I have the metric version of the Warco Super Mini Lathe but needed to cut an imperial thread for a project. The lathe’s manual includes a table of gear combinations for cutting imperial threads on the metric machine. However, it specifies 55 and 65 tooth gears, which the lathe doesn’t come with.
I wanted to see if there are combinations of the gears that the lathe does come with that could approximate imperial thread pitches. I wrote a small program which tries all the gear combinations, discarding any which are infeasible, to find the closest match. It turns out that it is possible to get quite close using the supplied gears.
There are 4 gears arranged as follows:
Since the lathe has a 1.5mm pitch leadscrew, a combination (A, B, C, D) of gears produces a thread pitch of: 1.5mm × AC / BD
The centre point of gears B & C is adjusted so that all the gears mesh. Its position can be calculated given the gear pitch circle radii. There may be no solution which means the gear combination is infeasible. We also need to ensure that gear C doesn’t collide with A’s shaft and gear B doesn’t collide with D’s shaft. Finally, the B & C centre point must lie in the adjustable region of the adjustment bracket which pivots around D - which can be characterised in terms of the min and max angle and min and max radius with respect to D.
I needed to cut a 20 threads-per-inch (tpi) thread. The program gives this gear combination: A=60, B=40, C=45, D=80, which has a 0.35% error. I was able to successfully setup the gears in this combination and cut the thread and it worked fine for my purposes (a camera mount).
I’ve included a table of the program output at the end of this post. I have not physically tested any combinations other than 20tpi so cannot be sure that they will all work in practice.
It’s possible to 3d print 55 and 65 tooth gears which reduce the errors. While not as robust and accurate as metal gears they do seem to work fine. I also replaced the 20 tooth metal gear A for the power feed with a 3d printed one so that if I crash the lathe, hopefully it’ll destroy the plastic gear first instead of a more important part. Here are STL files for suitable gears: 20, 55, and 65. Here are STL files for holders to organise the gears: small, large.
| TPI | Pitch (mm) | Gears [A B C D] | Actual Pitch (mm) | Error (mm) | Error (%) |
| 6 | 4.2333 | [80 50 80 45] | 4.2667 | +0.0333 | +0.79% |
| 7 | 3.6286 | [80 50 60 40] | 3.6000 | -0.0286 | -0.79% |
| 8 | 3.1750 | [80 30 40 50] | 3.2000 | +0.0250 | +0.79% |
| 9 | 2.8222 | [60 40 50 40] | 2.8125 | -0.0097 | -0.34% |
| 10 | 2.5400 | [80 35 45 60] | 2.5714 | +0.0314 | +1.24% |
| 11 | 2.3091 | [60 35 45 50] | 2.3143 | +0.0052 | +0.22% |
| 12 | 2.1167 | [80 45 40 50] | 2.1333 | +0.0167 | +0.79% |
| 13 | 1.9538 | [45 80 80 35] | 1.9286 | -0.0253 | -1.29% |
| 14 | 1.8143 | [80 50 45 60] | 1.8000 | -0.0143 | -0.79% |
| 15 | 1.6933 | [50 40 45 50] | 1.6875 | -0.0058 | -0.34% |
