Drill bit size for 5/16-18 tap

[jarhead]

I looked at a tap chart and it says for 5/16-18 tap to use "F" for drill bit size. The chart legend says that "F" falls between 1/4" and 17/64" bit size. Is it better to drill a smaller (1/4") or a bigger (17/64") hole?

[sbs]

17/64

http://www.boltdepot.com/fastener-in...rill-Size.aspx

[Tom Slick]

what is the tapped hole for? if you don't need maximum strength it will be easier to run the tap though the 17/64" hole. use the 1/4" hole if you need max strength but it will be harder to run the tap through.

[jarhead]

what is the tapped hole for? if you don't need maximum strength it will be easier to run the tap though the 17/64" hole. use the 1/4" hole if you need max strength but it will be harder to run the tap through.

I am about to install a blade detensioner on my HF BS. See this tread.

[LCHIEN]

I was going to make a similar comment as Tom.
F is 2.570 which is .070 over 1/4 and .086 under 17/64 (.2656)

.070 or .086 can be a lot in thread strength. It may not seem to be much distance but it a large fraction of the thread depth.

If the load is not critical then I would use the 17/64 and it will be a lot easier to tap.

If the load is really critical I'd get the recommended tap size; you should be able to buy that "F" bit in a well equipped auto supply store or hardware store.

A 1/4" tap hole might be really hard to tap depending upon the depth and material.

[jarhead]

Loring,
I am following in Popeye's footsteps and ned to tap 3 holes in my BS. Can I use the 17/64" bit for this application?

[GregW]

either one would would work...personally if I wasn't going to use the right size bit, I would use the 17/64 (.008 oversize) because you would have less of a chance of breaking your tap

[Black wallnut]

http://www.blackwallnutjigs.com/tapchart.asp

For 75% thread engagement which should prove plenty for what you have in mind.

[ted van halen]

either one would would work...personally if I wasn't going to use the right size bit, I would use the 17/64 (.008 oversize) because you would have less of a chance of breaking your tap

What he said. The 17/64 drill is .008 per side. I do tool & die design for a living. The machine shop @ my plant routinely uses the next size up fractional bit for tap strength. It's not such an issue once you get above 3/8 but the necks on smaller taps are pretty thin. Especially if tapping by hand & the twisting force is not consistent & smooth as it would be on a mill or a lathe. Also, I'd use some locktight on the bolts w/ lock washers instead of really crankin' down when you torque 'em. If they're seeing shear stress & you have room you might consider drilling for a dowel pin. I'm not real hip to the exact application so I don't know if that's possible. Heck, if the threads fail it's not a big deal. They make heli-coils all day long.

Ted

[Sam Salter]

...or you could buy a set of letter drills for about $20 and be set for life!
sam :-)

[LCHIEN]

What he said. The 17/64 drill is .008 per side. I do tool & die design for a living. The machine shop @ my plant routinely uses the next size up fractional bit for tap strength. It's not such an issue once you get above 3/8 but the necks on smaller taps are pretty thin. Especially if tapping by hand & the twisting force is not consistent & smooth as it would be on a mill or a lathe. Also, I'd use some locktight on the bolts w/ lock washers instead of really crankin' down when you torque 'em. If they're seeing shear stress & you have room you might consider drilling for a dowel pin. I'm not real hip to the exact application so I don't know if that's possible. Heck, if the threads fail it's not a big deal. They make heli-coils all day long.

Ted

I like ted's idea, use the 17/64ths and secure it with Locktite.
In your application (blade tensioner bracket holder), the bolts will the in shear mode and the threads not carrying the load directly.

[ted van halen]

I speak from experience about the tap neck breaking when hand tapping BTW. I was just chasing a thread to clean it up when the tap siezed instantly & w/o warning. Try as I might to avoid breaking it, it broke anyway. What a pain. If you do indeed use the locktite, DO NOT use red. I don't know the actual # for it but once it's set, it's set. Heat & patience are the only tools I've seen break it loose. The blue stuff is more than adequate in all but the most extreme cases.