My lathe is the METRIC version, it doesn't have a Norton quick change threading gear box. On the indicator table the two wormwheels for the thread chaser dial(on the saddle) show 56 tooth and 60 tooth. The ones that came with my lathe are 42 tooth and 45 tooth. The packing slip mentions a 45 tooth wormwheel. I wonder if the chart above is wrong or if I got the right wormwheels, I havn't tested it out yet. To date I have been threading metric and inch by keeping the half nuts engaged and reversing spindle to run back to the start. Update, it appears that the thread chaser indicator table on the metric lathes is a misprint. 56T should read 42T and 60T should read 45T. I checked a metric lathe on the showroom floor and one of the wormwheels was 42 teeth so am assuming that the one that was on it's thread chaser was a 45, thus giving it a 42 and a 45, just as my lathe. When I went to the sales place and was checking on this they looked at me kind of strange, wondered what the hell I was on about but did accomodate me by counting the teeth on the wormwheel that was in the tool box of the metric lathe they had on the floor. After verifying this I have machined a couple of different metric threads with disengaging and re-engaging the half nuts at the positions as indicated in the indicator table above and all worked fine. It's a bit of a pain though that the wormwheel of the thread chaser dial has to be changed when doing the different pitches. I just noticed another error on the indicator chart, they've spelt pitch as pitcm.
Underside of thread dial, sometimes called thread chaser, showing the 45 tooth worm wheel and 42 tooth is in my hand. I always disengage the wormwheel from the leadscrew when I am not thread cutting, this saves metal chips/swarf from getting crunched up between wormwheel and leadscrew.
Different part now. To screw 1.25mm pitch a 52 tooth gear has to engage(according to the chart) with the 80 tooth at top right. I had to make the modification shown in the next photo otherwise the 80 tooth gear (behind the 75) would clash with the 80 tooth one above it. I did this mod some many months ago when I first wanted to cut 1.25mm pitch. See update further down.
I had to mark the correct position and drill and tap the swing arm to locate the stud for the 52 tooth gear so it was up a little higher, this stopped the two 80 tooth gears from clashing. See update further down.
Pic of the stop/start and reversing switch. With a bit of use you can get pretty good at stopping spindle, flipping the reverse switch, starting, running back to the beginning of the thread then stopping, flip reversing, then wind cross slide in and starting to cut thread again.
UPDATE (17thNov2002)
On the plate chart in the metric thread section wherever it shows a 52 tooth gear for position "a" on the swing arm you can use a larger gear as it acts only as an idler. Therefore I needn't have drilled and tapped a special hole so that the 80 on the swing arm didn't clash with the 80 above it. You can check the final ratio by multiplying each Driver over Driven right through to the final driven. Also, if you want to cut 4mm pitch thread, which is the thread that is often on the spindle of the machine you can achieve this by substituting the 80 tooth gear with a 60 tooth in the 3mm pitch column. My lathe, the metric version has a 2mm pitch leadscrew. Remember if you are cutting inch threads on a lathe with a metric leadscrew you have to keep the half-nuts engaged(closed).
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