Newbie- Centering flat stock in 4 jaw chuck

[daanmuller]

Hi,

what would be the way to precisely center a piece of flat stock in an indepent 4 jaw chuck?

I want to do 4 sides indexed machining and for that my stock has to be precisely centered in both y-axis and z-axis for the faces to match up I think?

[John Welden]

If say your stock is .5 thick, you could first clamp a .5" diameter into the chuck and dial it in with an indicator. That will get you close in one direction. Take the bar out, swap in your flat stock and you'll be close in one direction.

They key I find is to get close in the first place. It can be difficult to get things centered when you start a mile off. Even if you only spend 30 seconds eye balling it, you're better off than if you start in outer space.

Lets say you've got the bar centered in the y axis. Rotate the chuck 90 degrees, come down in Z and touch off on one side. Zero your indicator and zero out your z axis. Back the indicator off, rotate the bar 180 degrees and then come down and touch off the other side. Come down until you indicator reads zero. If the bar is centered, your Z axis should also read zero. If it reads say, .125", you know you've got to move the bar .0625 to get it centered.

You have to monkey around with 4 jaws to get things perfect. Get close in one direction, get close in the other and then go back and dial in the last little bit. Be patient and take your time.

Hard to explain in text, easy to show in the shop.

[Geof]

Are you centering this in a four jaw mounted on a 4th axis? It sounds like it.

Center the flat stock as good as possible by eye in the four jaw.

Put a dial gauge in the spindle, rotate the 4th axis until one face of the stock seems very level. Now sweep across this with the dial and rotate the 4th axis until the stock is level within the limits of your dial gauge.

Make a note of the machine coordinate reading for the Z axis and just raise the dial gauge straight up but do not touch it.

Rotate the 4th axis 180 degrees and come down until the dial shows the same reading it had on the other side of the stock.

Make a note of this machine coordinate for the Z axis; you need to adjust your stock by half the difference between the two reading which you may be able to do carefully with the dial in contact with the part.

Rotate the 4th axis 90 degrees and repeat everything.

Keep on repeating this procedure until you get the same Z coordinate reading when the dial gauge shows the same reading on all four sides of the stock.

[John Welden]

Originally Posted by Geof Are you centering this in a four jaw mounted on a 4th axis? It sounds like it. Center the flat stock as good as possible by eye in the four jaw. Put a dial gauge in the spindle, rotate the 4th axis until one face of the stock seems very level. Now sweep across this with the dial and rotate the 4th axis until the stock is level within the limits of your dial gauge. Make a note of the machine coordinate reading for the Z axis and just raise the dial gauge straight up but do not touch it. Rotate the 4th axis 180 degrees and come down until the dial shows the same reading it had on the other side of the stock. Make a note of this machine coordinate for the Z axis; you need to adjust your stock by half the difference between the two reading which you may be able to do carefully with the dial in contact with the part. Rotate the 4th axis 90 degrees and repeat everything. Keep on repeating this procedure until you get the same Z coordinate reading when the dial gauge shows the same reading on all four sides of the stock.

Thats what I was trying to say. Wish I could just show the guy in the shop.

[venomgrrrl]

if your on a lathe?? you could also lay out your center and use your tail stock to "get it close...i could be way of base if your in a mill..

[daanmuller]

Thanks! That makes sense.

So the dial mounted in the spindle first is used to level the stock by sweeping it and adjusting with the rotation of a-axis.

Then I touch of the top side with the dial to say 0.5 mm
zero z on dro
lift spindle
rotate stock 180 degrees
lower spindle till I have the 0.5 mm reading on the dial again
The amount on the dro greater or larger then zero is half what I need to adjust the stock.

Right?

[venomgrrrl]

exactly..the hardest parts getting it close, concentrate on two opposite sides at a time.

[Geof]

Simple but tedious if you want to get within a thou or so.

[John Welden]

Originally Posted by daanmuller Thanks! That makes sense. So the dial mounted in the spindle first is used to level the stock by sweeping it and adjusting with the rotation of a-axis. Then I touch of the top side with the dial to say 0.5 mm zero z on dro lift spindle rotate stock 180 degrees lower spindle till I have the 0.5 mm reading on the dial again The amount on the dro greater or larger then zero is half what I need to adjust the stock. Right?

Yes exactly. You explained it better than I did.

[Rich Kay]

My method might be crude, but here I go again with my ".19 dia. X 3" long ground to a point held in a Jacobs chuck" setup cheat. If you have a piece of flat plate, blue the center, scribe criss crossing lines from corner to corner, If you can, prick punch a mark at scribe intersection, and gently jam the point into the prick mark, and draw in your chuck jaws? This will work verticle, but not sure on a lathe chuck... also I am not sure exactly "how" close you need to be initially (I am much more versed verticle machining, than lathe) Good luck !

Rich