Pratt No. 58 5" 3-jaw stuck backplate

Hi,

I've got a Pratt No. 58 5" 3-jaw chuck with a backplate for another machine which I am having trouble removing. I believe this type has a recess in the back that the backplate is machined to fit and I've tried warming the chuck on a radiator to see if I can expand the recess so it comes off but no luck.

The chuck was fastened to the backplate by 3 off 1/4" Whitworth screws which are clearanced through the backplate and threaded into the backside of the chuck, can anybody tell me whether it is safe to try to drift the chck off of the backplate by setting a 6mm bar into the bottom of the screw holes and wellying it ? Or would that damage the scroll or something else inside the chuck ?

Any better ideas for getting it off would be appreciated - I also tried getting a chef's knife blade between the two but no joy there either...

<Edit:> Ideally I'd like to keep the backplate rather than machine it off because I think there's enough meat there to re-machine the thread for the new purpose which is to fit the 1.125" x 11 tpi nose of my Boxford dividing head </Edit>

Thanks,

Edited By Mike Davies 3 on 13/01/2022 01:15:05

Hold the chuck horizontal in the vice by the backplate, possibly with soft jaws if you're concerned about damage, put a large adjustable spanner on one of the jaws and try twisting the chuck with the backplate held in the vice.

I would be tempted to try Pete's method, albeit I'm paranoid about damaging stuff so I would probably use a strap wrench on the chuck body first. I assume you're trying to save the backplate?

I would first remove the screws, clean the joint-line and apply ‘dismantling fluid’ in small quantities … allowing it to gently seep in.

Next day … I would take a nylon-faced ‘dead blow hammer’ to it : hitting radially at many points around the circumference. This should open the joint sufficiently to apply more fluid and give it another beating [et seq]

MichaelG.

Hybrid method.

• Put the chuck face down on the bench and fill the bolt holes with penetrating oil (cf MG)
• Next day hold the backplate in the vice and chuck a bit of large hex bar, and apply wrench to said bar (cf MK and Pete)

Mike, assuming the thread in the backplate is larger than the centre hole in the chuck body, I would make a threaded plug to fit the backplate then a drift through the chuck to remove it. Use longer screws in the body to support it whilst using the drift. And use a large hammer 4lb with a gentle tap increase if needed.

Its unusual for this to happen usually they are easily separated, I appreciate my method is time consuming but it is less likely to cause damage to the chuck IMO !

John

I get it moving by slacking the bolts a coils of turns and giving them a crack with a small hammer and drift. Once the gap opens loosen another turn and go again.

One thing not to do is pry the halves apart with a hard edged prybar or screwdriver. If you bruise the edge you might cause the chuck fave to run out. Use aluminium shims if you have to pry the halves apart.

If all else fails, mark out for two holes diametrically apart.

Drill and tap, for say M6, or 1/4 or 5/16 BSF or UNF You will, need to drill far enough for the bottoming tap to go right through the backplate.

Then screw in two jacking screws, and tighten nequally.

That should prise the backplate off the chuck

Howard

As Pete says, leave the screws/bolt in but about one turn slack. Hit the head of each bolt in turn (decent wallops) with copper/ali faced hammer and the joint should separate easily.

Ian P

For what it's worth, I damaged the cast iron backplate for my mini-lathe 4-jaw chuck by using jack screws (through the threaded mounting holes to press on the back of the chuck). It was that tight. Still usable, but a piece broke off the registration surface.

On my chuck there are three screws from back and three other screws from the front. The back screws are counter bored into the back plates and the other front screws are counter bored in the chuck and tapped in the back plate .

David