From: djb@invalid.com   
      
   On 20/09/2025 00:01, Jim Wilkins wrote:   
   > "Snag" wrote in message news:10akfb6$ns86$1@dont-email.me...   
   >   
   >   I finally got around to wiring my lathe for reverse today . I'm   
   > finding the need to make metric threads , and on my lathe that means a   
   > set of transposing gears and you can't disengage the half nuts or you   
   > lose register . Gotta back out of the cut and rewind back to the   
   > beginning .   
   > Snag   
   >   
   > -------------------------   
   > This says you can disengage them, but it still isn't as convenient as   
   > quickly moving the carriage back by hand and setting up the next cut   
   > while waiting for the thread dial mark.   
   > https://conradhoffman.com/metricthreading.htm   
   >   
   > "*When you reach the stopping point, release the half nuts just as you   
   > always do, but immediately shut off the lathe. The thread dial will go   
   > a bit beyond the chosen number as the lathe coasts to a stop.   
   > *Back the tool out of the thread just as you always do, preferably   
   > while the spindle is still turning.   
   > *Reverse the lathe motor while watching the thread dial. Engage the   
   > half nuts as soon as the original number comes back to the mark."   
   >   
   > Instead of releasing the half nuts I pull up the belt engagement lever   
   > which stops the spindle and leadscrew almost instantly. After the   
   > motor coasts down I finish the cut by hand to the end hole or stack of   
   > previous chips by moving the exposed cone pulley, easy enough with   
   > back gear reduction. When done I V-chisel out the chips and smooth the   
   > thread end with a triangular file. For me the well spaced controls on   
   > a South Bend are very convenient and I appreciate the smoothly   
   > sculpted castings over bulkier square machined components. I don't ask   
   > it to give the power of a gear head lathe.   
   >   
   > On my inch lathe with the 70 position Quick Change gearbox I need two   
   > transposing ratios to get all the metric threads I want. They are the   
   > standard 100/127 plus 120/127 for finer threads for optics. As the   
   > threading label was missing I made one up as a spreadsheet and printed   
   > it. Linking the transposing ratio into the cell formulas of a copy of   
   > it gives the metric modulus for each gearbox setting. In Britain pipe   
   > thread pitches are different and the spreadsheet won't be correct.   
   If you're referring to BSP threads why mention it as they're inch   
   threads and typically differ from NPT by 1TPI, excepting 1/2" and 3/4"   
   which have the same pitch. The ISO pipe thread is BSP.   
   >   
   > Boston lathe change gears are meant to go on a double splined center   
   > bushing that couples a pair of them together. The bushing bore for the   
   > thread pitch that matches my lathe's tumbler and QC box input gears is   
   > smaller than the sliding post on the banjo so I machined a new post   
   > and base. I could also have made or bought a plain bushing with the   
   > right OD and bore but no splines and bolted the gears together.   
   > https://www.bostongear.com/products/open-gearing/stock-gears/s   
   ur-gears/change-gears   
   >   
   >   
      
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