Saddle play when direction of cut is changed in my ML7

I was testing my ML 7 with a test bar and DTI

I have noticed that when I change direction of saddle travel there is about 1 thou "jump" on DTI reading, as if saddle "wiggles" slightly when dragged by a leadscrew right and left. Gib strips are adjusted

Of course more "jump" is close to headstock.

How much of such "jump " is acceptable before some bedways correction is called upon?

At the moment lathe is cutting clean, no chatter etc.

Edited By Martin Dowing on 04/09/2017 08:14:12

Most, All ? Lathes will do this that is why we back the cut off to return to start point. Some of your very acceptable one thou or so movement will be from the DTI, it does need some working clearance to work without jamming.

Thanks for your comments.

As I have written, lathe is cutting clean, so no alarm yet but It is nice to know that situation is "very acceptable".

I just wondered how much "wiggling" is acceptable in industry, if machine is to be considered "precision".

I have seen big lathes where saddle was wiggling like a drunk man walking on the curb and still operated.

Martin,

Have you tried deliberately wiggling the saddle during your testing?

eric

As has been said, it isn't really a concern if it doesn't affect your results.

It's the downside of the British pattern flat bed, the American style V-flat sends to self-compensate as when the V wears you don't have any significant effect on alignment. Obviously the v-flat costs more to regrind, but probably lasts longer before needing one.

@Lambton,

Yes, I did. This 1 thou is what you get this way.

With some force applied (within reason) it will go 1 1/2 thou but then spring back to 1 thou.

@ Neil Wyatt

The nice thing with flat bed is, that even amateur with moderately advanced skills could scrap it back to shape once need arisen.

But of course it is better if there is no call for it at the first place.

Don't panic Captain Mann'ering. According to the old Myford brochure on reconditioning services for their lathes, the bed ways can have up to five thou of wear before they are a candidate for a regrind. You have plenty to go yet.

The thing that usually seems to wear first/most is not the bed ways but the matching surface on the carriage. The quarter round piece that sticks down from the carriage base and runs along the inside vertical surface of the front shear or way. That is particularly narrow on the Myfords (up until the fairly late model ones), about four or five inches from memory. So with a bit of wear there, combined with at least one thou of working clearance to allow for the oil film, when you reverse the feed direction, with the halfnuts mounted outboard from one side of the carriage, it is natural for a slight pivoting action to take place, hence the DTI movement you are seeing.

If the wear on the narrow guide on the carriage gets too bad (my ML7 had at least .030" of wear at one end, virtually none at the other) it can be rectified by performing the "wide guide conversion". This consists of attaching a strip of 1/16" x 1/2" gauge plate along the equivalent position at the rear of the carriage, so it bears on the rear vertical surface of the rear way, or shear. This surface is machined when new at teh factory, but does not bear any load in the standard set up. So you now have the carriage guided by an as new unworn surface. The surface on the carriage at this rear point is about twice as long as the front one, giving better guidance and hence the name "wide guide conversion".

If you lathe is turning nice and clean and true, carry on as is and file this away for when you have worn it out in 20 years' time.

Edited By Hopper on 05/09/2017 12:08:53

Yes.

This "wide guide conversion" is a good idea and quite often discussed.

Presumably regular gib strip in front side of saddle need to be shaved a bit to widen gap on the back. Probably unused shear will need scraping to provide better bearing surface. What do you think about it?

Anyway at the moment my lathe doesn't need it albeit I do wonder if doing so would allow me to get rid of a little bit of stifness noticed while carriage is winded down to tailstock.

Edited By Martin Dowing on 06/09/2017 23:24:22

Yes it probably would get rid of stiffness at the tailstock end, but how often do you use that section of the bed? Enough to muck about with a lathe that otherwise is working well?

I don't know that the rear vertical shear face would need scraping as an absolute necessity. I don't know that the front one was scraped from the factory. Didn't appear so on the unworn sections of mine. But who knows what they did back then. In my case, the fresh machined surface of the rear shear is certainly going to be better than the well worn front shear, which is having a fairly close look at the five thou wear limit mentioned by Myford re bed regrinding in their old brochure.

No need to shave the front moving way. It moves away from the bed shear when the extra strip is inserted on the rear moving way. So a small strip of previously unused surface that bears on the top of the lathe ways will need to be scraped down so the carriage does not ride on that. Or on a very worn carriage, remilling the horizontal surfaces may be in order, followed by a bit of scraping. Maybe some realignment of the half nuts or apron needed, via enlarging the mounting screw holes, but possibly not. Although it's a well known mod, I have not been able to find any indepth, specific how-to articles on it. Quite a few generalized "I put a strip of gauge plate in the rear guide and that worked well" kind of mentions in various articles but no step by step how-to's. This includes JA Radford's book on lathe mods, which I think was regarded as the origin of the idea in ME circles, or certainly close to it.

I have a strip of gauge plate in my shed waiting to do this conversion on an ML7 I am (very) slowly rebuilding so will document it when I do it. No idea when that will be though. Shed time is limited these days.

.

Chapter 19 of Radford's book [ Improvements and Accessories for your Lathe ] is titled "Renovating a Myford Lathe" and he describes the modification in reasonable detail ... His lathe was a Super 7B long-bed.

MichaelG.

Rather you than me

Presumably regular gib strip in front side of saddle need to be shaved a bit to widen gap on the back. Probably unused shear will need scraping to provide better bearing surface. What do you think about it?

It didn't on either count when I did mine. There was enough clearance in the apron screws to get the half nuts engaging correctly as well (slacken apron screws, engage halfnuts as close to the end of the screw as possible, tighten apron screws).

Rather you than me

Indeed. It took me two months of lunchtime sessions where I last worked to do the top of a Super 7 bed that had around 0.008" wear. Milled the vertical faces on a large Zayer bed mill as a "test cut" after it was rebuilt & retrofitted after a fire.

Nigel B

If you compare early hobby lathes like Drummond with later 'up market' lathes the saddle increases in length to help with this aspect. The ultimate is the CVA which if you look at a picture has a saddle almost half the length of the total bed.