Centering square bar in 4-jaw chuck

Plenty of info on YouTube and this one:- **LINK**

is worth a look.

Nick.

Maybe I should have said parallel to the lathe axis for that is what I meant :D

So all previous discussion is moot.

The question is how to hold square bar so that, once centred and running true at the chuck, it sits parallel to the lathe axis. IE, how to get the far end running true also.

Good question. A lot depends on the quality, accuracy and condition of your four jaw chuck and the jaws therein. And of the surface finish on the square bar, and the straightness of the bar. Are we talking about bright bar, black bar, ground bar?

You might be able to persuade the far end of the bar to sit more centred by carefully drawing two diagonal lines across one end and then carefully drilling a centre hole at the intersection, in the drill press. (Or in the lathe without marking the lines if your spindle hole is big enough to slide the square bar right in.) Then by using a tailstock centre to hold the bar in position while tightening up the chuck jaws and setting the bar at the chuck end to run true radially in the methods discussed at length in previous posts, it might come out more on centre at the far end. Depends on the condition of the chuck jaws and the piece of bar.

An alternative might be to use a female centre, as long as the end of the square bar has been faced off square

Another way to do it, possibly with more accuracy, might be to make a "cat's cradle" to allow the far end to be held in a fixed steady. The cat's cradle is a piece of pipe, machined nice and round on the OD with four holes drilled and tapped around the OD at each end. Four bolts are put in through these holes and tightened up to grip on to the square bar. Then by holding the cat's cradle OD in the fixed steady and manipulating the bolts you can adjust the square bar to run on centre at the far end.

BUT... if you have the square bar gripped in the four jaw running true at that end, then use the force of the cat's cradle bolts to make it run true on the other, you may well be bending the piece of bar, or springing it at least. So after it has been machined, when you take it out of the lathe it will spring back to its original banana shape. You can avoid this by turning the square bar between centres after drilling a centre hole in both ends by methods outlined above.

Edited By Hopper on 06/01/2017 02:52:44

Edited By Hopper on 06/01/2017 02:53:33

My first thought was exactly the same !

There are that many videos on YouTube about this it isn't funny , try some of these channels as watching it being done is easier than trying to explain in text.

Abom79

Keith fenner

Mrpete222

Joe pieczynski

Interesting stuff this. I'm just thinking about a few possibilities here. One is, if the application requires the level of precision we are talking about here then why would you choose to use black bar or similar, which is bound to be far from perfect? I assume we are talking about long lengths here. Second, a very short piece of square bar is highly unlikely to be bent by any significant amount.

Thirdly, there is no reason to suppose that a piece of round bar of similar length and quality to the square stuff we are talking about wouldn't be bent either.

So a bit more detail from the OP on what is trying to be achieved would be helpful.

If it is something special and you want square bar that is really square and to dimension and straight, you could try using a length of key steel.

Get a copy of a book called "Simple workshop devices"by Tubal Cain.

There is a picture on the cover that is so simple that it will make you cry ! it struck him dumb !!

The idea was by Mike Chrisp.