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.

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.

With great care, the Grip Tru can be dial in to very small amounts of concentricity. The limit is actually the roundness of the part and the roundness and clearance of the headstock of the machine tool itself.

I prefer the Grip Tru chuck when setting up my piston turning mandrels, over using a 4 Jaw. Mainly because the Swash stays in the same plane with the Grip-Tru.  It does require some really good indicator to achieve a good result if trying to set up and remove a very small amount of material.

Neil

When truing the Jaws, I first set the outer body of the chuck , running true to the backplate.

The over all outcome, any size on the chuck now is quite close, and is closer than the standard 3jaw chuck that came with the lathe. The runout with my rebored GripTrue is about 0.06 tir on various sizes. In my case 25mm is the zero diameter and every 1/4 inch diameter smaller from there is very close to Zero, and every 1/8 inch is off by upto 0.06mm.

This suggests to me, that if I rebore it again, and correct the body by 0.03, still won’t be any better, as that is the wear or the mechanical limit to this particular chuck.

On 6 August 2025 at 08:38 cogdobbler Said:

I thought the point of a Grip-Tru was not that it was a set-and-forget deal that would then always run true for all diameters? But it allowed fine adjustment on individual jobs of different diameters to set each job true?…

 

 

Yes and no judging by Pratt Bernerd’s instructions!   If I owned one, it would almost always be used to fine adjust individual jobs, but according to PBI they can also be trusted for repeat work.

I’d bet the farm accurate resetting in a Griptru is only possible when the components are already truly round, which is rare in my workshop because I mostly work straight from stock.  Off the shelf, it’s usually slightly bent, dinged and oval with only nominal dimensions.  And the inaccuracies vary randomly along the length.  It would be unwise to expect a Griptru doing repetition work to hold raw stock  accurately, but it should work if the job was trued up earlier.

If the job is inaccurate, then the Griptru has to be tweaked to hold it true, and the instructions warn this may not be possible.

Judging by past posts, the Griptru is not well understood. Second-hand items are found to be jambed, or the new owner struggles to get the expected accuracy.      The manual claims “Operation of the adjustment feature is simplicity itself”  but this is followed by a page of advice and warnings!   How a Griptru is adjusted isn’t obvious!   Pratt Bernerd warn: Never overtighten the adjustment screws, which a newcomer might well do.

Rightly or wrongly I’ve assessed Griptru chucks as a poor match to my needs and, although “fit for purpose”, they fail my “value for money” test spectacularly!

• I mostly work to a shade under ±0.02mm accuracy (about a thou)
• When matching parts to better than 0.02mm, I do it by fitting, not by measuring
• An ordinary 3-jaw will round stock just as well as a Griptru
• An ordinary 4-jaw will set stock just as accurately as a Griptru, except it takes longer
• I rarely do repetition work
• When resetting work is necessary, as when moving from mill to lathe and back again, I use collets

If only allowed one work-holding method, what would would you choose and why?   My choice is a 4-jaw, because they are both versatile and accurate,  Definitely not a Griptru because an accurately settable 3-jaw chuck is a specialist.  Tools that don’t do what I need are rejected, even if everyone else loves them!

Dave