|In building my "Senior 20" Organ I've generally followed John
Smith's plans closely as far as dimensions go, but I've adopted a few
alternatives in assembling the drive. I report them as methods which fit
with my parts availability, skills, and tools, not as "improvements." Since
I'm building in the United States of America, I'm working primarily in
old-fashioned inch measurements and raw material sizes, and so I have used
inches in my descriptions.
I decided to fabricate a 2-piece crankshaft. This is to facilitate parts
installation. The main part of the shaft from the handcrank through the three
bearings is brazed into one piece for rigidity. For easy alignment, the portion
of the main shaft extending through the central throw was cut after brazing.
I used ¾ inch by ¼ inch steel side plates, rather than the 1 inch
by ¼ inch pieces described, in order to reduce the width of the slot
required in the pressure box front panel. I could not find a specification
for the rod diameter in the instruction package, so I chose 3/8 inch steel
I made the front single-sided crank throw as a separately brazed part so
that it could be removed for assembly. It is made from 1 inch by ¼ inch
steel and 3/8 inch rod, and is drilled to clear a 6-32 machine screw through
the main shaft and one side and tapped on the other. Since the front
throw can be removed I was able to use a solid wood drive wheel. A small
hole drilled in the throw and a brass nail driven in the drive wheel to provide
the driving force. The drive train photo shows the removable throw,
the wood drive wheel, and the pin assembled in place. It also shows a minor
variation in the idler wheel construction.
I used a 3 inch hole saw to cut the disk, which resulted in a diameter of
about 2 7/8 inches. I then grooved the outside of the disk and installed
an O-ring with a 3 inch OD as tyre. This rubber ring came from a tractor
parts supply house, and is sold as a repair part for hydraulic cylinders.
I decided to use a ¼ inch steel shaft for the idler. I drilled the hole
in the idler wheel slightly oversize and press fit a short bushing of 9/32
inch OD, ¼ inch ID brass tube in the hole. I used the same size tube
to bush the holes in the pressure box sides for the take-up spool shaft.
The 2-piece crank also permitted the installation of a simple spacer cut
from plastic pipe between the pressure box side and the center throw. The
drive wheel and this spacer hold end motion of the crank to a small value.
Removable Hand Crank
I constructed the arm of the hand crank from 1/8 inch by ¾ inch steel.
I drilled a 5/8 inch hole in the crankshaft end and brazed a short length
of 5/8 inch OD, 3/8 inch ID tube in place. The arm was bent at two places
before brazing in order to provide good clearance. I drilled the tube and
crank to take a "hitch pin" so that the crank can be removed without tools
for transportation and storage.
Take-Up Drive and Spool
I thought John Smith's drive clamp, disk, and "cam" pin on the take-up drive
wheel was more complicated than necessary. I cut a plywood disk with a hole
saw and used a classic "set screw and flat" drive. I used a milling machine
to put the flats on my shaft, but they can easily be filed. My "set screws"
are what we call "dry wall screws" in the US. I drilled a pilot hole in the
disk and ground the point off the screw before installation. The brass pin
sticking out of the wheel is to drive the arm of the animated conductor featured
in the "Senior 20" design.
I could not find the 68 mm diameter drain tube recommended for the take-up
spool, but I discovered that I had a considerable stock of 2 ½ inch
OD, ¼ inch wall cardboard "mailing tube" already on hand. I soaked an
appropriate length of tube in varnish and let it dry for several days before
using it to construct the spool. I used a "set screw and flat" drive here
as well, drilling a 3/8 inch diameter hole through the tube and grinding
the screw point until the screw head was fully recessed. Instead of fastening
the drive tongue to the tube with screws I cut some springy aluminum strip
long enough to go about 2/3 of the way around the inside of the tube, curved
the strip to approximately the outside diameter, and bent the hook so that
spring action holds it in place.
Cutting Disks on a Drill Press
I used a variation of the "pivot and band saw" approach to cut several of
the larger wooden disks needed. I drilled a hole in a length of scrap wood
and inserted a short length of wood dowel in the hole. This jig was then
clamped to the drill press table, and a router bit installed in the chuck.
A pivot hole was drilled at the center of the disk to be formed and the plywood
placed on the pivot. The drill press table is then rotated until the bit
is accurately located and locked in place. After starting the drill press,
the bit is lowered to plunge-cut an initial hole and then locked in position.
The wood can then be rotated to cut the rest of the disk profile. For the
1/8 inch thick spool ends, the disks could be cut in a single rotation. For
thicker material it is advisable to step down through the wood in cuts of
1/8 inch or so. I think this technique gives a smoother circle, and the
plunge-cut means you don't have to work into the edge as you must with a
I turned two solid crankshaft bearings from squares of ¾ inch thick
oak. They are about 1 3/8 inch square, 7/8 inch OD, and drilled for the 3/8
inch shaft. They have oil holes in the top of the square. They are a light
press fit in the panels and retained with small wood screws. I made a similar
split bearing for the front by fastening two rectangular pieces of wood together
into a square before turning and drilling. The purpose of these bearings
is not to reduce friction, which will be minor in any case, but to increase
bearing life by reducing contact pressure and increasing surface hardness.