Griptru wil not ad just

Richard said:

But - I can relatively easily adjust the chuck using light taps on the body with a plastic-headed hammer.

So TonySimmonds comment :

The screws fastening the chuck body to the backplate are too tight, they need to be just snugged up.

is invalid - the chuck is not 'stuck' due to the mentioned 'screws'

In fact , is is quite clear in the Grip-True and in the ROHM ZS Hi-True ( works in the same way as the Grip-True) documentation that the bolts fastening the chuck body to the spindle backplate are NOT to be loosened whilst adjusting the chuck concentricity.

Per the ROHM Manual:

Adjusting the chuck for concentric gripping

No mounting screws to be loosened for this adjustment

1. Chuck a workpiece or test mandrel and determine the
maximum indicator reading.
2. Depending on the position of the eccentricity, loosen one
or two of the adjusting screw spindles located opposite
the point of the maximum indicator reading.
3. The retighten the remaining one or two screw spindles
until the gripping centre has been corrected by an
amount corresponding to half the indicator reading.
4. Check radial run-out again and repeat the adjusting procedure
if necessary.
5. Lightly retighten the previously loosened

Richard, do the adjustment screws rotate easily? Can you loosen all three up completely, and lightly tap the body over in the direction of one of the adjusters, as far as it will go, then gently screw in that adjuster as far as it will go and observe if the body moves away from the screw?

Joe

Hello

Read my article in MEW issue 292 about adjustable chucks, more will be said in the following issue about the tightening screws.

J Maurel

I'm looking forward to the solution to this "great mystery"!

One small mystery I can solve is the disconnect between the text on MEW 292 pages 67 and 68: on close examination, the first line on 68 has been near-obliterated by the red line under the pull quote at top left, which has been repeated on page 70.

Edited By ega on 03/04/2020 13:59:55

I have recently acquired one, which has barely been used, if at all. Mine is the bespoke Myford one, so no backplate as such.

The adjustment is brought about by conical screws in the body bearing upon tabs which project forward internally from the base. If the body and base are clamped together tightly, it is difficult to see (i.e. IMHO impossible) how one could be moved over the other; but how tight is too tight?

On my particular chuck the screws which unite the body and base enter from the back, but with a separate baseplate, I imagine that they must enter from the front.

I have found that 1 Nm allows adjustment to be made with quite gentle force. 2 Nm makes the force a little more than I would like, but it is possible.

The next question is how tight do the screws need to be when turning? Possibly no more than 2 Nm. Remember that the manufacturers intend these chucks for grinding, so certainly not for roughing out. IMHO 5 Nm is plenty for a 4" chuck. The difference requires barely any movement of the screws.

If you don't have a torque driver, I would suggest that for adjustment, the screws be nipped up holding the short arm of a hex key between finger and thumb; afterwards just the normal good twist holding the long arm in the palm of the hand in the normal way.

HTH.

My UK made Hardinge has one and the screws under the backplate so there is no way of adjusting on the run so to speak. If the instructions are followed and all taper screws are tight, as long as the mounting screws are tight enough to prevent endplay everything should be ok. No eccentric movement possible.

Edited By Alan Crawley on 19/10/2020 14:15:21

Well it seems i have the same problem with chuck runout. I found this thread while mosing around, & wow. My blooming chuck is adjustable. I have had the machine a while now & have just worked around the fact that it has a 9thou runout.

Now thanks to this amazing forum & members i can adjust it (hopefully).

Steve.

Would have been a better video if you had shown close up of movement if the dti pointer during adjustment.

George.

I have a 4" Griptru which came ready mounted on a Boxford backplate. I can't say that I'm wildly impressed. It's tedious to adjust and only seems to hold its accuracy for diameters very close to the one it's adjusted to I use it basically as a general purpose chuck. I wouldn't buy another one.

Well i have been i the workshop for a few hours , trying to adjust the chuck. I was having no luck at all. So i took it to pieces to see how it all worked. Gave it a good clean , although it was clean inside.

Put it back together & started to adjust. As someone said further up the thread , you need to loosen the backplate or it will not move. Once i did that away she went . I have gone from 9 thou runout to literaly zero. I have taken plenty of pics & video of doing it. I will compile it all tonight & hopefully get it uploaded either tonight or tomorrow.

I also turned a 1" stainless bar 8" long down to check for runout along the bed. Pretty much perfect all the way.

Once done i will post a link for anyone who has one & having trouble adjusting it.

Steve.

I have uploaded a video for anyone having trouble with these chucks. Hope it is of some use.

Steve.

3 Jaw adjustable chuck

Perfect timing , I have one on the bench right now that came with an ETA lathe aquired earlier this year. Never seen so much swarf in a chuck before , preventing that is something I will address on reassembly.

I'm seeing 2 points in the previous posts and the PB instructions that are as I see it key to getting the desired result.

1. The correct fitting to the backplate is vital , Blindingly obvious ? No , not if you have no idea how the job was done in the first place so I'll give it a thorough once (or twice) over.

2. Damage is done to these chucks when adjusting if the first step in the sequence , slacken off all the adjusters , is ignored.

Finally. The idea is to take up a few thou not half a yard like a 4 jaw. There should be no need to slacken the retaining bolts.

I'll be back later to eat some humble pie when all my theories are blown out of the water.

Have you not watched the video.

Steve.

Lee has summed it up neatly.

