Centering square bar in 4-jaw chuck

[PaulRParticipant @paulr]

[PaulRParticipant @paulr]

I'm trying to figure out the 'correct' way to do this using a plunger type DTI. I can get the bar running true with no problem but of course that doesn't mean it's centred.

Previously I've used a flat piece of bar mounted in the tool-post and run this alongside opposite sides of the workpiece, adjusting the jaws an needed so it will just slide past each side. Seems to work pretty well but feels a bit Heath-Robinson.

As I'm using a Myford lathe the chuck begins to unscrew as I rock it back and forth to find the low point on the bar when trying to use the DTI (i.e. when the plunger is perpendicular to it) which ruins the accuracy.

Can someone tell me or point me to the proper way to do this please?

[JasonBModerator @jasonb]

Put your chuck on a bit tighter and take the lathe out of gear then it should not come loose with the small load from rocking the chuck.

Or get a far eastern lathe with bolt on chuck even then best to put the switch into neutral so you don't get the braking effect from the motor which makes it easier to turn the chuck by hand

Edited By JasonB on 18/12/2016 16:09:03

[roy entwistleParticipant @royentwistle24699]

If you can, use two chuck keys, one each side

[Jim CParticipant @jimc]

On a Myford it is best to loosen the belt tensioning lever to allow easy rotation of the chuck and as Jason says, make sure the chuck is tight. Regarding setting the square bar true, I don't think there is a correct way. It depends on the accuracy you require. I usually use the edge of a hss lathe tool on the side of the square and then it's trial and error I'm afraid. If you've got lots of square work to do then you could invest in a self centering 4 jaw.

[AjohnwParticipant @ajohnw51620]

Just explaining Jason's method a bit more. The DTI will show min deflection when a face of the square is perpendicular to the end of the dti. It's a right pain to do and not that accurate in some respects as it may not be easy to spot that the face is at a very slight angle. The other problem is that when things need to be centred very precisely the last part is usually done by tightening the chuck up very tightly – often too tightly really to save slackening the opposite jaw and starting again. That can be more of a problem with square work. It can be done though.

Fortunately exact centring on square bar in a lathe doesn't usually need to be extremely precise so people buy a self centring 4 jaw. Then it's just a case of making sure that all jaws are closed nicely on each face of the work. Unless there is a specific need there isn't any need to buy a large one. A 100mm chuck is likely to be ok but do check it's bore to see if it's suitable for the sort of thing you may want to do with it.

John

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[mechman48Participant @mechman48]

Hi Paul, Not intending 'Granny & eggs' scenario here… depends what you mean by 'Centered'; if you have the square bar set so that the cutting tool tip is touching ea. corner then you will be reasonably centered, & what ever you drill will be true centered to the bar. To dial in your bar I would suggest that you draw a line from ea. corner of the bar using a centre finder from a combination set or other type & where these lines intersect will be the centre of the bar, be it oblong or square. Carefully centre punch this intersection, then use a centering bar, ( wiggler ) probably have to make one up i.e. longish piece of silver steel, 150mm should be long enough, machined to a 60* point one end, centre drilled the other, place a live / dead centre in you tailstock & fit the centering bar onto the tail stock centre with the point in the centre punched end of the bar, set your DTI up to read off the OD ( wiggler ) & adjusting your 4 jaws true up the centering bar to read '0' then you'll have the bar centered to your headstock, chuck,& tailstock.

George.

Edited By mechman48 on 18/12/2016 17:44:47

[PaulRParticipant @paulr]

Thanks all for the replies, much useful info there and I'm sure that's enough to get me going. Cheers

[JasonBModerator @jasonb]

I'm not keen on using the corners of off the shelf bar for any marking out, even bright bar can have corners that are quite rounded. So I use the DTI on the face and do opposite sides as it is also not always square but can be a bit rectangular. also trying to draw corner to corner on a bit of 6mm sq bar is not easy!

You should be able to get quite close initially by using the rings cut into the chuck face and getting the chuck jaws equal to these then using a bar in the toolpost will often do, and finally the dti if it wants to be accurate

Edited By JasonB on 18/12/2016 18:21:44

[alan-lloydParticipant @alan-lloyd]

If you're square bar is 1" put a round 1" bar in the chuck and set it to run true with your dti, then undo two of the jaws at 90 degrees to one another, place your square bar back in the chuck and do up the jaws you previously loosened,

[VicParticipant @vic]

As said the rings on the chuck can get you quite close.

Edited By Vic on 18/12/2016 19:03:32

[HopperParticipant @hopper]

Posted by alan lloyd 3 on 18/12/2016 18:50:05:

If you're square bar is 1" put a round 1" bar in the chuck and set it to run true with your dti, then undo two of the jaws at 90 degrees to one another, place your square bar back in the chuck and do up the jaws you previously loosened,

+1 on this method. Quick and dirty wins every time in my shop.

