Acme internal threading

Hi again

A few days back I cut myself an Acme thread as a test piece, 5/8” x 8 TPI. It went pretty well. To do it I made an Acme cutter (see pic).

Next step is to practice making an Acme internal thread to fit. I need a suitable internal Acme cutter. I made a “boring bar” a while ago (also see pic), which I’m quite proud of because looks a bit like the Loch Ness monster. It works however. I want something similar but with an acme cutter on the end. The reason for it’s strange shape was there was so much grinding I did the coarse grinding with an angle grinder and a flap disc.

Surely there is a better way to create an Acme internal threading cutter?

Steve

Have a look at 'Old Tony's ' video on the subject. Quirky style but full of very good advice. Enjoy

https://www.youtube.com/watch?v=11pcIJN1Gd8

Mike

Perfect, thanks Mike, I’d not seen that one. I will have a go with that technique.

Steve

YOu can buy boring bars that are a round shank held in a slotted square holder. The end of the round bar has a cross-ways square hole for a piece of 1/8" or 3/16" etc square HSS, which is held in place by a small grub screw coming in from the end. You then just grind the piece of HSS to the shape of your Acme profile. Put it in the boring bar and cut off the long unused tail of HSS sticking out. Easy peasy. Just be sure to grind plenty of clearance angle on the leading face to allow for the helix of the thread, which can be quite a bit on small acme threads.

**LINK**

You can of course buy ACME taps which for a 5/8" dia thread might be a better answer?

The first time I did an ACME thread, I made a matching tap out of W1 tool steel and gashed the flutes on a shaper before hardening it. It sort of worked, given that there was no relief on the threads at all. But it only had to work once.

That was 3/8"x10tpi

Acme can be tough threads to tap, requiring big muscles or pieces of pipe on the tap wrench. Roughing it out by screw cutting and finishing with a tap is probably the best of both worlds.

There are inserts available for Acme threads, and if a suitable holder can be found at a reasonable price I think I'd be tempted to go this route. I've seen a few videos of people using Acme taps and in general, I like to see the machine doing the work, not me!

I'm pretty sure any company that had a lot of internal acme threads to do would go for CNC thread milling. The cost of acme taps plus the risk of a tap snapping would not make sense for them. If I had to do some for myself I would make my own tool for this if it only had to do a few threads. You can do both left and right hand threads with the same tool if you are thread milling. Less stress on the tool and less stress on the workpiece.

Martin C

This is the 1 inch x 0.1 pitch thread I cut a couple of weeks ago. A round bar with HSS cutter held by grub screw in end.

https://youtu.be/jRL97vY7NmA

David

5/8 X 8 ACME internal threads would normally be done with a two stage tap about 10 inches long, as trying it on a lathe with any type of threading bar would be very difficult because there is so little room. I produced nuts in 3/4 X 5 ACME by using an 8 ACME laydown insert in a ground down bar to remove some of the metal, up to its depth limitation and finishing with a tap. This was because I was afraid of snapping the tap if I tried to do it in one go.

With 5/8 x 8 ACME, the starting hole is only 1/2" diameter.

Edited By old mart on 14/04/2022 22:40:28

As stated above, it is best to use a 2 stage tap because it takes a lot of muscle to cut an acme thread.

I have cut a 1/2" Acne thread in brass and I had to use extension bars on the tap handle to turn it. Luckily, I did not break the tap.

Edited By Paul Lousick on 14/04/2022 23:02:47

It does seem to be a major limitation of laydown internal threading tools that they're pretty well unusable for most normal internal threads. The only ones I've seen that get close to a reasonable pitch/diameter are made by Carmex and are rather spendy.

You can, but those bars are 3/8” diameter and the DOC is 1/8” for a 5/8” acme thread. That has to fit into a 1/2” hole as a poster mentioned earlier - impossible. I have a piece of 1/4” tool steel I could try as a boring bar, but realistically it’s still too close.

A tap is a possibility.

Or make it bigger. I am only trying to make a tail vice for my woodworking bench as a fun project. I don’t think it really matters what size the acme thread is, within reason. Looks like the ID for a 1” 8 TPI thread is 0.881.

Acme threaded nuts are available which could be used in a fabrication as an alternative to machining one but they are expensive. An alternative is to butcher a G-clamp and weld or braze the female threaded part into the job. G-clamps are cheap and you may even be able to utilize the screw.

Clamping units for making wood vices are available on ebay.

Hi Paul, I know, but I want to make one for “fun” to learn all the machining techniques needed. No cheating if possible.

Steve

It is always a good idea to be willing to learn new techniques, but the size to pitch ratio you have chosen would tax the most experienced machinist. The internal threading bar has to be small diameter and is therefore flexible and the coarser the pitch the higher the cutting forces, not a good combination. I have made internal threading bars out of Densimet, a tungsten sintered alloy made the same way as tungsten carbide, but machinable and just threadable with HSS taps. This is stiffer and denser than any steel and helps with the flexibility problem to some extent. I can just manage 3/4" x 8 ACME with one of these with an IR or ER 16 size laydown insert. Those special taps for ACME look like two double length taper taps in tandem, they are expensive.

I have the same problem .I have to cut the thread and then cut the nut.The thread is acme 5/8" 8 8Tpi,.The leadscrew is for a shaper I am making, with 275mm of thread in the middle of 595mm overall leadscrew.I have cut threads before but never this long ,daunting prospect but I am looking foreward to it.The threaded nut not so much,as I have a leadscrew from a milling machine and I thought I could use part of the unworn section and make a tapered tap and case harden it and then will buy a tap to finish.I don't know if this will work ,but I will have a go and would appreciate any thoughts on my idea.

Frank

A little bit on this on Homemade tools.net today