Myford ML7 Saddle Gib

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 21/06/2015 10:27:49:

… It's interesting, because if you draw out the geometry, the free play of the saddle is entirely down to the gap and the length of the guides, not their distance apart.

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Neil,

That is exactly the point; and is why I said it would have been more popularly understood if referred to as Long rather than Narrow. … The problem with Myford's implementation seems to be that the inner bearing is not actually very long.

… "Relativity"

MichaelG.

Edited By Michael Gilligan on 21/06/2015 11:29:17

[AjohnwParticipant @ajohnw51620]

Posted by Neil Wyatt on 21/06/2015 10:27:49:

Hornrich's book is a good find. Sadly it's all descriptive with no actual analysis of why and how the narrow guide is better.

It's interesting, because if you draw out the geometry, the free play of the saddle is entirely down to the gap and the length of the guides, not their distance apart. I think this is why it has more to do with freedom of movement than accuracy of alignment.

Strangely the narrow guide is often quoted as a British practice, but the American (and now almost universal) practice of inverted-V bed has an even greater length to width ratio for the guide.

Neil

Not sure what you mean about width Neil. The idea is bound up in aspect ratio really not width. The higher the better. Accuracy does come into it. Going back to my hole at 1 dia long it can tip by so much and if pushed it's likely to tilt by a certain amount causes wear in localised spots. Make it 10 dia long and it will probably take less effort to move it in practice as you point out but wear will be at a much lesser angle for the same clearance. In terms of accuracy there is no getting round the reduced tilt the arrangement can offer.

To me the correct term for an inverted V bed is a prismatic one. The other type that has been common is the dovetail style still used on the slides fitted to the saddle. Myford style – never come across a common name for it and it's not one that has been used that often. I should add that for some reason it works a lot better on a Raglan. I suspect that is down to a combination of factors. All sorts of odd things can go on when the saddle is moved when there has to be some clearance. For instance the tool may move inwards when cutting forces are applied and outwards when the rack and pinion is used to wind back to the start of a cut. On a prismatic bed the tool just stays were it is. Other than any oil film there isn't any clearance. Dovetail beds have the same problems as Myford style especially as they implemented them on their more recent lathes but like prismatic beds it's easy for them to offer larger wear faces than the ones on 7's. They could have made those a lot larger. I will always wonder why they didn't sort them out, maybe using a square section front rail in the extreme and even fitting wear strips to the saddle. Given their fame I feel that they would still be about if they did the right things to their lathes.

John

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[Michael GilliganParticipant @michaelgilligan61133]

Posted by blowlamp on 21/06/2015 10:51:34:

It's not something I personally thought of, it's what Myford did from new at their factory.

The procedure was to adjust the saddle centre gib screws first, working outwards, leaving the tailstock 'unsupported' end screw until last, which was then merely nipped to provide contact.

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Sorry, Martin; I still don't get it … What you describe is exactly the opposite to what's written in the ML7 book.

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Michael Gilligan on 21/06/2015 12:57:47

what's written in the ML7 book.

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[KWILParticipant @kwil]

Micheal,

I agree with you entirely, knowing the Myfords as I do (over the last 50 +odd years), I have always ignored those words as illogical and have set the gib centre screw first and then left screw, in balance. The right hand screw to engage but not force so as to counteract any potential carriage rotation with increasing tool pressure.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by KWIL on 21/06/2015 14:39:59:

… I have always ignored those words as illogical and have set the gib centre screw first and then left screw, in balance. The right hand screw to engage but not force so as to counteract any potential carriage rotation with increasing tool pressure.

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Well … [Ken and Martin] … That is interesting

This sort of insight is why we'e all here on this forum.

MichaelG.

[AjohnwParticipant @ajohnw51620]

I think Myford's statement makes a lot of sense if everything is perfect, less messing about. Trouble is following use things probably wont be perfect. There seems to be some ancient law about how lathe slides should be set up – rather loosely. That results in faster wear and even bent gib strips, same problem as guides that can tilt, pressures are applied to small areas. I suspect the main reason this is done is to reduce lead screw wear and if high class work is needed the slides can be tightened up just for that purpose and then loosened off again. I'd guess that some lathes never get tightened or even set correctly in the first place. Not that the usual grub screw and lock nut is that easy to set anyway. Taper gibs are much much better in that respect.

I set my slides on the firm side and expect to have several goes at it before things settle down. I start by setting all screws moderately tightly to take out play and then backing off. If the cross slide tightens a bit when wound right out I accept that as I feel it's a better option than more play and higher wear rates elsewhere. Ideally I think slides should be set with any lead screws out pushing by hand. Some lead screws provide a lot of mechanical gain.

John

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[KWILParticipant @kwil]

With care it is not a difficult task to remove the high spots etc to restore to a well set up state without tightness.

When Myford reground and fitted an "old" carriage they also, of necessity, reduced the level of the matching area for the apron casting, to allow for the metal removed from the shears plus the underside of the carriage so as to maintain the correct location as to the leadscrew.

In respect of the narrow guide, here again this can be restored as part of a refit process.

Edited By KWIL on 21/06/2015 17:34:18

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