# Trouble centering square stock in 4 jaw chuck

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• 09-16-2011, 06:33 PM
SwampDonkey
Trouble centering square stock in 4 jaw chuck
Im new to this and really need to get this done for a project. Any help would be much appreciated.
I just cant seem to center a piece of square stock in my lathe using any of the videos or written instructions ive seen. It just doesn't seem to work when I try it. Is there a decent diagram around that illustrates the steps to this?

• 09-17-2011, 12:24 AM
Delw
heres an easy way to get it close
scribe a line from one corner to the other and another line for one corner to the other.
stick the part in the 4jaw chuck, put a center drill in your tail stock, bring your tail stock close to part, then move jaws till the intersection of the 2 lines meet the tip of the center drill. ie "X" marks the spot ;)

Delw
• 09-17-2011, 04:55 AM
txcncman
How to: Centering square stock in 4 jaw chuck
• 09-17-2011, 08:57 AM
SwampDonkey

Yes, it needs to be fairly precise since I'll be machining a slot through the round section and it needs to be dead center.
And here's what im not understanding: Indicating using the flats and rocking the work to find the lowest reading. It just didnt seem to have any place in the process. I just ended up repeating this procedure for 45 mins. Something is being left out, but I have no idea what to ask in order to uncover the mystery. This is kinda why im looking for instructions with illustrations or a diagram.
Thank you though:)
• 09-17-2011, 10:28 AM
Geof
Truing square stock in a four jaw is difficult because you can't simply use the dial gauge to read directly on the material as it rotates.

Try this approach:

Eyeball it using the circles inscribed on the face of the chuck to get it as true as possible. I hope your chuck does have circles, most do, and all you do is adjust opposite pairs of jaws until they are both the same distance from a circle. Cinch up the jaws fairly tight but not gorilla strength tight.

This will get it within 1/32" or so, maybe better if you have good eyes.

Now mount your dial so it can read on the outer tips of a jaw with the chuck positioned so two jaws are vertical and two horizontal.

Use the cross slide t bring the dial in so that it deflects only about twenty (small) divisions when it contacts a jaw and adjust the bezel to 0. Make a note of the cross slide handwheel reading and back it off to move the dial clear of the jaw.

Rotate the chuck to the opposite jaw, i.e. 1/2 a rotation.

[B]Carefully[/B] bring the cross slide in to the same hand wheel reading as before watching the dial deflection. If it goes past the 0 this jaw is too far out and if it does not reach the 0 this jaw is too far in.

With the dial in place gently adjust this jaw in or out while watching the dial.

You want to move the dial half the distance between where it is and the 0. There is always a bit of spring in the chucks and if you are tightening the chuck in it will move but get tighter. If you are bringing it out the stock will move slightly but as the jaw gets looser it will stop springing and the dial will stop moving.

Either way you will have to move to the opposite jaw and move it also and move back and forth until both jaws show the same dial reading at the same cross slide position.

Repeat this on the other pair of jaws.

Now your stock is centered as good as the accuracy of the length of the jaws. This is probably pretty good and actually you can check how good it is by truing up a piece of round stock first and then checking how true the outer ends of the jaws are. If they do differ simply make a note of the difference and compensate when you are truing the square stock.

Finally adjust your dial against the stock and move the carriage along the bed. This will tell you if the stock is parallel to the centerline. If it needs truing up tap it with a hammer then go back and check the jaws again.
• 09-17-2011, 11:09 AM
SwampDonkey
[quote=Geof;994134]Truing square stock in a four jaw is difficult because you can't simply use the dial gauge to read directly on the material as it rotates.

Try this approach:

Eyeball it using the circles inscribed on the face of the chuck to get it as true as possible. I hope your chuck does have circles, most do, and all you do is adjust opposite pairs of jaws until they are both the same distance from a circle. Cinch up the jaws fairly tight but not gorilla strength tight.

This will get it within 1/32" or so, maybe better if you have good eyes.

Now mount your dial so it can read on the outer tips of a jaw with the chuck positioned so two jaws are vertical and two horizontal.

