Hi All,

Just need a confirmation from the members here that what I have geared the plain lathe drop gears will produce the thread I expect.

A little background is that the lathe is a Raglan Little John. This has an 8 tpi leadscrew and in backgear this equates to a 16 tpi leadscrew. for those that do not know the Little John the drop gears are arranged as A/B x X/Y x C/D. A/B gears are fixed centres and must add to 90, likewise X/Y are also fixed centres and must add to 96, whilst C/D can be a floating arrangement able to accommodate a fairly wide range of gear combinations.

The X/Y pairing are listed as 48 x 48 for English threads and 52 x 44 for metric threads.

I am looking to cut a 1.66 p Schaublin buttress thread for a tool holder for my mill. Previously I have used a set up with a 51 tooth gear but for some reason the workshop gremlins have hidden it. In desperation I sat down and tried to formulate the gearing using standard gears. I came up with the following:-

A = 45, B = 45, ~ C = 45, D = 40, ~ X = 44, Y = 52 – in each case it is Driver/Driven. with compounding of B~X and Y~C

this gives the gear ratios as A/B = 1.0; C/D = 1.125; X/Y = 0.846154

this gives 15.2307 tpi, or 1.6676p

I arrived at this combination by juggling the gear set up for 18 tpi which was given as A/B = 45/45, C/D = 32/36 and with the X/Y meshing pair set for imperial at 48/48. by using the metric tooth counts at X/Y (52/44) BUT by reversing them I got the reduction necessary to give the final tpi/pitch required. Note that for reasons outside my control the 36/32 C/D pairing fouls the gear boss so I increased the tooth count to 45/40 to maintain the ratiometrics (9/8 as an improper fraction)

Will it work or have I missed something basic?

Thank you everyone who responded.

The confusing part about the leadscrew pitch is that it gives the formulae and finalises the tpi by a multiplication of 16, the tumbler gear assembly does give a 2:1 ratio as mentioned by Martin Connelly; this rather than my incorrect assumption that it was achieved via the back gear, which I accept increases torque available by slowing the spindle speed rather than affecting anything else (a Simpson moment! Doh!!).

As the job is mounted in a 4 jaw and not wishing to break it all down, having taken all bar the finishing cuts to clean, up I was loathe to break it down for a trial cut. Then it struck me that I could mount a 'Sharpie' fine point felt pen on the toolpost and 'scribe' a trial thread on the component. Sure enough the thread was wrong – I had transposed the C/D gear wheels and managed to get a finer pitch thread! Getting them the right way around gave the correct pitch.

Thank you again everyone who responded – it helps to be able to discuss issues like this when you are a one man in a shed with only those who know how to bake delicious cakes in the household!