Barrel drill and tap?

[Cypress]

I am about to drill and tap a barrel for iron sights and I have found that for a 6-48 tap some places recommend a #31 drill and others a #32. Why is this and which one do I need?

 

[machinisttx]

It's just because two different people did the math and had differing opinions.

If it were me, I'd use the #31, a brand new tap, and sulfurized cutting oil.

 

[brickeyee]

The depth of threads cut with a tap can be adjusted by changing the size of the hole.
Many threads are only 60% of full depth, and in hard steel that is often adequate.
Machinery's Handbook has depth tables and the equations for finding thread depth.
Keep in mind that drilled holes are always slightly oversize, if only a few thousandths.

 

[ieszu]

The question is how much thread engagement you really need. The smaller the initial hole, the more engagement, but 100% is impossible to get and be able to screw something in I believe. Most situation call for 60-80%, and you should have no problems. So long as the necessary strength is there, more engagement will do nothing for you.

I believe a #31 and # 32 have so little difference in this case that it should make no difference.

Remember, the hole only has to be as deep as it wide, any more is meaningless, as that is the maximum strength possible, and you need at least 3 threads for optimum strength.

 

[highorder]

If it were me, I'd use the #31, a brand new tap, and sulfurized cutting oil.

That makes two of us.