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Pratt Burnerd 4 Jaw Chuck

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Len Morris 224/10/2021 10:21:28
57 forum posts
29 photos

Hi Everybody,

I have a PB 6 inch 4 jaw chuck in need of some TLC. I'm not sure how to remove the forked pins that retain the jaw screws. They have no obvious fixing on the chuck back face. All that's visible are the flat ends of the pins. I suspect they just press out but would like to know for sure before I start forcing things. Can anyone help?

Thanks and regards,

Len

DC31k24/10/2021 10:38:23
586 forum posts
1 photos

They are a tight fit in their holes. No other method of retention.

It is prudent to keep pins, jaw screws and slots together so they go back in the same way they came out. If really fussy, mark rotational orientation as it would be possible to reinstall 180 degrees out.

Examine them well and stone off any burrs resulting from the removal process before reassembly.

Beware on reassembly that if the forks are twisted (rotated) it can make operation of the screw very tight.

Reassembly can be made a little easier if the chuck is warm and the pins are cold.

Dave Halford24/10/2021 11:03:14
1820 forum posts
19 photos

I tapped mine out with a hammer and knackered screwdriver.

Len Morris 224/10/2021 11:58:17
57 forum posts
29 photos

Hi,

Thanks for those quick replys. I can now get the pins out with confidence on the fly press. I'm not a fan of press fits on items that need regular cleaning and so intend to lap the pins for a sliding fit and lock them axially in place with cap screws leaving them free to rotate for fork alignment. Petty much the same as with larger 4 jaw chucks.

Best regards,

Len

Dave Halford24/10/2021 12:03:22
1820 forum posts
19 photos

Len,

The backplate holds them in.

Brian Wood24/10/2021 13:51:58
2475 forum posts
39 photos

Len,

Be aware that they are hardened, it might well influence your holding method and it would be interesting to know how you intend to hold them for lapping.

You might want to change your mind. the method used by Pratt is long established.

Brian

Len Morris 224/10/2021 14:32:06
57 forum posts
29 photos

Hi all,

Agreed the back plate stops the pins dropping out but it would allow too much axial slop for my liking. I know the pins are hardened but I've found that modern tooling takes the fear out of that. I've not got the pins out yet so don't know how I'll hold them. For an easy life I might just polish them and relieve the holes in the cast iron body. As far as I can see, the method used by Pratt on this chuck is cheap and nasty and just allowed them to save a few pounds on its production.

Anyway, thanks for all your comments. I now know how to get the pins out and will post some pics and comments on the final job.

Best regards,

Len

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