Granville lathe leadscrew change wheel

[cogdobblerParticipant @cogdobbler]

But all those combinations require two oddball gears. 63/80 only one, and that is available for some lathes as part of the standard set.
<p style=”text-align: left;”>The Myford ML7 uses an oddball 21T for metric conversion as standard kit. Not sure of the math there from memory, although 21 is a third of 63, so could be run with a fellow 20 and 60 gears to triple its value possibly.</p>

[cogdobblerParticipant @cogdobbler]

Here is a handy chart showing how to gear to cut metric threads on any lathe with an 8TPI leadscrew, eg Myford, Granville etc. All using the standard gear set “by fives” and the standard 38T used for cutting pipe threads. As the third column heading shows, error rate is usually between 1 in 3000 and 1 in 8000 or so. IE, one thou error over 3 to 8 inches. Close enough for anything outside aerospace, especially on 50-year-old mass produced hobby-grade leadscrews!

Disregard the final column “Translator”. It refers to the math used to calculate the gears in the chart and has no practical bearing for the end user. Just set up the gear train as shown in the column “Quadrant Gearing” and carry on (metric) screwcutting. I have used this method for years on two different lathes with wholly satisfactory results.

 

[duncan webster 1]

On duncan webster 1 Said:

… also how to measure PA

This rather cluttered illustration might help, Duncan

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Easiest if you get some tracing-paper, or re-work it in CAD

MichaelG.

[Martin of WickParticipant @martinofwick]

…..but can anyone explain how the necessary compounds are calculated?

Strictly for the ‘die-hards’ amongst us, life is too short, I just use this…

Change gear calculator

simple and to the point (Other calculator sites are available).

Usual caveats apply – and not all combinations may ‘fit’ your lathe.

[cogdobblerParticipant @cogdobbler]

Yes, I have not calculated change gears from first principles since sweating over final trade school exams as an apprentice. Out in the real world there was always a chart on the lathe, or in the lid of some old boy’s toolbox, Machinery’s Handbook or these days online calculators.

 

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