Wont they both go up by the same %?
My field is/was electronics I love working on the lathe If I had a decent size miller I would make the Quadrant this is my tiny milling machine I may sell it and get a bigger one I mainly make stuff for my classic Rover or Clocks.
Sorry John I am not familiar enough with the ML7 gearbox to be able to answer that. There are plenty of owners out there though someone will surely respond.
The gear that I'm on about changing, is the small (24 toothed) steel gear, below and in front of the red tufnol tumbler gears, in the top/mid left hand side of your photograph, held in place with the hexagonal bodied oil nipple.
Swap the 24 toothed gear for a 30 toothed gear, set the gearbox selector arms as shown on the gearbox for 20 tpi and with a bit of luck, you should get a 15 tpi thread. If this setup does give you 15 tpi, then an 18 tooth gear instead of the 30 tooth, should give you 25 tpi.
The Myford 18 tooth gear that I linked to earlier is hardened an needs modifying to fit (I've found one in an old box of odds and ends from my fathers old ML7).
If I get a chance in the morning I'll try all this myself on my Myford Super7 and report back.
Thanks guys for your help its much appreciated! I will recount my input gear just in case it's a 75 certainly not a 70, if I hadn't fitted a gearbox I could have done it lol!! but I wouldn't be without the GB now!
Would you get an 18tpi with the shaft dia Nick?
0k the 72 tooth input gear is standard for all myford gearboxes, the charts that gaz1 linked to are for Myfords without gearboxes.
Tried the 30 tooth gear this morning and it appears to work, the thread pitch measures (as best I could using a magnifying glass and Mitutoyo digital caliper) around 0.065", 15 tpi is 0.0666666".
Spured on by this result, I've gone ahead and done :-
Heated to a dull red hot and cooled slowly to soften, then bored out to 0.6255" and then broached. But I only had a 3mm keyway broach and no sharp needle files to open the keyway out to 1/8" wide, so it will have to wait until monday at work. There's not much material left between the tooth root and top of the keyway, but the gear is good quality steel, so hopefully will still be up to the job. This gear is apparently used inside the gearbox hence the reason for the hardening, smaller bore and narrower width than standard gears.
Scrol down and one chart lists a 25 tooth gear fitted in place of the 24 tooth (in the same place as I intended to fit the 18 tooth gear) and the gearbox set to 26 tpi gives you the 25 tpi you need.