Tap drill size for 1/2 -10 Acme

WCraig said:
You have a set of letter drills, right? An X drill is nominally 0.397 inches, Y is 0.404 and 13/32 is 0.4063. The actual size they create is likely to be a little larger, as @John O says. Why not drill and tap a few test holes in some scrap and see how you like the resulting fit with your acme rod?

Craig
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Yes, I have number drills.
Your suggestion is a good one. I’m definitely going to make some test holes and see how they go.
 
Considering that your are building a vise, I would be FAR more concerned with breaking the tap than getting the maximum percentage of the thread. Speaking from experience.
 
Machinery's Handbook states the tolerance for a 1/2-10 Acme class 2G internal thread minor diameter to be 0.4000-0.4050.

I would use a Y or 10.2 mm twist drill. If none of those are available to you, a 13/32 would be the next best option. The fit of the thread will most largely depend on the pitch diameter of your tap (i.e. class 2G) and not the size of the pilot drill, unless you drill way undersized or way oversized.
 
MrWhoopee said:
Considering that your are building a vise, I would be FAR more concerned with breaking the tap than getting the maximum percentage of the thread. Speaking from experience.
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tkalxx said:
Machinery's Handbook states the tolerance for a 1/2-10 Acme class 2G internal thread minor diameter to be 0.4000-0.4050.

I would use a Y or 10.2 mm twist drill. If none of those are available to you, a 13/32 would be the next best option. The fit of the thread will most largely depend on the pitch diameter of your tap (i.e. class 2G) and not the size of the pilot drill, unless you drill way undersized or way oversized.
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Breaking the tap is definitely my concern too. I think that I'll start on the larger end of the scale and see how it goes.
 
Most drills will drill oversize, so I would use an "X" lettered drill (0.397"). If this is too small of a diameter, it is close enough that your tap will correct for it.
 
The tap should be here in a day or two so I'll be able to test it out.
 
DavidR8 said:
Both are class 2.
What are your thoughts on tap drill size?
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My concern was that your tap and leadscrew match in terms of fit class. If your leadscrew is a general and the tap is centralizing then you are not going to be happy. Given the expense of an Acme tap, just wondered if you made sure of your fits when ordering your stuff. As for tap drills, I would consult with the tap manufacturer and ask them for a recommendation. If you drill it to the size they recommend and your tap snaps you may have some recourse.

If you haven't use an Acme tap before, I think you will be surprised at how much force it takes to cut that thread. Even the dual taps that rough and finish require a LOT of torque. Expect that the stock may spin in the chuck.
 
mikey said:
My concern was that your tap and leadscrew match in terms of fit class. If your leadscrew is a general and the tap is centralizing then you are not going to be happy. Given the expense of an Acme tap, just wondered if you made sure of your fits when ordering your stuff. As for tap drills, I would consult with the tap manufacturer and ask them for a recommendation. If you drill it to the size they recommend and your tap snaps you may have some recourse.

If you haven't use an Acme tap before, I think you will be surprised at how much force it takes to cut that thread. Even the dual taps that rough and finish require a LOT of torque. Expect that the stock may spin in the chuck.
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Fortunately (?) the stock is rectangular so it will be tapped in a vise. I plan to try different size holes to find the best combination of fit and least
tap-snapping potential :oops:
 
DavidR8 said:
Fortunately (?) the stock is rectangular so it will be tapped in a vise. I plan to try different size holes to find the best combination of fit and least
tap-snapping potential :oops:
Click to expand...

The area of contact with an Acme tap is comparatively huge. Having broken them before, and having to pay for the original and the replacement, I just wanted to suggest that you ask the maker for suggestions. Yes, I know how to look stuff up but I've done this stuff before and paid real money for the mistakes. It will take just a little time to ask, which may save you time and bucks.
 
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