View Full Version : Centering Odd Size Parts

05-13-2007, 11:03 PM
I had to chuck a piece of metal 1" by 2" by 3" to do some turning on it. The 1" by 2" sides were held by my 4 jaw chuck. Squaring them up is not as easy with round stock. I quickly determined that the dial indicator would not work on the edges.

My solutuion was to mark the center line of each side that would, have a jaw on it and then center the line with the center of the jaw. This worked to get the part fairly centered. I then marked the center of the 1" by 2" end by constructing diagonals and then using the dead center in the tail stock, to further adjusted the part until it was centered. I spent quite a bit of time and I never felt that it was really perfectly centered. After I started turning the piece, it was several thousands out.

Is there a better way to center odd size items in a four jaw chuck? I only have a four jaw chuck and don't have any of the fancy tooling such as collets that would hold the items. Appreciate any help. Thanks.


05-13-2007, 11:16 PM
If you have a milling machine you could drill a reference hole in the part with a center drill, use the tailstock center to hold the part in place while positioning it in the chuck, then indicate the hole for final location.

05-13-2007, 11:24 PM
You did just fine up to that point. That's the way to get centered within about that tolerance. If you do need the stock centered closer you can use an indicator, but just a little differently than you're used to for round stock.

What I do at that stage is get the indicator held on the toolpost (or similar), then work two jaws at a time. Bring the indicator up to a flat, rock the chuck slightly to find the minimum reading. Zero the indicator and also zero the crosslide. Move the indicator off the work (usually move the saddle down the ways). Rotate 180 degrees, bring the indicator back and check this opposite side. If you back off the crosslide, then move in until the indicator is again zero, you can read the double error off the crosslide dial. Assume you're past zero by .016 on the dial, meaning that this side of the stock is short by .008". Back off the dial, move it forward to read .008, then back off the front jaw and crank in the rear jaw until the indicator again reads zero.

Recheck, then do the other two jaws. Since you know which direction to move, and exactly how much to zero the indicator, you can get centered quite fast.

05-13-2007, 11:34 PM
Hi Bigboy1,

I use a dial indicator and check each side and when they match I feel that its as good as I can get it. So far this has worked for me.

I check sides 1-3, then get 2-4. This may not be the exact center, but I find it very close. I hold the dial stem back when I turn the part and them ease it back and see if it checks the same from side to side.

05-13-2007, 11:57 PM
Scroll down the page here: http://homepage3.nifty.com/homeshop-tools/boring_head/boring_head-e.htm and see what he did. I just did that very thing today for a base I'm making for a ball cutter.

05-14-2007, 03:27 AM
All you need is a piece of wire with a point and a surface to indicate off. I use a brass set-screw electrical connector.
Chuck the wire in the tailstock jacobs and rest the point in a centre pop-mark. Indicate off the block/connector/wotever. Easy.

Before the subject arises, the point on the wire does not have to be central. The wire can be bent like a corkscrew, as long as the point is centred in the punch mark. You are clocking offset here (and hopefully, eliminating it)

05-14-2007, 07:45 AM
Nice trick SS. I'll remember that one.

05-14-2007, 11:09 AM
Thanks Evan, praise indeed :)
Now, if I could just work to your standards.....
dunno how you do it man, but keep up the good work.

Frank Ford
05-14-2007, 11:28 AM
I made myself a long flexible center for just that purpose. It was an easy project, and it comes in handy from time to time:

Wiggly Center (http://www.frets.com/FRETSPages/Machining/Tooling/WigglyCenter.jpg/wigglycenter.html#Hardening)


Frank Ford
FRETS.COM (http://www.frets.com)
Gryphon Stringed Instruments (http://www.gryphonstrings.com)
My Home Shop Pages (http://www.frets.com/FRETSPages/Machining/index.html)

05-14-2007, 12:31 PM
Thanks for the ideas on how to center. Now I just have more things to make so that I can make more things!!!!


05-14-2007, 01:26 PM
That's the beauty of this one Bill, it takes all of 60 seconds to make :D
If you've got a jacobs in the tailstock, you're off and racing.

05-14-2007, 03:57 PM
I'm sure if I put a little thought into it I can make it much more complicated (and shiny). :D

John Stevenson
05-14-2007, 04:19 PM
Provided that the centre needs to be central to the work one way that works with no gadgets, tools or expensive measuring equipment is the MKI eyeball.

This way works with square, round or rectangular work and after a few goes you will get to a thou.

Remember when using a 4 jaw chuck it's best to realise that it doesn't have 4 independent jaws but two sets of two opposite jaws, in other words it's two 2 jaw chucks with opposed jaws.


So first off centralise the work in the 4 jaw using the rings on the face of the chuck and a fixed point on the jaws or the edge of the work. Work two jaws to push / pull the work into place using the rings as beat you can by eye. Then do the next pair, with a bit of experience that should get you to within 10 thou or so.

Now to get knob on. Again using just two jaws set one face of the work vertical so one jaw is pointing towards you and carefully touch the lathe tool on the work, note the reading or zero the dial if you machine allows that, wind off, rotate the work 1/2 a revolution so that you are now working on the opposite jaw, again carefully wind in and note the reading.
If the first reading was zero and this one is plus 10 then you need to go back to the first jaw and tighten it a bit so it reads plus 5 and re-zero. When you go back to the opposite jaw it should then read zero as well.

Once you have two jaws set then treat the other two the same.

It sounds a long winded method but believe me it takes far more to write this up than to do :D
Once you get used to this method and you realise it works on most regular shapes it's quick.
The purists can resort to a dial gauge if needed for the last lap.

Next post deals with irregular centres in either regular or irregular work.


John Stevenson
05-14-2007, 04:26 PM
So now you want to pick up a centre popped mark that could be anywhere on a block of work ?

To start off slack the jaws off to give clearance to the sides, offer the work up to the chuck and locate with the centre in the tailstock whilst nipping the jaws up.
This should get you close.

Now if you have previously made one of these:-


All you have to do is offer the point up, watch the dial and work TWO OPPOSED jaws just as in the post above.

If the picture isn't clear enough here's a sketch of the workings.
Nothing is critical as regards sizes, material etc. Also not relevant to a given machine and can be swapped between machines regardless.



05-14-2007, 10:49 PM
John, why do you hold the dial indicator at an angle? I would have thought it would be difficult to get a reading, or at least consistent readings, that way?

John Stevenson
05-15-2007, 12:50 AM
As most people are standing over their lathes it's easier to read than set vertical but it doesn't matter.
Even though the reading won't be accurate to the dial with being on an angle all you are looking for is a consistent reading so it could be calibrated in anything.

Unlike a micrometer which is a measuring instrument a dial gauge is only a comparator and needs to work from a reference.

What does matter is you get the tip of the DTI as close to the end as possible. That was the MKII in the pic, the MKI below didn't work as well as it wasn't close enough.



05-15-2007, 02:04 AM
If you want it very accurate you need to use setting buttons and slip gauges, set up on surface plate then clock it in in lathe.

05-15-2007, 08:43 AM

Spin Doctor
05-19-2007, 10:52 AM
another way to do it. If (amazing how that word always pops up) you have an edge finder with the pointed end put that in a drill chuck in the tailstock. Eyeball the center point close and then engage the point into the spot. Indicate the .500" diameter of the centering end. It is the most useful thing I have ever you could use that end of the edge finder for anyway. In the mill I always prefer the single end models. It sucks when the other end gets hung up in the collet.