Myford Metric Lathe Leadscrew

Hi guys, I've lurked on this forum for some time and finally joined, seeing I recently acquired a metric Myford Super 7.

I've many questions about these lathes. Firstly I was wondering what kind of leadscrew I have, seeing the tables look quite different to pictures I've seen online?

I'm after 1.5mm on this table

Zoomed out

Sorry the pics aren't orientated correctly they look fine on my phone.

Edited By JasonB on 27/12/2022 06:49:45

Myford used the same 8 tpi leadscrew on both imperial and metric lathes, only the cross and topslide screws are metric pitch on the metric version. Myford's screw cutting chart is therefore the same for both the imperial and metric lathes. The metric pitches are not precise in the mathematical sense but are very close approximations, way beyond the accuracy that the machine is capable of.

Rod

The chart tells you what change wheel train to set up and you will need a 21T which may not be in a standard set but is available. Noel.

Is it a metric Super 7, or just an Imperial Super 7 with a metric screwcutting chart? Seems odd for a metric lathe to list imperial threads first, metric second.

Are the cross slide and top slide collars graduated in metric or imperial?

The metric pitches on the chart are extremely close approximations for a metric pitch, there never has been a Super 7 with a metric leadscrew . Don’t worry about the threads being approximations, unless you have high end metrology equipment you will not measure the error. For most practical purposes it will be satisfactory. The only way to cut a true metric thread is to use a 127 tooth gear in the gear train. 5 inches is exactly 127mm which is where the conversion comes from. Using the metric setups in the table will mean not disengaging the half nuts and reversing the lathe to start the next cut. There are other methods but not disengaging keeps it simple. Plenty has been written on screwcutting both online and in print.
Mike

You can always check the pitch of the Leadscrew, by measuring the travel for ten turns of the Leadscrew.

If it is an 8 tpi Imperial, the travel will be 1.25 inches (31.75 mm).

If it is Metric Leadscxrew, the travel will be ten times the pitch, (So 15, 20 or 30 mm, as examples )

Lots of Myford 7 Series owners on here to help with details.

More general questions can be answered by lots of other model engineers.

Find and join a local model engineerinbg society for face to face contact, and advice from like minded folk.

Don't be surprised if some of the advice is conflicting!

Different ships, different splices. What works for one, may not work for another!,

Howard

Hi there, the lathe is an original metric one with 0.05mm micrometer graduations and all the metric listed gears are included. That's why I was wondering about the leadscrew.

The 20 driver and 20 leadscrew with idlers between giving a 0.125" pitch is a good indication of it being an imperial 8tpi leadscrew. as that is a 1:1 gear train

Any of the common metric leadscrew pitches need a far more complex gear train so not likely to be a metric leadscrew.

Edited By JasonB on 28/12/2022 19:09:53

Thanks Mike, I think the only option is to wind the chuck back by hand for each pass? I imagine if I engage the tumbler gears to reverse it, backlash would cause thread misalignment?

It's not just the backlash, you would lose the indexing between the gears in the drive chain.
As far as I'm aware, Myford never made a S7 with a metric leadscrew; all are 8TPI
The cross and top slides had metric screws, with the appropriately marked thimbles.
On the end of the leadscrew, you might have a hand wheel; on an imperial lathe it has 125 divisions, whereas on a metric one, it will have a different number. (I'm not sure what off the top).

When screwcutting metric threads on the Myford, it's convention to run the lathe motor backwards, after withdrawing the tool a bit, to return for a fresh cut.
If you don't give the tool clearance, by withdrawing it a bit, that's when you strip the previously cut thread pass, due to backlash.
Don't disengage the feed, or the tumbler.

Bill

N.B. read my previous post first, and get your head around why you need to maintain the indexing between gears from the spindle to the leadscrew drive.
It doesn't help that the terminology often used is misleading; whilst yours may be described as a metric lathe, the leadscrew is imperial.

If you have a thread dial indicator on your machine, there is a way to make your life easier, when cutting up to a shoulder, but reversing the lathe to run back to the start must be done electrically via the motor on a Myford.

This video provides a good explanation, and would work equally well for cutting imperial threads on a lathe with a metric leadscrew.

