On Mon, 02 Mar 2020 12:26:59 -0800, Stephen Dixon wrote:

Hi, Ive just been looking at the questions above and notice someone may
have gear chart for screw cutting imperial & metric. Any chance you
could send it to me or point me in the right direction. Thanks Ste

As Jim Wilkins mentioned, you can make a spreadsheet. Or, look online
for program CHANGE by Marvin Klotz, and adaptations of it by others, eg
David Forsyth. It is worth looking those up so you can read Marvin's
comments about driving and driven gears, putting gears on banjo, etc.

Eg: "Sets of gears need only be placed with 'drivers' driving and driven
gears being driven. The actual order is not important, just select
pairs so that they fit on the banjo. So long as the correct ones are
driving, the ratio will be correct. Always cross check by threading
(just a surface scratch is needed) some scrap and measuring the pitch
if at all possible."

In pictures of Tyzack lathes, it looks like some models have four
change gears for setting the reduction ratio, so the program mentioned
above is applicable. Below is a somewhat simpler program, written in
Python 3. It accepts either an inch thread count or a metric thread
pitch (in micrometers), with numbers below 150 considered tpi and 150
or more as um. For example, the command ./gearset.py 1000
looks for gear sets to make a 1 mm pitch (1000 um) and after a few
milliseconds prints out:

Target is 25.4 tpi = 0.0394" = 1.0 mm
tpi: 25.422 = 0.9991 mm ABCD: 45 55 50 65
tpi: 25.455 = 0.9979 mm ABCD: 20 35 55 50
tpi: 25.333 = 1.0026 mm ABCD: 40 50 45 57
tpi: 25.319 = 1.0032 mm ABCD: 35 45 65 80

From that output, you would pick out which set you like best (or have).
You can modify the gears list in the second line of the program
to match whatever set of gears you actually have. The list there is
similar to the set that comes with mini-lathes from HF etc. (Note,
to look at the program, turn off line wrapping in your newsreader, or
in an editor remove extra line wraps and restore any that are missing.
Python syntax is sensitive to leading spaces.)

#!/usr/bin/env python3
gears = sorted([20,20,35,40,45,50,55,57,60,65,80,80])
from sys import argv; from itertools import combinations
rn = 0; pre = (-1,-1); candi = []
# Metric thread pitch gets entered times 1000, eg 500 means 0.5 mm
rn+=1; tpi = float(argv[rn]) if len(argv)rn else 16 # Desired threads
per inch
rn+=1; eps = float(argv[rn]) if len(argv)rn else 1 # Allowed error in
%
rn+=1; Lead = float(argv[rn]) if len(argv)rn else 16 # Effective Lead
Screw ratio
target = tpi if tpi150 else 25400/tpi
ftpi, itpi, mtpm = round(target,4), round(1/target,4), round(25.4/target,
4)
print (f'Target is {ftpi} tpi = {itpi}" = {mtpm} mm')
errmax = target*eps/100 # Convert % error to tpi error
for gdex in combinations(range(len(gears)),4):
gset = [gears[x] for x in gdex]
for b, d in combinations(gset,2):
b, d = min(b,d), max(b,d)
gtwo = gset[0:]; gtwo.remove(b); gtwo.remove(d)
a, c = gtwo; a, c = min(a,c), max(a,c)
t = Lead*(b/a)*(d/c); err = abs(t-target)
if errerrmax: candi.append((err, t, a,b,c,d))
for item in sorted(candi): # Remove duplicate candidates
if item==p candi.remove(item)
pre = item
for err,t,a,b,c,d in sorted(candi)[:4]: # Print best 4 candidates, if
any
print (f' tpi: {t:0.3f} = {25.4/t:0.4f} mm ABCD: {a:3} {b:3} {c:3}
{d:3}')



--
jiw