Post by ChrisCrosskey on Jan 25, 2008 23:03:26 GMT
Hi folks, this is an unashamed brag I'm afraid.... just got out of the workshop where I've been making a mount to attach an old photographic multi-image prism to an adapter to mount it on my various optikinetics (old fashioned oil-wheel liquid lightshow) projectors..... most of it was the fairly routine boring of a short piece of 3 1/2" x 1/2" ally tube, the hard bit was cutting an M62x0.75mm thread to screw the prism mount screw into ....on a 1948 Myford with an early version of their gearbox on that doesn't have the 2:1 reduction on the drive from the output shaft to the leadscrew.... on these early boxes the 25:12 pinion set that Myford now sell as the "ultra-fine feed" kit is actually required just to make it cut the threads on the plate..... It also means that a lot of published gearbox mods don't work because you end up needing to double the size of a driven gear or halve the size of a driver and it just doesn't work out..... so what I've done is worked out what range of gears I can fit on the tumber end shaft and built myself a little spreadsheet that calculates what the threads they cut are both on standard feeds and also if the fine feed pinion in the middle of the gearbox is turned around..... The best result would be if I had a 17-tooth gear on there, and indeed building some sub 20-tooth pinions with captive sleeves is on the list of things to do, but I calculated that if I put a 35-tooth gear on there, put it into fine feed and set the box to 11tpi I get a pitch of 0.748mm.... or 2 one-thousandths of a millimetre out per thread.... good enough for Jazz I thought.... and I was right.... bored the mounting hole out to 61.25mm, set the threading tool in with the right tip on it, put the lathe in fastest backgear, gave it a 15-thou cut (approx 0.375mm in the new money) and let fly.... Result was the prism screwed in first time..... I've calculated some other ones, including most of the worthwhile metrics and the bigger BA's.... from measuring the banjo I reckon you can get up to a 48-tooth gear in there but you only really need a few.... 17-tooth would be the one with the most going for it but you'd have to make it.... a 34 and a 35 get you quite a few.... the thing is all these smaller Myford gears are fairly cheap these days, but I probably wouldn't have thought about it if I hadn't got a DAG Brown gearbox banjo, discovered the problem with th early boxes lack of gearing and gone on a mission to obtain every single Myford gear they've made.... as far as I know the only ones I'm missing is the 91 and the 96, and the only gears that are missing in a complete run from 20 to 66 are 41 and 49 and Myford don't make either of those sizes..... I guess I ought to write this up properly and send it into one of the mags.... although there are a lot more late boxes than early ones, such is the "survivability" of Myfords that I suspect most of those made are still in use, even if you scrapped the lathe you'd be able to sell the gearbox on.... The early ones are easy to spot as the drive to the leadscrew is off the right-hand (tailstock) end of the gearbox instead of being behind the banjo (almost) at the headstock end, the drive has a little cover over it and also restricts how far the saddle can move towards thhe headstock.
/brag
Chrisc
/brag
Chrisc