If you can't currently reverse your motor electrically, shout up and you should get some assistance on the wiring and switching.
For the time being, don't try machining with the lathe in reverse; there is a chance you could unscrew the chuck, this is only about returning the carriage for another cut.

Bill

Edited By peak4 on 28/12/2022 20:01:59

Thanks for your help, Bill. I have a thread dial indicator with the usual 1234 on it. I'm guessing the little gear would have to have a special ratio or something.

Also, more importantly, I do appear have a motor capable of reverse. Here's the motor info plate. I would definitely need some sort of switch preferably a lever mounted on the lathe or stand enabling easy reversal for each pass, as you say. Any safe solutions you can suggest would be much appreciated. Regards, Nick

Edit: Although I've created an album, after hunting around a bit, it doesn't seem to have an edit option to add more images or tidy up existing ones, so I don't want to create a new album for every time I post. Anyway, the motor does appear to be reversible by switching the polarity.

Edited By Nick DoubleYa on 29/12/2022 03:48:54

Nick, click "Albums" on the green bar at the top of the page. you will get your album (s) come up and at the bottom of each is "edit photos". Click that and you get the option to add more etc.

Rotation is probably due to holding your phone in portrate position, hold it sideways with shutter button on the right and photos will come out right way up.

That's a clever little trick and a good way of doing it if you are machining up to a shoulder. Othewise, it is just as easy to wind the tool out a turn and hit the stop switch then reverse switch without disengaging the half nuts.

If you don't have reversing on your lathe motor, a hand crank in the spindle is very good for most hobby screwcutting jobs. They are usually short and you want to go slowly anyhow. Easy enough to make a hand crank that slides inside the far end of the spindle and grips via a tapered plug on the the end of a bolt that expands the split end, like a concrete anchor bolt in principal. Also a fantastic aid when cutting threads with a tap or die. Soooo much easier than wrestling the chuck or the tap handle etc. I love mine. Best thing since sliced bread. Along with the resettable cross and top slide dials it is the best accessory I have made for my lathe. The shown tailstock die and tap holders are gold too.

Hi, The OP Nick did not say whether his machine has a screw cutting gearbox or not ? Looking at the charts he has posted probably not so this link to a previous post relating to the Myford-metric-threading may not be be appropriate to the OP ? Nevertheless it is an interesting post on the subject.

https://www.model-engineer.co.uk/forums/postings.asp?th=125630

Regarding the video showing the method of using a thread dial indicator, I was taught this back in the early 1960's by my mentor during my apprenticeship and have passed it on many times - so many I "wrote it up" some years back and later added a link to the excellent video by OX Tools.

The method can of course be used on any machine fitted with a thread dial indicator and works from imperial to metric or vice versa.

John

Can't see the motor but there are switches, most notably dewhurst or santon but also others . the change in the wiring is (or should be ) easy. BUT when reversing, the motor MUST have stopped turning before reversal and keep an eye on the temperature of the motor as repeated starting of a single phase motor will in time cook it. If you can post the motor diagram then help will be forthcoming . I'm assuming that you have a single phase motor not 3Ph and VFD. Noel.

+1 for Hopper's Mandrel Handle, and Sliding Tailstock Die and tap holders.

The time to make them is well spent

They save a lot of broken tools when taking a thread upto a shoulder or into a blind hole.

Howard

I am wondering if you grind all your dies specially. None of the ones I have will thread up to a shoulder, even if used from both directions, due to the lead/chamfer that is on them from the factory.

I've ground the backs on a few of mine particularly the ME ones so I can get close on things like steam fittings. Tend to use a DCGT insert to put a small radiused undercut as a face the shoulder. Ideally you want to do them on a surface grinder so the back remains true so it seats properly in a die holder.

I save breaking tools when screwcutting by cutting away from the chuck

In Martin Cleeve's little book Screwcutting in the Lathe (highly recommended) there is a discussion of how to pick up threads accurately by calculating the "synchronous distance". IIRC you can use this to just cut your thread in the usual way, disengaging the leadscrew at the end of the cut and winding the carriage back, and being able to find a position for starting the next pass which will re-engage with the thread exactly. I think it involves marking the spindle so you can put it in a known rotational position and possibly having a carriage "back stop". It seems it may not be practical for all threads but it might mean that you can avoid having to reverse the spindle in many cases.