Squaring in the lathe

[Iain DownsParticipant @iaindowns78295]

I have a tiny mill (150W) and a beefier lathe(500W).

I'm about to embark on trimming some largeish pieces of stock (mainly cast iron) and it will take forever on the mill.

As an example I've got a 50x50x65 chunk of cast iron that I need to trim down and make two bearings out of.

I have a strong recollection that one of my machining books gave instructions on how to square up a block in the lathe. However, I can no longer find it.

Equally, the Interweb provides uncertain results. One you tube video purports to demonstrate this (though it didn't actually play for me), but the comments rubbished the approach.

Can I square up a block accurately? Could I just take off sizeable amounts on the lathe and finish up on the mill?

Iain

[Iain DownsParticipant @iaindowns78295]

I’m sure this is in one of my books

[JasonBModerator @jasonb]

Do it all in the 4 jaw, provided the face of the chuck runs true and you have a tri-square or 123 block its no problem.

[Neil WyattModerator @neilwyatt]

The book was probably the 'Model Engineer's Foundation Course' by Peter Wright.

Neil

[Iain DownsParticipant @iaindowns78295]

Thank you Neil.

The book is indeed the one you quote. And I did have a quick look before (I don't have THAT many engineering books), but couldn't find it.

More indexes! (or a bit more care on the part of the reader….)

Iain

[Ady1Participant @ady1]

Lathes actually cut a bit convex, or is it concave, and are designed to do that

There are also chuck jaw spring issues when the workpiece gets removed and re-positioned

You can do an okay job on a lathe but for a really proper flat square job I ended up getting a shaper

I came to the conclusion that Lathes do proper round stuff Shapers do proper flat stuff

 

Edit: for your job do most of it on the lathe then a nice finish job on the mill is probbly the way to go

Edited By Ady1 on 12/12/2018 09:36:32

[not done it yetParticipant @notdoneityet]

I expect there are lots of demos on u-toob.

Enot put one up recently, I recall

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ady1 on 12/12/2018 09:32:32:

Lathes actually cut a bit convex, or is it concave, and are designed to do that

.

Concave [very slightly]

MichaelG.

[JasonBModerator @jasonb]

I'm sure that 1 or two micron difference will make or break Iain's engine design, an out of tram mill could do worse than that.

Spring back in the jaws will have no effect on squaring up a block, only an issue if you are putting previously clocked work back into the jaw and expecting it to stay concentric with the lathes axis.

As for You-Tube, Iain has already said they it gives varying results.

May be a good subject for Neil to cover in his Lathework for beginners, I've done it for the Mill already.

[HopperParticipant @hopper]

Pffft! A lathe MIGHT have turned half a thou concave on a 10" diameter as required by Herr Schlessinger when it left the factory. By now it could be anywhere, the same as any milling machine. Chuck your cast iron up and turn away. As you say, you can always do final finish in your milling machine if all else fails.

[Anonymous]

The face of my 4-jaw independent runs pretty true, a thou or two. But squaring in the lathe hasn't worked for me; I end with thickness variations of a few thou. So I'd be inclined to rough in the lathe and finish off in the mill.

Andrew

[Neil WyattModerator @neilwyatt]

Posted by JasonB on 12/12/2018 10:16:31:

I'm sure that 1 or two micron difference will make or break Iain's engine design, an out of tram mill could do worse than that.

Spring back in the jaws will have no effect on squaring up a block, only an issue if you are putting previously clocked work back into the jaw and expecting it to stay concentric with the lathes axis.

As for You-Tube, Iain has already said they it gives varying results.

May be a good subject for Neil to cover in his Lathework for beginners, I've done it for the Mill already.

Good suggestion, Jason. I want to cover some less common but useful operations.

[JasonBModerator @jasonb]

I think on of the main things to watch is that any jaw that is holding an unmachined edge can have a tendency to try and move the work away from the chuck face, so much like milling put something between jaw and unmachined edge in the same way as you do between moving vice jaw and work. Also tap the work back against the chuck face if using that as a ref surface.

[Iain DownsParticipant @iaindowns78295]

I shall give this a try today (all other projects being equal) as soon as my shed has warmed up enough .

