Morning, I am looking for the 4x large grub screws that adjust the jaws on this 4 jaw chuck.
only quote we have is £150 each and we need all four!

Thanks!

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Do you mean the chuck actuating screw (the one you stick the chuck key in)? If so it is a bit more than a grub screw and I am not surprised that spares are expensive although £150 sounds a bit OTT.

John

Have you tried M/s Rotagrip for a quotation?

Alan.

Try Ebay search for:

Pratt Burnerd Operating Screw For 200mm 4 Jaw Metric Chucks 2500-20365

Other sizes also listed

Edited By Phil JOHNS on 16/02/2020 12:37:36

All the dimensions are HERE.

Doesn’t look too difficult to make with your sort of kit? Silver steel, if hardened - or something less exotic, if not?

£600, less material, tooling and time would be a good saving?

I made a set out of silver steel for the Pratt lightweight 6" four jaw. They had LH acme threads, although some of their chucks used square threads. Unfortunately, they are still in my cupboard, after I found another more serious problem with the chuck. I broached the hole hexagon, as it was easier than having a square hole. Much too small for a 16" one, however. When the blocks which hold the screws in are drifted out, check them carefully for cracks.

This damage is common with Pratt chuck screws, they are not able to withstand abuse. Ham fisted people wrench the key sideways, putting great strain on the square. This is even more prevalent with screwed on chucks. Then it is easy to put the key in and whack the base of the key with a mallet to start undoing the chuck from the spindle. At the museum, we have a strap wrench for removing chucks and never use any other method.

It is surprising how many chuck keys and jaw screws are ruined by using them to unscrew chucks! Ditto for gears in Headstocks when Backgear is engaged to lock the Mandrel, for the same purpose.

If only the square is damaged, would it be possible to turn it away and weld on some form of replacement?

Or even to weld up the "hole" and rebroach it back to a square?

Howard

Looks like the chuck is fitted to a Colchester lathe so I assume it is a camlock mounting, therefore the damage has been caused purely by over tightening, a couple of foot long extension to the chuck key springs to mind.

Right there Baz, there are not many lathes with a 16" chuck and a screwed spindle.

Old Mart, I don’t suppose there are many lathes with a screwed spindle 16” chuck, I certainly have never seen one, the point is that all the damage has been caused by someone over tightening the jaws, not removing the chuck. Makes you wonder what other abuse the machine has been subjected to.

Thanks for the replies, we’ve phone Rotagrip and they are £147 each!

Looks like we will make our own.

When I had a Colchester I had the same issue, I TIG welded the top of the splits up. They lasted 20 years till I sold the lathe.

Cheers

John

+1 for welding.

TIG by skilled user is, objectively, the best way but over the years I've done about 6 or 7 jaw adjusters with similar issues using a Fronius (good quality) inverter stick welder. 1 mm disk in the angle grinder to Vee out the cracks then carefully weld up at around the lowest current that would produce a good arc and proper weld. I suspect a conventional transformer (buzz box) stick welder wouldn't have the low current control needed. The ones I did were smaller being off Myford appropriate size four jaw chucks.

If welding works you have saved a fair bit of effort. If not you are no worse off as the screws were scrap anyway.

If you do decide to make your own its easier to use square holes. Cut a U channel in the end and weld a cover plate on the side before turning stock down to size and cutting thread. Theoretically you need to take account of material weldability but I've never really bothered about such things. Just accepted that my made versions won't be quite as strong or long lasting as factory.

On a job like that I'd probably go for thicker walls making a stronger job and accept that a smaller key would be needed reducing the ultimate holding power of the chuck.

Clive

Thanks, it’s actually at work rather than something at home so I’d rather buy some than let the tool room make them, they have better / more important things to do unfortunately! As much as they’d like to try a left hand acme thread!

I’ll probably make a clamp to go round the broken part to hold together while it’s welded, then use the EDM to make squares or hex’s.

Are they hardened?

Are they hardened?

Test with a file?