I have watched a couple of videos about these chucks and have to wonder if the users have read the maker's instructions. It only claims to repeat accurately at the diameter set, and also that the chuck key should be used in the same hole every time. You have to forget the habit of checking all three. And the same number should be used to unloosen it.

I have used one professionally for a good number of years without problems. The retaining screws are never touched. And I believe I may have stated earlier, if ham-fisted users strain them so they are bell-mouthed the jaws can be corrected carefully on a surface grinder. No need to grind the bore and make the jaw face concave.

Plus one for Alan's comments, good bit of kit if used correctly.

Tony

My take for what it's worth!

Whether or not the fixing screws need to be slackened to adjust the chuck has been argued since they first appeared. Myford didn't know, and it took a long time to get a statement out of Bernerd. Their view is repeated as a correction to the article posted by ega earlier in this thread:

So the maker says the fixing bolts should not be touched. Yet the suggestion they need to be slackened pops up in Model Engineering as regularly as Xmas. Owners don't believe Bernerd.

I think the answer to the paradox lies in the question: "What could possibly go wrong?"

Several things!

First, the range of adjustment is tiny, perhaps smaller than common-sense suggests is reasonable. A Griptru might be forced bending one or more of the wedging screws.

Second, Pratt Bernerd's instructions (also in ega's post) make it clear that the adjusters are worked in a particular way. If the chuck is operated by the sort of optimist who never reads instructions, then he might well work the adjusters in opposition, jambing the device. If he also has gorilla instincts, the adjusters could be forced. Maybe many times in the hands of a persistently heavy-handed owner.

Third, some people can't stop themselves dismantling their toys, even though they don't know how to put how to put them back together! Or after several years service a chuck is taken apart for a clean. My guess is the fixing screws are tightened to a particular torque in the factory, and some amateurs tighten them excessively. Some Lads believe tighter is better and ensure it by whacking their spanner extender with a big hammer. Ignorance is bliss.

Fourth, years of corrosion and gunge is likely to gradually increase friction between the chuck body and backplate causing the mechanism to stiffen up. Again, some owners will force the adjusters, maybe causing damage.

If the fixing screws are too loose, the chuck will rotate slightly independent of the adjusters, again with risk of damage.

The answer? I suggest the fixing screws are torqued to a specification such that friction between chuck and backplate is somewhat lower than wedging screw force. As a screw-driven wedge is an excellent force multiplier, I'd expect the fixing screws to be more than finger-tight, perhaps another turn or so, but not vigorously tight. I'd experiment until as Bernerd say "no great force is needed to operate the adjusting screws".

Although the mechanism is solidly built, make sure the wedge screws haven't been bent by an enthusiast. If bent the mechanism won't work properly. Otherwise I suggest there's no need to touch the fixing screws provided they've been tightened correctly and the moving surfaces are clean.

Dave

Well SOD i bought my lathe with the extra's supplied so i cannot comment on the tightness of the rear backplate fixings. I only know what i had would not move. After stripping it & seeing how it operated it seems i was turning the wrong side. I did 1st take the tension off the 3 adjusters so no binding could occur.

However the body was solid to the backplate , hence stripping it too. I can only say that yes the instructions maybe correct for a new or untouched chuck. When one is very old & probably passed through many owners then who knows what has been done.

I just wonder how many you have found to be hard to adjust.

Not sure about the dig of people taking things apart & cannot put them back together. Surely it cannot have been at me.!!!.

I just hope the guys who have a go at adjustment find their screws not so tight or stuck up.

Try following a Haynes manual.

Steve.

Edited By Steviegtr on 20/10/2020 16:26:09

It's back together on the bench. I previously mentioned that it was choked with swarf there was even some in the bottom of the backplate retaining bolt holes. That leads me to concur with SOD Dave on the points he makes.

I'm now at the trial and error stage with the torque. I should add that mine is mounted on the adaptor plate mentioned in the PB instructions and no slackening or adjustment is possible once it's mounted on the lathe.

No 3 jaw is a mighty clamping tool (some seem to think they are  and this one I see as a light duty chuck with the benefit of being able to clock it in like a 4 jaw for every job.

I have read this complete thread over twice now. Conclusion right or wrong is.

There are 2 types of this chuck. The main one is a chuck mounted to a back plate by long screws from the front face of the chuck approx 2" long. These go through clearanced holes the full thickness of the chuck to threaded holes in the mounting plate. The Myford one which i have has a dedicated backplate & is bolted directly to the rear of the chuck by means of threaded holes flush with the rear of the chuck.

I think logically that the front entry bolted type would be much easier to adjust without any slackening of the said bolts. C on the diagram further up this thread.

On the Myford type i can see why it would be much harder to push the chuck sideways when boited directly with short bolts , straight into a threaded entry. No flex The front mounted type has around 1 5/8" of free shank to be able to deflect.

Waiting for the gunfire.

Steve.

Mine has the adjustable backplate , this then has an adaptor backplate secured from the rear so that it covers up the disputed loosen or not bolts , so it is clear that they are not intended to be loosened.

I have it on a non running lathe now and I have it to a half thou. The bolts have been taken up to a light torque on the short side of a normal allen key. This light touch is a bit counter intuative untill you remember that it is the adjustment cones that are keeping things together.

As for the adjustment procedure it's easy once you get the routine in your head. Loosen opposite before you tighten and numbers on the chuck help too.