GRIPTRU Chuck adjust

[Speedy Builder5Participant @speedybuilder5]

I notice in the News Tab here that Neil has entered the information (Still blurred) for the adjustment of these chucks. This may help.

Bob

[peak4Participant @peak4]

In conjunction with the above diagram, the official manual is available here
https://prattburnerd.com/wp-content/uploads/2022/01/PBI-GRIPTRU-MANUAL.pdf

Bill

[BazyleParticipant @bazyle]

I remember this coming up in the readers pages of ME repeatedly some 20-30 years ago. The big failure in the instructions which resulted in people often stripping the adjuster threads or being completely unable to use set up the chuck was that you MUST first slacken the screws holding the chuck to the backplate, and obviously tighten them again after setting.
I think this was left out because they mistakenly thought the users had the same level of basic mechanical experience and feel as an indentured machinist.

[Kiwi BlokeParticipant @kiwibloke62605]

In the interests of avoiding (continued) confusion, the chuck adjustment moves the front part of the body of the chuck, carrying the scroll and jaws, with respect to the back part of the body. The backplate – if fitted – doesn’t come into it: the back part of the chuck body remains fixed immovably to it. As delivered, the screws securing front and back parts of the body are often too tight for proper adjustment, and should be eased just enough for the adjusting screws to be turned without the buttocks needing to be clenched. They should then be left alone, and, in any case, won’t be accessible if the chuck is fitted to a backplate.

In my experience, it’s often almost as quick to adjust a 4-jaw independent chuck, but, of course, that doesn’t provide the excellent repeatability of the Griptru – which, in practice, may turn out to be a somewhat theoretical requirement.

[Neil LickfoldParticipant @neillickfold44316]

Both of my GripTru chucks happen to have Bellville washers in them. One was new from the NZ importer in 1989, and the other came with my S7, that was imported in 1972. I have replaced the washers on the chuck I use all the time.

I think the secret is not to be over tightening the cam screws. Every once in a while, I remove the cam screws, and I re grease tapers. The grease I am currently using is a ant seize grease. A light coat on the threads and on the taper pins.

Every few years, I take it all apart, clean, lightly grease everything and reassemble. The backplate to body and scroll etc all get bearing grease. Remember to align the key slots , so they all are in the same alignment.

I don’t do heavy or big interrupted  cuts using the griptru chuck. I use it for finish turning and for things that require a very high precision, like re trimming pistons for model engines etc. With care, you can dial in a round part to better than 5um tir quite easily, if the spindle is set correctly.

[Charles LamontParticipant @charleslamont71117]

I find having a Griptru means I rarely feel the need to use collets other than for milling in the lathe. I have the retaining screws just nipped while adjusting, and re-tighten after. George Thomas has nearly two pages on the Griptru in The Model Engineer’s Workshop Manual.

[Clive Brown 1Participant @clivebrown1]

Agree with KB and Neil. The backplate screws are not invoved in any adjustment process. It’s not necessary, nor reasonably possible, to adjust the tightness of the body clamping screws, as said they are normally inaccessible and should be left at an optimum tightness when the chuck is assembled. After setting, the adjusting screws can be “snugged up”.

A very useful bit of kit if properly used.

[old martParticipant @oldmart]

I can see the best use of a Griptru would be for fine finishing cuts because the halves cannot be fastened together rigidly. Almost every scroll chuck at the museum has a loose register to allow fine adjustment if necessary, and the ones which are likely to get the hardest treatment have 6 screws holding them to their backplates. The exception is the 6 3/4″ Pratt with the serrated jaws which uses soft top jaws, the hard jaws we have for it would be originally intended for gripping the rough unmachined ends of sawn stock for a first operation during production runs.

[Clive Brown 1]

On Clive Brown 1 Said:

It’s not necessary, nor reasonably possible, to adjust the tightness of the body clamping screws, as said they are normally inaccessible and should be left at an optimum tightness when the chuck is assembled.

On old mart Said:

I can see the best use of a Griptru would be for fine finishing cuts because the halves cannot be fastened together rigidly.

I have been using a Griptru as my only 3-jaw chuck for over 50 years and I don’t understand either of these comments. The screws that hold the chuck together are perfectly accessible, and if required can be tightened firmly after adjustment.

[Charles Lamont]

On Charles Lamont Said:

On Clive Brown 1 Said:

It’s not necessary, nor reasonably possible, to adjust the tightness of the body clamping screws, as said they are normally inaccessible and should be left at an optimum tightness when the chuck is assembled.

On old mart Said:

I can see the best use of a Griptru would be for fine finishing cuts because the halves cannot be fastened together rigidly.

I have been using a Griptru as my only 3-jaw chuck for over 50 years and I don’t understand either of these comments. The screws that hold the chuck together are perfectly accessible, and if required can be tightened firmly after adjustment.

Same here

Martin

[Clive Brown 1Participant @clivebrown1]

Are we talking about the same arrangements? Pictured is my 4″ Griptru factory fitted to a Boxford backplate. The allen screw in the picture is solely to hold the chuck to the backplate. It’s permanently fully tightened. To access the screws holding the 2 halves of the chuck together, it’s necessary to remove the backplate. Joint between the 2 halves of the chuck can be seen as a faint line.

Is it possible that a directly mounted Griptru does allow easy access to these screws by eliminating the backplate?

Incidentally, this chuck is more or less my “everyday” chuck, and I’ve never had problems with it going out of adjustment under cutting forces, but I try not to abuse it.

