Acme 7/8-6 tap threading questions

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dml66

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I want to tap a piece of mild steel, or maybe 12L14, about 1/2" thick, a through-hole, acme 7/8-6 thread pitch.

Taps for this pitch seem to fall into two camps: $15-$25, and $150+. Since I only need to tap one hole, I'm inclined to go with a less expensive one.

Any experience with these taps?

Also, what size bit is recommended for this size bolt? A standard tap drill chart shows 7/8-9 uses a 51/64" bit, not sure if that's close enough for acme 7/8-6.

Thank you.
 
dml66 said:
I want to tap a piece of mild steel, or maybe 12L14, about 1/2" thick, a through-hole, acme 7/8-6 thread pitch.

Taps for this pitch seem to fall into two camps: $15-$25, and $150+. Since I only need to tap one hole, I'm inclined to go with a less expensive one.

Any experience with these taps?

Also, what size bit is recommended for this size bolt? A standard tap drill chart shows 7/8-9 uses a 51/64" bit, not sure if that's close enough for acme 7/8-6.

Thank you.
Click to expand...
Can you post links to the taps you're looking at?

Acme taps are available in a couple different styles, each having their own peculiarities.
 
I've noticed the acme taps that I saw on McMaster (high priced) had two sections. Is the first section to hog out material and the second to form the trapezoid? The pictures of the taps weren't good enough to tell.

When single pointing acme, I was taught (here) that one cuts a vee thread first, then followed by a (resynchronized) acme. This has worked quite well for me. So my question of the first paragraph is coming from that experience.
 
I've used a 10-$ 1/2-10 acme tap. Feels terrible but the results are good. I would avoid mild steel, though
 
RaisedByWolves said:
Can you post links to the taps you're looking at?

Acme taps are available in a couple different styles, each having their own peculiarities.
Click to expand...
They're a lot of them on eBay, all around $50-ish, from China of course. The expensive ones are on MSC Industrial, McMaster-Carr etc.
 
This is what I'm after. I have asked the seller if a 7/8-6 is available but I doubt it.

I figure I can buy a steel disc from McMaster, drill and tap it acme, then add my own spokes.

I'm open to suggestions if there's a better way.

Screenshot 2024-11-23 134923.jpg
 
If you can get the part in your lathe, single point the threads. Make the threading tool.
 
Acme tap drill chart on Victor Machinery shows 7/8-6 closest drill is 23/32. Actual minimum diameter for the male thread is 0.7083 so that should be close.
 
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