Setting up a Hi-True 3 Jaw Chuck - How to?

Hello all.

I have a ROHM Hi-True 3 jaw chuck and cannot find definitive instructions anywhere on how to set the 3 adjusting spindles that are used to true to near-zero runout.

The chuck is brand new, and the 3 taper adjust spindles seem very stiff to adjust. Since adjustment relies on the chuck body being able to move relative to the the chuck backplate, the operators manual indicates that the rear plate mounting screw torque is critical, and a figure is given against which I have verified said bolts torque.

I have tried and succeeded in adjusting a test bar in the chuck to 'no' discernible runout, but the adjusters are very stiff. The user manual implies that the adjusters have to be 'tightened' up against the ring ( inside the chuck, against which the spindle operates) when done, but the stiffness destroys any feel as to when tight is tight. I have dropped ROHM germany an email, 5 days and 2 days ago, and no response..

In essence, you treat the adjuster as you would one jaw of a 3 jaw independent chuck, ie, loosen an opposing one or two, and tighten the other, etc. But there is just no 'feel' in the process - the adjuster taper spindle loosens up when you unscrew maybe 6 to 8 turns, but the first 4 or so unscrew turns are as tight as anything.

I understand the adjusters need to be tightened up again, else I suppose the chuck could move on the rear plate in a heavy or interrupted cut, but I cannot say when tight is tight - I fear over-tightening and mucking the taper spindle!

How to do it?

Joe

I don't know if they work in a similar fashion to the GripTru, but this article and the two embedded links are worth looking at.

**LINK**

Also, if you have, or can get access to , a copy of George Thomas' "The Model Engineer's Workshop Manual", you will find a good explanation on P280/281.

I'm not sure about reproducing it here for copyright reasons.

Bill

Edited By peak4 on 13/07/2017 16:20:46

You do realise that you can't just keep on doing up the adjusters but need to back off all 3, adjust one as needed, THEN BACK IT OFF before using the next one round. This is why the main securing screws need to be set specifically to keep it fairly firm. At the end you put a bit of torque on all the adjusters just to stop them coming out in use.

This was (and still is) always the problem with the Myford set-true chuck and people before the internet were often writing in to ME with problems because the instructions were not clear enough.

edit - meant Grip Tru as Bill says.

Edited By Bazyle on 13/07/2017 16:19:12

Also it isn't so much about feel as 'view' of the dial indicator. Sensible lubrication of mating surfaces and screws will avoid the dial jumping.

Edited By Bazyle on 13/07/2017 16:22:56

After re-reading I see you are doing it correctly so the real question is why is it so stiff for the 4 turns at the end of its travel when you would expect it to immediately slacken off due to the taper.
Have you examined the contact point of the taper screw on the backplate? Rather like the problem with an ill fitting cotter pin (on an old bicycle crank) if the taper screw is too small / worn relative to the clearances it will not be working on the taper itself but on the 'heel' of the taper.

Hi Bazyle,

Yes I have inspected them - I have completely removed the taper adjusters and find no binding marks that I could perceive to be the problem. Maybe I will try cleaning them of all grease and blue them up and then see where they rub and maybe why..This is a rather expensive 200mm top of the range chuck, around 2500 BP and I find this both confusing and surprising, also that they do not give clear user instructions as to how tight is 'tight'

It is very difficult , almost impossible, to use only one hand to loosen these adjusters those 3 or so turns, using a T -Bar tool with a T handle around 80mm long.

This in the 'manual'-

Adjusting the chuck for concentric gripping (valid only for chuck ZG Hi-Tru)

(No mounting screws need 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. Then 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 adjusting
screw spindles and check again.

Lightly Indeed....

Joe

Bill, the Grip-True info you linked to is in essence the same as my Hi-True. Unlike the Pratt'n'B these do not need the slacking up of backplate screws, which is nice. The link referred to has pretty much the same user setup procedure as the ROHM, so I begin to suspect that it is in fact that simple indeed! Maybe my chuck is so new that the adjusters fit is still tight...I will remove and blue them and inspect how they fit, just to pacify the Head...

Thanks all.

Joe

I have a couple of adjust true chucks with 4 adjusting screws. The process for adjusting them true is essentially the same as adjusting a 4-jaw to run true. I have one chuck (Yuasa) with three adjusting screws. Is there a procedure for adjusting the 3 screws? At present I just keep fiddling with the screws until it runs true.

If the adjusting screws are hard to turn there may be insufficient clearance between the spigot on the back plate and the ID of the chuck. I had one Chinese adjust true that wouldn't adjust. I had to bore a little out of the ID of the chuck to get clearance for adjusting the chuck.

My Grip Tru has what look like Bellville washers under the screws.

It works so did nit remove them the last time I was servicing the chucks.

Neil

John Reese - If the three screws operate in the same or similar fashion to the Grip-Tru/Hi-Tru, then in essence the chuck and workpiece will move towards the screw that is being tightened. The link provided above by Bill has in the lower text 2 links to gif images - scans of Grip-True setup procedure - and the second gif image from section 12 explains how to do it. Your chuck's implementation may differ somewhat, but I believe the basis would be the same.

Joe

Joe and John, looks to be the same principle;

**LINK**

See also the following "Tubal Cain" video, but note that the Buck chuck he's showing has 4 adjusting screws, whereas the GripTru has 3.

For those who aren't aware, this is a different Tubal Cain to the one who sometimes features in literature this side of the pond (AKA Tom Walshaw).
It's a pen-name adopted by both folks from the son of Lamech ;

https://www.collinsdictionary.com/dictionary/english/tubal-cain

Bill

Edited By peak4 on 14/07/2017 08:40:27

Edited By peak4 on 14/07/2017 08:44:58

Edited By peak4 on 14/07/2017 08:51:10

Joe and Bill,

Thanks for the reply.

I have the adjustment of a 4-jaw chuck or an adjustable with 4 adjusting screws down pat. Been doing it for nearly 70 years. With 4 jaws (or screws) if you adjust jaws 1&3 it leaves the work position relative to jaws 2&4 unchanged, and vice versa. It is just a matter of getting the same indicator reading on 1&3 then doing the same on 2&4.

With three adjusting screws if you adjust screws 1&2 it changes the relationship with respect to 2&3 and 1&3. In other words when I adjust one pair of screws it throws of the setting of the other screws. What I do now is determine the TIR of the workpiece. I adjust screws 1&2 to get the edge of the part to the mean of the TIR. I repeat the process with screws 2&3, then with screws 3&1. After that I go around again however many times it takes to get acceptable runout. The 3 screw process seems a lot messier than the 4 screw method. I was hoping for a more direct method of adjustment