Pratt Bernard Grip true issues

[teamrickyParticipant @teamricky]

Hi Folks,

 

Its possible I’m going over old ground here but I thought I’d share some experience with an old in need of restoration and set up of a 4” Grip true chuck, In principle the chuck by design can be slipped around on its integral back plate leaving the final coupling and registers fixed as per normal to the spindle nose.

In my case the chuck is old and a bit of a project to attempt to restore to a usable condition.

Its 4” 3 jaw 100mm diameter

 

One thing to note is they are still available and expensive

 

In stripping down the chuck cleaning comes first and and few checks made measurements wise, My problems by design start to come into effect from here !

 

Most I believe are being used incorrectly to start with by not properly providing access to tighten the rear intermediary bolts once you have adjusted the total runout via the 3 cheese wedge bolts, This leads to the chuck having terrible rigidity and it hard rides on the three pins further creating point loads , In my case it was nesasary to extend the thread on each taper bolt to allow adjustment to re-occur

Once stripped the body in the y direction was not precise enough so the registers were re established square once more, All good so far so with everything tightened down I took a light cut from the inside of the jaws with little success, I had got rid of the slight bell mouths on the jaws but soon realised the following that you have work around sadly…..

 

The 3 wedge bolts have more than enough influence to distort the front face of the chuck body towards the Z

It will also be able bolted together in an symmetrical and simply Aim the jaws off square again due to distortion, I spent a fair bit of time stoning off these high spots on the two body half’s to get them to take up flat and not rock on 3 points

 

In this condition it’s possible to dial in stock at the chuck end but say 8” out with non bell mouthed jaws it’s just a lottery as to how the far end runs, Typically 0.125-0.2mm or worse, I spend more time thinking looking and resolving the problems

 

Main problem solved was to stone off the back of the chuck until the bolts when tensioned gave an even force to the point that the stock in the chuck didn’t deflect from axial alignment…..Very time consuming…… last it would be nice use a too post grinder to finish but I had good carbide so used that to take a very light skim, Finally to get close to perfect co-axial alignment I removed the jaws and honed them until the work mounted first time with 20 microns or so at a good amount of stick out!

 

Up shot is I had a spongy in accurate un adjustable chuck but knowing where the downsides are hopefully this can help other folk struggling with longer stock in these chucks

 

[old martParticipant @oldmart]

I have always been suspicious of having to leave the screws anything but tight, when I mount a standard scroll chuck on a backplate using the “poor mans set tru” method with a loose register, I add 3 more SHCS or bolts and after truing up the chuck at any given diameter, all 6 get tightened fully.

[bernard towersParticipant @bernardtowers37738]

When you say you took a light cut from the jaws did you use a petal plate as this loads the jaws in the correct manner(as it would be when holding a workpiece). Otherwise the jaws will end up more bell mouthed.

[Neil LickfoldParticipant @neillickfold44316]

My two came with bellvile type spring washers under the capscrews. One came with the lathe when I bought it used in 1987, and the other chuck New from the importers closing down in 1997.

On my old chuck, I closed the edges of the jaws onto a pieces of material the same size. Then carefully took a skim cut until a minimum clean up. the bore was about 25mm or so.

A petal plate is effectively what i used to hold the jaws.

Found the picture taken at the time.

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