‘New lathe chuck jaw screws’

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‘New lathe chuck jaw screws’

This topic has 27 replies, 18 voices, and was last updated 26 January 2013 at 13:33 by JOHN BRIDGE 1.

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21 March 2011 at 13:42 #65780
Ted 047
Participant
@ted047
Having read Norman Hurst’s article, I wondered if there are really so many chucks, industrial or otherwise, with the broken screws he describes.

If this is the case, surely it must be due to the use of added leverage, using lengths of conduit, box spanners etc., about which we have all been warned in our early days!

I’m sure Norman’s re-designed screws will be entirely satisfactory, and, as he says, the load will be shared by six corners, as opposed to four, but it seems to me that even better protection from failure and abuse will be provided by the ‘torsion-limited’ Allen key, which has been subjected to welding heat!

Nice article though!

Ted
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21 March 2011 at 13:42 #38039
Ted 047
Participant
@ted047
21 March 2011 at 14:28 #65781
Terryd
Participant
@terryd72465
hi Ted,

I’ve come across quite a few of these broken screws in my time. most seem to get damaged when someone tries to remove a tight on chuck by hitting an installed chuck key with a mallet or hammer.

Regards

Terry
21 March 2011 at 19:13 #65789
KWIL
Participant
@kwil
That’s it in one Terry, hitting the key provides a moment sideways which splits the square socket as opposed to a rotational force.
13 April 2011 at 17:47 #66846
michael chalmers
Participant
@michaelchalmers19148
Having also got a broken screw,does any one know where these 1/2 inch whit grub screws can be obtained.

Thanks
13 April 2011 at 18:41 #66848
Steve Garnett
Participant
@stevegarnett62550
Posted by Ted 047 on 21/03/2011 13:42:18:

Having read Norman Hurst’s article, I wondered if there are really so many chucks, industrial or otherwise, with the broken screws he describes.

When I got my Kerry, it had a 4-jaw Pratt-Burnerd chuck with exactly this damage, and I’m sure that it was caused exactly the way that terryd says. I was fortunate – I found somebody selling an identical chuck, and just transferred it to the original backplate. I still have the old chuck, and only a couple of the screws are damaged, so I’m keeping those as spares for the new one – and as far as the old one goes, I’m also wondering where you get these socking great grub screws from… and also trying to figure out what the safest way of removing the chuck is, in the longer term.

This particular Kerry is an ex-university one, so whilst most of it is remarkably unworn, some clumsy students at some stage have done for the aforementioned 4-jaw adjusters, and also managed to destroy the end of the compound slide by repeatedly running it into the chuck. I don’t altogether blame them for that though – with the original toolholder this was a stunningly easy thing to do. Compound is fixed now, though.
13 April 2011 at 21:42 #66861
Robbo
Participant
@robbo
Michael,

I have got a few of these 1/2 whit grubs, I could let you have 4 (unless you want to use 2 types of chuck key!). Send me a message if you want them.

I got mine from David Bateson in Cumbria, email d.bateson130@btinternet.com if you want to see if he has some left. Very reasonable price.

Phil Robinson

Chorley, Lancs
13 April 2011 at 21:56 #66864
Nicholas Farr
Participant
@nicholasfarr14254
Hi, most supplyers of nuts and bolts to industry should be able to get any type and size grub screws you require, most of them will accept cash sales these days.

Regards Nick.
13 April 2011 at 22:58 #66867
Lathejack
Participant
@lathejack
I thought Norman Hurst did a fine job of remaking the jaw screws, and as i have a six inch Pratt chuck with a damaged screw i was quite interested.

But, if i remember correctly, the chuck used in the article has screws that are held captive in the jaws with the matching half thread form cut into the chuck body. So these will use a standard right hand thread, so that turning the screw clockwise will close the jaws onto the workpiece.

All the indipendant four jaw chucks i have, and have ever had, use screws that are held captive in the chuck body with the thread form cut into the back of the jaws. So these use a left hand thread to close the jaws while turning the screw clockwise.

I am not sure how easy it will be to get left hand threaded grub screws, as standard right hand threaded ones will unfortunately be no use in a chuck of this type. What a shame, as i was all set to modify the grub screws i have for my chuck as Norman did.

