Harrison L5 3HP Lathe

[Sideways]

A single middle aka idler wheel has no effect on the gear ratio, same in, same out so to speak.
If you stack two wheels in the middle, then the ratio of these two becomes part of the chain.
Making metric threads with an imperial Norton gbox, the middle has a pair of wheels in a 2:1 ratio
You then need spacers to line the top and bottom wheels up with the appropriate one of the middle wheels.

Easiest for noise reduction would be to have both wheels on the middle pin made from delrin.
I only figured this out because on my original 9" lathe with abc gearbox I was short a 60T and having had trouble finding one. I bought a delrin wheel from the ebay guy Fergie linked above.

The "spacers" are simple sleeves with maybe a 7/8" bore but they need to have a keyway cut to clear the key on the shaft.
The spacers aren't driven by the key which is used to drive or take drive from the cogs, but they need to slide over the keys. You could file a clearance slot in the inside of a home made spacer - a rattling good fit - as long as the bore is a neat fit. Maybe even make the outside diameter bigger than the original.

Outer of my spacers are knurled to make it easier to pull on and off but they must get tight because mine also have plenty of bite marks from mole grips or waterpump pliers.

 

[Sideways]

So in your second video there was another wheel behind the one on the intermediate shaft engaging with the top shaft?

Yes. Half the tooth count of the wheel you can see.
There's a key in the middle idler shaft so both of it's wheels rotate together with no slip.
It acts to speed up the teeth per rev by a factor of two.
The big 127T wheel has the spacer behind it.

 

[Slooby]

I'm getting myself in a muddle here.

My current set up goes 50 Tooth top, 50 Tooth (fibre) on the idler and 100 on the Norton gearbox which from what I understood is standard for a Imperial thread cutting

To go metric I should replace the 100T with a 127T, then have either 40T or 60T (or 63T if I want to go mad and cut a 4mm pitch thread...) on the idler when using the original 50T on the top

 

[Fergie 307]

Haven't used mine for metric for a while but as far as I recall the intermediate has a stacked 40 outer and either 60 or 63 inner (63 for 1.75, 3.5 as well). So in the set up charts the first number quoted is the outer gear. The 127 runs on the outer. As Sideways says the spacers can be a bit tight, usually seem to get a burr where the gear bears against them. I just put a bit of a chamfer on mine on the inner bore and have been ok since.

 

[Fergie 307]

Oh and spray motorcycle chain lube is good for the gears. Dries to a waxy consistency so doesn't flick all over the inside of the covers.

 

[Slooby]

Ahhh...sorry took me long enough to realise what was being said and now of course it's obvious! And of course I can now se how I could run a Delrin 127 gear with a steel outer idler and a Delrin inner going to the top shaft. Chain lube is a good shout, I'm well used to applying it.

Didn't get to bench testing the motor today, was busy tidying Mega-Shed at my girlfriend's place for its new dual purpose life of not being her ex's woodwork shop but a space for exercise and Scalextric when we lower the 3x2m table i built down...he had a metalwork shop to, in the garage with Warco single phase Mill and Lathe...my Sunbeam and track wheels are filling that now...