Renovating vice screw and nut

I have just bought a Qualters & Smith power hacksaw that needs renovating. On inspecting it, I find that the long screw that operates the vice (which by the looks should have an acme thread), and the thread in the block of cast iron that it screws into, are hopelessly worn. See images here and here. What is the best way of renovating this? I guess I need a new screw, the threaded part of which is about 19mm diameter, 335mm long and about 6 threads per inch. But what do I do about the thread in the block of cast iron? Can I buy or make a block of metal (mild steel? brass?) with the right thread in it that I can bolt to the back of the block? Or can I buy a helicoil this big? Or do I need to bore out the cast iron block and fit a new threaded insert into it? Or what? There isn't much meat to allow a threaded block to be inserted, unfortunately. Not enough, for example, to insert this.

Any suggestions on how best to repair this would be most welcome.

Thanks - Rowan

Edited By Rowan Sylvester-Bradley on 19/03/2020 17:00:01

To keep the speed of opening and closing it will be desirable to keep close to the 6 tpi.

I would favour you bolt a block/plate on the back or even a bronze standard available lead screw nut with a flange and screw to suit. Take a look at places like Automotion Components for off the shelf parts, more precision than the machine requires but cost effective.

I note they make (made) a range of sizes.

Keeping it all metric or imperial is your choice. It looks like you have several options that may fit the bill. That bush you linked to could be turned down a bit and still work.

Thread form is not particularly important - as long as appropriate. Even fitting a 5/8”/16mm screw might be good enough if you are never going to use the full capacity of the machine. An alternative is just to make a new part rather than repairing the current item.

I would likely turn down that bush, leaving a flange of full diameter, and fit it, if there is enough ’meat’ in both parts when completed. I might even make my own bush (particularly if a 16mm screw was laying around unused🙂!).

No need to purchase a screw full length of the original - the old can be fitted with a new threaded section.

You have so many choices...

Whatever you do check it's not a left hand thread because it most likely is.

FWIW, the vice on my 60+ yr. old Rapidor has a 3/4" dia. 10tpi screw, RH; ie 3/4" BSW. Easier to cut than Acme and taps quite cheap.

I don't think that the thread form is all that critical.

Looks like you're right Clive. Looking at pics of other Q&S saws the moving jaw is furthest from the handle so the thread would indeed by RH.

Thanks for all the advice.

It's definitely a right hand thread. As you say, the moving jaw is the furthest one. I'm wondering whether I can cut out a section of the moving part of the vice to make room for the nut, something like this.

This would have the added advantage of giving the nut something to push against when opening the vice.

What is the best way of fixing the nut to the vice jaw? Can it be welded to cast iron? Or would some kind of epoxy metal filler be strong enough? Or what?

Thanks - Rowan

If I were tackling that job I would simply turn a new nut from bronze and set it into the cast-iron nut with threadlock. Turn a round nut with a flange on one end and make sure that it's 'pulling' into the hole you bore in the cast iron nut when the vice is tightened. The threadlock will be more than sufficient to hold it for opening the jaw.

Unless the old screw is knife-edged it'll work for many thousands of cuts.

Posted by Pete Rimmer on 20/03/2020 13:15:48:

>turn a new nut from bronze

Bronze will work better than mild steel will it? In what way?

>Unless the old screw is knife-edged

Not quite sure what you mean by this. The old thread is in a pretty awful state (see this). I'm loth to try to reuse this and have it wreck the new nut that I have just fitted.

Thanks - Rowan

Are you sure that screw is ACME form? It looks suspiciously like it could be a buttress thread-form in that pic.

First thoughts

Being legacy British, the thread will be 3/4" So work in Imperial; the rest of the machine is going to be. Trying to match your non integer metric size exactly will prove a fruitless and frustrating waste of time.

Since the thread is only expected to provide thrust in one direction, it could very well be a buttress thread,

However unless you are absolutely intent on refurbishing to original spec, a 3/4 BSW would probably suffice.

If it is Right Hand, obtaining, or making, a replacement thread and nut will be so much easier.

3/4 BSW is 10 tpi, so for the same torque applied to the handle, the gripping force will be increased.

Howard

But at the right hand end of the piece in the picture, you can see a section which is less worn, and looks more like Acme. At some point in the past, someone has replaced part of the screw (there's a new piece welded on). The bit the other side of the weld (which I didn't show in the picture) is definitely Acme. But of course I don't know what the working part of the screw looked like when it was new. I am proposing to replace both the screw and the nut (since both are badly worn), so I can use anything that works (metric, imperial, BSW, acme, buttress or whatever).

Thanks - Rowan

You can use literally anything you want. Some M16 threaded rod and a long nut would be the most basic way. That's how the cheaper saws do it.

I'm with Pete Rimmer for the easiest way to repair it. Studding is easy to get and long coupling nuts as well.

**LINK**

The only suppliers of 3/4" 6 tpi studding that I can find (in Google) seem to be in USA, and the shipping cost is outrageous. Do you know of anyone in UK who will still be able to supply this?

If I use anything other than 3/4 6 tpi acme, then I can't use a long nut to join the pieces. In this case I will have to use a sleeve turned to match the diameters with pins through, I suppose.

Thanks - Rowan

Edited By Rowan Sylvester-Bradley on 21/03/2020 16:42:30

Edited By Rowan Sylvester-Bradley on 21/03/2020 16:43:00