Edited By Lathejack on 13/04/2011 23:04:39

Edited By Lathejack on 13/04/2011 23:05:57
19 April 2011 at 08:50 #67122
methusala
Participant
@methusala
Hi lathejack,

Its just a thought, why not screwcut the jaw screws, drill a

suitable size hole in one end. Then turn a spigot onto a caphead screw and

silver solder it in position.

hope this helps,

Colin.
19 April 2011 at 13:49 #67136
Clive Foster
Participant
@clivefoster55965
I have successfully arc welded broken square sockets back together in the past. My welding skills are adequate -, at best, so its a pretty practical proposition. Needs a steady hand and concentration. A decent inverter style welder, like my Fronius, is a great help as its much easier to exploit small rods. But I did the first one with the SIP 140 buzz-box I had previously. Albeit with considerably more verbal encouragement.

A simple corner crack isn’t too bad to stitch back but if you have to replace a completely broken away side a good deal of care is needed to avoid getting significant quantities of weld inside the square. For the third one I had to make a complete new side. A piece of key steel in the remains of the socket helped teach things manners and careful attention to piece overlaps helped me get things together without welding the key steel in place. I did have to do a fair bit of filing round the outside tho’. Diamond files are nice for this sort of thing. Arc EuroTrade gave me a set of small “store in the handle” ones as a freebie with an order which are far better than the price indicates and very useful for jobs like getting inside the square. Heck, when they wear out I’ll probably buy some more!

Clive
19 April 2011 at 14:02 #67137
Steve Garnett
Participant
@stevegarnett62550
Posted by Colin Stewart on 19/04/2011 08:50:24:

Its just a thought, why not screwcut the jaw screws, drill a

suitable size hole in one end. Then turn a spigot onto a caphead screw and

silver solder it in position.

Since mine is exactly as lathejack described, I might have a go at something like that – it sounds like a much easier thing to do than trying to find enormous LH threaded grub screws, and on the face of it is a Good Idea – thank you.
19 April 2011 at 17:55 #67154
Steve Garnett
Participant
@stevegarnett62550
So I took one out. In this particular chuck, the screws are located by a yoke that projects forward from the rear of the chuck, fitting around the groove. This can be driven out with a small drift, so at least it’s easy to replace afterwards. In other words, the jaw looks like a normal chuck jaw when you’ve removed it, and the screw bears some sort of resemblance to the one shown in the article:

The eagle-eyed amongst you will realise that this doesn’t have a Whitworth thread at all, but appears to be a LH Acme thread. The diameter of the groove cut into the thread is just over 8mm, so drilling it out for a spigot, grinding off the top and silver-soldering a suitably turned-down hex-head bolt looks like a distinct possibility.
20 April 2011 at 19:45 #67237
Steve Garnett
Participant
@stevegarnett62550
Slight snag – apart from the broken bit at the top which appears (presumably because of repeated fatigue) to be quite soft, the centre of the screw is rock-hard, and won’t drill at all without softening it. I suppose that this won’t matter if I can grind off the rest of the broken top, and solder a much shorter hex-head into the small recess. I’m open to other suggestions though. How else could you realistically do this, preferably without having to soften the screw – or is that the best thing to do? I don’t have an EDM, unfortunately, and I have four of these to do.
21 April 2011 at 10:45 #67278
John Olsen
Participant
@johnolsen79199
Can you get at it with an air die grinder or a dremel type of tool?

regards

John
21 April 2011 at 12:42 #67282
Steve Garnett
Participant
@stevegarnett62550
Thanks for replying, John. I suppose that the answer is that I’m not sure. I’ve had another go at photographing what’s really happened to the socket at the end, but I don’t know how much clearer it is – perhaps a bit:

The socket is square down to the bottom, which is about 2mm below the flat surface I filed at the end of the thread, which is the softer part. You can just see the tip of my attempt to centre-drill this if you look carefully.

Having thought about it some more, what I’m now wondering is two things; One is that I’m intrigued as to how the hell they make these square holes in the first place, because I’m reasonably confident that they didn’t use a wobble-broach as a mass production method, and the other is whether I actually need to extend the hole down deeper anyway. The idea that I currently have is that if I can find a suitable size hex-head bolt, I could clean the rest of the screw surface flush with the broken side, and reduce the bolt thread so that it’s only 2mm long, and file it square, and then silver solder it in place. That is, assuming that I can get everything clean enough, and don’t screw up the soldering.

