I picked up a 1951 Singer Featherweight sewing machine recently. It's
in great shape, except that even after applying lots of oil, the shaft
which rotates the bobbin assembly resists rotating through about half
its travel. Presumably this means the shaft is very slightly bent,
although I can't detect any distortion visibly. So, while I can still
turn it by hand, the motor isn't quite up to the task. Here's a photo;
the problem is the light shaft running down the middle:

http://www.lunkwill.org/tmp/singer.jpg

As you can see in the photo, it's about a 1/2" dia shaft held in place
by two cast iron mounts at either end, with cast iron caps at either
end connecting to the bobbin assembly and motor rod. The caps are held
in place by about 1/10" dowel pins, which unsurprisingly can't be
pounded out.

Here are the solutions I can think of, but I'd appreciate your
comments:

1. Remove the shaft by drilling out the dowel pins or perhaps by
careful heating of the caps with a torch to loosen the pins. Then
either:
a: turn the shaft on a lathe so it moves more freely. (How much play
might this introduce?)
b: cut a new shaft and make sure the caps end up at exactly the right
places. (Apparently, bobbin position and rotation needs to be rather
precise)

2. Leaving the shaft in the machine, heat the center of the shaft until
it just starts to become plastic, rotating the shaft continuously as it
cools, so that the mounts themselves ensure smooth rotation. Potential
problems would be twisting of the shaft causing the caps to be at the
wrong angles, and expansion of the shaft preventing any rotation while
it's hot.

-J

If you have a lathe, you probably have a dial indicator. I would
verify that the shaft is indeed bent with the indicator - if not,
then a little investigation is in order as to all the components
connected to the shaft - do they all operate freely? If the shaft is
slightly bent, I would suspect that it would be binding 360 degrees,
not just half of its rotation. You may be able to place the bend
close to the "cast base plate" and gently pry to straighten - checking
over and over with the indicator to know how much correction you are
gaining.
Ken.