Lathe Setup / Tests

Thanks Howard.

To be honest some people have said you're better of with inserts, some people say you're better off with HSS. So...there you go. I had some Sandvik tool holders and inserts already, and have used them in conjunction with various other types of insert sourced from Cutwel (DCGT, DCMT, cast iron grade, aluminium grade, larger or smaller nose radii etc.) for different applications. I've had no particular issues with them when building a Stuart 10V. I didn't use HSS turning or fly-cutting tools at all.

One reason I didn't use HSS was because I have no experience of grinding tools, and assumed that adding the variable of poorly ground tools into the mix at this stage of the game would only end badly.

Cheers!

Carbide tools are designed to generate heat to soften the material locally. They are not sharp (Except the ground variety. Which most of us do not use )

The ML7 was designed many years ago, and launched in 1947. It is somewhat flexible machine, so not the perfect for carbide inserts, although folk who know what they are about do splendid work with such combinations.

A correctly ground HSS tool has a razor sharp cutting edge and should not rub in the way that a carbide tends to do.

I have just compared a shear tool to a freshly sharpened HSS bit in a tangential tool holder, and am pleased ,with the results of a 0.002" cut with a 0.0023"/rev feed. There is not much difference betweenn the two, whether dry or musing neat cutting oil. Not a ground finish, but pretty good, although I would not run a highly loaded white metal bearing on either.

So sharpen a HSS bit, (as large as possible, for rigidity ) and try the dumbell again.. Do ensure that tghe cutting edge is on the centreline, or it will not cut properly.

It is possible that your problems stem from the lack of clamping and rigidity in the Tool Holder / TopSlide interface, so that is one variable to remove before you start looking for greater precision..

Howard

Thanks Howard. Yes, the next job is to sort the toolpost issue.

I thought it was a minor issue, but there are apparently no minor issues with an old lathe!

As Hopper said elsewhere on the forum, owning an older machine and fixing the problems can become a way of life in itself.

My wife says that "Fixing one problem unveils at least three more"

You try to slacken a nut. The stud shears off, flush, so then you have to remove the manifold to drill out the remains. And the Easyout (!!!!!) breaks in the drilling!

And so on.

You fix what appears to be the biggest problem and then work your way down the list of the smaller ones. By the the time that you have eliminated all the problems, the machine has worn some more, so you start again.

Forgive my cynicism! But you are less horrified by events that way

Howard

I’ve now put the toolpost back, and tried the unsupported test again.

Its 1” diameter EN1A mild steel, turned down with about 5” between the test bosses.

I used the 4-jaw chuck.

I had to mess about with feeds and speeds a bit, but eventually got very good cuts on both bosses.

Im getting a consistent difference in diameter of 0.0002” (according to my Mitutoyo micrometer).

Should I twist the bed to get it perfect, or leave it? I’m not sure I could get it much closer, but is it worth trying?

The other thing is that I’ve centred the test bar to try a between centres test. I’ve never turned between centres before and the only drive bar I’ve got is too small a diameter to fit over the bar. Are there any tricks to use a makeshift drive bar some other way?

Thanks all.

I assume it is two tenths larger at the tailstock end, if this is correct leave it well alone.

Ok so I used a nut splitter as a makeshift drive bar.

I'm getting 0.0003" difference in diameter over the same 5" boss distance between centres this time.

It's very similar to the unsupported test (0.0002" )so I'm wondering if they are both due to slight bed twist?

Any thoughts?

Thanks all.

Edited By Dr_GMJN on 15/11/2020 18:26:23

Looking at the Myford Inspection sheets:

test 16, work held in chuck, permissible error .0000" to .0008"

test 17, between centres permissible error 0000" to .0004"

So I think you are doing well with your test results.

Edited By ian j on 15/11/2020 19:09:00

Thanks Ian, completely missed those on the sheet, I thought it was just DTI limits.

The between centres test lols like it’s over 12”, my test piece is only 5”. So I suppose pro-rata, that limit should be 0.00017”? So I’m about twice that limit?

...or is it half? Hmmm.

The “held in chuck” seems OK.

Cheers.

Edited By Dr_GMJN on 15/11/2020 19:30:37

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.

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

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!

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.