The dremel is a good idea though – I have some diamond burrs, so even if it only cleans it up, it’s worth it.
21 April 2011 at 13:43 #67287
Ian S C
Participant
@iansc
Could you, weld or get someone to weld a piece on the side of the hole, a well done job would be almost invisable. Ian S C
21 April 2011 at 14:56 #67290
Steve Garnett
Participant
@stevegarnett62550
As in repair the original square socket, you mean? That might work on two of them, but the third one is way worse – it’s broken out on two sides, not just one. The other thing that’s not visible from either photo is that this one is starting to crack in another corner, as well. Which will make it as bad as number three in no time…

The thing I’ve now realised is probably how they make these. You drill a hole, and notch the corners with something rather powerful – which makes me think that perhaps the manufacturing process leaves the material with four relatively weak spots. The wall thickness in the corners is only about 2/3 of the side thickness around the rest of the socket head, after all.

But I’m bearing all these suggestions in mind, certainly. Fortunately there is absolutely no rush to fix these screws, as presently it’s a spare chuck anyway. If I can get it sorted one day, it will most likely get used on a rotab, and not for higher-speed turning as such.

Any more suggestions or comments are welcome – I still haven’t got the best solution sorted in my mind yet.

Edited By Steve Garnett on 21/04/2011 15:22:46
21 April 2011 at 18:38 #67298
Les Jones 1
Participant
@lesjones1
Hi Steve,

The temperature required for silver soldering may well soften the screws so softening them to enable you to drill them may not weaken them any more than the silver soldering.

Les.
21 April 2011 at 19:14 #67299
Keith Long
Participant
@keithlong89920
Silly suggestion? Could the last posting from Gray suggest a fairly easy repair – cut some key stock or something similar, position in whats left of the square socket so it sticks out, silver solder/braze in place and make a “box” spanner to do the tightening?

Keith
21 April 2011 at 21:33 #67305
Nicholas Farr
Participant
@nicholasfarr14254
Hi Steve, following Keith’s suggestion you even be able to silver solder/braze a head with a short section of the shank from a suitable sized hex bolt and then get a socket wrench and weld a T bar into it for the tighening.

Regards Nick.
21 April 2011 at 22:42 #67314
Steve Garnett
Participant
@stevegarnett62550
Thanks for the replies, guys. I’m pretty certain that I’ve got to do something which involves silver-soldering, but it’s a bit of a moot point as to what. The idea of reversing the polarity of the driving sections so that they are male instead of female is intriguing – the only thing that concerns me about that is that it would inevitably leave parts of the shank projecting beyond the circumference of the chuck, if I only used the 2mm available, and that they’d have rather sharp edges!

OTOH, Les’s point about what the soldering temp would inevitably do the the steel does leave me thinking well, what the heck I might just as well soften them anyway. And if I did that, then I could drill them, and it would then be possible to insert some hex bar or whatever rather further in, so that it wouldn’t have to project beyond the circumference at all. Since the screw has that groove in it, it might well be possible with a bit of care not to soften the entire screw by using a heatsink around the longer part of the thread. Then I’d only have to choose whether to make them male or female driven.

All these points have been useful – I will sleep on it!
22 April 2011 at 02:01 #67323
John Olsen
Participant
@johnolsen79199
Steve, I don’t think it is going to matter if you do soften the screws. Unless you are planning to use the chuck for mass production work, a mild steel screw would last a lifetime.If I were to find myself making something like this at home, I would probably try to find a piece of good steel, say high tensile bolt or similar, but I would not try to harden it.

On the possibility of reversing the sex of the screws by brazing in a piece, you might not want to do this for a lathe chuck, where the bits sticking out would be a hazard, but if it is for use on a rotary table or dividing head it would not matter.

regards

John
15 March 2012 at 20:18 #87293
John Mills 2
Participant
@johnmills2
I've just seen this thread and it is probably a bit late for you now but G & M Tools (Ashington) have a box of jaw screws from which I found the exact one I needed to repair my Burnerd 4-jaw. It needed to be dressed and carefully fitted but the 'repair' is permanent. Best to visit with broken item and then match it up from the many in the box!

John Mills
17 March 2012 at 09:47 #87367
jomac
Participant
@jomac
Hi, whats wrong with TIG welding a similar piece of metal in place, if you do one weld then let it cool down, (anyway TIG welds that small dont get that hot), then grind away the excess, or use another chuck or collet to machine down the offending bits. By the way as said before DONT!!! hit the key or but a length of steel in the chuck jaws and belt that too, not only do you damage the screws and the chuck jaws, you can also put the chuck out of alignment with the back plate, even bending the holding down screws.

If you have some spare time, drill three or four holes into the outside rim of the back plate and make a HEAVY duty C spanner, to remove stubborn chucks, if you drill the holes properly they make good indexing marks.

John Holloway.
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