How to cut square threads?

[drummerdimitri]

I've been looking around for square thread cutting taps and so far haven't found any to purchase.

Are those a thing or is the only way to create square threads with a lathe?

I read those are stronger than the triangular shaped threads and would like to know how to make them.

 

[APEowner]

Try looking for an ACME thread tap.

 

[matt_i]

On a tap of Acme or square (buttress) threads you don't hog all of that material out in 1 pass like you do with a 60 deg V thread.

There are 2-3 tapered sections on the tap that are all cut on the same "lead" (not Pb), with tooth forms that take out gradually more and more material.

So an Acme tap might be 18-24" long and cost you a couple of grand as a thru hardened edge tool.

You can cut the external thread on a lathe, just like a 60 degree V, but you need a Lathe that weighs more than your car (lets just say a 5hp motor) to even get started in this game.

I've seen people try to tap the Acme in a single pass with a shop-made tool but it always snaps or twists the tool.

Tl;dr....the 60 degree is a well-optimized sweet spot between (good/strong, fast and cheap).

 

[HoosierBuddy]

Whenever I've need to use an acme thread for any of my builds, I wimp out and just buy acme threaded rods and nuts. Maybe that's cheating, but if you need an acme thread, say in a 1" steel plate, you can generally accomplish the same thing with a through hole and a 1" nut. You can even weld the nut to the plate if you want to.

I built a wood vise (tail vise) for my shop a couple of years ago by doing exactly this. Works great.

Phil

 

[drummerdimitri]

Whenever I've need to use an acme thread for any of my builds, I wimp out and just buy acme threaded rods and nuts. Maybe that's cheating, but if you need an acme thread, say in a 1" steel plate, you can generally accomplish the same thing with a through hole and a 1" nut. You can even weld the nut to the plate if you want to.

I built a wood vise (tail vise) for my shop a couple of years ago by doing exactly this. Works great.

Phil

Seems like you read my mind

I would like to eventually make my own "machinist" vise and one of the reasons why I wanted to know how to make square threads is to tap them into a solid block of metal.

Will see if I can get my hands on said ACME taps else I would take your advise and get a rod and weld a bolt into the block after it's been drilled out.

 

[JackOfDiamonds]

Acme threads are not square, or even close. They're nearly the same angle as UNC threads, just with a flatter top and root.

Edit: I was wrong, Acme threads are 29 degrees and UNC threads are 60 degree. Metric leadscrews are 30 degrees which is probably what had me confused.

 

[drummerdimitri]

Acme threads are not square, or even close. They're nearly the same angle as UNC threads, just with a flatter top and root.

If that's the case, my problem hasn't been solved really...

How to get perfectly square threads?

 

[MushCreek]

The included angle of an Acme thread is 29 degrees vs. 60 degrees for a regular thread.

We had a job (before the days of CNC) where we needed to cut a square thread, and it had to start and stop in a specific location. The boss dug out an attachment for the B'Port that I didn't know we had, or that such a thing even existed. It was a horizontal rotary fixture that was geared off of the X axis lead screw. You took the handle off and put a gear on, and replaced the handle. By changing the gears, you could set the number of threads per inch. We cut the threads with an ordinary end mill, taking several cuts to get a smooth, accurate thread.

In a CNC, you use a thread mill, which can have any thread profile you want, and program the diameter and pitch.

I've cut male Acme threads on a lathe- even on a small lathe, but I always just bought a nut and modified it to suit. They sell a variety of Acme threaded nuts for various purposes. I'm not sure why you would need true square threads; Acme threads have been used for these purposes for a long time with excellent results. Places like McMaster-Carr have a wide variety of Acme threaded rod and nuts. I made a crossfeed screw for a lathe out of Acme threaded rod, and a bronze nut machined to fit the lathe.

 

[matt_i]

If you look at a Kurt milling vise (thinking a D688), it has a triangular thread but its a special where the threadform is "rotated" -- its optimized for bearing area on one face.

I don't think those guys are messing around, despite the proprietary nature it looks easy to cut and perfect for its job.

In other words I trust a milling machine vise more than I trust a C-clamp

 

[BukitCase]

Matt, almost sounds like you just described a Buttress thread - a couple of my older pullers have those, had the pullers for about three DECADES before I found out what the name of the thread was. Now I'm gonna hafta go look at my Kurt vise, didn't really look that close... Steve

 

[dutchgray]

Buttress threads are a standard thread form, English quick release vices use them for their screws.