Imperial Thread Cutting on a Metric Warco WM250V : Some Questions

A quick intro : for the last 40 years, I've been using my WW2 Lend-Lease Sheldon (Chicago) 10" lathe to restore pre-war British motorcycles. A couple of weeks ago, I bought a new metric WM250V and sold the Sheldon to a friend. Time will tell whether I'll regret this......

Got a few questions on the WM250V if that's ok? BTW: I've already read many of the excellent threads on this subject and have downloaded NthreadsP and PICCLOCK's "WM250 Change Gear Calculator" but my favourite is this online tool, **LINK**

1) The marketing blurb said the range of imperial thread is 8 to 56 and I foolishly forgot to see what actual threads fell within that range. Initially, I was a little alarmed not to see 26tpi as one of them in the gear change tables in the manual or on the front of the machine but Googling led me here and all is well. My first question concerns the recommended (as in the facia table) change gears for 20tpi which are stated to be 40 (spindle), any idler, 80-75, 60 (leadscrew). All the above software programs show this combination to have an error of 1.575% and yet another combination for 20tpi of 40 (spindle), any idler, 70-50, 45 (leadscrew) only has an error of 0.012%. What am I missing here? Is it that some combination won't physically fit on the machine? I've now created my own table covering all TPIs between 8 to 40 focusing on the least error and using the minimal leadscrew gearbox on the 250 (A 1:1, B 2:1 and C 0.5:1), this range can be expanded to 4-80.

2) Second question concerns the engagement of the horizontal leadscrew. The 250V introduced power cross feed but this level can also engage the horizontal feed but at half the speed (I think) to the main lever. Is there any reason why I should not use this new lever for thread cutting? If not, it means I can set up the change gears (for example) for 20tpi on the A (1:1) leadscrew gearbox setting and cover 10tpi on B (2:1), 40tpi on C (0.5:1) and then 80tpi if I use this new lever for leadscrew engagement?

3) I have no idea how to use the thread dial indicator when cutting imperial threads on a metric lathe...can anyone advise please? In the meantime, I'll play safe and not disengage the leadscrew....

4) Why does the leadscrew have a groove running down its length?

Man thanks for your help and advise

Hello Paul,

Do you know what pitch the leadscrew is please? That will help enormously in checking your screwcutting results

Item 4 The groove down the leadscrew is to provide power take off for powered cross feed

Regards Brian [ An imperial Myford man]

Thanks for the reply Brian. It has a 2mm pitch.

Afaik you can't use thread dials when cutting imperial on a metric (or visa versa). Personally I've never bothered with them for metric.

I think this too. I've been using a Metric WM250V for about 8 years now, and the screwcutting I've done has been mostly threads for railway parts where the Imperial pitches I remember have been 11, 14 and 19 TPI. As far as I remember I couldn't find another lathe in their workshop that was listed on its plate to do 19 TPI.

Frankly, I think the threading dial Warco have put on there is probably just to satisfy a marketing requirement that there be one visible to the punter as they make up their mind to buy, irrespective of whether it's any good, which I suspect it isn't. It's a helluva lot safer and simpler to just keep the nut engaged and reverse back, and unless you're doing a long thread for a time-deadline, that's not much of a problem.

I have generally a pretty high opinion of the 250V. It doesn't have the power of the industrial lathes I used for my daily bread in the 70s, but OTOH that's also saved me a few broken tools. It's quiet, smooth, versatile and reliable - the only problems I've had have been of my own making, and fixable.

Thank you for the links to off-table pitch gear calculation routines - I haven't come across requirements to do any such yet, but of course it's possible I may.

Hello Paul,

Now that I know the leadscrew pitch [thank you] I can confirm that the second gearing choice you listed will indeed produce a 20 tpi thread of 20.0025 tpi, close enough for most purposes I think. The version of 40/80 as the first reduction stage will produce a 20 tpi thread if followed by gearing of 47/37, hardly handy!

40--idler--63 on the leadscrew works well [error 0.003 tpi ] and with a 63T gear you can get most imperial pitches. My book on lathe gearing covers lathes with 2 mm pitch leadscrews. Available through Google with change from £15 it might solve several problems for you. [Gearing of Lathes for Screwcutting] ISBN 978-1-78500-250-2

Regards Brian

The "groove" along the Leadscrew is a keyway that provides rthe drive for the Poawer cross Feed

If there is a rack fixed to the lathe bed, the keyway will also provide the drive to the piunion that engages with that for the longitudinal feed

In which case, there will be laver to engage either the power feed OR to close the half nuts onto the Leadscrew for screwcutting.

To cut Imperial threads on a lathe with a Metric Leadscrew, there will be na 127T gear in the train for a perfect translation, or a m63T which will be slightly inaccurate in many instances; but then eroor will actually have minimal meffect in many cases.

If in doubt, buy and study Brain Wood's book "Gearing of Lathes for Scxrewcutting) That will contain variious changewheel set ups, and the error rhat they may intruduce.

HTH

Howard

I have just started cutting metric/imperial threads on my Warco GH1322, and found the above by Fulmen to be absolutely correct.
I find leaving the half nut engaged and using reverse works just fine but it does require a greater level of concentration, for me at least.

Thanks Mick....that's good to know about the 250V. I loved my 80 year old 10" Sheldon but it lacked many features that a modern lathe has like electronic speed control, reverse motor, power cross feed.

I put the following table together using data from the bilar website link posted above, so I'm not responsible for the calculations but having checked a few of them with other software programs that do these calculations, they match. I'll be using this table rather than anything in the manual or on the front of the lathe. I have it as a PDF but couldn't find a way of posting a PDF file on this Forum

Well, thanks again for that table, Paul - I've saved it and printed a copy to put in my manual.

I have found that threads I've cut as above - using the gear settings in the manual - have fitted existing components quite nicely, but I'll certainly use your table if I come across pitches not in the manual.

There are two shearpin issues I've come across, both resulting from errors of mine, that it's as well to be aware of:-

a) The leadscrew shearpin on mine (just right of the sleeved flange where the leadscrew drive comes out of the headstock) was made of not-very-mild steel, and failed to shear when I engaged the wrong feed by mistake, resulting in a bend in the apron pinion (or its shaft) that drives the saddle feed. I was able to straighten it, but lived with tight spots for over a year before I found a way to do that. I then replaced it with brass.

I had a chance to examine and test the very similar (and more recent) Chester DB10VS a few months back, and it looked as if it had a brass shearpin in the analogous position, so the factory where both machines probably originate may have done a change here.

b) If the crossslide leadscrew shearpin (which is brass) breaks, it's likely to fall into the apron gearbox, and you'll likely need to take that and the longitudinal leadscrew off to get out the broken bit - which in my case jammed the gearbox most perfectly!

Both of these issues could be fixed with no lasting damage once understood.

Time will tell whether I'll regret this......

Still undecided? Please let us know when you do, and why.