Tap drill size for 1/2 -10 Acme

DavidR8 said:
What size is that?
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Not sure you saw my suggestion above, but if you still have a hard time getting it started, 7/16-10 Acme taps are cheap and should be much easier to start, as they are cutting only about 1/2 their thread depth with a .405 drill. You should then be able to chase it with your 1/2-10 tap with a lot less material to worry about, which would reduce the force.
 
I cant find a 7/16-10 acme tap only 7/16-12...
 
ErichKeane said:
I posted an amazon link to one above.
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Thanks Erich, I did see that. I bought the tap I have from the same company.
I'm going to try your suggestion.
 
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Just spent some time in the shop.
Using a new piece of stock I drilled a hole with a 29/64 (.453) bit.
I started the tap in the mill and went till it stalled. I left it in the stock and removed the tap from the chuck.
Put it in my bench vise and using my 12" Patience and Nicholson tap wrench I went at it. Definitely tough going for the first part of the tap. I could see the tap twisting and I feared it might snap. Lots of Tap Magic and I made it through!

So I think that starting with the 7/16-10 tap on a 13/32 hole ( the size spec’ed for the 1/2-10 tap) will give me a good chance of tapping the hole and achieving the full thread depth.
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I see about 8 threads in your test piece. I believe I read somewhere that a nut develops maximum strength in only 4 threads, give or take. I'm pretty sure they were referring to 60 degree V threads but I can't imagine that acme threads would need more. So you could likely clearance drill part way through your stock so you have less to thread.

Also, your test piece doesn't seem to have been countersunk. If you are going to hand tap your vice jaw, the bevels may make it easier to get the tap started. Or not; I don't have any hands on experience with acme taps.

Craig
 
Thanks Craig, this piece (and the eventual vise jaw) is 1" thick. I thought about countersinking, the starting end of the tap is about .450" so slips right into the hole. Would countersinking ease the entry of the rest of the tap?
 
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