Lathe Setup / Tests

[Howard LewisParticipant @howardlewis46836]

The ML7 bed is a bit flexible, so should be shimmed / adjusted to remove any twist. before making any other checks of accuracy.

Turning "dumbell" test pieces and adjusting until both diameters are the same is the method recommended by Myford.

See pages 41 and 42 of Ian Bradley's "Myford 7 Series Manual", or Pages 27 and 28 of "The Amateur's Workshop" by the same author, or pages 36 and 37 of "The Amateur's Lathe" by L H Sparey..

Myford tell which fixings should be shimmed to eliminate any difference in diameter between the two ends of the test piece.

If your machine is on raising blocks, adjustment is made easier.

Howard

[Dr_GMJNParticipant @dr_gmjn]

Posted by Howard Lewis on 16/11/2020 10:18:23:

The ML7 bed is a bit flexible, so should be shimmed / adjusted to remove any twist. before making any other checks of accuracy.

Turning "dumbell" test pieces and adjusting until both diameters are the same is the method recommended by Myford.

See pages 41 and 42 of Ian Bradley's "Myford 7 Series Manual", or Pages 27 and 28 of "The Amateur's Workshop" by the same author, or pages 36 and 37 of "The Amateur's Lathe" by L H Sparey..

Myford tell which fixings should be shimmed to eliminate any difference in diameter between the two ends of the test piece.

If your machine is on raising blocks, adjustment is made easier.

Howard

Thanks Howard. I've done the levelling checks, and adjusted the slight twist error out.

Now I'm turning between centres, and need to offset the tailstock to correct a silght error there. I think this is done by loosening and tightening a couple of opposed screws at the base of the tailstock, a bit like when you adjust a 4 jaw chuck.

[Howard LewisParticipant @howardlewis46836]

Exactly! Time well spent.

It is nice to have a machine where you can be confident of what it can produce. (Rather than relying on my skill, or lack thereof! )

Am always amazed by the work that real old timers produced on machines which did not even have graduated dials. Yet with a six inch rule, and callipers they worked to fine limits. In all honesty, they were making one part to match another, rather than for interchangeability. But SKILL, just the same.

My turning instructor, once checked a length with his very knarled thumb nail and his visibly worn six inch rule. "You've got about another ten thou to take off" The depth mic said 0.008"

Is the surface finish improving now that the toolpost is now solidly clamped?

It would be, by my reckoning, time well spent to make up a Centre Height Gauge. Good experience and a time saver when setting up a new tool for the first time.

May your swarf, rather than the air, be blue!

Howard

[Dr_GMJNParticipant @dr_gmjn]

Thanks Howard. I think all the critical things I'm making will be made to fit, rather than individual items that must be made to a tolerance. But yes, knowing the lathe is set up to as close to new spec. as possible removes one source of error.

I'm much happier with the surface finish of the free-end of the test bar:

In total it's about 6" from the chuck jaws:

I think the tightened tolerances I can now get on the gibs, the more secure toolpost and Andrew's suggestion that the chuck was shagged combined to sort it all out.

I'm still puzzlng over the best 3 jaw chuck to get – or 4 jaw. I might call Arc and have a chat.

Definitely getting there with teh late re-furb though. Very happy with that test bar finish.

Thanks!

[Dr_GMJNParticipant @dr_gmjn]

So…I'm getting frustrated with this whole thing now. Yesterday, after hours of work, I got the tailstock offset such that I couldn't detect any difference in turned diameter on two c. 1" bosses spaced 5" apart – turned between centres. Measured with a 0.0001" micrometer.

I also re-checked with the Myford test bar between centres – fine.

Just now, I decided to do a quick check: I removed the test bar, and both centres.

Then replaced the centres and turning test bar in random rotational orientations and took a 0.001" cut. Result: a difference of 0.002" in diameter.

I checked the centre in the headstock, and it's true. Then substituted the tailstock centre, and there's about 0.0005" – 0.001" runout.

What are you supposed to do about a brand new dead centre that's out of true? Are you supposed to mark it such that it goes in the same orientation in the tailstock?

It just sems at the moment that it'll be impossible, day-to-day, to get consistent results out of this lathe. If it's not a perfect dead-centre that puts the offset out, then no doubt it would be tailstock ram position, or tailstock on bed position, or probably some other small detail.

It's so frustrating.

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