For the following discussion, please refer to the sprocket prototype
located at:
http://www.rctankcombat.com/tanks/T079/005-large.jpg
We started by choosing a dimension for the tread and a slope for the
angled edges. With that cross-section, we then laid out the sprocket
dimensions using a compass and ruler. We choose 8 treads around the
sprocket so that the diameter of the sprocket was close enough to scale
(and because 45 degree angles are easy to lay out).
The angle and height of the sprocket teeth fit the angle and thickness
of the tread. The top-width of the sprocket (which equals the distance
between treads on the belt) was set to approximately 3/8" based on
experience with previous TTS treads. If it's too small, the sprocket
tooth might break off and the belt may be too hard to bend around the
sprocket. If it's too large, it looks dorky. Once those values are
chosen, the rest is just math and a little fudging. I laid out the
initial design using modeling wax and cut it by hand to make sure it was
correct.
Someone (I can't remember who) then drew up the design in CAD and then
we sent it off to one of the 3D printer services to make a prototype.
The prototype included a 6-sided 1/4" high spacer on one side of the
sprocket. The spacer diameter and height were chosen to allow a 1/2"
wide x 1/2" high tooth molded onto each tread to pass around the
sprocket. The spacer also had dimples to locate the center of the
sprocket and an alignment point. We also drew up an inner tread
prototype and had that 3D printed at the same time. (The outer tread is
easily prototyped by hand out of wood/plastic/wax so we didn't waste
money having it 3D printed.)
When the prototype came back, we made a rubber mold of it and then
produced 4 pieces per tank. The 6-sided spacer was used to drill out
the sprocket center in the lathe and a drill press used to drill out the
alignment hole in each sprocket. Mounting holes were custom drilled in
each pair of sprockets to fit whatever axle mount was being used. In
each case, the axle was used to ensure that the sprocket was centered
properly before drilling any holes. Notches were cut into each mount
flange to ensure that the proper sprockets went back on the proper
flange and in the proper orientation while working on things.
We also made rubber molds for the inner and outer treads, with four
copies of each to make the molding process go faster. A couple hours
were then spent cranking out molded parts using polyurethane resin and a
small amount of dye per batch (Never, Never, Never use too much dye).
The exact volume of resin used by each mold was determined by filling
them with water and then marking that level on the cups used to pour the
resin. The total number of treads needed was calculated by measuring
the wheel assembly, with the tension mechanism at it's center position.
With all molded parts made, we made a jig to hold the treadmill belt and
then started laying out spacers to fit the sprocket. With the first
tread attached, we slid the belt along one notch, then attached the next
tread. This kept all the treads properly spaced to fit the sprocket. A
3-person assembly line was formed to drill, glue and rivet the
inner/outer treads to the belt. The final closing of the track was done
using the jig to ensure proper spacing.
On 10/24/2012 5:44 PM, Neil Rochford wrote:
I have an interest in this track and drive sprocket, I would very much
appreciate a few more detailed "build pages" on the track construction.
Im about to benefit from access to a cad controlled laser cutter, so
making multiple drive sprockets like these will be the way ahead for FTF
future vehicles. Im also wondering how your track guide teeth were
constructed.
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