Acme LH 7/8" x 4 tpi nut

[bob ward]

I need to make a replacement nut for the vice screw on my power hacksaw, its a LH 7/8" x 4 tpi and presumably acme profile. The nut is about 40mm long.

I know I need to consider flank clearance when I'm grinding the threading tool, and I know I need to hold that tool in the shortest and thickest possible threading bar to counteract chatter. Anything else I need to be aware of, any thoughts?

http://i237.photobucket.com/albums/ff28/sirrobertthegood/IMG_0055_zpsgof5rqij.jpg (http://s237.photobucket.com/user/sirrobertthegood/media/IMG_0055_zpsgof5rqij.jpg.html)

[Michael G]

Have or make up a piece of matching thread to try for size;
Make it in a piece of round and once to size, put it on the threaded part and machine square (that is, use the thread as a mandrel so that the nut is aligned with the working surfaces);
Make your blank a little bit longer than the thread needs to be and use the extra length for a counter bore the major diameter of the thread. The tool will deflect away while cutting but the counter bore will give you a reference so you know when you are close as well as providing a surface to line up on if something moves.

Michael

[bollie7]

You are going to be starting with a hole some where around 5/8" ID (if my extremely rough, mental calc is somewhere close) so you might have trouble with a std boring bar.
I made a nut like this many years ago except it was 5/8" od. I ended up grinding up the tool out of some 3/16" hss and then welding that onto the end of a hi tensile bolt long enough and small enough in dia to do the job. In my case I was making the nut out of Phos bronze so I wasn't too concerend about heat from welding affecting the HSS too much.
The nut was about 1&1/4" long so I had to take a lot of spring cuts. Took a bit of time but it worked out really well.

bollie7

[franco]

Bob,

Two possible alternatives to avoid machining a new nut which might be worth thinking about are:

1. If there is enough metal in the existing nut bore the remains of the old thread out and and cast a new thread in the old nut in Babbitt metal using the least worn section of the existing screw as a mould after blackening it with soot to stop it the Babbitt sticking to it.

2. Make a new nut in Acetal plastic, see:

http://bbs.homeshopmachinist.net/threads/43645-Making-Acetal-leadscrew-nuts-the-easy-way

Frank.

[Stustoys]

Given you're not doing production, how about you make a quick release nut?
My bandsaw has one. Now while I cant recall how its done off the top of my head I don't think it looked that hard to do. Once loosened the jaw can slide freely in and out.
If you're at all interested I'll take mine to pieces at take some pictures...... maybe its not as simple as I recall :D

Stuart