[Charles LamontParticipant @charleslamont71117]

Ah, I see. Yes mine is a threaded body job for direct mounting. GHT shows a diagram of of a chuck for backplate mounting having the holding-together screws going in from the front. I suppose you could put holes through the backplate to allow access to the holding-together screws.

[Clive Brown 1Participant @clivebrown1]

Yes, I’ve also just looked up GHT’s notes on ME3662. He apparently owned 2 types, both different from mine, ie a direct mounting type and one with backplate screws fitted from the front. He suggests it could be beneficial to slacken these screws to allow easier adjustment, but PB are silent on the matter.

[Charles LamontParticipant @charleslamont71117]

In another thread Michael Gilligan asks for a definitive answer to the slackening question. To which I can only say “Don’t be silly, I can give you at least three different definitive answers.” (‘Maybe’, ‘it depends’, and ‘it is up to you’.)

[Michael GilliganParticipant @michaelgilligan61133]

🙂

[SteviegtrParticipant @steviegtr]

I did a youtube video on the griptru for my Myford S7. I found i had to ever so slightly loosen the plate screws as the force i was exerting on the adjusting screws was , I thought too much.

It made it much easier to do the whole operation.That was a few years ago. Not needed to adjust since.

Steve.

[Charles Lamont]

On Charles Lamont Said:

In another thread Michael Gilligan asks for a definitive answer to the slackening question. To which I can only say “Don’t be silly, I can give you at least three different definitive answers.” (‘Maybe’, ‘it depends’, and ‘it is up to you’.)

Whilst Charles’ witty rejoinder made me laugh out loud, can I ask him to try harder.

Historically many problems reported adjusting Griptru and some of them have come up again here!  Let’s face it: something is wrong.   Looking at Pratt Bernerds diagram, I think the difficulty is fairly obvious.  I see a clever mechanism, that within limits, can be tweaked to reduce run-out.   This is a good thing, but it’s a fine adjustment requiring the chuck to be operated correctly.

Unlike a plain 3 or 4-jaw, both robust mechanisms, the Griptru adjusters are rather delicate, and an untrained operator could get into trouble by overtightening one or more.   This might lead him to believe that releasing fixing screws to allow more movement is the answer, when that shouldn’t be necessary.  Griptrus are vulnerable to operator error!

Bill provided a most helpful link to the manual, which I recommend reading carefully.   Here’s what Pratt Bernerd say about the operator:

Note the word ‘only’.     I translate this as “Pratt Bernerd have been bothered with chucks returned damaged or believed faulty by owners who didn’t understand them. Pratt Bernerd need to cover our sorry corporate bottom because some customers mess up.”  Judging by the number of times Griptru comes up in old mags and on the web, quite a few customers…

Never set a Griptru myself, but Bill’s instructions show a process like that used to centre a 4-jaw.    Not rocket-science, but centring a 4-jaw requires understanding jaw movements, plus an  acquired skill.  Practice needed.

Is it worth owning a Griptru?   In my case, no!   What I already own covers the ground adequately:

• Most of my turning is held in a plain 3-jaw, because all the necessary cutting is done without taking the job out of the chuck.
• When I must reset work in a chuck:
  • A 4-jaw and DTI allow accurate centring.  Fairly quickly, though I guess not as fast as a Griptru.  But I’m rarely in a hurry.
  • When speed matters, I look to collets.   ER32, nothing special, but apart from bumping the collet chuck  into alignment on first fitting, collets are fast and accurate.  Already owning an ER32 collet set for my mill means I don’t have to spend money on a Griptru,

If the only choice was either a Griptru and a 4-jaw, I’d take the 4-jaw.   In addition to allowing accurate centring 4-jaw chucks are more versatile than the Griptru in that jobs can be deliberately rotated off-centre, which is useful for cams and ovals etc.  In contrast the Griptru is less useful.

Though I can’t justify owning a Griptru myself, they’re advantageous for others.   When an unusual level of accuracy is needed, and parts have to be made against the clock, and the workflow requires parts to be repeatedly taken on and off the chuck.   Or perhaps in an ordinary workshop a Griptru can be used most of the time as an ordinary 3-jaw (not adjusted), but tweaking is handy if a job requires resetting.   Down to the owner to decide if that’s worth it!

In conclusion, although the Griptru fails my “value for money” and “fit for purpose” tests, it might well be the bees-knees in your workshop.   A potential problem though is that the operator has to learn how to adjust them, and – like a 4-jaw – that’s not quite as easy as it seems.   As with all tools, my view is don’t waste money on them unless there’s a specific need.  And if you do buy one, read the instructions, and practice setting it up.  If there’s trouble, first suspect the operator!  Then, check that a previous owner hasn’t damaged it…

Does the above make sense?  Hope it’s more helpful than ‘Maybe’, ‘it depends’, and ‘it is up to you’.  No jokes I’m afraid.

Dave

 

[Charles LamontParticipant @charleslamont71117]

So that boils down to: “A Griptru owner need to know how to use it.” “You need a 4-jaw first.” and “I don’t want one.”

[Michael GilliganParticipant @michaelgilligan61133]

Although genuinely amused by Charles’ response to my suggestion … I maintain that it is valid: A definitive/authoritative explanation of “all aspects” [to use the decorators’ favourite phrase] would be a good positioning-statement for the new Model Engineer xxxx Magazine.

MichaelG.

 

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