Metric Threading with QCGB not using 37/47 gears

Thanks everyone for your responses. Somehow notifications on this thread didn't get to me so I'm sorry for the delay.

@Flyinfool I understand that none of the conversions for an English lathe to cut metric threads are going to be perfect other than the 100/127. I don't often need metric pitches, but want to have something that works for many common pitches in my arsenal when needed. As you said with the short length of thread on a nut (probably all I'll ever need) the accumulated error is negligible.

@Canuck75 With your 63/80 setup, what settings do you use on the QCGB to set metric pitches? Do you have to install other gears to open up more options? Maybe I'm missing something because I don't see many common metric pitches in the converted chart using this ratio.

@KyleG I can cut English pitches just fine. There should be a 2:1 ratio from the spindle to the gearbox input. Mine came equipped with 24/48. You can see from Canuck75's picture that he has the same 2:1 ratio using 28/56. I'm working from a spreadsheet that I made to try and help myself to understand the challenge. I cannot guarantee it is free of mistakes, which is why I asked for some advice here. I may be totally wrong or missing something.

@mickri I'll look again for threads in the Craftsman/Atlas forum on this. I have one of these machines and many people use 56/44 to transpose for metric pitches. I've never used the Craftsman to cut metric threads either. I did look at Halligan's video on this and it seems that he was doing what I was thinking of by trying to effectively transform the 8 tpi leadscrew into a 6mm pitch
 
The 60 gear in your gear train is an idler gear. It does not have any affect on the ratio. What you need to do is to replace the 60 with a combo gear to change the speed of the lead screw to a metric pitch. On my craftsman lathe a 44/52 combo changes the lead screw from 8 tpi to 7.5 mm. If you have a craftsman 10 or 12 lathe I would use it to cut metric threads. Very easy to switch between imperial and metric threads on a craftsman lathe. All you have to do is move the sliding gear. In the out position you cut imperial threads. In the in position you cut these metric threads. 7.5, 6, 5, 4, 3, 2.5, 2, 1.5, 1.25, 1, .75 and .50. If you should need to cut any other thread, imperial or metric, the manual for the qcgb does all of the math for you. It lists the gears needed to cut several thousand different threads. To use the manual for metric threads you need to convert the metric to tpi. For example. One of my projects needs a 1.80 mm thread. 25.4/1.80 = 14.1111 tpi. The qcgb manual lists the gears for a 14.1137 tpi which is close enough for a 1/2" long thread.
 
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