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| US and old Imperial threads are defined in threads per inch, usually whole threads per inch giving often odd pitches, inches per thread. Metric threads are defined as pitches, usually (except for BA threads) using integer mm, 1/2, or 1/4 mm steps for pitch.
Thread cutting on metal lathes can be mixed metric and fractional in several ways. Generally if you shift the change gears from US to metric throwing in a factor of 2.54 or 1.27 (classically a 100 to 127 gear pair) the threads per inch now become threads per centimeter and if the quick change gear box is very well equipped some are useful. Like 10 threads per inch becomes 10 threads per cm or 1mm pitch. Sometimes the quick change selection has been cobbled to give 12.5 threads per inch which fits no standard US screw but with the change gears shifted becomes 1.25 mm pitch.
Most US metal lathes stack the selecting gears 8 through 15 teeth per gear (driven gears) on the shaft coupled to the lead screw, and change ranges in factors of 2 on the input side of the quick change gear box. A pure metric lathe has those 8 through 15 teeth gears on the driving side with the factors of 2 change on either side of the quick change box, just the selections within a range change integer pitches, not integer threads per inch.
Some metric/US thread quick change boxes have two sets of fine gears. Inputs driving for metric, outputs driven for US. And with the magic of the 127/100 gear pair its possible to get US threads from a pure metric quick change gear box with a few extra none metric selections.
Some western Pacific combination lathes take care of the lead screw pretty well, but on the cross feed make it a compromise between .1" per turn and 2.54 mm per turn, but show only 25 divisions on the dial so for one its always a little wrong. Either the thread of the cross feed is .1" and 2.54 mm per turn or its 2.5 mm and .098425" per turn. So in the first case when you depend on the dial for cutting to size, it works out for inches, but is undersize for metric, or safer with the second version and the metric feed screw it cuts right for metric but leaves the work oversize with US which you learn with your final micrometer check that the part is oversize, so you have to cut another pass.
Without a quick change gear box, my lathe is most versatile (but very inconvenient), there are many possible sets of gears for most every thread, metric or US though its handy to have that 100 to 127 ratio for the metric threads, but with the collection of gears there are often several, up to a dozen, different gear trains that will get an approximation of the desired thread. Just depends which one you use if you have the right gears and how picky you are about the accuracy of the resulting thread. Years ago I found a windoze program on line that will show the possible combinations given the available gears and the threading lead screw thread and show the error of each. Its not in this computer. I've not looked for something like it lately, probably there's a web page and a dozen apps for that.
Gerald J. | |
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Units of measure
