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Here is the back of the sander's cabinet; left-over BC plywood, butt joined with screws and glue, nothing special.
Before you assemble, lay out the centers of the axle in the top and bottom, exactly in the same vertical plane. This
is the only precision called for in the project, and even this isn't super critical; 1/16" more or less shouldn't matter.
In the photo you see a very small (2" - 2 1/2") pulley driving a very large one (15" - 16") which drives another
very small pulley via the jackshaft (a "mandrel") which finally drives another large pulley on the final drive axle.
This arrangement will get the sanding speed down to a comfortable and safe 25 -35 rpm.
The jackshaft is mounted on a swinging plywood plate anchored with a length of allthread, also known as the lower belt
tensioner. Slide the motor on it's mounting bolts to tension the upper belt.
Large pulleys cost a fortune, so I made mine out of plywood. Clamp a vertical fence above your tablesaw and saw
a vee in the rim with the blade tilted. If you screw up the cut and wreck the rim. reduce the pulley's diameter and
repeat. A fraction of an inch will have a very small effect on the final speed.
Here is the bottom bearing - plywood, of course - on the lower end of the drive shaft.This disc is about 6 " in diameter
because I have a 6" hole saw, and I could use an adjustable circle cutter to match the disc to the hole in the square plate,
which is screwed to the bottom of the cabinet.
Notice the tiny steel nub in the center of the hole. I drill 1/4" holes everywhere I want good alignment, and then
use short sections of 1/4" round rod to position the parts temporarily before I cut or bore them for final assembly.
It also helps that many hole saw mandrels and adjustable circle cutters use 1/4" pilots and drill bits as centers.
The drive axle is a 30" length of 3/4" black iron pipe with a floor flange threaded and loctited on each end. This
length will determine the height of your machine, so you can adjust to your best working level. You should gather as many
of the parts together as possible before you begin cabinet construction; there's nothing wrong with a little mocking-up to
help with visualization.
The end grain of wood is very wear-resistant - as much as 50% of the edge of a piece of plywood is end grain, and simply
loading the edge and bottom face of the bottom disc with white grease will provide all the lubrication needed. See the rub
marks from the bottom bearing; this machine has sanded the rib of a couple of dozen guitars, and the wear hasn't obscured
all of the plywood's grain pattern.
This is a view of the top bearing protruding above the cabinet. The cabinet height was planned to align the top bearing's
guide - same arrangement as on the bottom bearing - with the edge of the top bearing. The top bearing carries a
side load only, with the bottom bearing carrying both side and down thrust.
Notice the three steel pins in the face of the ply disc. The middle one is one of those ever-useful locators,
dead center. The two outer pins are the actual drivers; they match holes in the bottoms of the various sanding
dishes and plates I use.
Now check out the ball bearing in the corner of the picture. It's an "Alwayse" and it comes from England. If
these are unavailable a standard lazy susan bearing or a sheet of UHMW polyethylene will substitute, and you could even try
wood running on wood; this machine has so much spare torque that it is nearly impossible to bog down.
Here is a view of the top of the machine, ready to accept a dish. The bearings are installed in pairs arranged as opposites,
on different diameter circles, the first pair at 12 and 6 o'clock laid out at 18", the next pair at 18 1/8", and the final
pair at 18 1/4". This spreads out any possible wear, and seems to work, since no grooving is apparant in the back of
the dish carrier.
Back inside the cabinet, the axle has been cross-drilled to accept a length of 1/4" rod, which fits up into
a routed slot in the underside of the drive shaft pulley. I trapped the pulley down onto this pin with a shaft collar.
Nearly the most important components of this machine are the v-belts. Standard belts are too stiff to travel snugly
around the small pulleys without pulling in excessive tension. COGGED v-belts will transmit far more torque before they
slip, and they have the added virtue of transmitting power with less vibration. They can be set up with less tension
than standard belts, so putting lower side-load on ball bearings; consequently, everything runs smoother and quieter and lasts
longer. All my machinery has been treated to COGGED belt replacements.
Here is a side view of the hollow dish carrier: two plywood disks carefully centered to each other and separated by a
glued-and-screwed plywood box. This won't twist or warp and keeps the MDF dish to it's original configuration - an r15' form
won't become 13' or anything else with changes in the weather.
See also the carriage bolt; it's an adjustable stop to control the mold height above the sandpaper. The wooden dowel
is a locating pin to keep the workpiece from spinning with the dish.
Top view showing the leveling jacks (stops) and locating pins. Load a set of ribs in the jig, turn on the motor, and
walk away; sanding ends when the rib-holding jig touches the stops and can't drop any closer to the sandpaper.
At home in the new shop
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