Problems fitting soft jaws to a 3″ Pratt lathe chuck

[GreensandsParticipant @greensands]

[GreensandsParticipant @greensands]

The photo shows my 3.25" diameter Pratt 3-jaw lathe chuck the jaws for which have been tapped 2BA to allow for the fitting of soft jaws. The problem is that only one hole per jaw has been provided and this lies very close to the centre line of the chuck. My original intention was to make up a set of jaws from 1/2" square aluminium fitted with (counter bored) 2BA socket screws but if the jaws are machined with a square end as is normally the case the end face is then very close to breaking through to the counter bore. Has any else come across this problem? The Pratt chuck jaws are 3/8" wide

[Tony Pratt 1Participant @tonypratt1]

Move the c/bored hole further away from the end face of the ally block??

Tony

[JasonBModerator @jasonb]

possibly drilled to take hexagonal soft jaws which will meet in the middle. Or simply cut 120deg point onto your square ones

Edited By JasonB on 12/10/2020 13:10:12

[Howard LewisParticipant @howardlewis46836]

You could mill a slot on the back of the jaws, so that they fit over the chuck jaw, to locate, and as already suggested, have the soft jaw over hang the end of the chuck jaw proper.

If the end of the soft jaw coincides with the end of the main chuck jaw, you will only be able to bore it for a short distance before the tool hits the main jaw.. This will be less of a problem if the soft jaws are long, and stick mout a long way ahead of the jaw proper.

Howard

[old martParticipant @oldmart]

If you visit the ROTAGRIP website, they sell soft jaws for these small chucks at just under £30. The code is EJB and the exact dimensions are listed, as there are so many variations with chuck jaw sizes. I have bought lots of the much commoner soft jaws for the 5" size chucks on ebay, in various remaining ammounts of usability. Even when a totally used up set was included in a job lot, they got turned into base jaws for aluminium and brass tops.

If you can mill down the jaws illustrated to be level with the lower step, then they have potential for making base slot and tenon jaws to hold a variety of tops.

Edited By old mart on 12/10/2020 14:00:08

[GreensandsParticipant @greensands]

Ah, being drilled and tapped for use with Hex stock makes sense. I will try it out. Thanks for all the helpful advice provided.

[GreensandsParticipant @greensands]

Can anyone explain why the 2BA holes in the (hardened) chuck jaws illustrated in the photo of my OP have been drilled in a non concentric fashion which on the face of it precludes the use of concentrically drilled hexagon soft jaws?

[old martParticipant @oldmart]

2ba is hardly going to be able to hold any type of jaw extensions onto your existing jaws. The holes were probably drilled freehand which would explain the oddly positioned one. I would not trust less than three off 2ba fixings on each jaw to hold work securely, even on a small chuck like yours. Evwn then I would go for 6mm with 12.9 screws.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by old mart on 13/10/2020 15:46:23:

[…]

The holes were probably drilled freehand which would explain the oddly positioned one.

[…]

.

… or, just possibly, drilled very carefully for a special purpose

either way, I doubt they were done by Pratt.

MichaelG.

[GreensandsParticipant @greensands]

These are hardened jaws we have here and so I tend to favour the idea that the 2BA holes have been added for a special purpose, presumably by someone or some authority with full hardening facilities. Close examination of the chuck reveals that the three holes are on their own PCD replicating perhaps the staggered start associated with the scrolls on the the back of the jaws as required for operating the 3-jaw self centreing mechanism.

[JasonBModerator @jasonb]

I would think the single 2BA upto the job, I know a very capable member here who's soft jaws are retained by JBWeld

You can also buy hex softjaws that use a single screw so can't see why more would be needed unless for very heavy work.

[DiogenesIIParticipant @diogenesii]

..Hm..it's a bit of a long shot, but will the jaws go in backwards..? ..it might explain the eccentric offset and perhaps leave you with location for three pins that'll grip a disc the same size as the chuck..

[Author]

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