For my midterm, I modeled and fabricated Mechanical Movement 26, the crown gear. This movement consists of two gear shafts that transmit power through a transition between a spur gear (the gear you normally imagine as a ‘gear’) and a crown gear (right of image). A crown gear’s defining characteristic is that the teeth protrude from the face of the gear instead of along the outer edge. Industrial production typically does not use this type of gear in favor of the more strong helical bevel gear, however,  this gear can be fabricated much more easily in low-resource environments when necessary.

Moving into the early design stage, I rendered all the parts to be fabricated: the spur gear, the crown gear, and the assembly enclosure.

I made each piece separately using the principle of parametric dimensioning. These models reflect the final piece and did not account for kerf in their placement. To create the tool paths that accounted for kerf, I created sketches on each parts’ finished surface and created an offset path that directly accounted for kerf on each machine.

I then fabricated the two gears. The spur gear was made using the WaterJet and a sheet of stainless steel. The path offset used for this material on the WaterJet was 0.02″ and fabricated the gear at a very precise quality.

The crown gear was a bit more involved. It was fabricated on the laser cutter using a kerf offset of 0.007″ from 0.20″ birch. The toothed face was the first one that I created (visible to the right) and then I fabricated a circular backing to provide the support and geometry which defines a crown gear. I finished both of these using a polyurethane-stain blend, and then glued the pieces together using wood glue and clamps (shown) to form one solid piece.

Next, I fabricated the enclosure using the same birch sheet as the crown gear. These pieces fit together using finger joints to create a simple box-frame for the mechanism. One issue with the box was the tolerance for the gear shafts. The holes allowed the shafts to fit through, but they caused substantial resistance when I attempted to turn them, so I used a drill bit of just slightly larger diameter to increase the tolerance of the hole. This relieved the pressure on the surface while still restricting the rod’s motion the way it was intended.

(I later refabricated the entire box to account for this as well as lower the spur gear by 5/1000″ to improve meshing)

I then attached the gears to the wooden-rod gear shafts and completed the assembly of the mechanism.

I coated all of the rotational surfaces with graphite. This further reduced the friction between parts and allowed rotation more easily.

To top the assembly, I fabricated a diamond out of aluminum on the plasma cutter and used the angle grinder to hone and sharpen up the edges.

 

I resurfaced the piece by sandblasting it and etched my name and the class title into the metal using the laser cutter.

To attach the piece, I used the band saw to cut a thin slit into the top of the crown gear’s crankshaft (shown left).

For finishing the enclosure, I decided that graphite looks really cool on wood. So, I spent the next 50(ish) minutes coating the entire outer surface of the enclosure in graphite to yield the final product.

 

Evaluating the cost:

• Design Cost
  • File preparation for parametric dimensioning, file conversion, and rescaling in Adobe Illustrator took me approximately 3 hours to prepare due to experience from the box project. This time was primarily spent in determining the algebraic representation of distances throughout the design sketches.
    • I would charge a total of $50 for this step because the value of parametric part files is still substantially more than that of numerically dimensioned parts, but this part is much simpler than the box project.
  • Access to Fondren Library for a Rice Alumnus is $75/year. Fondren Library has computers with the most up-to-date Adobe Creative Cloud Software and Autodesk Inventor.
    • I would argue that this assignment constitutes 2/52 of the Fondren Library cost due to the proportion of time which I’ve spent doing file creation ($3).
  • Assembly and finishing accumulated to 4 hours.
    • I would charge a total of $30 for my time because this is relatively simple, non-specialized work.
• Consumable Cost
  • 24″x32″x1/4″ birch sheet = $5
    • This is an appropriate size sheet to do this part fabrication on a laser cutter.
  • Wood Glue 4 fl. oz. = $3
    • Only a few drops used
  • Graphite 6.5g Tube = $4
    • Completely consumed
  • 12″x12″x1/32″ aluminum sheet = $20
    • This is an appropriate size sheet to fabricate the plasma cut piece.
• Machine Time
  • Access to a CO2 Laser Cutter
    • Laser Cutting took 30 min, and CO2 Laser cutting costs approximately $20/hour ($10 total)
  • Access to a Plasma Cutter and angle grinder
    • This entire process took approximately 30 minutes to complete at $20/hour ($10 total)
  • Access to a WaterJet
    • All path-making software is open source and free. The machine time including consumed abrasive defines the cost. The cut took approximately 25 min at $20/hour and set up took about 5 min ($10 total).

Total Expense: $145