For our midterm, we were tasked with creating a physical mechanical model from one of the examples in 507 Mechanical Movements. I selected movement 127, the reciprocating rack and gear combo that I modeled for my 2D drawing assignment. Looking at 127 made me immediately think of a see-saw, with the two racks being people moving up and down. 127 had potential (and would also save me a lot of work by starting with a familiar mechanism), and so after some deliberation, I decided to venture into the land of gears and racks.

My first challenge was to determine how the machine would actually be implemented in real life. Instead of being suspended in air, there must be something to support and stabilize the components while moving, as well as a connection point for the gear shaft. After browsing past ENGI 210 midterms involving racks and gears, I decided to create a backboard to place the rack and gear against, while a central hole for a gear shaft connection and slots along the sides to guide the motion of the racks. The lever would be mounted on the shaft and in front of the gear. By pushing the lever, the user would be able to move the racks up and down.

With the basic idea in mind, I needed to create a 2D drawing for an actual first prototype. I knew the gears/racks I traced for the 2D drawing assignment had nonuniform teeth and would not actually work in reality. I used the website http://hessmer.org/gears/InvoluteSpurGearBuilder.html (thanks to Kelly L for writing about it last year!) to automatically generate a rack and gear set. I duplicated the rack in Illustrator and added a basic lever and two backboard pieces with slots (2 pieces would let me adjust the rack spacing). With that, I created my first cardboard prototype.

Cardboard, to no one’s surprise, is not that great for gears. The gear teeth were easily bendable and often got stuck when turning. The racks also wobbled when moving up and down, making me realize that having only one dowel stuck in the middle of the rack as the support was a dumb idea. I edited the Illustrator file to add long slots on the rack and rectangular guide pieces that would insert into those slots, to control the movement of the entire rack.

I made my 2nd low-fi prototype with wooden components and a cardboard backboard. The rectangular rack supports allowed the rack to move up and down without wobbling left or right. However, turning the gear sometimes pushed the racks out of their slots. Furthermore, I had trouble getting the gear to turn at all. The wood made lots of unpleasant creaking sounds when the gear moved, and the racks often got stuck in the slots. I verified that the teeth of the gears and rack could mesh properly by rolling the gear between the racks on the table. I messed around with the prototype for half an hour before I tried sanding the insides of the rack rectangle support slots, magically fixing the problem.

What happens next is a neverending process of optimization. Thanks to cardboard, I determined that building a model for 127 may actually be possible. Now, it was back to Illustrator to make something a bit more refined. A long list of changes and tests happened between cardboard prototype 2 and the medium-fidelity. These included:

• Deciding to glue the lever to the gear and allow the gear to rotate freely along the shaft dowel, instead of making the dowel the source of movement and press fitting the gear.
• Trying to cut out an acrylic backboard so the racks could move against a surface with less friction (ended up cutting at way too high a power and leaving some nasty burn marks)
• Actually putting my rack and gear into Solidworks to get a number for the spacing between them
• Lots and lots of Illustrator modifications

 

After all that, I finally made my medium-fidelity prototype! Featuring: lever that is finally functional (will turn gear when pushed), a single wooden backboard with intentional slot spacing, and racks that can move with minimal wobbling.

What comes next is even more optimization and Illustrator work. Now that I had a functional model, I turned my attention to the question of aesthetics. After a long period of brainstorming, I thought that the racks would look neat as little palm trees, with the gear as the sun and animals sitting at the ends of the lever see-saw. The lever would be my plasma cut piece. I also wanted the whole device to stand vertically and would need a way to mount and support the model. I decided to combine finger joints with triangular back anchors and rectangular front anchors.

I spent way too long adjusting anchor points and dimensions in Illustrator. At last though, my design work was done, and I had a composite file of every wooden piece I needed to laser cut.

 

By then, it was Saturday. On Sunday, I entered the OEDK with a mission: laser cut ALL the pieces. It was not to be. I heard the news that the Epilog was having issues cutting completely through, but I held on to hope and started a 40 minute cut anyways, without doing a single test cut beforehand. When I opened the Epilog lid after the end of 40 minutes, I felt betrayed.

I was in a mild panic mode at that point, but it was dinner time by then and I wanted food. On Monday, I marched into the OEDK again, straight after my 1o AM class. The Epilog was back up! I got all the pieces I needed cut in the morning.

The afternoon was a whirlwind of assembly and postprocessing. I filed and sandblasted my lever, stained or painted all the wood, and used a respectable amount of clamps while wood gluing. I had to enlarge the size of the holes in the lever using the dremel or drill press in order to fit the necessary dowels. I used honey maple gel stain for the backboard, mahogany stain for racks (tree trunk), and paint (thanks to Brian for letting me borrow his green paint) for the rest. It’s amazing how many different stains and paints you can find in the OEDK cage.

Waiting for paint and glue to dry takes a long time. Because my pieces weren’t completely dry by the time I began clamping and gluing, a lot of dust and particles got stuck on the surfaces. I attached the lever to the gear and donut supports using epoxy. When fully assembled, the device functions as it is intended. However, the movement of the racks isn’t as smooth as I would like, perhaps because of the stickiness of the paint or the dust trapped on the surfaces. The paint job could’ve been neater overall, and there are a few specks of paint in random places. This midterm certainly took a lot of work, but it is incredibly satisfying to have a full working machine built at the end. It certainly combined all the previous projects in ENGI 210 and taught me valuable things about mechanical design.

Cost Analysis:

• 48”x23” plywood sheet (1/2” thick): $14
• Aluminum sheet: $30
• 1/4 ” OD Dowel: $2
• 3/4” OD Dowel: $5
• Gel Stain: $8 * 2 = $16
• Paint set: $12
• Paint brushes (to apply stain/paint): $11
• Wood glue: $11
• Epoxy: $10
• CAD/Illustrator design labor: $17/hr * 10 hrs = $170
• Machining labor (laser cutting, plasma cutting, sandblasting, sawing): $22/hr * 5 hrs = $110

Total: $391