Adapting an imperial lathe to cut metric threads

摘要

For some time, I have been interested in adapting my American MONARCH CK tool room lathe to cut Metric threads. Unfortunately, there is not much room in the gear train section on the left hand side of the headstock to use the 127/120 tooth compound gear required to give accurate metric pitches. What is usually done is to use a compromise compound gear of approximately half the size with 64/63 teeth. Besides not giving exact pitches, just dismantling the existing imperial gear train and replacing it with the 64/ 63 compound gear (which may need to be reversed 63/64 to give greater accuracy in some of the pitches) and setting the gears to mesh properly is a time consuming, awkward and oily business which needs to be done again when the job is finished.