Precisely Holding Long Square Bar in Lathe for Turning

Hi, i am after some pointers on how i can precisely mount a large and long square bar in a lathe for turning?

i have a 300mm length of 70mm square bar that needs a boss turning on one end, the boss needs to be very accurately centred and running parallel with all the sides.

The only method i can think of (with the kit i have) is to use 2 independent 4 jaw chucks, one in the head stock and the other freely rotating in a very accurately aligned tail stock which would allow me to adjust both ends precisely

is there another or better way of doing this? i dont have a tailstock chuck so will need to make that however i am sure there are other probably better ways of solving this problem

thanks in advance ) Tim

Careful marking out or a DRO in the mill will mark the centre of one end to within 0.001", center drill that punch mark and support with a live ctr in the tailstock, drive the other end with the 4-JAw

Can't see how you would turn a boss on the end if you are holding it with a rotating tailstock chuck

J

Edited By JasonB on 05/03/2014 15:23:31

The way I'd do it would be to make a flanged collar that is bored just big enough to take the square and then mount four screws to bear on each face of the square. The collar would then be mounted at one end and supported by a fixed steady. The other end would be set up true in the 4 jaw independent chuck. Using a DTI mounted on the saddle you should be able to set everything up using the screws and/or the steady supports to get it correct for turning and centering the boss.

Rod

Hi As J B says
The one centre drill as accurate as you require in your mill wobble edge's and pitch over 35 mm and centre . You can also clock the bar in the centre before drilling using a DTI . but you know that .!!! and 4 jaw at chuck end . revolving or dead centre in tail stock grease dead centre . fit carrier, fit on lathe . and away you go
Nobby

Rod, I did think about a cats cradle but it would need a big fixed steady, that bar is going to be about 100 across the corners.

J

Hello Tim,

Do you have a boring head and the ability to mount a suitable sized vice on a vertical slide? If so it would be feasible to turn the boss from the headstock end with the tool swung round the outside of the bar.

Without a boring head it gets rather tedious but you could still index a boring bar held in a 4 jaw chuck to do the same job.

Some far end support would be wise I think to mop up stray vibrations and stop the bar from being pushed away from the cut.

Regards

Brian

Yeah, your right. Comes down to drilling an accurate centre hole then. Easy on my mill which, although quite small has a horizontal spindle so can be used as a jig borer. A similar set up could be done using a vertical slide in the lathe. Holding 300mm vertically and drilling is more of a challenge in a Model Engineer's workshop, headroom wise.

Rod

Hi
Correction no carrier needed in my set up I was thinking in terms of between centre's
Nobby

Thanks for all the suggestions; i have just received my copy of "Workholding in The Lathe" so i will give that a read for some more ideas

i dont have a DRO or a mill big enough to hold it for drilling as for turning a boss with a chuck there i had planned to turn down a small section that could be held in a fixed steady while i drilled the accurate centre hole

i wonder if i swapped a flanged collar for a way to hold a smallish 4 jaw chuck in the fixed steady i would then be able to drill the centre with the tailstock for turning

i might also do a few experiments to see how closely i can get a punched centre hole... my only worry is using that centre mark to drill a hole big enough for a dead centre... i might have to drill it by hand and generally i am utterly useless at getting them anything like square

thanks again

I think the only way I'd be able to do on my smallish lathe is mounted on the cross -slide and a boring tool or similar in the 4jaw. As others have said, mark out and drill a center hole, hand drilling is not too bad, Wonder if the bell shaped center drill would be better ?

You could flycut the end square, then drill and ream a hole.

If you then made a separate barrel with a peg on the end you could bolt, weld (or both) the end on.

I don't thank I'd have the patience to keep stopping the lathe to keep adjusting the tool.

I have a feeling you can get an automatic boring head, but I've never understood how those work.

Doing a good between centres job on a round bar is a big challenge

Doing a good between centres job on a SQUARE bar is a heck of a challenge

I've never seen one done before in anything I've ever read

Workholding would be made a lot easier with a longer than required square bar

Turn each end into roundbar and hold between centres with two four jaw chucks?

Each chuck jaw should be parallel with one side of the square bar

None of the Above can happen until you get a spot-on centre hole at BOTH ends, or disaster will happen, the sides of the square bar will be misaligned when its on the lathe

Truly accurate 2nd operation jobs are a PITA

Bear in mind that you can also combine Two bespoke dead centres with the 4 jaw chucks, since they will fit down the centre of each chuck into the mandrel taper, this may help....

A heck of a challenge IMO

GL

Edited By Ady1 on 07/03/2014 08:43:59

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Tim,

There have been some good suggestions so far, and [aside from the labour involved] I don't think there was much wrong with your original suggestion.

Before we try go any further, it would be worth knowing:

How accurate is the square bar that you are using?

• are all sides equal within 0.1mm, or 0.01mm, or 0.001mm ?
• how straight is it [have you checked on a surface plate] ?
• is there any measurable twist [or "wind" as a carpenter would say] ?
• are the ends just "saw-cut", or are they machined ? <etc.>

... and, incidentally, what diameter is the proposed boss ?

I ask these questions not for the sake of pedantry but, simply to better understand the scale of the problem ... you say "the boss needs to be very accurately centred and running parallel with all the sides." but responders can only guess what you mean by that.

MichaelG.

Once the bar is mounted between centres fit a DTI to the cross slide, set it against one end at (say) 2.0 and run it along the length of the bar

Flip bar 90 degrees without moving the DTIs position and do the same again, it needs to be at 2.0, as do the other 2 remaining sides

The DTI will need to be set at 90 degrees to the centreline  of the square bar or you will get goofy readings

lol, good luck

Edited By Ady1 on 07/03/2014 09:03:28

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Sorry, Tim,

I had better withdraw that comment ... Obviously, if you want to turn a boss on the end of the bar; holding the workpiece in a chuck at the tailstock end would cause a few access problems.

MichaelG.