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.

 

[Howard LewisParticipant @howardlewis46836]

No need to worry about gear tooth dimensions, particularly, as long as the DP or Module and Pressure Angle are the same for all the gears involved, in a simple train.

Basically, if you want to cut a thread, you need to compare the pitch required with that of the Leadscrew on the lathe, and set up a gear train with the required ratio.

If the lathe has a 8 tpi Leadscrew, and you want to cut a 16 tpi thread, the Leadscrew needs to rotate at half the speed of the chuck. So you set up say, a 20T Driver, an Idler, and put a 40T gear on the Leadscrew.

For a 12 tpi, you would put a 30T on the Leadscrew.

For 24 tpi you would put a 60T on the Leadscrew.

The idler is only there to “fill the gap” between the Driver and Driven, and to ensure that the Leadscrew moves the saddle TOWARDS the chuck, for a Right Hand Thread.

It does not affect the overall ratio.

If you wanted a Left Hand Thread, you would insert another Idler, so that Chuck and Leadscrew rotate in opposite directions.

For some pitches, you may need to make up a compound Idler to get the required ratio.

Things can get interesting if trying to cut a 19 tpi pipe thread, or a Metric thread on an Imperial machine. (For the latter, ideally a 127T gear gives an exact translation, but a 63T will suffice with a few slight errors, for most short threads

For threads, 3/8″ or 10 mm or smaller, or even 1/2″ or 12 mm, Taps and Dies will make life easier than calculating the gear train for one of the standard pitches.

For instance for a finer feed rate, rather than screwcutting, can be obtained by setting, 20T Driver, 80T / 20T Idler to drive another 80T on the Leadscrew.  This gives an overall reduction of 16:1, so that with a 8 tpi Leadscrew, the feed per rev of the chuck is 0.0078″/rev.

If you could introduce another 20T/60T Idler, and use the Tumbler Reverse, the feed rate would decrease to 0.0026″/rev, which should be fine enough for most purposes; especially if the tool is sharp and has a small radius on the nose.

HTH

Howard

 

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