Iain

[Ady1Participant @ady1]

But squaring in the lathe hasn't worked for me.I end with thickness variations of a few thou

I reckoned my chuck jaws weren't quite right, no matter how hard I tried, hugely frustrating and blatantly in error if you really need good squareness

The workpiece just got smaller and smaller and laffed at me

This eventually forced me to get out and get a shaper

A decent mill should also do the same job

[not done it yetParticipant @notdoneityet]

“As for You-Tube, Iain has already said they it gives varying results.”

But the comments will usually provide an insight of how to improve the outcome. Most certainly did on one poor video, that actually showed cutting sides parallel to each opposite side but not square to the other axes.

A bit like kW, kWh and kW/h. The first two are power and energy, but the third is not a sensible unit of anything.  Many people seem to get that mixed up and need straightening out by messaging or comment.

Checking with a dti, or whatever, is simply a necessity, even when setting up angle plates, face plates, etc. Ady1’s chuck would surely have either checked out good, or not, by careful investigarion.

There are not so many parts that require to be made with more than two pairs of absolutely square faces, I would guess, apart from measuring or setting tools? A bit like woodwork where the face and side are selected and every other measurement/setting is made with respect to those.

Edited By not done it yet on 12/12/2018 12:40:55

[Howard LewisParticipant @howardlewis46836]

Carried out in the right sequence, and with reiterations, (hopefully each one will improve accuracy), it should be possible. Certainly a bit of clocking and shimming should set you moving in the right direction.

I've seen a good uniform hexagon produced in a lathe, as a test piece.

Given determination, time, ingenuity and facilities, most things are possible

Howard

[KettrinboyParticipant @kettrinboy]

To the OP don't be afraid to skim up the face of your 4 jaw chuck if its out at all, a couple of thou runout is too much if your trying to get faces on a block parallel and square, done the once it should stay true for years.

regards Geoff

[Martin ConnellyParticipant @martinconnelly55370]

A small angle plate on a faceplate and a square (or 1 2 3 blocks) should work. Just needs care with balancing everything and making sure nothing can fly off. Remember the cube does not need to be on centre.

Martin C

[Gas_mantle.Participant @gas_mantle]

I used to square items off regularly in my lathe before I bought a mill so I know its easy to do and still maintain accurate results if care is taken when setting up in the 4 jaw.

One item I would suggest if you haven't got one is a hide faced hammer or some sort of soft lead slug to keep tapping the work solidly against the chuck. As mentioned earlier tightening the jaws can have a tendency to lift the work away from the chuck face so it is vital to keep 'knocking' it back when getting close to the final settings.

A few parallels could useful depending on your particular requirements

[J BENNETT 1Participant @jbennett1]

Can any one explain why lathes turn very slightly concave? I have observed this phenomena but could never work out why it happened. I am also intrigued by the suggestion that they are designed to do this. Please enlighten me!

[SillyOldDufferModerator @sillyoldduffer]

Posted by J BENNETT 1 on 12/12/2018 17:14:07:

Can any one explain why lathes turn very slightly concave? I have observed this phenomena but could never work out why it happened. I am also intrigued by the suggestion that they are designed to do this. Please enlighten me!

It's because turning in the slightest bit convex means the resulting face cannot sit flat, whereas a concave surface will. Given that it's impossible to get and keep lathes exactly at 90 degrees through life, it's safer to deliberately allow a small concave error.

Dave

[Michael GilliganParticipant @michaelgilligan61133]

Posted by J BENNETT 1 on 12/12/2018 17:14:07:

Can any one explain why lathes turn very slightly concave? I have observed this phenomena but could never work out why it happened. I am also intrigued by the suggestion that they are designed to do this. Please enlighten me!

.

Two very good reasons:

1. A face that is concave will sit without rocking on a flat surface or on another similar face.
2. As lathes wear [and also under the pressures of working] they tend to face less concave.

The intentional concavity is [should be] very small, and is basically 'not convex'

MichaelG.

.

@ Dave … You beat me to it [nice to see that Schlesinger's work is appreciated] 

 

Edited By Michael Gilligan on 12/12/2018 17:43:56

[J BENNETT 1Participant @jbennett1]

Thanks for the great explanation of why, but how is it built into the lathe?

[Chris TriceParticipant @christrice43267]

I think I read on this forum somewhere, we're talking maybe .001" in 12".

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