Jo,
I had not thought of a double thick gear but I think we can get very close with only cutting a 37/47 set of gears and making a single keyed bushing.
I took the compound set of gears out of the standard set I was using in NthreadsP. To keep the error small I used the 16/32 gear in four of the solutions. The 16/32 gear will change the effective pitch of the lead screw to 0.5 or 2. I just used the standard set and changed the pitch of the screw to 0.5 for the solutions. I did not find any use for the 20/56 gear.
Adding the 16/32 will work fine for the right hand threads but the left hand combination will need an idle gear which might be a problem.
To make sure we are on the same page the gear set I am using is the standard set with the auto-traverse gears and the screw cutting kit, 20,25,30,30,32,35,40,50,64,
37 & 47 and 16/32 and 20/56 compound gears.
I will check the combinations graphically and if there is a problem I can go back to NthreadsP for alternative combinations.
0BA Driven gears 30 Driver gears 30 Pitch 1 error % 0 (Simple)
1BA Driven gears 40 25 Driver gears 30 30 Pitch 0.9 error % 0
2BA Driven gears 20 Driver gears 25 Pitch 0.8 error % 0 (Simple)
3BA Driven gears 47 35 Driver gears 30 40 Pitch 0.729 error % 0.070
4BA Driven gears 35 40 Driver gears 25 37 Pitch 0.660 error % 0.108
5BA Driven gears 47 40 Driver gears 30 37 Pitch 0.590 error % 0.072
6BA Driven gears 35 40 Driver gears 20 37 Pitch 0.528 error % 0.270
7BA Driven gears 40 47 Driver gears 30 30 Pitch 0.478 error % 0.266
8BA Driven gears 64 47 Driver gears 35 37 Pitch 0.430 error % 0.120
9BA Driven gear 64 Driver gear 25 Pitch 0.390 error % 0.159 (Simple)
10BA Driven gears 40 50 Driver gears 20 35 Pitch 0.35 error % 0
11BA Driven gears 30 47 Driver gears 25 35 Pitch 0.310 error % 0.091 (use 16/32 also)
12BA Driven gears 40 50 Driver gears 32 35 Pitch 0.28 error % 0 (use 16/32 also)
13BA Driven gears 40 50 Driver gears 20 25 Pitch 0.25 error % 0
14BA Driven gears 37 47 Driver gears 25 32 Pitch 0.230 error % 0.007 (use 16/32 also)
15BA Driven gears 64 37 Driver gears 20 25 Pitch 0.211 error % 0.544
16BA Driven gears 64 37 Driver gears 30 30 Pitch 0.190 error % 0.017 (use 16/32 also)
26TPI Driven gears 32 40 Driver gears 25 50 TPI 26.00 error % 0.036
32TPI Driven gears 40 37 Driver gears 25 47 TPI 31.99 error % 0.021
40TPI Driven gears 40 37 Driver gears 20 47 TPI 39.99 error % 0.021
60TPI Driven gears 35 50 Driver gears 20 37 TPI 60.06 error % 0.112
Dan