Sam,
There are also a few SBL discussion groups on Yahoo where you could get some great info. The Southbendlathe group has a summary of what to do for the A to B or C conversion, authored by paul_hvidston:
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South Bend 9” Lathe Conversion to Model “A”
Introduction:
This document is provided as a guide to those who have an interest in upgrading their Model “C” or Model “B” lathes, to Model “A” configuration. It is by no means a definitive work, but it is a compilation of practical information provided by the South Bend Lathe, Yahoo Group, which provides a good understanding of the hardware and modifications necessary to complete the transition.
The Models:
Model A: This model is equipped with a quick change gear box, which allows quick changes in lead screw rotation rates relative to headstock rotation. The Model “A” apron (attached to the saddle) has a clutch fed power take off (from a slotted lead screw). The apron has a detent lever with three positions, which may be selected to either power the saddle lengthwise along the lathe, drive the cross feed assembly, or have no action at all (neutral). When in neutral, the operator can manually manipulate the cross feed and move the saddle by either engagement of the half nuts onto the lead screw or through the use of the hand wheel.
Model B: This model is equipped with the apron features of the Model “A”, but has no quick change gear box. Gear changes are accomplished by manually replacing selected gears from a supplied set and realigning the gear meshes.
Model C: There is no quick change gear box, gears are changed manually as in the Model “B”. The apron only supports half nut engagement or hand wheel drive of the saddle. There is no power cross feed drive.
What do I need (parts wise)?
To convert to Model "A" from Model "B":
20 tooth stud gear
40 tooth stud gear
Quick change gear box w/ 56 tooth gear
Lead screw w/ drive gear
To convert to Model "A" from Model "C":
20 tooth stud gear
40 tooth stud gear
Quick change gear box w/ 56 tooth gear
Lead screw w/ drive gear
Model "A" or "B" apron assembly
Cross feed screw (gear driven)
Can I use my existing gears and/or lead screw?
The only gears that are used from the Model “B” and “C” units are the 80 tooth, which is utilized as an idler, and the reverse gears on the reverse bracket assembly. The existing 24 tooth stud gear on the reverse assembly is not utilized and is replaced by either the 20 or 40 tooth stud gears (see below). An existing 72 tooth gear may also be used be employed as an idler in place of the 80 tooth.
The required 20 and 40 tooth stud gears and 56 tooth gear box input gear do not come with the Model “B” and “C”, nor does the lead screw gear which engages the gear box output gear. The two stud gears are manually interchanged depending on the required operating range of the gear box. The stud gear not in use is stored on the input shaft of the gearbox, as a spacer, next to the 56 tooth gear.
Since a newly installed gear box will take up a portion of the span formerly occupied by the lead screw, the lead screw becomes too long to work in conjunction with the gear box. Again, the Model “C” lead screws do not have the longitudinal slot, which allows the apron to take power off the lead screw. Some people have supposedly cut down the Model “B” lead screws to work. Remember, the proper lead screw will be determined by overall bed length of the lathe (3”, 3 ½’, 4’ etc), gearbox/ no gearbox, and slot for apron power take off.
What other modifications do I have to make to my lathe?
The gear box mounts to the lathe bed through three counter sunk holes through the lathe bed. On Model “B” and “C” lathes, two of the three holes (located next to the headstock) are already present. A third hole must be drilled to properly mount and support the gear box/lead screw assembly. Most of the hole is drilled through relatively soft cast iron, however, part of the area of countersinking is through a piece of hardened angular way material adjacent to the hole. Drilling through two different consistencies, vertically, at the same time, may cause the drilled hole to be misaligned. Some people have used their milling machines to achieve acceptable results. The rest of the project should be a straight “bolt-up”.
Where do I find the parts?
Newspapers, user groups, wanted postings, E-bay, and second hand parts dealers (check the back of Home Machinist).
How much?
It depends on how much you need and where you get your parts. Several hundred dollars is probably average for a “C” to “A” upgrade. Some people have good access to parts and others don’t.
Can I add the gearbox now and add the apron later?
Absolutely.
Where can I get more information?
Read the past postings on this group.