View Full Version : centering a square in a 4 jaw chuck

01-09-2007, 10:08 AM
totaly a newbee question how do you center a square, rectangle piece of steel in a 4 jaw chuck in the lathe? I know a round piece you use a dial indicator but how about square.


Al Messer
01-09-2007, 10:37 AM
Same way: DTI clamped in the toolpost with the tip of the indicator on the edge of one face of the workpiece with it roughly vertical to the bed, adjust the workpiece until you get the same reading of the DTI when you touch each face in turn. I do it by roughly setting the piece by going by the circular rings on the face of the chuck, it will get you in the ball park.

01-09-2007, 10:37 AM
First you have to decide where the center is, or where the hole is going to be. Then a narrow pointed center can be used in the tail stock to allow jaw adjusting/fiddling to move the designated center to the desired locus as defined by the center. Machining manuals such as Moltrecht have illustrations and discussions of methods to approach this. Mounting indicators on tool posts to center up off center holes to be bored to size in castings is another approach. Using scribes and right angles on face plate work is another way, depends a great deal on what you are mounting. A true square would be straight forward, though tedious, to indicate off the sides, lots of cranking involved.

01-09-2007, 10:38 AM
The way I go about it for a piece of rough material is as follows:

1. scribe two lines corner to corner on the front face
2. center punch the intersecting point
3. mount the part by eye in the chuck
4. put a pointer in the tailstock chuck (or use the dead center)
5. bring the pointer or center up to the face (nearly touching)
6. adjust the jaws to suit.

If you need more accuracy than this then a wiggler or a dial indicator is required. The center point also has to be measured more carefully by using a scriber on a surface plate (flip until centered) and then using a center punch.


John R
01-09-2007, 10:40 AM
You can center mark the end and use a wiggler and if the block is truly square or rectangular you can use your dial indicator on the edges, looking for the highest readings . Be careful to protect the plunger on the indicator. If the difference between the long and short sides of a rectangle exceed the range of the dial indicator then two set ups can be used. When the center looks good then run the indicator down the side using the carriage to determine if the stock is cocked. Correct with a few hammer taps then check center again. A few times back and forth will do the job.
John R

01-09-2007, 11:29 AM
This subject came up not too long back. One good suggestion was to use a piece of shim or flat spring stock clamped so it was at 90* to the square stock and rode over the corners as you rotate the chuck. DTI on the shim stock and it's almost as easy as centreing round.

01-09-2007, 11:35 AM
This has been discussed before, so a search of the archives ought to come up with additional hints.

A couple of other ideas:

Check to be sure what you have is truly a square and not slightly a rectangle, otherwise you may spend an inordinate amount of time chasing 5 thousandths of an inch is not mis-centering at all.

Somebody else suggested once that it helps to think of a 4-jaw chuck as two two-jaw chucks at 90 degrees to each other. Get one opposing pair of jaws correct, then do the other pair.

01-09-2007, 11:42 AM
SGW, that's the beauty of the 'spring and guage' method. It doesn't matter if the stock is rectangular, as long as you get the same reading on opposing jaws.

01-09-2007, 11:45 AM
...but you don't want to expect the same reading from adjacent jaws....

01-09-2007, 11:50 AM
soitanly not! But as long as spring and guage have enough travel, you can centre stock that is well off square.

John Stevenson
01-09-2007, 12:06 PM
Dial indicators are OK on round work but a pain on square or rectangular. OK you can back off or hold the plunger back at the expense of possibly disturbing the setting.
I never bother using an indictor unless I need less than a thou accuracy which isn't often.

You can get carried away with all this tenths mumbo jumbo. Real world a thou's plenty.

Take a piece of square as an example.
Clamp it in the jaws and use the concentric rings and the mark one eyeball to get as accurate as possible.
Then treating the chuck as two 2J chucks bring one jaw facing you, touch the tool onto the work and either zero the dial or note the reading, back off and turn 1/2 a turn to bring the opposite jaw to face you.
Touch the tool on the work and note the reading. True centre is the difference between the two.
Slack one jaw off and move the other.
Only wok these two opposite jaws until the reading is the same
Moving to the second set of jaws you repeat the procedure but this time you have a reading to aim for.

Rectangular is the same you just work on two opposite sides at a time.

Once you have done this a couple of times it's an easy method to master and you will probably not use an indicator again it's that quick.

Remember you don't have a 4 jaw, you have two 2 jaw chucks and this will get you to a thou on any shaped material.

John S.

01-09-2007, 01:50 PM
A picture is worth a lot of typing - here is how our friend Frank Ford does it:


And Ishimura in Japan does it thus:


See half way down the page.

I built a co-axial centering tool - a spring rod inside a cylinder and which mounts to the chuck on my tailstock. The dial indicator clamps to the cylinder and the probe rides on the rod. I'll have to take a picture of that one some day.

John Stevenson
01-09-2007, 02:15 PM
If you are working on a centre punch mark this is a self contained setup of the previous idea's that can be moved from machine to machine easily.


Build details:-



01-09-2007, 03:03 PM
Saves me taking a picture, John - that is exactly what I have here, too. It works rather well.

01-09-2007, 03:44 PM
Jason, take a look at this thread.