How to dead center a cylinder on indepedent 4 jaw chucks

[bebob1]

I have an independent 4 jaw chuck and a cylindrical extrusion. How do I dead center the cylinder in the rotational axis using the independent jaw? Its not as easy as self centering jaws. I am indicatoring and then adjusting, but it takes forever and Im only to within reasonable accuracy

[packrat]

1) Get the part as close to true as you can without the indicator.

2) number your jaws 1-2-3-4.

3) indicate opposite jaw positions, 1-3 , 2-4. NOT 1-2 or 2-3, etc.

4) it really does get easier the more times you do it.

[shrtwrec20]

Use a dti and put the needle on the work piece. Turn the chuck by hand and take note of which jaw gives the highest reading. This is the jaw that need tightening. You may need to slacken the opposite jaw depending on how far out it is. Continue this process till the work is within a thou or so. Then you will need to check the run out on the face of the work and adjust as required. Then back to the axial run out until both run outs are clocked in as needed

[neilw20]

Get it approximately right by eye.
Don't take the reading at the high spot.
Take the low reading at a jaw, and loosen it a bit.
Take note of reading. Zero the DTI.
Turn half a turn to high side.
The DTI will move an amount say 10 units.
Tighten 5 units - half of what it moved. No more, no less.
Keep repeating to lowest jaw and loosening and tightening the other side.
After you get good at it 3 - times around and you will spot on every time,
unless the job is uneven or rough.

Tell us when you've mastered this every day task.

[bebob1]

Thanks for the tips guys. Ive managed the best I can to get to 0.001" max deviation on the indicator. Is this good enough? I guess not technically, considering the part I am just doing actually was machined with a lathe to a lower tolerance. What do you normally achieve in error when you try to center a round piece using 4 jaw independent chucks?

It is ridiculously figgity though since if you were say hard tightening the part at the end after adjusting all jaws to a good tolerance, you may end up shifting the part when you do a final hard tighten. Maybe I need to tighten both jaw teeths from both ends simultaneously to prevent this from happening.

[neilw20]

Once you have it true, still with the DTI on it you tighten each one just small bit.
The indicator will move!!
Then the other side.
Just like above now in ever smaller steps.
The DTI needs to be exactly lined up with the jaw so it is not indicating the other jaw at 90 degrees.

As well as all this, often the part changes shape as you squash it, or worn chucks make it a bit more difficult.

Often putting scrap ally under each jaw, so that it it crushes nicely helps.

Be aware that it may end up wobbly on the axis, so moving the DTI to a different place on the job, it will start to indicate crook again!
This is where the bits of ally can be put non central in the jaw to help get the pressure where you want it.

[shrtwrec20]

This is why I tighten from the high spot of you can rather than loosen the low spot first. If you now tighten your highest point without slackening its opposite you should be able to take up that 0.001" no problem and the work will be right also.

[bebob1]

I find sometimes the error is not aligned along the direction connecting two opposing teeths, but rather at an angle.

When that is the case, to adjust it will require loosening of other orthogonal direction jaws. This just throws everything off sometimes. If the error is only in 1 axis, then maybe, but say its on one axis only, and the other normal axis is correct and tightened, then to adjust the axis with error will require loosening the teeth representing the axis normal to this. Its kinda frustrating.

[neilw20]

As I said, don't chase the lowest LOW spot.
It is the low spot in line with the jaw.
Treat the jaws a 90° to each other separately, and find the one that is has the biggest LOW error and do it first.
When almost tight, you can do the HIGH spot and tighten that jaw.
I find that if you set the DTI near the top, and not horizontal, then you can adjust the jaw in the same position with the handle at the top.
You will always get a better feel with both hands. Don't jerk.
Even temporarily extend the handle with a tube for smoother control.
I often stick an indicator bar on the head with magnets to line up with the jaw exactly at the same place each 90°.