Square block material hows that aligned on a lathe?

[John S]

It is possible to set a bar up in a 4 jaw chuck to 2 thou easily or 1 thou with a bit of care using no specialised equipment at all.

Works on square, round or rectangular all the same method.

First thing to remember when using a 4 jaw independent chuck is to forget it has 4 jaws and tell yourself it has two sets of 2 jaws even to the point of marking opposite jaws with a different coloured marker pen.

On the front of a 4 jaw chuck is a series of concentric rings these are guide rings so you can line opposing jaws up using the rings for a guide, some of the cheaper, smaller chucks may only have 3 or 4 rings on, widely spaced.
There is no reason why you can't remove the jaws and turn some intermediate rings on with a Vee shaped tool.

Now put your piece of material in the chuck, we will assume it's square although the operation is the same for square and round and only slightly different for rectangular.
Using opposite jaws just nip the work up trying to get the ends of the jaws in the same position relative to the rings, this should get you within a post code of centre.

Now take any lathe tool and turn it round in the holder, blunt end out. This is your secret weapon, everyone has one of these but mat not have a dial gauge or other expensive toys.

Now with one face of your square bar vertical wind the blunt end in until it touches the bar and note the reading, if you can, zero the dial, now retract, turn half a turn to the opposite jaw and wind in to get a reading.

If you are dead lucky both readings will be the same, chances are they will differ by 10 thou or so. In which case work out the low side, that's the side you have had to wind in more and you will need to slack this jaw off a tad and tighten the opposite one up.

Repeat taking the readings on these two jaws, forget the other two, until the reading is the same on both sides.

Now deal with the other two jaws, this is easier as you know what the reading should be from the first two unless it's rectangular and you will have two different readings.

Sounds a bit long winded but believe me it takes far longer to type this than do it, after a few goes it's easy to get to a thou by this method, if you want better then break out the DTI at this point.

The main thing is to remember to just work on a pair of opposite jaws, beginners make the mistake of trying to work all 4 jaws at the same time which together with inexperience causes untold problems.