Myford ml7 21 tooth change gears

[Jon CameronParticipant @joncameron26580]

For reference my lathe has a 20 tooth gear on the spindle and the available gears are, 20, 20, 25, 30, 40, 50, 55,, 60, 65, 65, 95, not including the back gears, or the two gears on a ML7 tumbler that I have.

[Nick HulmeParticipant @nickhulme30114]

I don't "approximate" threads, if it's right it's right, if not then it's wrong.

I know lots of people are happy with a very close approximation, God bless them for settling! 

Where else will they "Settle" ? 

Edited By Nick Hulme on 02/02/2018 01:28:48

[HopperParticipant @hopper]

Posted by Jon Cameron on 02/02/2018 01:06:12:

For reference my lathe has a 20 tooth gear on the spindle and the available gears are, 20, 20, 25, 30, 40, 50, 55,, 60, 65, 65, 95, not including the back gears, or the two gears on a ML7 tumbler that I have.

My same-era Drummond M-type has the same gears as that, but no 95T. It will cut all metric threads using those standard gears, set up as compound gearing. My standard reference is the the charts in Martin Cleeve's book "Screwcutting in the Lathe". He gives all the needed combinations to cut metric threads with 8tpi leadscrew lathes, within about one thou per eight inches or so!

I think Rod's chart above and Michael's linked calculator will give the same result.

[HopperParticipant @hopper]

Posted by Nick Hulme on 02/02/2018 01:27:46:

I don't "approximate" threads, if it's right it's right, if not then it's wrong.

I know lots of people are happy with a very close approximation, God bless them for settling!

Where else will they "Settle" ?

Edited By Nick Hulme on 02/02/2018 01:28:48

Sounds like what Martin Cleeve in his book "Screwcutting in the Lathe" refers to as the enthusiams of "overzealous amateurs".

In the real world, his "approximation" gearing is accurate to one thou in several inches. Mostly per eight inches or so, in other cases three inches.

How accurate is your lathe's leadscrew? Six thou per inch variation is not unheard of. Using "perfect gearing" on such a home hobby low-cost lathe (whether Myord etc or Chinese made) is perhaps something like waxing a dirt floor. The "approximate" gearing works just fine for anything one is likely to do at home, or in the average commercial machine shop for that matter.

[Nick HulmeParticipant @nickhulme30114]

Posted by Hopper on 02/02/2018 05:23:03:

Sounds like what Martin Cleeve in his book "Screwcutting in the Lathe" refers to as the enthusiams of "overzealous amateurs".

In the real world, his "approximation" gearing is accurate to one thou in several inches. Mostly per eight inches or so, in other cases three inches.

How accurate is your lathe's leadscrew? Six thou per inch variation is not unheard of. Using "perfect gearing" on such a home hobby low-cost lathe (whether Myord etc or Chinese made) is perhaps something like waxing a dirt floor. The "approximate" gearing works just fine for anything one is likely to do at home, or in the average commercial machine shop for that matter.

I have the gears for a gear train that gives me the option to work without gearing errors in metric threads, and it's quicker to set up than the approximation options ;D

Do you introduce errors if you have the option not to?

[Neil WyattModerator @neilwyatt]

Posted by Nick Hulme on 02/02/2018 01:27:46:

I don't "approximate" threads, if it's right it's right, if not then it's wrong.

I know lots of people are happy with a very close approximation, God bless them for settling!

Where else will they "Settle" ?

Edited By Nick Hulme on 02/02/2018 01:28:48

The biggest fault in production engineering is to work to a greater tolerance than the job demands.

Working to higher than required precision is a luxury hobbyists can enjoy, but they shouldn't be scorned if they choose not too.

Neil

[Nick HulmeParticipant @nickhulme30114]

Posted by Neil Wyatt on 02/02/2018 18:00:33:

Working to higher than required precision is a luxury hobbyists can enjoy, but they shouldn't be scorned if they choose not too.

Neil

Neil,
It's usually the other way around as you see above where those who prefer working to as close a tolerance as possible are pilloried for it

[Mike PooleParticipant @mikepoole82104]

Unless you have some very good measuring equipment I doubt the threads produced by the approximations would trouble most people most of the time. It would be interesting to measure how good the threads are produced by a theoretically perfect setup.

Mike

[Jon CameronParticipant @joncameron26580]

Thank you for the replies, for my accuracy it will be if it works it' close enough, if it dont then it' back to the drawing board. I'l never be able to get accuracy down to microns on an ML4 but if I can get it to under a thou accuracy on any given part then that will be good enough for me. Others might disagree but most drawings are only to +/-0.001" tolerance, others 5thou. So for most application I should be set.

[HopperParticipant @hopper]

Posted by Nick Hulme on 02/02/2018 16:27:02:

Posted by Hopper on 02/02/2018 05:23:03:

Sounds like what Martin Cleeve in his book "Screwcutting in the Lathe" refers to as the enthusiams of "overzealous amateurs".

In the real world, his "approximation" gearing is accurate to one thou in several inches. Mostly per eight inches or so, in other cases three inches.

How accurate is your lathe's leadscrew? Six thou per inch variation is not unheard of. Using "perfect gearing" on such a home hobby low-cost lathe (whether Myord etc or Chinese made) is perhaps something like waxing a dirt floor. The "approximate" gearing works just fine for anything one is likely to do at home, or in the average commercial machine shop for that matter.

I have the gears for a gear train that gives me the option to work without gearing errors in metric threads, and it's quicker to set up than the approximation options ;D

Do you introduce errors if you have the option not to?

Ah, well if one has the luxury of the correct gears, then of course they are the ones to use. I lie in bed at night and dream of such luxury!

[HopperParticipant @hopper]

Posted by Jon Cameron on 02/02/2018 21:33:17:

Thank you for the replies, for my accuracy it will be if it works it' close enough, if it dont then it' back to the drawing board. I'l never be able to get accuracy down to microns on an ML4 but if I can get it to under a thou accuracy on any given part then that will be good enough for me. Others might disagree but most drawings are only to +/-0.001" tolerance, others 5thou. So for most application I should be set.

And your threads may well still be more accurate than mass produced hardware store nuts and bolts that seem to work perfectly well. Have fun with that old ML4. There is a perverse pleasure that comes from doing accurate work on such old machinery.

[Mike KavanaghParticipant @mikekavanagh49516]

Hi all, stupid question but I've just bought the 34/33 teeth gears to cut metric on my imperial lathe. I'm presuming you have to leave the half nuts engages and run the saddle backwards after each cut?

[Neil WyattModerator @neilwyatt]

Posted by Nick Hulme on 02/02/2018 18:03:56:

Posted by Neil Wyatt on 02/02/2018 18:00:33:

Working to higher than required precision is a luxury hobbyists can enjoy, but they shouldn't be scorned if they choose not too.

Neil

Neil,
It's usually the other way around as you see above where those who prefer working to as close a tolerance as possible are pilloried for it

Well perhaps we should respect personal choice either way.

Neil

[Neil WyattModerator @neilwyatt]

Posted by Mike Kavanagh on 09/11/2020 08:38:29:

Hi all, stupid question but I've just bought the 34/33 teeth gears to cut metric on my imperial lathe. I'm presuming you have to leave the half nuts engages and run the saddle backwards after each cut?

That's the usual approach.

It's possible to fit a device that declutches the leadscrew while ensuring relative alignment is kept, but that's a choice for those doing many threads.

Neil

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