| 16 | 1.5875 | [40 60 80 50] | 1.6000 | +0.0125 | +0.79% |
| 17 | 1.4941 | [40 80 80 40] | 1.5000 | +0.0059 | +0.39% |
| 18 | 1.4111 | [60 40 50 80] | 1.4062 | -0.0049 | -0.34% |
| 19 | 1.3368 | [50 45 40 50] | 1.3333 | -0.0035 | -0.26% |
| 20 | 1.2700 | [60 40 45 80] | 1.2656 | -0.0044 | -0.34% |
| 21 | 1.2095 | [40 60 60 50] | 1.2000 | -0.0095 | -0.79% |
| 22 | 1.1545 | [80 45 35 80] | 1.1667 | +0.0121 | +1.05% |
| 23 | 1.1043 | [50 45 40 60] | 1.1111 | +0.0068 | +0.61% |
| 24 | 1.0583 | [60 50 35 60] | 1.0500 | -0.0083 | -0.79% |
| 25 | 1.0160 | [60 50 45 80] | 1.0125 | -0.0035 | -0.34% |
| 26 | 0.9769 | [50 45 35 60] | 0.9722 | -0.0047 | -0.48% |
| 27 | 0.9407 | [30 60 50 40] | 0.9375 | -0.0032 | -0.34% |
| 28 | 0.9071 | [60 50 40 80] | 0.9000 | -0.0071 | -0.79% |
| 29 | 0.8759 | [50 50 35 60] | 0.8750 | -0.0009 | -0.10% |
| 30 | 0.8467 | [50 50 45 80] | 0.8438 | -0.0029 | -0.34% |
| 31 | 0.8194 | [50 40 35 80] | 0.8203 | +0.0010 | +0.12% |
| 32 | 0.7937 | [60 50 35 80] | 0.7875 | -0.0062 | -0.79% |
| 33 | 0.7697 | [50 80 50 60] | 0.7812 | +0.0116 | +1.50% |
| 34 | 0.7471 | [20 80 80 40] | 0.7500 | +0.0029 | +0.39% |
| 35 | 0.7257 | [50 60 35 60] | 0.7292 | +0.0035 | +0.48% |
| 36 | 0.7056 | [50 60 45 80] | 0.7031 | -0.0024 | -0.34% |
| 37 | 0.6865 | [30 80 60 50] | 0.6750 | -0.0115 | -1.67% |
| 38 | 0.6684 | [80 45 20 80] | 0.6667 | -0.0018 | -0.26% |
| 39 | 0.6513 | [60 60 35 80] | 0.6562 | +0.0050 | +0.76% |
| 40 | 0.6350 | [60 80 45 80] | 0.6328 | -0.0022 | -0.34% |
| 41 | 0.6195 | [40 60 50 80] | 0.6250 | +0.0055 | +0.89% |
| 42 | 0.6048 | [80 50 20 80] | 0.6000 | -0.0048 | -0.79% |
| 43 | 0.5907 | [45 50 35 80] | 0.5906 | -0.0001 | -0.01% |
| 44 | 0.5773 | [40 60 35 60] | 0.5833 | +0.0061 | +1.05% |
| 45 | 0.5644 | [45 60 30 60] | 0.5625 | -0.0019 | -0.34% |
| 46 | 0.5522 | [35 80 50 60] | 0.5469 | -0.0053 | -0.96% |
| 47 | 0.5404 | [50 60 35 80] | 0.5469 | +0.0064 | +1.19% |
| 48 | 0.5292 | [50 80 45 80] | 0.5273 | -0.0018 | -0.34% |
| 49 | 0.5184 | [40 50 35 80] | 0.5250 | +0.0066 | +1.28% |
| 50 | 0.5080 | [45 50 30 80] | 0.5062 | -0.0018 | -0.34% |
| 51 | 0.4980 | [40 60 40 80] | 0.5000 | +0.0020 | +0.39% |
| 52 | 0.4885 | [60 80 35 80] | 0.4922 | +0.0037 | +0.76% |
| 53 | 0.4792 | [40 80 50 80] | 0.4688 | -0.0105 | -2.19% |
| 54 | 0.4704 | [40 80 50 80] | 0.4688 | -0.0016 | -0.34% |
| 55 | 0.4618 | [40 80 50 80] | 0.4688 | +0.0069 | +1.50% |
| 56 | 0.4536 | [60 50 20 80] | 0.4500 | -0.0036 | -0.79% |
| 57 | 0.4456 | [20 80 60 50] | 0.4500 | +0.0044 | +0.98% |
| 58 | 0.4379 | [35 60 40 80] | 0.4375 | -0.0004 | -0.10% |
| 59 | 0.4305 | [30 60 35 60] | 0.4375 | +0.0070 | +1.62% |
| 60 | 0.4233 | [60 80 30 80] | 0.4219 | -0.0015 | -0.34% |
| 61 | 0.4164 | [50 45 20 80] | 0.4167 | +0.0003 | +0.07% |
| 62 | 0.4097 | [35 80 50 80] | 0.4102 | +0.0005 | +0.12% |
| 63 | 0.4032 | [35 80 50 80] | 0.4102 | +0.0070 | +1.73% |
| 64 | 0.3969 | [35 80 50 80] | 0.4102 | +0.0133 | +3.35% |