ML4 screw cutting change wheels

[Paul BarterParticipant @paulbarter66156]

Hello all I need some assistance and clarification please regarding gear train arrays for screw cutting. My ML4 has a 20 tpi gear on the mandrel that looks permanently fixed,with another 20 tpi gear on the output of the tumbler assy I need to cut an 8tpi thread. Westburys helpful book gives a gear train for 8 tpi leadscrew Myford lathes as the first gear of 35 tpi, does this take account of the 20 tpi mandrel gear or do I need to find two 35 tpi gears for the spindle and out of the tumbler set? the array quoted is 35tpi "driver ",20 driven,40 driver,idle wheel 70 leadscrew. Any advice will be most welcome thanks in advance for any assistance.

[Paul BarterParticipant @paulbarter66156]

Advice please for screw cutting change wheel array

[Les Jones 1Participant @lesjones1]

If you have an 8tpi leadscrew and want to cut an 8tpi thread then the gearing just needs to give a 1:1 ratio between the headstock spindle and the leadscrew . I have no idea with tpi numbers in relation to gears. If you can't work out haw to get a 1:1 ratio yourself then list the number of teeth on all the change gears you have and the number of teeth on the gear on the end of the headstock spindle. If there is any other gearing that you can't change then post a sketch of it with the number of teeth on all of these gears. Or a photograph of the change wheel area of your lathe.

Les.

[Paul BarterParticipant @paulbarter66156]

hello Les, Thank you for your swift response, there is a 20 tpi gear on the spindle and a 20 tpi gear on the tumbler assy output. I have the following change wheels:

20 tpi x 3, and one off of 30,35,40,45,50,55,60,65 tpi gears. what gear train would you suggest for the i:1 relationship to the leadscrew? Thanks again for your help

best regards Paul

[Paul BarterParticipant @paulbarter66156]

Hello again Les to spare your patience, do I simply need to arrange a set of meshing wheels between the 20 tpi tumbler output and a 20tpi gear on the leadscrew?

Paul

[BazyleParticipant @bazyle]

You have two options.

If you mount 20 gears on the tumbler reverse and the leadscrewas you suggest you can then bridge the gap between them. You will probably find one gear doesn't reach though. So you would need two studs and two gears. these intermediate ones are set as one gear per stud and mesh with each other. You do not put both on one stud and it doesn't matter what the tooth count is just that they fit.

Option two would be more normal. Generally you use larger wheels where possible to reduce the stress and make things smoother as a 20 on the leadscrew is not too good for that. So first you do a speed reduction with 20 on tumbler and 40 on a stud. then you need to speed it up again so have to use a 60 on the same stud ie connected to your 40 and on the outside of the pair, and a 30 on the leadscrew.

[BazyleParticipant @bazyle]

By the way double check that your leadscrew is 8tpi and not 10tpi. I'm not familiar with the ML4.

[Paul BarterParticipant @paulbarter66156]

Bazyle, Thank you your most clear explanation, I am most grateful, the scales have been lifted from my eyes, until the next time.

best regards and thanks again

Paul

[Les Jones 1Participant @lesjones1]

Can anyone explain the significance of "tpi" in respect to a gear wheel ?

Les.

[Brian WoodParticipant @brianwood45127]

Just to clarify the point Bazyle raised. The Myford ML 4 most defintely was fitted with an 8 tpi leadscrew,

The feedscrews for cross slide and top slide were both 12 tpi with collars marked for 80 divisions which gives a movement error of 4 thou in one inch. The handbook that came with the lathe rather charmingly says that for most work that degree of error can be ignored!

Regards

Brian

[Paul BarterParticipant @paulbarter66156]

Thanks to all of you for taking the trouble to enlighten me, my apologies to Les for using the wrong term to describe gear wheels, got confused thinking about threads.

Paul

[Les Jones 1Participant @lesjones1]

Thanks Paul,
Am I correct in assuming the number was the number of teeth on the gear or was it the number of TPI on a chart and the gear was one of the gears in the chain ?

Les.

[Paul BarterParticipant @paulbarter66156]

Hello Les, Yes I was referring to the number of teeth on the gear wheels quoted in the Westbury chart and the number of teeth on the gear wheels I have. What confused me also was the spindle gear appears to be fixed with a grub screw between the gear bore and the spindle and thus not readily exchangeable.The 35 tooth wheel must be the start of the chain quoted by Westbury, not the 20 tooth wheel fitted to cut an 8tpi thread with an 8tpi leadscrew.

regards and thanks Paul

[Brian WoodParticipant @brianwood45127]

Hello again Paul

This is the thread chart in the Myford handbook.

Your 20T gear pinned by the grub screw to the lathe spindle drives another 20T gear on a short stud; which is the one listed as the Mandrel via the reversing cluster of Tufnol gears so that it is driven at spindle speed. The gears you fit start from there.

Your collection of gears is missing a 25 and 38 gear to give you all the ratios the table lists.

Regards

Brian

[Paul BarterParticipant @paulbarter66156]

Hello Brian, Thank you very much for that chart now printed for close scrutiny, and for clearing up my confusion as to where the sequence starts,it has taken a while for it to sink in that the tumbler set reproduces the spindle revs but allows for reversing at the same speed.All this is so I can cut a trial 8tpi square thread on a dummy spindle for the Hobson H7 lathe I have refurbished, hopefully I can summon up the courage to cut the internal thread on the backplate I have machined to size!

Thank you again

best regards Paul

[Brian WoodParticipant @brianwood45127]

Hello Paul,

Only too glad to help, I was an owner after my father died for about 20 years which gave me a familiarisation time from when he bought it in 1945 of about 40 years.

The thing to bear in mind with gearing is that a simple chain of gears, all in line with each other, is that the ratio is determined by the first and last gear in the chain; just as would be the case of a pulley system with the belt linking the two pulleys. The 'belt' in that case is idler gears that merely provide the link and can be any size for convenience.

Compound gearing however, examples are in the fine feed and metric table I sent, alter the overall gearing ratio.

Get a pencil and paper and lay out such a chain with the driver wheels on the top line, divided by driven wheels on the bottom line, multiply the result by 1divided by 8 [the leadscrew tpi] for imperial or 1 divided by 8 times 25.4 for metric to get the resulting pitch of the thread produced.

Returning to your original question, cutting an 8 tpi thread. Before you actually cut metal I have always found it useful to do an 'air-cut' first just to be sure you are following the expected track and that all the subsequent cuts to depth are in phase.

Please get in touch if I can be of further help

Brian

[Author]

Posts