When I was lathe shopping last year, I had a list of features I wanted in a lathe: cross feed, tumbler reverse, QCGB and the option of cutting metric threads with change gears. The SB 10K I ended-up purchasing met all those needs well (and I really like it) but I was left scrounging around for the metric gears. South Bend manufactured a set specifically for these lathes (the 9A & 10K) back in the day but they are hard to find. Tools4Cheap sells a set made overseas, but they have been out-of-stock for most of the year. About a month ago I got the email saying they were back in stock, so I bit the bullet and made the purchase. Here's what $200 gets you:
It's a box-O-gears! The transposing gear is 2 large cast gears (127T/100T) pinned together. The rest of the gears are steel and everything is nicely cut and fits well. The only clean-up needed was to grind the pin down in the transposing gear a bit with a Dremel as it was a bit proud. From the pictures I've seen, these look identical to the original SB set, except here you get another 40T gear.
You'll need a spacer between the 56T gear and the gearbox to clear the 127T gear in the transposing set. You could use one of the small gears and some washers, but I made a simple aluminium spacer with clearance for the key cut with a file.
The next pic is what the gear cover on my lathe looks like and there's enough room inside to close it with the metric gears installed. The cover was hitting the thin shield at the gear box which is usually about a 1/2" inside the gear cover. The shield is adjustable, but the gear cover was closed enough so you didn't have to worry about getting your fingers caught in that buzzsaw of gears so I left it as is. After all, the metric gears are only installed temporary until the threads are cut and then it's back to the factory setup.
Next is what the stock gear setup looks like. Judging from the goop and filth on the gears, I don't think mine was ever touched in 40 years, the awesome advantage of the QCGB.
The last pic is what the lathe looks like with the transposing gear and a 48T stud gear. I was cutting a 1.50 pitch thread for a M10 bolt, but you could also cut a few other pitches with changes to the QCGB with this same stud gear. With this gear set, you can cut all the standard metric thread pitches and then some.
I couldn't find a really nice metric chart to print for in the shop, so I made one. You can download it here:
http://doug.sdf.org/files/metric_chart.pdf
If you want to get your freak-on about cutting metric threads and have a few extra change gears laying around, you can grab this large chart for almost every conceivable combination of gears for various threads:
http://bnordgren.org/files/metchart2.pdf
If you are cutting metric threads with a inch lead screw, you often here "Don't move the half-nut lever until you are done". This isn't exactly true, you can pull the lever, you just have to catch the same thread on the lead screw when you are winding-backwards in reverse to the beginning of the thread you are cutting. I've read about this technique, but it didn't really make sense until I saw this video were Tom demonstrates how it works:
It's a really handy technique when you are cutting threads to a shoulder, which seems to be about 99% of the time. Worked great for me the first time.
While I try to stick to using SAE threads, I still have to dabble in the metric world occasionally and this will only become more common in the future. Yes, these gears are expensive to have in a box and only break them out a few times a year, but they are there if needed. Besides, if I wanted to not spend money, I would have taken-up knitting instead!