Acme Thread Tap Drill Size

devils4ever

devils4ever

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I'm just about ready to tap a 1/2-10 Acme thread in steel. I have a tandem tap ready. However, I'm not sure what size drill to use for the hole.

I know the formula for minor diameter is Dmaj - Pitch = 0.500" - 0.100" = 0.400". That's the basic size, but to make tapping easier, I'd like to go for the maximum end of the tolerance range. If I'm reading my Machinery's Handbook correctly, is it 0.405"? That would be between a Y bit and 13/32" bit.
 
Acme thread cutting removes a lot more metal than a normal 60 degree screw. I would be inclined to measure the minor diameter off the tap and subtract .010 or so for .005 cut on the minor to form the thread. Or use Machinery's Handbook. Whatever you do, make your calculation specific for Acme threads and not standard screws. What material are you working with?
 
pontiac428 said:
Acme thread cutting removes a lot more metal than a normal 60 degree screw. I would be inclined to measure the minor diameter off the tap and subtract .010 or so for .005 cut on the minor to form the thread. Or use Machinery's Handbook. Whatever you do, make your calculation specific for Acme threads and not standard screws. What material are you working with?
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Would you not ADD .010 to the minor diameter of the tap to allow for clearance? Otherwise, the tap would have to cut on the root, increasing cutting forces and causing interference with the screw. The acme thread allows .oo5 clearance in the root and on the crest of the screw and nut.
 
Yes, I think I need to add not subtract so the hole is bigger and easier to tap. I believe MH is showing 0.405" for the maximum, but I just want to be sure. I may go up a little more. I never tapped an Acme thread before, so I'm a little intimidated.

This is in 1.2" of 1018 steel for a vise I'm making.
 
devils4ever said:
Yes, I think I need to add not subtract so the hole is bigger and easier to tap. I believe MH is showing 0.405" for the maximum, but I just want to be sure. I may go up a little more. I never tapped an Acme thread before, so I'm a little intimidated.

This is in 1.2" of 1018 steel for a vise I'm making.
Click to expand...
A bit of research in Machinery's Handbook would settle the issue, but I think that .005 clearance is standard, that is .010 on the diameter.
 
I used a Z (0.4130") drill bit to drill the hole for the Acme tandem tap. It was slightly larger than the 0.4100" diameter recommended, but I'm glad I went bigger. See this thread for more details on the process.
 
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