Myford ML7 screw cutting limits

[Gareth Richards 1Participant @garethrichards1]

Hi,

just purchased my 1st lathe, a Myford ML7 .

I bought it mainly to learn how to do square cut threads ,but need to cut them at 6TPI.

However, the threading chart indicates it will only go a low as 8TPI.

Is this the final limit for this machine ,or is there an aftermarket gear available ?

Many Thanks

Gareth

[Gareth Richards 1Participant @garethrichards1]

[Brian WoodParticipant @brianwood45127]

Hello Gareth,

​Welcome to the Forum, I'm sure you will find all sorts of solutions here

​To answer this one, you can over drive the leadscrew in the ratio 4:3 to get a 6 tpi thread pitch. Go carefully as the normal recommendation is not to exceed the leadscrew pitch as that strains things, but with care you can get away with it

​The Mandrel gear should be 40 teeth, the leadscrew gear 30 teeth with suitable idlers to link them. Use the lowest back gear for powered cutting and take off only a few thou at a time per cut. Cutting fluid helps

Regards
Brian

Edited By Brian Wood on 13/07/2017 18:31:28

[Gareth Richards 1Participant @garethrichards1]

Hello Brian,

Thank you very much for your welcome .

Your information is a great relief and now look forward using your expert advice when I get started.

Regards

Gareth

[MeunierParticipant @meunier]

Hi Gareth, welcome,

Brian's expert advice is encapsulated in his recent book 'gearing of lathes for screwcutting'. I have ordered a copy since my screwcutting knowledge is minimal, and as an indirect thanks to the author for being a great source of info on this forum.

DaveD

[Gordon AParticipant @gordona]

Hi Gareth and welcome,

I have cut the equivalent of a 6 TPI worm gear on my ML7. You need to go very carefully as Brian said, I tried to take more than a couple of thou or so cut and ran into trouble. Luckily I had plastic tumbler gears fitted; needless to say the gears were destroyed. Go steady and you should be ok.

Gordon.

[BazyleParticipant @bazyle]

It is often better with such high leads to put a handle on the leadscrew and use that to turn the whole job so that the gear train is still overall a reduction.

Although you said it was your first lathe is it your first experience of using one? You might want to start by making a mandrel handle anyway rather than using the motor and cutting a more regular thread like 18tpi. To start screwcutting on a difficult job might not be such a good idea.

[Brian OldfordParticipant @brianoldford70365]

ICBW but I seem to recall the technique of driving the spindle from the lead-screw via the change-wheels was a suggested in the "words and music" for making the column of the Dore Westbury mill.

[Brian WoodParticipant @brianwood45127]

Brian,

​The Dore Westbury column is a very coarse pitch indeed at 3/4 inches and by any standards should be regarded as a special case for hobby sized lathes.

​I have successfully cut a 5 tpi square thread into half shaft steel without great drama using a Myford lathe, but I did first rough out a standard Vee form thread to reduce the material quantity somewhat, taking care to be sure the shoulders of the vee remained inside the material that would be cut away with the square form tool. It helps a great deal if you have cut a decent run out groove at the end of the thread as well.

You do need a sharp tool, cutting fluid and light cuts for success

Regards Brian

[BWParticipant @bw]

Hello,

I've never understood why cutting threads below "X" strains and hurts a lathe. Is it a bit like trying to drive a car uphill in the wrong gear ?

Bill

[KWILParticipant @kwil]

When you are attempting to cut threads coarser than the leadscrew you are trying turn the leadscrew faster, ie gearing UP, this is where the straining begins.

[Ian S CParticipant @iansc]

It's sort of like climbing a steep hill in top gear with the brakes on. If you are from NZ, think of Baldwin Street Dunedin, generally known as the steepest street in the world, I know it was hard work driving my Fiat 500 up it 50 years ago. The angle  on that one is 1:2.86, 19*, or 35%, what ever, it's a good workout, and course threads are a good workout on the lathe.

Ian S C

Edited By Ian S C on 23/07/2017 13:31:40

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