change gear claculation

Charts with standard gear set for metric mini-lathes: 20, 20, 30, 35, 40, 40, 45, 50, 50, 60, 60, 80, 80.

When for B and C gears ANY is shown on tables, it means that  B and C gears are only necessary as a idler gear. On  these cases, use for B and C a pair of gears with the same teeth number and a suitable size to mesh with A and D.

To calculate change-gears combination for threading on any lathe you can use the following formula:  TPI = (B/A) x (D/C) x leadscrew pitch

Metric threading on lathes with metric 1.5mm pitch leadscrew

Simplified chart

For the mini-lathe banjo and with the original gear set. 

MM / PITCH	A	B	C	D	ERROR %
0.25	20	60	40	80	0.0000
0.3	40	50	20	80	0.0000
0.35	20	50	35	60	0.0000
0.4	20	60	40	50	0.0000
0.45	20	60	45	50	0.0000
0.5	40	60	30	60	0.0000
0.6	20	60	60	50	0.0000
0.7	35	60	40	50	0.0000
0.75	20	60	60	40	0.0000
0.8	40	60	40	50	0.0000
1	60	45	30	60	0.0000
1.25	40	60	50	40	0.0000
1.5	40	60	60	40	0.0000
1.75	35	60	60	30	0.0000
2	60	50	50	45	0.0000
2.5	50	60	60	30	0.0000
3	40	80	80	20	0.0000
3.5	35	60	80	20	0.0000
4	80	60	60	30	0.0000
4.5	60	80	80	20	0.0000
5	50	30	80	40	0.0000

Extended charts

Many of these combinations don't fit the mini-lathe banjo, though can be used with other banjo styles. 

MM / PITCH	A	B	C	D	ERROR %	TPI
0.25	20	45	30	80	0.0000	101.600
0.25	20	60	40	80	0.0000	101.600
0.25	20	80	40	60	0.0000	101.600
0.25	30	45	20	80	0.0000	101.600
0.25	40	60	20	80	0.0000	101.600

MM / PITCH	A	B	C	D	ERROR %	TPI
0.3	20	50	30	60	0.0000	84.667
0.3	20	50	40	80	0.0000	84.667
0.3	20	60	30	50	0.0000	84.667
0.3	20	80	40	50	0.0000	84.667
0.3	30	50	20	60	0.0000	84.667
0,3	30	60	20	50	0.0000	84.667
0,3	40	50	20	80	0.0000	84.667

MM / PITCH	A	B	C	D	ERROR %	TPI
0.35	20	50	35	60	0.0000	72.571
0.35	20	60	35	50	0.0000	72.571
0.35	35	50	20	60	0.0000	72.571
0.35	35	60	20	50	0.0000	72.571

MM / PITCH	A	B	C	D	ERROR %	TPI
0.4	20	50	40	60	0.0000	63.500
0.4	20	60	40	50	0.0000	63.500
0.4	40	50	20	60	0.0000	63.500
0.4	40	60	20	50	0.0000	63.500

MM / PITCH	A	B	C	D	ERROR %	TPI
0.45	20	50	45	60	0.0000	56.444
0.45	20	50	60	80	0.0000	56.444
0.45	20	60	45	50	0.0000	56.444
0.45	20	80	60	50	0.0000	56.444
0.45	30	50	40	80	0.0000	56.444
0.45	30	80	40	50	0.0000	56.444
0.45	40	50	30	80	0.0000	56.444
0.45	45	50	20	60	0.0000	56.444
0.45	45	60	20	50	0.0000	56.444
0.45	60	50	20	80	0.0000	56.444

MM / PITCH	A	B	C	D	ERROR %	TPI
0.5	20	ANY	ANY	60	0.0000	50.800
0.5	40	60	30	60	0.0000	50.800

MM / PITCH	A	B	C	D	ERROR %	TPI
0.6	20	ANY	ANY	50	0.0000	42.333

MM / PITCH	A	B	C	D	ERROR %	TPI
0.7	30	45	35	50	0.0000	36.286
0.7	30	50	35	45	0.0000	36.286
0.7	35	45	30	50	0.0000	36.286
0.7	35	50	30	45	0.0000	36.286
0.7	35	50	40	60	0.0000	36.286
0.7	35	60	40	50	0.0000	36.286
0.7	40	50	35	60	0.0000	36.286
0.7	40	60	35	50	0.0000	36.286

