Home-made Backgeared & Screwcutting Lathe
by Earl Young
The builder writes: This is a backgeared and screwcutting small lathe that I made over several years - though the bulk of the work was completed at "night tech" over 40 years ago when I was in my late 20s. The lathe was based on plans purchased through an advertisement in the Model Engineer magazine around 1978 with construction based on mainly standard steel stock. The backgears are in mild steel, the tumbler gears (this mechanism was designed and added by me to the original plans) are in cast iron and the drive train in aluminium. High-resolution pictures - may take time to open
I managed finished it off the winter before last (2020) with the final jobs being the making of changewheels and the countershaft drive system. The gears were made on my Prazimat 180 x 450 by the method used by "xynudu" about seven years ago on YouTube, this involving the use of cheap Chinese gear cutters.
Made from an old Viva half-shaft and finish ground to size, the headstock spindle had a register that was supposed to be the same as a Myford ML7, but the thread ended up as 7 t.p.i. instead of 12 t.p.i. - this being my fault for not reading the drawing properly Drilled through to clear a 10 mm diameter bar and with a No. 1 Morse taper socket, the spindle has bearing diameter of 1" and 13/16" and, like all the shafts on the lathe, runs in brass bearings made from standard plumbing fittings. Thrust bearing is a captive-ball type with home-made races machined from cast steel and hardened.
Spindle speeds are a bit of a mystery; the lowest I can count is 48 r.p.m. but, when calculated using the backgear and belt-drive ratios works back to a motor speed of 1350 r.p.m. instead of the 1425 r.p.m. it's supposed to run at. Still, it's near enough...
The pulleys sizes are: Motor pulley 45 mm diameter to layshaft pulley 170 mm diameter giving a 3.8:1 reduction.
Layshaft to spindle 100 mm to 54 mm, 75 mm to 75 mm and 54 mm to 100 mm giving ratios of, respectively, 1 : 1.85, 1 : 1 and 1.85 : 1
Backgears have 35 to 70 teeth and 70 to 35 teeth to give a reduction of 4 : 1. The overall reduction ratio then being 28.12 : 1 (28.12 x 48 = 1349)
All the pulleys were machined from castings with those on the spindle and layshaft from large blanks cast in "night tech", the motor and layshaft pulleys from the lost-foam technique, this being done recently at home. The fixed steady was also cast using the lost-foam method.
The finished job is a mixture of imperial and metric with a centre height of 3.25" and a a capacity between centres of 12". The leadscrew, fitted with a dog clutch, has a pitch of 3 mm, the cross-feed screw pitch is 1.75 x 12 mm and that of the top slide screw 1.5 x 10 mm.