Lathe Setup / Tests

The Chuck looks tired and knocked about, have the jaws gone bell mouthed? so not gripping the bar very tightly. What type of insert are you using not many like taking 1/2 thou cuts unless it’s a sharp silver one for aluminium.

Thanks George. The point of the test is that you need it unsupported, so that the cut isn’t influenced by the tailstock support.

Yes, I’d like a new chuck, but that’s another question: which is a good one to get? Some seem to need thecbaxkplate machining, some don’t. I’d appreciate advice on that too!

I’ve seen the test specified for 4” protruding, and 6”. Maybe I’ll have to try 4” next, although that must be approaching the state where the separation between bosses is so small it makes an evaluation of twist difficult.

By the way, the chuck came with the lathe - I didn’t knock it about like that.

Edited By Dr_GMJN on 09/11/2020 20:28:49

Out of idle curiosity I just tried an experiment with a 1" diameter bar of EN3B sticking 6" out of the chuck. I used the well worn CCMT insert that is on the lathe at the moment. Parameters were 800rpm, depth of cut 5 thou and feedrate 4 thou/rev. Absolutely no problem; no chatter or vibration. The finish is perfectly smooth to the touch.

The problem is unlikely to be resonance. I'd place my bet on the chuck; I was using a Burnerd EC collet with 6 fingers, so the work was held solidly.

Andrew

Thanks Andrew. I'll try again in the 4 jaw chuck tomorrow. I've also got another 3 jaw which looks much newer, but is far worse in terms of runout. They all came with the lathe. For most of my work on the steam engine I used a new collet chuck or the 4-jaw.

I suppose if your lathe is heavier duty than my ML7 that could make a difference. However I'm pretty sure I've seen steel cut on an ML7 for this test on YouTube, with no apparent issue.

This is a copy of the Myford Inspection sheet

Thanks for the Myford test sheets Ian. I might re-check mine over the 6” test rather than the 8” I did, just for a laugh.

Interesting that my maximum deviations from nominal on the bed wear measurements were all well within the specified limits, yet the saddle still locked solid at the tailstock end.

Well yes. If you have 3 thou of wear and set your gib to one thou clearance at the worn spot, it will not fit over the section that is 3 thou larger. You were expected to adjust your gib to suit. IE set it a bit tighter to fit the worn section most of the time, because you rarely need to move the carriage to the very far right end of the bed. If you were doing an unusually long job you set the gib to fit the less worn section of bed and live with the bit of extra slop on the worn section. You cant achieve perfection on a worn lathe. Its all about compromises and developing the skills to work with them.

Yes, luckily I never turned anything long enough to lock the saddle solid.

Still, for the sake of about 3 hours’ work with a file, some shim steel and Loctite, might as well get rid of the main wear, and remove the need to adjust it altogether.

For me, Id rather spend time between models getting the equipment right, so I can concentrate 100% on making parts when the time comes (and not wonder if any errors are mine or the machine - I know they’re mine now!).

Yep, that;s the beauty of the Wide Guide conversion, even with the extra touch up you had on your front shear. The return on the little bit of time and money it takes makes it the single best thing you can do for a worn Myford. (Along with setting the shimming on the headstock bearings.)

Don't worry, you'll find other things need fettling as you go along. ITs a bit like riding vintage motorbikes. Each ride something pops up that needs taking care of, Might be minor but always something.

Edited By Hopper on 10/11/2020 08:19:25

Andrew,

I just did a quick test with the 4 jaw, and it seems to work fine on steel at 6". Looking at the Harold Hall website, he suggests 4" unsupported for the outer boss, so that should be even better.

The odd thing is that I put the aluminium test piece in the 4-jaw, and only got a marginal improvement with the burnished effect on the outer boss. I wonder if it's the lower stiffness of the aluminium that causes it to move more than the steel under a cut?

So on to starting another thread on ML7 chucks...

Cheers.

Agreed.

I think the only comment I'd have on your measurement article would be that IMHO the overall bed width should be a key measurement. IIRC you suggest doing it as a double-check?

If you measure the individual bed widths, and you've assumed wear is predominantly on the inner faces, doing the wide-guide conversion will have negligible effect, and you might be left wondering what went wrong. I know you said mine was the first example of wear on the inside face you'd heard of, but at the end of the day, it was an easy fix either way, and it's a very easy check to do with a DTI on a base run against the un-worn rear bed edge - you don't need a large micrometer to do it if you've not got access to one.

Cheers.

Did you read the thread on vertical shear tooling? It is possible it will take off the fine cuts you are trying to do on the dumbbell.

Martin C

Thread 168702

Thread link added.

Edited By Martin Connelly on 10/11/2020 13:04:52

Rotate a jaw till it's at the top when gripping your bar and put a light behind it see if there's any light coming between jaws and bar where they should be touching.

If there is it might be worth keeping if the outside jaws are OK

I did an experiment at lunchtime with a 1" diameter 6082 aluminium bar sticking out 6". I changed the insert to an old CCGT one specifically for aluminium. As before I ran at 800rpm and 4 thou/rev feedrate. A DOC of 10 thou gave a smooth finish although there was a light mottled pattern. With a DOC of 5 thou the pattern had almost disappeared. With a 2 thou DOC there was no pattern, just a smooth finish.

Andrew

What lathe is it though?

Harrison M300:

Andrew

So...probably multiple times stiffer overall than my ML7? I wouldn't expect to have much problem machining a 6" aluminium test piece on that!

Correct, my lathe is much heavier and more rigid than an ML7. But we're talking light cuts here. So neither characteristic is significant. I did get a slight pattern on the aluminium at larger DOC. I suspect the issues are how firmly the work is held and the cutting tool itself. It may well be that a slower, or faster, spindle speed will help. Any sort of pattern implies a cyclic variation of some sort; albeit not usually resonance. The natural tendency is to reduce speed/DOC/feedrate, which can help. But counterintuitively increasing the parameters can also help. Experimentation is needed.

Andrew

I notice the reference to "A new insert", suggesting a carbide insert.

Carbide inserts do not universally like being used with small cuts. They were developed to remove metal rapidly., and many years after the ML7 was designed and launched

May I suggest repeating the dumbell test with a sharp HSS tool, carefully set on the centre line?

You may get a better result.

Howard