Hi Richard, welcome to the learning curve. Old machinery was designed and built using the British imperial scale of feet and inches still used in the USA. It works on a base 12 system which can be divided by 2,3,4 and 6. Unlike the metric 10 based system which can only be divided by 2 and 5. The beauty of the old system was that engineers and builders could do the maths in their heads, while to work in base 10 you need computers and calculators. So much for progress, we now have a system that no one can do in their heads consequently we are reliant on technology and have to pay through the nose for it! If you can learn you 12 times table and count to one thousand everything in imperial measurements makes perfect sense.

Good luck, enjoy the learning curve.

Bob