First make sure chuck backplate threads and spindle register are clean a bit of swarf can throw the chuck out, also scroll and the jaws should all be clean, people do regrind the jaws with a toolpost grinder to true them up, personally I nearly always use a 4 jaw independant chuck and dial in the work to run true

Mick

Hello Stephen,

Before you get into grinding the chuck jaws there are a number of other things to check which may be all that is needed.

First of all, is this a new chuck to you? Has it been fitted correctly on it's backplate? I would first use a soft faced dead blow mallet or stout block of wood to tap the chuck in the direction needed to reduce the error you report. It could well move very slightly on it's location to put matters right.

3 or 4 thou error is not bad really, certainly for reclamping a job in the jaws since it could well settle slightly differently every time you do it. Run round the chuck with the chuck key and tweak all the key points to get an even loading on the scroll.

Grinding jaws will not correct that sort of error without giving them a true rim to grip on to start with and it is a rather desperate method of correction

Regards

Brian

You will be lucky to get better than 0.001" even with a new chuck unless it is a good one AND scrupulously clean* – you only need one bit of swarf on the scroll to introduce such an error.

If you can make your OD 25mm it should come out of 1" stock with even a worn chuck.

Neil

*I have two 4" SC chucks (one Indian one Chinese) that can do 0.001" but it takes cleaning to achieve this. An in-situ clean is enough, but it MUST be done.

4 thou ain't bad.

Grinding the jaws should only be a last resort IMHO and only after careful checks have been made. If you think about it, jaws are hardened steel and a very compact and strong shape. Chuck face is essentially a flat disc of cast iron with three slots milled in it. If something's going to go out of shape when abused it's more likely to be the soft iron chuck not the hard steel jaws.

I had a 4 5" PB chuck with severe runout that varied between 8-15 thou and I suspected the jaws until I put them into another similar PB chuck and they were perfectly fine. Turns out the slots in the front face of the chuck were dished and it was holding the jaws bell-mouthed, probably from being abused by over-clamping something held at the extremities of the jaws.

Consider also the commonly accepted statement that grinding the jaws is only good for the diameter they are ground at. That is exactly what you would expect if the chucks jaw guides were dished or domed.

Edited By Pete Rimmer on 02/03/2018 18:24:30

Edited By Pete Rimmer on 02/03/2018 18:25:01

Personally I would never consider a 4jaw self centreing chuck for engineering. Woodwork possibly. The main reason being that many is the time I've found that metal isn't square

Roy

4-jaw SC holds round work very well.

Neil

That old chuck problem again.

Generally, I thought I wasn't bothered by chuck run-out as usually try to avoid second chucking and I was using a Burnerd and they are good so we are told, so wasn't expecting significant run-out.

On having some issues, I actually checked with a DTI and was shocked to find a 10 thou total run-out (on both sets of jaws vs. less than 1 thou run-out on the outside of the chuck body (using silver steel test pieces).

So strip down, clean, inspect, relube and reassemble. There was no damage or obvious wear steps in the scroll, jaws or chuck body. In fact the jaws were in remarkable condition, no trace of bell-mouth or jaw damage evident.

On reassembly, slight improvement was noted to 9 thou TIR on both sets of jaws, slightly less for smaller diameters of silver steel (1/4, 1/2 and 5/8 was all I have to test).

The chuck is a Myford having a backplate with threads inside the chuck body and actually quite a tight fit, so wasn't expecting to gain much by rotating the chuck body with respect to the threaded back part. In fact this did help and gave a slight improvement reducing TIR to between 5 and 6 thou.

Experiments with shimming the apparently low side jaw with strips of paper showed that TIR could be reduced further (between 0 to 2 thou (which would be acceptable).

Assuming the chuck was good once, how can a scroll or jaws selectively wear to end up like this?

and should I now resort to grinding? to be achieved by carefully skimming off some material from the two apparently high jaws (say half thou at a time in the tool cutter grinder, replace, check, repeat until TIR is within reason etc). Or just shell out for a new chuck?

By comparison, the Chinese chuck on my Seig C1 is still within 2 thou TIR after years of abuse. No idea how old the Myford chuck is, the machine is mid 70s and has not had hard use. It is possible the chuck is the original but may not be.

Would appreciate any thoughts on whether there is anything else I can try before the dreaded grinding option. Don't want to grind in situ from the tool post either!

Brian,

thanks for that.

I think I understand, but just to be sure, are you saying remove the chuck body from the internally threaded back plate and then check the fore and aft run-out on the outer mating surface of the back plate to test if that surface is truly normal to the spindle axis?

That hadn't occurred to me as a source of error.

Regards

Brian,

Thanks, I will check that later tonight and report!

Regards

Martin

Brian,

You Da MAN! !!!

The back plate of the integral Myford chuck comprises the register only as the threaded portion is in the chuck body. However, that register portion was a good fit on the mandrel nose and not going to wobble from the pressure of a DTI.

Even so I checked several times in differing positions and the result was always the same – 3 to 4 thou run-out in the plane normal to the spindle axis. Moreover, the low and high positions corresponded to the low and high chuck jaw run-outs, so well spotted.

I can see how this part not being truly normal causes the problem, but cant understand how it ended up in that condition – or was it like that from day 1?

Next thing to do is make up a slim tube/top hat 11/4"W12 nut arrangement to secure the register plate tightly to the mandrel for skimming – that will be an interesting internal threading project! If my dodgy geometry is correct skimming the chuck back register should eliminate 80 to 90% of the chuck run-out at the jaw location.

It is great to know what the real cause of the problem was and you have saved me committing the unspeakable crime of jaw grinding!

So thanks very much Brian!

Regards

Martin