How to remove four jaw chuck screws?

Folks,

I hope to repair the jaw screws in a four jaw independent chuck.

It came with a job lot of tools for a Myford M type lathe, and like them was covered in chicken poop and diesel exhaust carbon deposits. This must be some sort of unrecognised preservative; underneath the metalwork is in pretty fair nick.

This chuck however was the exception; all four jaw screws have some bits of the thread broken off and a couple have split sockets. The intention is to make a new set, but first I have to get the original ones out.

However on removing a jaw the screw just sits there, it's not free to move laterally and is firmly retained as per the pic below. About 2/3 of the circumference lies around the screw, so it's impossible to shift.

Another shot of a screw showing the yoke which also looks to wrap around the screw by better than half the circumference.

The chuck body is a single casting, it's not in two parts. The backplate bolts right through.There's nothing to be seen from the front face, but on the back are four blank plugs with no apparent means of removal. See below.

Any clues? Thanks in advance.

Eug

Edited By Eugene on 13/06/2018 20:56:06

Edited By Eugene on 13/06/2018 21:21:08

The clue is under the four blanking plates. Try tapping them out from the front, they are probably a press fit,

I'm thinking that those circular plugs at the rear of the chuck have some sort of forked section which locates in a groove in the screw holding the screw from moving along its axis. Maybe tap the plugs to take a length of screwed rod and then with a nut washer and a short length of pipe slowly extract the plug.

Hi Eugene, make a 'forked' punch from a suitable piece of bar that will fit in between the guide rails of the jaws and bear on the two top faces of the yoke and be clear of the screw, and then with the back of the chuck supported clear of the bench, drive the yoke out to the rear., the dust cover will come out first. I have several similar chucks, mine are old English 'Crown' make, and I mark each yoke to match its original hole.

Trevor.

Thanks lads,

vintagengineeer .... there's no way to tap them out from the front. The screw sits in the way.

stevelee .... I'd thought of tapping and threading the plugs and using a hot air gun and something like a hub extractor to pull them out too, but I wondered if anyone if the forum had any experience doing it.

It's really aggronoying; I've serviced another four jaw and it was dead easy, remove the jaws and the screws just fall out.

Eug

Trevor ...

Aie caramba!

Sounds like a job for a weeks soak in Plus Gas, and some nifty file work before resorting to t'ommer.

Thanks.

Eug

Eugene

Trevor's is the only way to get the yokes out and remove the screws. They will be a tight fit so as you say a good soaking in Plus Gas is a good idea. It can sometimes help to give the reverse side a tap to break the seal which will have formed. A little heat is usually a good idea.

They usually come out eventually. I have dismantled lots, and only had to give up on one which just wouldn't be shifted

I agree with Trevor Crossman1. I have 2 different size 4 jaw chucks and they both come apart by tapping out the yoke from the screw side. I made a forked tool many years ago to facilitate the removal process. I have also braze repaired badly damaged screws & recut the bad portions of thread in the lathe also remade the square sockets. Both chucks still give sterling service.

regards

My experience is taking one 4 jaw apart and it went very easily by just tapping through from the front with a punch. I think it is worth making some marks so each one can be replaced in its original position and also mark it so you get the orientation correct when refitting as they do not turn easily once they are tapped back in. If you have access to a press then that would be nicer than tapping but take care not to do damage if tapping them out.

Mike

I've always used a pin punch about 4mm dia. and tapped 'em out from the front. The punch will just fit along side the screw and clear the chuck body. Don't use a precision punch, a plain 'ol hardware store one works fine, as you'll probably have to bang it smartly to get the yokes out. Heck, even a stout nail works...

Pete

Eugene,

I am inclined to disagree with your statement that it is a one piece casting - unless you. are carefully ignoring it consists of a casting and a machined item It looks like those four fixings are securing a back plate to the main casting, Removal of those fixings may reveal how the screwas were fitted?

Eugene,

Yours looks like the 'backplate version' of the Pratt Burnerd 6"/150mm lightweight [with a Myford fitting?]

... In most respects this is similar in its design to the 'screwed body' version.

They are both a little simpler than the heavier versions [which have additional retaining screws] but, the first couple of minutes of this video should reveal what you need to know about the construction: **LINK**

https://m.youtube.com/watch?v=u9zcgoJJ_6A

MichaelG.

I have one of these screws to replace in Burnerd chuck like this and have taken advice....

The big clue I was given is to STICK IT IN THE OVEN before knocking out the yoke. And not to drive them in too deep when reassembling.

Cheers, Colin

In those chucks of this type I have worked on, the plugs and 'fingers' that surround the waist of the screw are hardened and I believe they are pressed in. I have knocked them out using a punch from the inside, but it is hard work and can swell the fingers.

Hillclimber's trick of heating the body in the oven to start with is not one I have tried and it might make all the difference.

Regards Brian

Thanks to one and all; I can see the way forward now. Plus Gas, oven or hot air gun, forked tool, bish bash bosh.

Just a special note to MichaelG ... you're quite right it is a Burnerd; if its contemporary with the lathe that accompanied it (and I believe it is) then its 68 years since those plugs moved. My right hip's about that age and I know how much that wants to shift around.

