Chuck Run Out Question

I’ve a 3 jaw chuck that I have mounted in my rotary table – with the chuck centred by means of an M2T taper sliding into the table (as supplied). I’ve not had great results when using it, and today I’ve discovered that whilst run out is < 0.1mm on the chuck body and outside jaws, run out using the inside jaws is a little over 1/3 mm as measured holding a bar of ground silver steel and just after the jaws. Before I get on to the supplier, does this seem quite normal or is it not very good ? My suspicion in that the inside jaws haven’t been ground too well ?

David

Edited By David Cambridge on 10/05/2015 20:39:37

Edited By David Cambridge on 10/05/2015 20:40:10

That's a lot, I'd even class 0.1mm as a lot but it all depends on the price, you do usually get what you paid for with chucks.
Not much help but I use a 4 jaw on my rotary table as it's usually holding something that has already had some work done on it so it needs to be clocked up to centre.

Steve

Hi David, are you assuming the taper in your RT has little or no run out. But do make sure that there is no dirt ect. trapped in the tapered hole or on the surface of the table where the chuck clamps down.

Regards Nick.

Hi David,
The first thing I would check is to see if the tapered hole in the RT is on centre.

You did not say how the chuck is then attached. Is it to a backing plate ? If so check if this is concentric with the taper. A photo would be good to see how it is mounted.

Are you able to mount the chuck on a lathe and check run-out ?

Paul.

Here is how I’m measuring my run out. Without the chuck mounted I’m within +- 0.1mm on the back plate, but even after a thorough clean I can’t do better than a run out of 0.2mm on the ground silver steel bar in the chuck. On my lathe I’d be an order of magnitude better than this.

The four jaw chuck sounds like a good suggestion, but I can’t figure out how to mount one on the 3 slot rotary table ? None the less I can’t help thinking I should be able to do better with the three jaw chuck , and that maybe something is up with the chuck ?

I've tried measuring run out with a 2MT centre held in the table centre taper, and it was too small to measure.

David

Edited By David Cambridge on 12/05/2015 20:49:05

Is it possible the jaws are out of their correct slots? Are they numbered or is the body of the chuck have a number denoting one of the slots?

Clive

Clive – I did wonder the same thing as that would have been an easy fix. Sadly no luck!

Just a word of caution about using ground silver steel as a standard:

a) It can be lobed, not something I've come across myself but a possibility.

b) It can be bent, something I certainly have come across.

Either of these conditions would lead to a false indication of run-out. The shank of a milling cutter, as mentioned by Bogs, might be a better choice as a standard.

HTH,

Rof

OK – now I’m completely mystified. I’ve tried with the shank of a milling cutter and both a 10 mm and 16 mm shank show 0.25 mm runout. But, a 12 mm shank shows 0.04mm runout. I don’t understand! It was repeatable as I tried all combination many times.

David

.

David,

My guess would be that the scroll of the chuck is running eccentric, and 12mm just happens to be its optimum.

As mentioned earlier, a four jaw is often more useful on the RT, if you can work out how to mount one.

MichaelG.

Hi David,

If your 4-jaw independent chuck is smaller in diameter than the rotary table you could make a backplate that is larger in diameter than the chuck. I have tried to make a sketch:

Thor

> But, a 12 mm shank shows 0.04mm runout. I don’t understand! It was repeatable as I tried all combination many times.

Sounds like time to strip the chuck and clean it up and re-lube.

Neil

The chuck if new is a bad one. With different sizes of stock samples 6,8,10,12,16mm , an error or difference of 0.05mm is inside the normal range for most chucks these days. I have noticed that there are some very cheap chucks coming onto the market that are just not up to snuff at all. Tightening the chuck on the marked or zero key does make a big difference. Sometimes the best key position for the chuck is not the marked one. So try each key position and see if that improves it at all. But I still suspect it is a dud chuck.

Neil

I had a RT where the taper was a pressed in insert and that item was not accurate in itself. Checked with a morse taper test bar and it had a wobble built in due to the taper being off axis. Cured by exchanging the RT for another new one!!

Well, today I took delivery of a new chuck and far better. So I guess you get what you pay for!

.

It's worth remembering that the raw material is [or should be] only a small proportion of the cost [rather like the Paint in the Mona Lisa ?] ... The real money goes into the engineering; manufacture; & quality-control.

MichaelG.

Might be worth regrinding the old one? assuming the error is similar at various diameters?

.

John,

I have quoted David's post, to make it clear ... the error is not similar at various diameters.

I remain convinced that the Scroll on David's chuck was mounted eccentric.

MichaelG.

I see, I stand corrected.

John,

I've just re-read my posting from last evening.

It was rather brusque ... my apologies.

MichaelG.