[Russell FurzerParticipant @russellfurzer50760]

I have tried hopper's method in the past with satisfactory results.

An alternative is to find a bit of tube that is close in ID to the diagonal size of the bar (i.e. root 2 x dimension), make one longitudinal split, put into the 3 jaw, bore to the diagonal size, pop in the square, gentle tighten and Bobs your mamas brother.

[AjohnwParticipant @ajohnw51620]

Posted by alan lloyd 3 on 18/12/2016 18:50:05:

If you're square bar is 1" put a round 1" bar in the chuck and set it to run true with your dti, then undo two of the jaws at 90 degrees to one another, place your square bar back in the chuck and do up the jaws you previously loosened,

Neat. Any error should be down to difference in sizes and how well it was set.

Centre is via the corners – I don't think so. For the corner reading to be true it needs to be on centre. Bit of a problem if it isn't like which jaw to adjust and holding it via the corners is something I wouldn't do.

I didn't give it much thought and bought a self centring 4 jaw, However the lathe I bought shortly after came with one.

John

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[old AlParticipant @oldal]

on a bit of 1" square it should only take a minute to centre the bar using the tool and edge method already described.

A tip. if you put a scrap piece of stuff, brass, mild steel, etc between jaw and work, you wont damage the edges of the work.They do fall out at the most inconvienient times when you are loosening and doing up the jaws.

I also use the jaws adjacent to the makers label as the jaws to take the work out. When i put the bar back in for a second item, they are nearly set for the next one. Never been rich enough for a self centering 4 jaw and practice make you quicker

[roy entwistleParticipant @royentwistle24699]

I personally wouldn't entertain a four jaw self centering chuck. Unless your bar stock is perfectly square and the chuck is in excellent condition you will find it only grips two faces. I understand that they were originally made for woodwork

[Martin ConnellyParticipant @martinconnelly55370]

The method outlined by George above is the one I would use. The only other thing I would add is that if your part has a poor outline for marking and centre punching you can use a suitable flat surface and a scriber set to about the mid point to mark where to punch. For example if you have a 12mm square bar set a scriber to about 6mm. Place the work piece on each side on the flat surface and mark the end with the scriber. When you have done all four sides this way there should be a very small square marked on the end of the workpiece that shows where to centre punch it.

Martin

[Allan BParticipant @allanb]

Just a way that we use with students, not the most accurate if you have any sort of runout in the chuck, but start by using the rings on the chuck to roughly set it to centre, then using digital vernia working on opposite sides, set each pair of jaws to the same measurements from outside of the chuck, using this method even a 16 year old can set a piece of rectangular steel up central to within 0.05mm in about 25 min, this is on the understanding that the school they went to actually taught mm which for some reason most don’t

Edited By Allan B on 05/01/2017 09:11:18

[colin hawesParticipant @colinhawes85982]

I use the following accurate way:-

(1) Ensure you can't accidentally start the lathe! Set the tool at centre height. Wind the tool clear at each stage.

(2) Nip the square bar somewhere near right by eye in the 4 jaw chuck.

(3) Rotate the chuck so one side of your square bar looks vertical.

(4) Wind in the cutting tool to just touch the bar and set cross slide dial to zero.

(5) Rotate the chuck 180 degrees and touch the bar again.

(6) Adjust jaws to half the difference so that the dial reading is the same on opposing flats. The bar is now central in one axis.

(7) Set the dial to zero again and rotate the chuck 90 degrees by eye Small error here doesn't matter.

(8) Adjust the chuck jaws so that the bar touches the tool.

Any final small error can be seen by touching the tool against each side of the bar in turn and any necessary adjustments made until the bar shows zero on all faces.

The above steps can be performed quite quickly. Colin

[MWParticipant @mw27036]

I've used mechman's method of getting started each corner near the tool on square stock, and i'm usually within .2mm each time.

Another good method to get near to size on odd shaped work is to center punch where you want the hole, and then use a scriber tip in the tail stock drill chuck as a guide.

Michael W

Edited By Michael-w on 05/01/2017 10:57:56

[Carl Wilson 4Participant @carlwilson4]

OK. I’m maybe being a bit thick here. It’s been a long and gruelling week so far.

If the square bar is running true, how can it then not be centred?

[Nick HughesParticipant @nickhughes97026]

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

is worth a look.

Nick.

[PaulRParticipant @paulr]

Posted by Carl Wilson 4 on 05/01/2017 11:55:31:
OK. I'm maybe being a bit thick here. It's been a long and gruelling week so far.

If the square bar is running true, how can it then not be centred?

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

[Carl Wilson 4Participant @carlwilson4]

Ah ok got you now!

[HopperParticipant @hopper]

Posted by PaulR on 05/01/2017 12:04:02:

Posted by Carl Wilson 4 on 05/01/2017 11:55:31:
OK. I'm maybe being a bit thick here. It's been a long and gruelling week so far.

If the square bar is running true, how can it then not be centred?

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

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

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