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Making a new screw for a bench vice

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Dog on wheels02/09/2021 09:03:16
6 forum posts

I have come upon an old Parkinson No.7 bench vice which has a bent and damaged screw. It would seem unlikely but all I can think that could have caused the damage is that it has been over-tightened and this has pushed the screw away from the half-nut, bending it in the process. The damage on the screw threads matches the length of the nut. Surprisingly, the nut looks fairly okay, so I was pondering on making a new screw for it.

What I've been wondering is what would be the most appropriate grade of material to use. It would surely have to be something better than EN1A, although the original doesn't seem very tough at all. My thought is that EN24 would be best for the job. It would be an expensive piece of material though (probably more than the vice is worth) and so I wondered what other people's thoughts might be.

Tony Pratt 102/09/2021 10:05:24
1704 forum posts
8 photos

It's got to be personal choice hasn't it? EN1A will be fine but EN24 will be better but more moneysmiley

Tony

peak402/09/2021 11:44:46
avatar
1504 forum posts
162 photos

Re steel prices; if you have a steel stockholder locally go direct.
The last piece of EN24 I bought was 3/4" @ about £8 per metre, so not that dear.

Bill

Howard Lewis02/09/2021 13:16:00
5348 forum posts
13 photos

Presumably a Buttress thread to cut?

Brian Wood02/09/2021 14:09:14
2454 forum posts
37 photos

Have you considered using wooden vee blocks at each end and straightening the screw with a hardwood block and suitable weight hammer? I have successfully corrected a bent shaft in a lathe screwcutting gearbox and a lathe leadscrew by such means..

It would be my first choice but it maybe you prefer the challenge of making a replacement!

Regards

Brian

Dog on wheels02/09/2021 14:33:17
6 forum posts

Thanks for your helpful replies.

Unfortunately, for me to keep the original shape of the 'bulb' where the tommy bar goes through, it would need to be made from 2" diameter material. Unless I make it from 2 pieces, possibly making a new screw and fixing it somehow to the original 'bulb'. I'm not so keen on doing this because I can't see how I'd be able to do it without introducing a new weak-point. It would save rather a lot of roughing out (and material cost) though.

I'm not sure I fancy going to all of the effort to make it from EN1A, unless I was sure that it would be good enough (which I'm currently not). I'd be very curious to know what the original is made of. It clearly isn't anything special to have bent as it has done, and it files very easily. Maybe it is just plain old mild steel, perhaps it is intended to fail before the main castings under heavy loads?

Yes, the thread is 1" x 5tpi buttress form.

I would be able to straighten it well enough for it to work (it's hardly a precision instrument after all) but the thread is damaged in one place so there would always be an area of motion where it would not be engaging very well on the nut and would likely jump the threads under load. The original thread doesn't look to be too well-made either. The peaks of the thread are sharp on one side and truncated on the other, so it's probably eccentric to the outer diameter.

Tony Pratt 102/09/2021 14:54:33
1704 forum posts
8 photos

So Dog what are you going to do?

Tony

Howard Lewis02/09/2021 15:13:45
5348 forum posts
13 photos

Sounds like cutting a new thread may make the vice better than original.

If you make the new thread out of ntwom pieces, may I suggest:

Cut then thread on 1" bar.

Make the "bulb" a clos fit on the plain end of the thread.

Press / Loctite the "bulb" onto the thread and then drillm through the assembly to take the handle.

In this way the handle will provide ma solid drive onto then thread, with the bar reinforced by the "bulb"

Strangely, kempes Yeasr Book does not mention Buttress threads, but A C parkinson in First yrear Engineering Drawing quotes the thread as having a 45 degree flank angle , with mthe thread being truncated by P/8 at bnoth root and creast, giving the actual thread as being 3/4P. So the depth of cut will be 0.15" for a 1" x 5 tpi thread, with a flat at root and crest of thread of 0.025"

HTH

Howard

Dog on wheels03/09/2021 09:28:33
6 forum posts

I had thought about something like that, Howard, but there is a flange on the screw that is 1 1/2" diameter so it wouldn't be of a huge advantage to do so. Besides, with the tommy bar being loose in the cross-hole, the pressure from tightening is taken on the edges of the hole, which would be the Loctited part and so the new bit would only be taking any pressure if the Loctite slipped anyway.

I think I've decided that it would just be simpler to make the whole thing from scratch and EN16T is a bit cheaper than EN24 so I'll probably use that. I was drawn to EN24 because I have used it in the past and know that it machines well. I've never used EN16 but the technical information I can find suggests that it'll be okay to turn. I suppose I'll see how it goes.

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