Lads, I've got a couple of projects on the boil just now, but when I tackle this chuck I'll let you know how I get on. Again, thanks.

Eug

I had to make replacement screws for a 12" 4 jaw many years ago. My process was a bit different than shown in ENOT's video. The stock was chucked at one end and supported by a center on the other. I threaded a blank long enough to make all 4 screws. I chucked up the threaded blank using aluminum shims on the chuck jaws. I then parted off the individual screw blanks. I chucked each screw and faced and chamfered it, machined the groove, and drilled a pilot hole in the end. I flipped the part in the chuck and faced to length and chamdered the end. My chuck wrench had a square end so I transferred the work to the mill. My final cut in the square hole was with a 1/8" endmill. I finished out the corners using the mill spindle as a slotter. The material I chose was ETD 150.

Well, it was easier and quicker than I imagined.

Twenty minutes with the heat gun followed by a good soaking in penetrating oil overnight and number three jaw came out as sweet as a nut; I've no doubt the others will do the same.

It's a bit weird to feel sorry for inanimate objects, but I think we'd all have some sympathy for number three, as below.

There's other chunks missing at the far end on the obscured side. As the late, great, Spike had it "Seen better days but not taken part in them".

A point of interest ..... the yoke horns each had the distinct imprint of a punch mark; not me M'Lud, I used a piece of turned down silver steel with a flat face. Also there was no debris from the broken threads, and some of the flakes would be a bit big to find their own way out. My conclusion is that whoever savaged the chuck disassembled it and removed the rubbish, then put it back together with the screws as is. Shining.

I may have to come back to you guys for advice on the threading exercise, but thanks to you I'm now out of the woods.

Eug

Edited By Eugene on 17/06/2018 18:16:09

Eugene,

The jaw screws are usually soft so you can make a matching square form screw,, they are not ACME form, drill a hole in the end to suit the A/F size of the chuck key and cut out the corners with a triangular notching tool laid on it's side, working as a keyway cutter.

Check the square form screw with the jaw to be sure they fit snugly before finally turning the 'waist' to fit the fork in the holding plug

I would though use decent steel for the screw, something that you can harden to light blue temper at the square socket end to resist splitting out of the corners in use, don't make it so hard it is brittle.

That was a rotten trick to play by the unscrupulous seller of the chuck in the first place, I hope the rest of it is in better shape and it didn't cost you top dollar

Regards Brian

Brian,

That's an excellent observation, thank you.

I've just checked with a mini hacksaw blade; the core dia. under the yoke is soft, and the socket end is hard and won't take a cut.

Again, the part under the yoke has a punch mark in it; I guess it was reassembled too tightly and the guy relieved it by knocking it back a bit. Fair enough if you use something soft, but pretty crude work with a hardened punch.

The original seller didn't cheat me; I bought a long bed Myford M type and a large quantity of tooling and accessories, sold as seen for £300. It had been lying around in a barn since the year dot and I think this chaps Dad had been the last to use it. All the rest of the stuff is fine, even if very dirty. The extended change wheel set alone is probably worth what I paid for the whole issue, so no complaints.

With the socket I'm going to try a wobble broach as per Mike Cox; it's all experience.

I'm a bit torn as to just what to do about a cutting bit for the threads; it's an 8 tpi so a 1/16th cutter should be used but that implies getting the helix angle correct to the nearest gnats. Perhaps I should make it a bit fat, say + .008" to give some  clearance. Backlash isn't a problem so looser fit won't matter.

Thanks again,

Eug

Edited By Eugene on 17/06/2018 20:45:40

Eugene

Congratulations on getting the screw out. I just finished re-assembling the 11" four jaw that came with my wartime Pratt & Whitney. Seriously neglected, quite rusty and crudded up to hell and gone but fortunately just enough oil in the crud to largely protect internal things. Those screw location forks clearly hadn't been moved since they were put in about 75 years ago but punched out OK. Albeit with about as much hit as I cared to use before going away and making proper pusher set to use with a screw force device.

Tricky bit on re-assembly is to get the forks exactly 90° to the jaw guides. The forks on mine were about 20 thou narrower than the jaw guides so it could be assembled from the front. Slipping the screw into the fork and eying up against the jaw guides before things were too far home worked well enough to get it all lined up using a spanner to turn the forks.

If yours is the same as the similar small one I did a fair few years back the forks will need to go in from the back so alignment is much harder. I made a screwdriver thing with a blade a nice fit in the forks and worked by eye twisting as required with the forks only just far enough in the body to be secure. Got there in the end but if I ever do another one I shall make up a guide with a similar blade on a part circle extension narrow enough to be nice sliding fit between the jaw guides. Jaw guide end probably needs to be rather wider to give nice long flat for good alignment. Needs to be made long enough so the forks can be slid well onto the blade before they start to go into the body.

With the forks mechanically aligned you can concentrate on getting them to go dead straight into the body. Its a knack. Odds are they will be determined to go bit off square. Naturally you will get the hang of it for the fourth one!

Clive.