Acme Threading

[devils4ever]

I plan on building a drill press vice as my next project. I could probably buy one cheaper than making it, but I want the experience and fun of making it myself.

I would like to use a 1/2"-10 Acme thread in my design. I've never cut an Acme thread before. My mini-lathe is probably not up to the task. I don't think it's rigid enough for such a big cut for either internal or external Acme thread cutting. So, I'm planning of buying an Acme threaded rod which is fairly cheap rather than machine it on my lathe.

However, I'll need to tap a 1.25" thick piece of mild steel for the rod to go through. I've seen various taps available for Acme threads, some are tandem. Would this be the preferred way to go about this? I imagine the cutting forces are much higher with Acme threads since much more material needs to be removed.

Another option is to buy a brass flanged nut and fit it into my steel block. This is a fairly expensive option. Is brass okay for a vise or will it wear out quickly from the higher forces?

The last option is to just use a standard 60* V-thread. I can cut either 1/2-13 or 1/2-20 easily. I've done this many times and have no issue doing this. I'm not sure a V-thread is the best option for a vise though.

Comments? Opinions?

 

[projectnut]

I would go with a single lead tap. Multiple leads are generally used where the parts are regularly disassembled and reassembled. The multiple leads aid in the speed of reassembly. The screw on a vise is permanently captured. Since it (almost) never disengages the "nut" there is no need for multiple leads.

As for the internal thread I would go with your first option. Brass would be a poor choice of material given the clamping forces likely involved. They wear over time with even light pressures. Think of a milling machine vise versus a table. The table generally uses a brass nut, and the only forces applied are the resistance of the table being moved and the cutter against the material. Even then they wear over time as can be witnessed by the play in the handle. A vise on the other hand sees clamping pressures multiple times that of the table lead screw. They rarely wear.

I have a Bridgeport mill and vise that were purchased together by a local high school in the early 1970's. The vise still works fine. There's probably some wear, but nothing that renders it unusable. The table on the other hand had nearly .030" of play when I got it. I was lucky in that it had a split nut configuration rather than 2 individual nuts for the lead screw. I was able to cut the nut in half (as recommended) to remove the play. If I had the dual nut configuration I would have had to replace both since there is no provision for tightening them due to wear.

A 60* thread will likely not last long you will either strip or disfigure the thread with the forces needed to solidly clamp material in the vise.

 

[devils4ever]

So, you're recommending I go with a tandem tap?

 

[SLK001]

If you take small bites when threading, there is no reason why you can't single point the nut. If you're not sure on your abilities, single point to about 90% then finish with a tap. This also helps if your tool isn't exactly on spec for threading. If you try to tap the entire thing, just know that the tapping forces are HUGE.

 

[projectnut]

My error. By tandem I thought you were referring to a "multiple lead" tap. In this case I would think the "tandem" as in combination roughing/finishing tap would be appropriate IF you don't feel you can turn the profile on your lathe AND have the means to hold the stock accurately and securely.

 

[devils4ever]

Yes, the tandem tap is a combination roughing/finishing tap in one. See this.

My mini-lathe seems to struggle cutting V threads on a course pitch near the end of the cut. I typically take 1 thou cuts after about 30-40% of the way through. I can't imagine cutting an Acme thread on it. I'd be taking a 1 thou cut and a spring pass every time and I'm not sure it would work. I'd love to try, but I'm not sure I want to invest in the tooling for it if I can't get it to work.

 

[MrWhoopee]

Get a steel acme coupling nut, turn it round (not full cleanup) with a shoulder and press/weld it in.

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That's what I did on my table/verti-vise.

 

[devils4ever]

Interesting idea. I can't weld, but I could press it in. When you say, "not full cleanup", do you mean don't make it completely round?

 

[MrWhoopee]

Interesting idea. I can't weld, but I could press it in. When you say, "not full cleanup", do you mean don't make it completely round?

Yes, the distance across the flats is 5/8, leaving 1/16 wall thickness if turned fully round. I just turned off the points to make it round enough to press into a hole, leaving about 1/4 in. full hex to create a shoulder which gets recessed into a counterbore. Be sure to put the shoulder on the right side of the jaw to take the thrust and prevent the nut from pushing out.

 

[devils4ever]

How tight a fit is required so it doesn't spin?