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South Bend 9-inch "Workshop" Lathe
Screwcutting
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Original 1934 9-inch Model 5    In the Factory

Both the Model C and Model B 9-inch lathes used changewheels for screwcutting whilst the Model A was always fitted with a screwcutting gearbox.
Although all but the very first lathes (the Model 504) were fitted with a tumble-reverse mechanism it was not until the late 1950s that South Bend include a spring-loaded tumble reverse selector lever in the specification. Until then, to reverse the direction of the carriage movement, or to disengage the drive, a square-headed spanner (wrench) had to be used to slacken the pivot arm before it could be moved. The English copy by Boxford employed a much more convenient and quicker acting spring-loaded mechanism from its first production model in April 1948.
Although the vast majority of 9-inch Workshop lathes for the American market were supplied with 8 t.p.i leadscrews numbers were manufactured for export with a 6 mm pitch metric leadscrews in both Model B and C form (with changewheels) and as Model As with proper, all-metric screwcutting gearboxes. The English and metric screwcutting gearboxes were different externally, and very easily recognised the former having the diagonal line of indent holes on the right hand half of the box's front face, the latter with them across the left-hand half.   
"English" threading Lathes with an 8-t.p.i leadscrew and changewheels for screwcutting (i.e. without a screwcutting Gearbox) were supplied with the following changewheels when they left the factory: *16, 24, 36, 40, 44, 46, 48, 52, 54, 56, 60, *80,  *72/18 compound,  *80 idler (with boss)
(* gears on machine as dispatched from factory for standard feeds)
To convert a Model B or C (non-gearbox) English-specification lathe to cut metric threads the following gears are needed: 20, 100, 127/100 combination
In addition, to cut the following five pitches extra gears are required as follows: 0.45 mm = 18t gear,   0.55 mm = 22 t gear,   0.65 mm = 26 t gear,  0.7 mm and 3.5 mm = 28 t gear. 
Metric Threading Lathes with a 3mm pitch leadscrew and changewheels for screwcutting (i.e. without a screwcutting gearbox): were supplied with the following gears as standard. *16, 24, 28, 30, 36, 40, 44, 48, 52, 56, 60,  *80,  *72/18 compound,  *54/18 compound,  *80 idler (with boss)
(* gears on machine as dispatched from factory for standard feeds)
To convert a non-gearbox Metric-leadscrew lathe to cut English threads the following gears are needed: 18, 22, 26, 38, 54, 64, 88, 135/127 compound, 48/24 compound.
"English" threading Model A lathes with an 8 threads-per-inch leadscrew and a screwcutting gearbox had a standard ex-factory drive train consisting of: 20t,  40t, 56t and an 80t idler. To convert this gearbox to cut Metric threads the following gears are needed: 24, 26, 28, 32, 36, 44, 48, 127/100 compound
Metric threading Model A lathes with a 3 mm pitch leadscrew and a screwcutting gearbox had a standard ex-factory drive train consisting of: 20t,  45t,  50t and an 80t idler. To convert this gearbox to cut English threads the following gears are needed: 38, 40, 44, 52, 56, and a 135/127 compound
As an alternative to the 127/100 gear for metric conversion with a standard changewheel lathe it is possible to use a 37/47 transposing gear and then use a formula to calculate the necessary gear trains, as follows:
Pitch of thread in mm = 2.5 x (stud gear teeth divided by leadscrew gear teeth) or, to find the gear necessary to fit on the leadscrew: leadscrew gear = (2.5 x Stud Gear) divided by Pitch in mm
All the gears necessary to generate metric and other pitches are now available at a good saving on the factory price.
The gear on the end of the spindle - and the two gear tumble-reverse gears beneath that match with it - are 20 DP. The tumble-reverse output gear, and the train of changewheels it drives, are 18 DP and the gears inside the gearbox 16 DP.

The Model B and C South Bend 9-inch lathe used changewheels for screwcutting. The tumble-reverse arm had to be released by a square-headed nut before it could be moved to reverse the direction of rotation of the leadscrew. The earliest lathes, made in 1934, had a single-arm bracket and a simple "reversing" stud by which means an addition gear could be incorporated in the train to cut left-hand threads.
Because there is no shear pin in the changewheel drive it is a good idea to just "nip" the bolt that secures the changewheel bracket to the boss on the headstock-end bearing hanger; this will allow the gears to push them selves out of mesh before mechanical mayhem sets in.

English pitch screwcutting chart for the changewheel-equipped Model C and Model B. A chart for the gearbox-equipped lathe can be seen here
It's likely that all Pre-WW2 South Bend screwcutting and other data plates were produced by a process called chemical milling - instead of rolling through a die or stamping, processes that leave an imprint on the back - all South Bend Plates are smooth on the rear. Whilst it's possible that during the 1950s a cheaper silk-screen process was used before then enamel was used to colour them. A paint in power form was applied in the low areas and then heated to melting point; the paint ran to its own level producing a superb, durable finish. A final touch was to lightly polish the raised letters and numbers. South Bend would not have produced their own plates, this was a process best left to a specialist undertaking.

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South Bend 9-inch "Workshop" Lathe
Screwcutting
Stands & Drive Systems    Headstock Details    Carriage Assembly    Screwcutting   
Accessories   9-Inch Home Page   
South Bend Home Page   South Bend 9-inch Clones
In the Factory  Original 1934 9-inch Model 5