Hi Dude,
The biggest problem is that inch and metric threads are opposite philosophies - inch are in threads per length, metric in length per thread, if you see what I mean?
Some manufacturers swap between the two by swapping drive to the QCGB between the first shaft with the cone of gears and the third shaft (taking the sliding gear on the tumbler as being on the second shaft) then picking up the leadscrew drive from the other shaft - well, at least Holbrook do it this way!
As you probably can't do this on the Monarch (I guess, I've been proven wrong before!) you'll have to set up the geartrain for each fundamental pitch of thread
- you can probably use the "range" selections to get multiples (e.g. 1mm, 2mm, 3mm, 4mm) as you would for inch (e.g. 28, 21, 14, 7tpi all use the same tumbler position on the QCGB) but the fundamentals (e.g. .2mm, .3, .5, .75mm) will require different change-gear setups on the quadrant...
Whatever you do, you're probably going to need a 120/127 or 100/127 tooth compound gear to translate between inch and metric (based on 127mm = 5") although there are some approximations that come *close* to the correct ratio and use smaller (less costly!) gears.
The book "screwcutting in the lathe" by Martin Cleeve in the workshop practice series (an UK model-engineering publication) goes into the ratios that are possible, inch-metric conversion and the calculations necessary - well worth a read
Just my ha'pennorth,
Dave H. (the other one)