MM / PITCH	A	B	C	D	ERROR %	TPI
0.75	20	ANY	ANY	40	0.0000	33.867
0.75	30	ANY	ANY	60	0.0000	33.867
0.75	40	ANY	ANY	80	0.0000	33.867

MM / PITCH	A	B	C	D	ERROR %	TPI
0.8	20	45	60	50	0.0000	31.750
0.8	20	50	60	45	0.0000	31.750
0.8	20	50	80	60	0.0000	31.750
0.8	20	60	80	50	0.0000	31.750
0.8	30	45	40	50	0.0000	31.750
0.8	30	50	40	45	0.0000	31.750
0.8	40	45	30	50	0.0000	31.750
0.8	40	50	30	45	0.0000	31.750
0.8	40	50	40	60	0.0000	31.750
0.8	40	60	40	50	0.0000	31.750
0.8	60	45	20	50	0.0000	31.750
0.8	60	50	20	45	0.0000	31.750
0.8	80	50	20	60	0.0000	31.750
0.8	80	60	20	50	0.0000	31.750

MM / PITCH	A	B	C	D	ERROR %	TPI
1	20	ANY	ANY	30	0.0000	25.400
1	30	ANY	ANY	45	0.0000	25.400
1	40	ANY	ANY	60	0.0000	25.400
1	60	45	30	60	0.0000	25.400

MM / PITCH	A	B	C	D	ERROR %	TPI
1.25	50	ANY	ANY	60	0.0000	20.320
1.25	40	60	50	40	0.0000	20.320

MM / PITCH	A	B	C	D	ERROR %	TPI
1.5	20	ANY	ANY	20	0.0000	16.933
1.5	40	ANY	ANY	40	0.0000	16.933
1.5	80	ANY	ANY	80	0.0000	16.933

MM / PITCH	A	B	C	D	ERROR %	TPI
1.75	35	ANY	ANY	30	0.0000	14.514

MM / PITCH	A	B	C	D	ERROR %	TPI
2	40	ANY	ANY	30	0.0000	12.700
2	60	ANY	ANY	45	0.0000	12.700
2	80	ANY	ANY	60	0.0000	12.700

MM / PITCH	A	B	C	D	ERROR %	TPI
2.5	50	ANY	ANY	30	0.0000	10.160

MM / PITCH	A	B	C	D	ERROR %	TPI
3	40	ANY	ANY	20	0.0000	8.467
3	60	ANY	ANY	30	0.0000	8.467
3	80	ANY	ANY	40	0.0000	8.467

MM / PITCH	A	B	C	D	ERROR %	TPI
3.5	35	20	60	45	0.0000	7.257
3.5	35	20	80	60	0.0000	7.257
3.5	35	30	80	40	0.0000	7.257
3.5	35	40	80	30	0.0000	7.257
3.5	35	45	60	20	0.0000	7.257
3.5	35	60	80	20	0.0000	7.257
3.5	60	20	35	45	0.0000	7.257
3.5	60	45	35	20	0.0000	7.257
3.5	80	20	35	60	0.0000	7.257
3.5	80	30	35	40	0.0000	7.257
3.5	80	40	35	30	0.0000	7.257

MM / PITCH	A	B	C	D	ERROR %	TPI
4	80	ANY	ANY	30	0.0000	6.350

MM / PITCH	A	B	C	D	ERROR %	TPI
4.5	60	ANY	ANY	20	0.0000	5.644

MM / PITCH	A	B	C	D	ERROR %	TPI
5	50	20	60	45	0.0000	5.080
5	50	20	80	60	0.0000	5.080
5	50	30	80	40	0.0000	5.080
5	50	40	80	30	0.0000	5.080
5	50	45	60	20	0.0000	5.080
5	50	60	80	20	0.0000	5.080
5	60	20	50	45	0.0000	5.080
5	60	45	50	20	0.0000	5.080
5	80	20	50	60	0.0000	5.080
5	80	30	50	40	0.0000	5.080
5	80	40	50	30	0.0000	5.080
5	80	60	50	20	0.0000	5.080

Source :  http://www.toolsandmods.com/change-gears-charts-for-metric-threading.html