 bit which is 0.2010" or 13/64" is 0.2030" that is listed in the charts I have. The extra .002" probably gives the tap a little wiggle room rather than the having the tap actually cutting the hole diameter needed.

All my 1/4" taps are made in the USA, and are 0.255" OD. The 17/64" is for the bolt clearance hole at 0.265", not for tapping.

The difference between 13/64 and 17/64 is 1/16" or .064". It is also .012" larger than the .250 bolt size. ......... Jerry (in Tucson)

 

[woodtickgreg]

I agree with you on that Jerry, sorry I'm at work and it was a quick answer of the top of my head. I still use the Starrett chart. Only reason I might go a little smaller on the aluminum is because it's easier to work with, steel being a little harder.
But you are correct, 1/16 is a lot!

 

[Steve in VA]

Thanks guys and my instincts were to go with the smaller size to start and go up a size if needed.

It still puzzles me as to why if a #7 or 13/64 bit is correct, which is the same info I have from multiple sources, why the tap would have "Drill Size 17/64" on it?

 

[DLJeffs]

Educational. So what does the 46-17 on the tap mean?

 

[Wildthings]

Educational. So what does the 46-17 on the tap mean?

Out of 46 answers on WB only 17 of them will be correct. Mine included

 

[Steve in VA]

Out of 46 answers on WB only 17 of them will be correct. Mine included

What's hilarious about that is I almost didn't ask the question at all figuring I'd get multiple answers and be even more confused. But thought it would be interesting to get different perspectives and the collective minds would lead me in the right direction.

When I got home to took the set screw I bought and put it in a sizing gauge. 17/64 is about the perfect fit for the outside diameter. Which still leads me to ponder why the manufacture would print "Drill 17/64" on the tap if the screw would essentially fall through a hole that size

 

[Steve in VA]

@woodtickgreg @Nubsnstubs

I think I've solved the mystery!

I was driving myself nuts as I knew a 17/64 bit would not work for 1/4 x 20 set screws. I measured everything with my dial calipers, researched it, and took the advice here and everything said no way a 17/64 bit would work despite it being imprinted on the tap itself.

I ordered a new tap off Amazon and got what I expected. Drilled a 13/64 hole and had my set screw hole tapped in no time. Perfect!

I did a bunch of research and found there is something called an STI Tap. STI = Screw Thread Insert. These taps are oversized to allow an insert to be put it the "oversized" hole so that the resulting thread from the insert is the size noted on the tap. These are often used with EZ Coil helical coil inserts. I got the tap from my Uncle, and when I asked him about it he wasn't sure so it's doubtful he ever used it.

Now you know!

 

[woodtickgreg]

Man talk about a curve ball, lol. Now it makes sense.