Tap drill size for 1/2 -10 Acme

ErichKeane said:
Woah, that is such an interesting profile! It looks like a normal 60 degree tap until the 1/2 way point with squared-off tops, then 'squares' up the holes in the 2nd half.
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Yup, it looks pretty much exactly like a 60 deg tap for the first part. If there was such a thing as a 1/2-10 60 deg tap I'd buy it.
 
that looks about right for a tandem tap. The V-shaped 1st part isn't a 60deg thread form, it's a narrow acme thread form. The idea is for the 1st part of the tap to remove the bulk of the material and the 2nd part generate the correct thread form.
 
Although I didn't do ACME thread before and I know it is harder, my experience shows that a good tap and lubrication and doing it correctly, it goes very easy and smooth, while bad tap is very hard and breaks. So translate from that, I would be very cautious to put excessive force just because it is an ACME thread. But how much is excessive would be up to your determination.
 
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I was just thinking... is there any overlap in size with a 7/16-10 acme thread (https://www.amazon.com/Acme-Speed-Steel-Choice-Sizes/dp/B018A3WR3Q?th=1)? You could start by tapping 7/16-10 with a massively-oversized start-hole (since 7/16's drill size is 40 thou under what you're cutting).

The result would be a pretty solid start-thread that would significantly reduce the load on the 1/2-10 tap.
 
DavidR8 said:
Yup, it looks pretty much exactly like a 60 deg tap for the first part. If there was such a thing as a 1/2-10 60 deg tap I'd buy it.
Click to expand...
It just looks like a 60º tap. Because of the force required, only part of the tap is a complete ACME profile (ie, it cuts half the profile). Later, another part cuts the other half. Only a gorilla can tap a large ACME profile by hand.
 
Just checked and my internal threading tool is larger than .5" So I need to make/buy a smaller boring bar.
 
that's why I ended up using a tap. It's a fairly deep thread relative to the minor ID, so making/ grinding a tool that could fit in the hole and still be able to cut the profile was not trivial.
 
What size is that?
 
3/4-10 RH, but it should look like yours.
I have a couple other sizes but not with me.
 
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