Myford ML7 metric threads.

I have an elderly Myford 7, (Serial No. K14509) but have never needed to do screw cutting, as I've always been able to use taps and dies. I decided to upgrade my skills and to create an 8M metric thread. I have the standard Imperial gear set, but found that I needed a 21-tooth wheel, which I had no trouble getting from Myford. Struggling a bit to build the gear train, and following the table for metric gears in the Myford 7 Handbook (page 27), I also looked at the plate attached to the lathe table. They don't match.
The handbook gives:

Pitch 1.25
/Driver 45
/1st stud driven 45, driver 21
2nd stud idle 50 wheel.
lead screw 50.

The plate gives:

Driver 20.
1st. stud Idle wheel.
2nd stud driven 60, driver 65.
Lead screw 55.

My lead screw is definitely Imperial, and it has a thread dial indicator to prove it!

Having seen the earlier correspondence in the Forum about Myford ml7 21tooth change gears, I wondered whether the second plate was for a Metric lead screw, since it doesn't mention any of the non-standard metric gears such as 21T?

I'd set up the first combo from the book and then measure with a dial indicator how far the carriage moves in one revolution of the chuck, just to make sure.

To add to your confusion ( ) Ian Bradley's ML7 manual lists for 1.25 pitch as follows:

45 -- 40/21 -- idle 50 wheel -- 60.

And Martin Cleeve (Screwcutting in the Lathe) lists as thus:

35 -- 50/45 -- 40/30 -- 60 to give an accuracy of 1 in 8,000, which ain't bad. (One thou in eight inches!)

Myford Mann,

The second arrangement with the 20 driver will give you a pitch of 1.25 mm. There are many mathematical arrangements of pitches, other writers will quote what they know.

Regards

Brian

In my copy of the Super 7 handbook the geartrain is as follows

Driver 45---driven 40----driver 21-----Idler----Leadscrew 60 which also gives a pitch of 1.25 mm

Brian

And out of curiosity I checked the result from the driver of 45.

That gives a pitch of 19 tpi [1.333 mm] so definitely something very odd about the entry you found in the Handbook

Brian

Thanks for your replies. I suppose that if I'd had the patience to learn how to work out the gearing (cf. Martin Cleave 'Screw Cutting in the Lathe' I could have come up with those answers. However, I took more notice of somebody else somewhere - I forget who, but very expert and respected, who advised to read all about it but then forget it and use the tables. All you really needed was awareness. Anyway, I'll poke around bearing your comments in mind. I'm certainly gaining awareness!

Myford Mann,

It really isn't rocket science.

The basic calculation is that Pitch P equals the ratio of gearing to the leadscrew input multiplied by the pitch of the leadscrew If a gearbox is a fitment then there is a further multiple by the selected gearbox ratio.

Taking the example that works in your case we have 20/60 times 65/55 times 3.175 equals 1.25075*** mm. It is not exact of course but the trivial error can be safely ignored

[3.175 is the pitch of an 8 tpi leadscrew in mm ]

Regards Brian

My apologies for not expressing my thanks sooner - other matters tend to get in the way! Anyway, as a result of your various bits of advice I have actually managed to cut a thread.

Whilst poking around, I came across references to mandrel handles, whereby one can turn the lathe by hand and gain increased control, especially when cutting to a butt, as I'm sure that everybody here but myself knew! I have ordered a Hemingway kit version, and wonder whether anybody has experience of that?

Regards,

Myford Mann.

Hi Myford Mann

Not too hard to make yourself...

Norman

Hello Myford Mann

Hemingway make decent kits, I'm sure it will do just what you want

A word of warning though. MOST EMPHATICALLY take precautions to either remove it after use or disable the motor power until you have done so

I very foolishly got sidetracked once and flashed up the lathe with my home made version in place and got away with it by using my panic rail across the front of the lathe that is my emergency stop; that was definitely not an occasion to reach over the lathe to the DOL starter. I know of another case which thrashed the change wheel cover to fragments before it could be stopped.

I now fit a cover to the ON button when using mine.

I'm glad to hear you triumphed with your metric thread in the end, I take it the nut fitted as well!

Regards

Brian

I have been re-reading this post as I need to cut a 1.25mm thread on my 1946 ML7.

The Plate on my ML7 gives 20, idle Wheel, 60, 65, 55.

What does it mean by "idle wheel"?

I have found 4 different Myford tables for screw cutting and all 4 have different gear ratios for 1.25 !

"Idle wheel" means you can use whatever gear you like. Number of teeth on it does not affect the overall ratio of the gear train. So you just pick one from the pile that looks like it will approximately fill the space then adjust the stud position or banjo so it meshes with the next gear if needed.

Cut away from the shoulder and out of the bore, saves all the mucking about with gadgets you just don't need :D

Cutting a right-hand thread away from the chuck entails running the lathe in reverse, so if using a screwed-mandrel lathe like the Myfords, beware of the chuck coming loose...