When I bought my Flexispeed lathe in the mid 1970's from the Norfolk lathe & Tool Company, I asked about screwcutting gears and was sent a leaflet showing how they had arranged the set up.

There was a 16 tooth gear fitted into the spindle in place of the worm gear.

This always meshed with the 32 tooth side of a double gear, the double gear had 32 and 16 teeth.

There was then an idler, either a 56 tooth gear meshing with the 32 tooth side of the double gear, or a 64 tooth gear meshing with the 16 tooth side of the double gear.

The idler then meshed with the lead screw gear, the tooth count of this gear was 24, 26, 30, 32, 36, 40 and 44.

There was a fixed plate fastened to the left hand side of the headstock and a conventional banjo or quadrant as they called it.

I hope you can follow all that?

The cost of the gears at the time was £12-50 + VAT carriage 60p. As I had only paid around £30 for the lathe kit, if I remember correctly, it seemed too much so I never bought them. The number of threads available did not seem to cover all the threads I thought I might need.

Some years later I made my own screwcutting attachment, I cut a set of gears from aluminium with a home made 24DP single point cutter in exactly the range that Bazyle has given, I cut the extra 20 tooth and also a 70 tooth gear. I should really have cut a 57 tooth gear for 19TPI, ( 1/4 and 3/8 BSP), maybe I will do that as an exercise sometime.

Neil

The above discussion seems to have a lot in common with the Cowells.

The Cowells set is 20T to 50T in steps of 5T, with two off 30T for achieving a 1:1 ratio.

The Cowells also has two permanently assembled compound gears – 16:32 and 20:56, mainly for slow feed although 16:32 has its uses for screw cutting.

The Cowells gears are 32DP. I'm not sure of the correct pressure angle but I've added some 20 degree PA delrin gears from HPC and they work OK once the centres are opened out to the right size and a keyway added.

If you want to make up your own compounds from the Cowells change gears then there is a challenge to overcome – you need a keyed idler bush and the 3/8 bore does not leave a lot of diameter to play with… essentially the key needs to be part of the bush because there isn't really room to cut a keyway in the outside of a bush.

The other limitation with the Cowells is the space available on the quadrant. This may make some of the higher numbers of teeth less useful than you might otherwise think.

I've added 37T and 47T (also from HPC) to mine for doing BA and imperial approximations. The inspiration came from this thread…

https://www.modelenginemaker.com/index.php/topic,1020.15.html

Hope this helps. Some of the above may apply to the Flexi but I know that some of them have a different drive arrangement for the feedscrew. The Cowells is a 1mm feed screw pitch, not sure if your Flexi feed screw is imperial or metric.