Can someone remind me how to cut a 1.5mm thread using an 8tpi lead
screw. I don't have a metric conversion gear, but I have the normal
steps of 5 up to 65 including many duplicates and a couple of 38 gears
(which I have never used so maybe this is what they are for). I think
its about 16.9tpi, so maybe 17tpi is good enough - but thats not in my
thread chart either.
Well if it helps at all (maybe you already know) the spindle needs to
turn 2.1167 times faster then the lead screw.
0.125 / 0.059055 = 2.1167
So just need to find a ratio close to that.
An 80 tooth on the screw with a 38 tooth on the spindle with direct
gearing would give you a ratio of 2.1053 for a lead of 0.0594" or
1.508mm should be close enough for most things.
I hope I got that right.
You don't say what lathe in particular, but if you can get three ratios
in there, you would want 38/45 40/65 50/55 which gives 1.4999223mm with
If you are restricted to two ratios then you want 65/50 20/55 for
1.500901mm, these are the same numbers posted by John Q.
I've written a piece of software that allows you to play some tunes on
the different division rations, you can download it at
from Duncan Munro
. . . . and an excellent piece of work it is too! . . . even though I
have a 127 tooth wheel it has got me out of 'centre-distance' problems
many times . . . if you have an extended set of wheels it will take
account of those as well.
The lathe is a 1950s Winfield, it has a single banjo slot rather than
the forked type you find on the Myfords (I bought a Myford banjo, but
it is a completely different fitting, and have put it to one side for
the moment). I vaguely remember looking at your excellent software,
and I think I concluded that it was a bit Myford specific, or maybe it
assumed I had gears that I didn't. That may have been a bit hasty on
my part - although it can't give the gear layout, it clearly can give
Can it be used for a single banjo like I have ? If the software allows
you to insist that all the gears are on the single long slot, then
that would work be perfect as the lengths are almost identical - its 4
3/4 inches between driver and leadscrew ! I sometimes have a real
struggle getting the gear train right - and I am just off to the
workshop to see how I can fit the ratios you have offered. I need to
turn up a radiator plug to get my Land Rover on the road.
Hi Steve, it was build around the forked style of banjo rather than a
parallel double or single.
However... I'll see if it's possible to fool the software into using a
single, if not I can add an option to take the Myfordesque checks out so
it will just concentrate on ratios rather than "will they fit".
It proved impossible to get the large gear combination on the single
slot of the Winfield, but I downloaded your excellent program and was
able to find 38/65 20/25 which I could manage and was (had to be) good
enough for the task in hand (1.485mm instead of 1.5mm). Job done.
I'll think again about modifying the Myford banjo to fit, but I should
be OK for now as I normally only have to cut imperial threads, and in
a month or two my Cromwell lathe will be up and running so I will have
a full thread cutting gearbox - at last.
On the ML7, you get 2 x 20t gears as standard, and also
a 38t, which foxes many people.
The 38t is your best friend when it comes to metric pitches.
0.6, 0.7, 0.8, 1.0, 1.25 and 1.5 pitches can all be cut
accurately with errors better than 0.02 leadscrew revolutions
using the 38t somewhere in the chain.
If anyone wants the chart I have prepared with all the
ratios and pitches, just PM or email me.
A 100/127 gear will work the magic on any Imperial lathe that can accept
it, because 127 is half of 254, and there are 25.4mm in 1". The trouble
is that some smaller lathes simply don't have sufficient space for such
a large gear. Unfortunately, 127 is a prime number, so anything smaller
has to be a compromise between accuracy and usability. A 38t gear does