Thread: 4 jaw chucks?

At the end of the day I wouldnt be to worried about what your indicators are showing.
After all it is a 4 jaw independent chuck.
If you are concerned about it not running true ( the back plate and chuck ) skim the back plate and recut the register on the back plate for the chuck.

I measured the inside surface of the spindle bore as Micheal said, this is the results.

Next I put a centre in the spindle and these are the results.

Ideally the needle should not have moved - but 5/10ths is not terrible. The indicator on the side of the chuck shows lots more variation so it is likely to be a dodgy backing plate (mounting thread not square to the plate surface.
The cheapest solution may be as Peter (pipeclay) suggests and take a skim off the backing plate & register to reset the chuck.

Michael

The backing plate.

The backing plate must be flat and perpendicular to the axis of the lathe. First job will be to take a skim of it (looks to be 8 thou run out, so minimum 8 thou skim). There should also be a step near the edge for the chuck to locate on which will also have to be done.

I've marked one face to be checked for run out. Normally this is a neat fit for the register on the back of the chuck. If it has run out, the chuck will be eccentric to the lathe axis, so you will have vibration creeping in. You may need to take a little off this surface too but then might have to introduce some shim to keep things concentric. See how much it is first - it may be a loose fit anyway given the standard of work on the rest of the backing plate (backing plates should be machined on the machine they are destined for for the best fit/ concentricity).

Michael

it nice to have everything parallel, but at the end of the day it is a four jaw chuck...

I bought a brand new Bison 200mm four jaw chuck expecting it to be all good and wonderful..

it will not hold any thing really really parallel with the spindle..

So I ask the question and the old wizened people told me, four jaws never do... Even the uber expensive ones.. That is why such things as steadies were invented..

Originally Posted by .RC.

it nice to have everything parallel, but at the end of the day it is a four jaw chuck...

I bought a brand new Bison 200mm four jaw chuck expecting it to be all good and wonderful..

it will not hold any thing really really parallel with the spindle..

So I ask the question and the old wizened people told me, four jaws never do... Even the uber expensive ones.. That is why such things as steadies were invented..

This ins not true. I have an Autrian Emco direct mount independant 4-jaw chuck, and it does hold workpieces parallel to to the spindle to a very high degree. But this was an expensive chuck, $450 30 years ago. In todays money, that would be one weeks wages.

That said, you need to understand why it once did make more sense to own a very accurate 4-jaw chuck. More sense than it maybe makes today. It is because 30 years ago not everyone could afford a milling machine. The lathe was used for milling. But milling in the lathe is not a very rigid affair, no matter if you use a vertical slide or a rear mounted milling column or a powered tool. On a 7 to 10" swing lathe, the workpiece enveloppe for a vertical slide is limited to the size of a matchbox or maybe a cigarette box. Only very small depth of cut are possible without undue chatter. And because the cross slide ways are ALWAYS at a little more than 90 degrees to the bedways (to gusrantee concave facing of the lathe) it is not possible to mill a cube with perfectly right angles. Not so in a large independant 4-jaw chuck, it is possible to machine cubes with each face at 90 degrees to each other and each surface very slightly concave (concavity being very desirable over the even slightst convexity). To a very high degree of accuracy, say a workpiece of 150x150 x100mm can be made accurate to better than 0.05mm each side just by clamping in the 4-jaw juck and facing. And that much faster than by milling in the lathe because it takes much less time to setup/clamp the workpiece, and a deeper cuts are possible. And with an impeccable surface finish superior to what is attainable by milling in a lathe. Even today, I often prefer to machine a rectangular workpiece this way, rather than doing it on the milling machine, for said reasons (I now have a standalonbe Sieg X3 mill).

Originally Posted by .RC.

it nice to have everything parallel, but at the end of the day it is a four jaw chuck...

I bought a brand new Bison 200mm four jaw chuck expecting it to be all good and wonderful..

it will not hold any thing really really parallel with the spindle..

So I ask the question and the old wizened people told me, four jaws never do... Even the uber expensive ones.. That is why such things as steadies were invented..

Are you talking about holding long lengths of stock?