Use the cross slide t bring the dial in so that it deflects only about twenty (small) divisions when it contacts a jaw and adjust the bezel to 0. Make a note of the cross slide handwheel reading and back it off to move the dial clear of the jaw.

Rotate the chuck to the opposite jaw, i.e. 1/2 a rotation.

[B]Carefully[/B] bring the cross slide in to the same hand wheel reading as before watching the dial deflection. If it goes past the 0 this jaw is too far out and if it does not reach the 0 this jaw is too far in.

With the dial in place gently adjust this jaw in or out while watching the dial.

You want to move the dial half the distance between where it is and the 0. There is always a bit of spring in the chucks and if you are tightening the chuck in it will move but get tighter. If you are bringing it out the stock will move slightly but as the jaw gets looser it will stop springing and the dial will stop moving.

Either way you will have to move to the opposite jaw and move it also and move back and forth until both jaws show the same dial reading at the same cross slide position.

Repeat this on the other pair of jaws.

Now your stock is centered as good as the accuracy of the length of the jaws. This is probably pretty good and actually you can check how good it is by truing up a piece of round stock first and then checking how true the outer ends of the jaws are. If they do differ simply make a note of the difference and compensate when you are truing the square stock.

Finally adjust your dial against the stock and move the carriage along the bed. This will tell you if the stock is parallel to the centerline. If it needs truing up tap it with a hammer then go back and check the jaws again.[/quote]

Thank you Geof, that was very thorough. I feel confident enough to take another whack at it:)
This is my first "real" project and its an insanely tough one. Remember the scene in Terminator 2 where Arnold shucks the skin off his arm revealing the endoskeleton underneath? Im building that, the working arm and every one of its 112 pieces. Seems like a decent project that will expose me to lots of problems to learn solutions to.
How good are these self-centering 4 jaw chucks?
• 09-17-2011, 11:27 AM
Geof
Self centering four jaws are good but I say do not get one. It does take a while to become proficient using an independent four jaw and sometimes you have to align things indirectly like I described. But with an independent jaw chuck you can also set the stock a known distance off center. Or you can grip an uneven piece of stock or a casting and get a particular point running true.

If you are doing production on square stock that has to run true then a self centering chuck is the way to go.
• 09-17-2011, 11:32 AM
txcncman

I don't know how else to help.
• 09-17-2011, 12:09 PM
SwampDonkey
Yeah that video was one of the problems. It starts with him chucking up and "finding the low spot" with his indicator by rotating the piece back and forth. I found that I could rotate the peice MUCH more than he could (or did) and the low spot kept getting lower as it rotated away from the indicator.

I don't know how else to help.[/quote]
• 09-17-2011, 04:52 PM
txcncman
That means you did not have your indicator on the center line of the spindle from what I can determine. The tip of your indicator should have been at the same height as a tool used for cutting would have been. No worries. I am sure you will figure it out soon enough.
• 09-18-2011, 09:31 PM
handlewanker
Hi swampey, my 2 cents worth....I just place a flat piece of 1/2" thick X 3" wide hot rolled steel bar, that smooth shiny stuff, across the ways on the lathe bed and use a dial indicator in a scribing block to indicate ON TOP OF THE SQUARE STUFF....makes it easier to move the indicator off the job when rotating the chuck.

If'n you use the indicator in the scribing block (never a magnetic base) you'll be able to slide it back and forth and have more control over the adjusting screw to get the indicator to zero on one side before turning the chuck 180 deg to check the other side.

You'll have to rock the chuck slightly to get the indicator to read level across the square, but you only want to get zero zero on two opposite sides.

Don't grip the bar too tightly when adjusting the jaws or you'll have a job to move it the last few thou.

If'n you haven't got circles on your 4 jaw chuck, start by just measuring the back end of the jaw to the outside diam of the chuck body....gets you to within .010" of centre for initial rough set-up.
Ian.
• 09-18-2011, 09:51 PM
eaglezsoar
See this PDF file:
[url]http://www.littlemachineshop.com/Reference/Centering4-JawChuck.pdf[/url]
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