Gearboxes and Screwcutting:
In standard form both lathes used changewheels for screwcutting and could generate (with the standard changewheel set) pitches from 6 to 112 t.p.i. (or 0.25 to 4 mm). However, both could be ordered (or retrofitted) with a screwcutting gearbox thus becoming, with the addition of a suffix to their model numbers, the ML7B and Super 7B. The gearbox generated 48 threads from 8 to 56 t.p.i. and the same number of feeds (by the simple expedition of pulling out a double gear mounted on the banjo) of 0.139" to 0.002" per revolution of the spindle. Only an Imperial box was ever offered, metric pitches being obtained by sets of conversion changewheels. However, on lathes sent to America the gear cluster inside the box was modified, 23T gear replacing the 19T gear normally used - the result being that the box could generate North American pipe-thread pitches.
Although in the late 1940s a kit-form gearbox had been designed and marketed by L.H.Sparey (author of The Amateur's Lathe) it was not until 1953 that Myford's first effort appeared. Lubricated by an oil-bath it was designed along long-established "Norton Quick-change" lines with a single-tumbler and a reversible gear on its left-hand face that allowed a change to be made between fine feeds and threads. Early boxes were fitted with unhardened gears and (hidden under a rounded, aluminium cover) a pair of external gears on the right-hand face (from which the leadscrew drive was taken) but in 1956, from box QC 2495 onwards, important alterations were made - with a change to hardened gears and a leadscrew that was much better supported by being allowed to pass all the way through the box with the drive taken from its left-hand end - so allowing the external gears to be dispensed with. It is worth noting that the Metric Conversion Set for the early box is different - the four gears needed being: 2 x 60t, 1 x 44t and 1 x 52t. Later boxes required five gears: 2 x 60t, 1 x 50t, 1 x 45t and 1 x 63t. The standard Metric Conversion Set was replaced during the 1990s by a different design of slotted quadrant arm on which could be mounted a greater variety of changewheels - and so enabled the generation an almost unlimited number of English, Metric and odd threads and feeds. The Myford gearbox was a beautifully made and very tough unit and, with a supply of lubricant in its base (not something that every maker of small lathes considered important) very reliable. The only signs of wear one should encounter are in the shaft bushes; if the gears show signs of damage then the box must have been mishandled or denied lubrication, in normal use it is simply impossible to wear them out.
Supplied as standard with an ML7 or Super 7 the changewheel set comprised: 2 x 20, 25, 30, 35, 38, 40, 45, 50, 55, 60, 65, 70 and 75t. To cut a wide range of metric threads requires, in addition, just two 21t wheels. Myford Series 7 10 changewheels of all years are 20 D.P. with a 14.5-degree pressure angle, 3/8" thick with a 5/8" bore and 1/8" keyway.
A reproduction of the gearbox mounting template can be found below..