Tap drill size for 1/2 -10 Acme

[SLK001]

I would mount the item to be tapped in a 4-jaw and single point the thread to ~90%, then tap the rest of the way. Depending on the material, the force required to tap it may be too much for the tap.

 

[DavidR8]

I would mount the item to be tapped in a 4-jaw and single point the thread to ~90%, then tap the rest of the way. Depending on the material, the force required to tap it may be too much for the tap.

It’s just mild steel but your idea is a good one.

 

[mattthemuppet2]

drill/ bore it oversize. There'll be plenty enough thread depth in steel for what you want to do and minor diameter will have next to no effect on fit of the screw to the nut. I did a 1/2-10 acme nut in bronze recently (tail stock mod) and bored it to 0.410". Tapping it was VERY hard work, using a single pass (very long taper) tap. It went easier once the full thread profile part of the tap was in the work. I wouldn't even consider 0.400" in steel.

 

[MrWhoopee]

Ok, I'm very nervous, bordering on terrified about doing this now!

Now you're getting it.
"If you ain't scared man, you ain't right."

I think I was tapping bronze when I broke a tap of about the same size.

@SLK001's suggestion to single-point rough it is excellent, even with a crudely ground tool. Just remove the bulk of the material.

 

[DavidR8]

Now you're getting it.
"If you ain't scared man, you ain't right."

I think I was tapping bronze when I broke a tap of about the same size.

@SLK001's suggestion to single-point rough it is excellent, even with a crudely ground tool. Just remove the bulk of the material.

I have a 60 deg internal threading insert bar. Would that work?

 

[ErichKeane]

I have a 60 deg internal threading insert bar. Would that work?

That would remove most of it. and would make the tapping easier. The biggest issue is going to be making sure you start the ACME tap in the right place, but if you end up getting 'most' of the material out (to the point where it is nearly square near the root) it should be enough to engage the first few teeth of the starter tap.

 

[MrWhoopee]

I have a 60 deg internal threading insert bar. Would that work?

Yes, but my first inclination is that it might be possible to go too deep, removing material that should be left.

 

[ErichKeane]

Yes, but my first inclination is that it might be possible to go too deep, removing material that should be left.

I'm not sure I'd be too concerned about 'depth' so much as 'width'. If you cut too deep (in the bottom of the thread) you are just hitting the 'relief' part of the thread. If you cut too wide, you are taking out the force bearing-surfaces.

 

[DavidR8]

Yes, but my first inclination is that it might be possible to go too deep, removing material that should be left.

That would remove most of it. and would make the tapping easier. The biggest issue is going to be making sure you start the ACME tap in the right place, but if you end up getting 'most' of the material out (to the point where it is nearly square near the root) it should be enough to engage the first few teeth of the starter tap.

The tandem tap should be here today or tomorrow. I have some 1" stock I to experiment with to see what works the best. I'm most concerned with the initial 'roughing' part of the tap.

1) drill with a 13/32 and try tapping.
2) drill with a 13/32 and try internal threading with a 60 deg tool for 2/3/4/? passes

 

[ErichKeane]

The tandem tap should be here today or tomorrow. I have some 1" stock I to experiment with to see what works the best. I'm most concerned with the initial 'roughing' part of the tap.

1) drill with a 13/32 and try tapping.
2) drill with a 13/32 and try internal threading with a 60 deg tool for 2/3/4/? passes

Could you just rough-grind an HSS tool for this? You could basically just 'undersize' a cutter and depth and let the ACME tools cut as little as possible.

Using the 60 degree tool you have to make sure you don't cut them 'too wide'.