Precisely Holding Long Square Bar in Lathe for Turning

[TimSParticipant @tims]

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

[TimSParticipant @tims]

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

[JasonBModerator @jasonb]

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

[Roderick JenkinsParticipant @roderickjenkins93242]

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

[NobbyParticipant @nobby]

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

[JasonBModerator @jasonb]

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

[Brian WoodParticipant @brianwood45127]

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

[Roderick JenkinsParticipant @roderickjenkins93242]

Posted by JasonB on 05/03/2014 15:49:43:

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

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

[NobbyParticipant @nobby]

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

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi TimS ,

Lot’s of ways of doing it accurately by making jigs or scrounging use of bigger equipment but I’d try just drilling a tiny hole on best estimate of centre position , mount bar in lathe , see if there’s any error , draw hole over as nescessary and drill slightly bigger each time and when satisfied drill the centre in .

Bet you could get it perfect in two goes .

Regards ,

Michael Williams .

[TimSParticipant @tims]

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

[Gordon WParticipant @gordonw]

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 ?

[jason udallParticipant @jasonudall57142]

Lets assume bar is square ( some times it is).
Take piece of stock say 5 mm bigger than across corners size.. bore the stock to fit bar.
Mark jaw one ( say) against “bush”..
Remove and split ( saw) choose position between jaws
Return to chuck..fit bar and close chuck. ..

But if you need to work at 300 mm from chuck..
Do the same but use steady on od of un split bush….

[jason udallParticipant @jasonudall57142]

Or use face plate mounted tool and square bar on cross slide..

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Make a disc same diameter exactly as square bar A/f .

Use any scrap plate but make it quite thick . Drill and ream a hole dead centre .

Place disc on end of square bar and centralise it on end using metal strips on all four faces . Drill through for centre .

Done .

MW

[jason udallParticipant @jasonudall57142]

If the challenge is marking center…why not
us center finder square thingy ( if you have one)..the ones on combination squares..included angle is 90 degrees. .thus line will fall diag to diag..do this from all four cornes and if all well you get what looks like two lines ( or at least very close depending on how regular a square the bar is) which cross..

[jason udallParticipant @jasonudall57142]

Or failing all that. .surface plate / height gauge..

[Andy AshParticipant @andyash24902]

Posted by Gordon W on 06/03/2014 15:54:59:

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.

[jason udallParticipant @jasonudall57142]

As to center hole. .
Start small..say 1mm by 1mm deep.check then enlarge. .say 3 mm..etc

[Ady1Participant @ady1]

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by TimS on 05/03/2014 15:06:42:

… 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.

.

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.

[Ady1Participant @ady1]

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Michael Gilligan on 07/03/2014 08:50:03:

Posted by TimS on 05/03/2014 15:06:42:

… 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.

.

and [aside from the labour involved] I don't think there was much wrong with your original suggestion.

.

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.

[Les Jones 1Participant @lesjones1]

I totally agree with MichaelG's comments as what is accurate in one context is not necessarily accurate in another. One suggestion about marking the centre is to use an adjustable set square like a depth gauge and scribe lines from all four sides. Even if the marks are not at the exact centre the square formed between the lines should enable the centre punch to be positioned reasonably accurately. (I would think within a couple of thou.) If this is not accurate enough then this two step approach would help. Set a length of the bar a little longer than the finished size between the four jaw chuck and this marked centre. Clock the end in the chuck to be true. Turn a circular section close to the chuck that will fit in the fixed steady. Cut off the square section that was in the chuck. Mount the other end in the chuck and true it up first at the chuck end then the round section at the far end. Now carefully set the fixed steady to support the round section. This should reduce any error due to the position of the original centre mark.

Les.

[NobbyParticipant @nobby]

Hi
In terms of home shop machinist with no CNC no Dro Set odd leg calliper to 35mm mark from 4 sides similar to Les's idea if you can get it under you drill clamped to an angle plate . put a sticky pin on centre drill pick up the line by moving into position then centre drill
Nobby.

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