Squaring in the lathe

[Neil WyattModerator @neilwyatt]

Posted by Chris Trice on 12/12/2018 18:02:53:

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

Schlesinger, Lathe faces within 0.02 in 300mm.

[not done it yetParticipant @notdoneityet]

Posted by J BENNETT 1 on 12/12/2018 17:51:59:

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

The cross slide will always have an error – if you proceed to measure to ever more significant figures – so they err on the concave side by an unimportant smidgen. Unimportant, depending on how many significant figures are considered – a spectrum ranging from cheap chinese to toolroom lathes (and beyond?).

[Chris TriceParticipant @christrice43267]

Posted by Neil Wyatt on 12/12/2018 18:43:21:

Posted by Chris Trice on 12/12/2018 18:02:53:

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

Schlesinger, Lathe faces within 0.02 in 300mm.

So eight tenths of one thou. Piece of cake.

[Neil WyattModerator @neilwyatt]

Posted by Chris Trice on 12/12/2018 22:01:41:

Posted by Neil Wyatt on 12/12/2018 18:43:21:

Posted by Chris Trice on 12/12/2018 18:02:53:

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

Schlesinger, Lathe faces within 0.02 in 300mm.

So eight tenths of one thou. Piece of cake.

Or even .001" in 15" to be exact

[Michael GilliganParticipant @michaelgilligan61133]

Posted by J BENNETT 1 on 12/12/2018 17:51:59:

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

.

Assuming that the headstock spindle is on-axis with the lathe bed …the cross-slde is set very slightly off 90°

MichaelG.

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It is important to note that Schlesinger's lop-sided tolerance allows 'zero to slightly concave'.

Edited By Michael Gilligan on 12/12/2018 23:25:41

[HopperParticipant @hopper]

Posted by Michael Gilligan on 12/12/2018 23:24:05:…

.

It is important to note that Schlesinger's lop-sided tolerance allows 'zero to slightly concave'.

So a brand new lathe, made within Herr Schlesinger's limits, may in fact face dead flat and not necessarily concave.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Hopper on 13/12/2018 00:21:04:

So a brand new lathe, made within Herr Schlesinger's limits, may in fact face dead flat and not necessarily concave.

.

Explicitly:

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MichaelG.

[Ady1Participant @ady1]

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 so you buy a shaper

The British are sneaky

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Michael Gilligan on 13/12/2018 00:36:23:

Posted by Hopper on 13/12/2018 00:21:04:

.

Note: the specific test chart that I sampled is of no great relevance to most of us

But the principle of the tolerancing is consistent for all the 'facing' specs.

MichaelG.

[Pete RimmerParticipant @peterimmer30576]

If you want to square small parts in the lathe turn one face flat in the 4-jaw then clamp something square like a short parallel or a good engineer's clamp onto your faceplate and clamp the piece against the square face of the parallel and start facing – running the lathe at a slow speed so you don't catch anything with your teeth.

[I.M. OUTAHEREParticipant @i-m-outahere]

That lazy machinist shows how to do this properly in the mill , ex toolmaker and teacher – nuff said .

[HopperParticipant @hopper]

Another way of doing it is a fly cutter held in the lathe chuck and the material clamped to the cross slide. You can then use a dial indicator to get sides set up dead parallel to the lathe axis before starting the cut. But again it depends on how good the alignment of the cross slide to the lathe axis is. (Not very good on most worn lathes and some newer ones of lesser quality.)

[not done it yetParticipant @notdoneityet]

Posted by Hopper on 14/12/2018 11:41:14:

Another way of doing it is a fly cutter held in the lathe chuck and the material clamped to the cross slide. You can then use a dial indicator to get sides set up dead parallel to the lathe axis before starting the cut. But again it depends on how good the alignment of the cross slide to the lathe axis is. (Not very good on most worn lathes and some newer ones of lesser quality.)

That might depend on the workpiece. The ‘dishing’ problem could well be resolved by doing it that way and opposite side parallelism need not be an issue – as long as the piece is turned around appropriately?

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