For nearly a month, we worked on our midterm project, which is creating a physical model of a movement from 507 Mechanical Movements.  For the project, I chose to do mechanical movement 93, which is colloquially known as a Scotch yoke (yes, the name played a major factor as to why I pick this movement). As per “507 Mechanical Movements,” it is described as “crank motion, with the crank-wrist working in a slotted yoke, thereby dispensing with the oscillating connecting-rod or pitman.”

First, I needed to create a 2D design for my piece.  I began making the piece by creating the concentric circles. for the crank wrist, and a circle towards the bottom of the large circle to represent the actual pin. Then I began creating the yoke. I began by using the rounded ellipse tool to create the part that actually touches the pin. I created inner surface of the yoke to be the size as the diameter of the pin, and gave the outer diameter a 5 points on every side. I then create began creating the shafts on each side of the yoke. I began by using the arc tool to create the arcs. I had the arcs touch the yoke 30 pt from the center, and used the add anchor and scissor tool to erase the parts in the yoke in between the arcs. I then added vertical lines up through each of the lines that filled the rest of the page, and then had horizontal lines close the gap.

 

2D Movement 93-268jns4 – If you want to see the initial design for my m0vement.

Then, I had to create a Low and Medium Fidelity Model (LFM/MFM) for my movement.  I decide to make a low fidelity out of cardboard and a medium out of entirely wood (as opposed to the final part, which would have a metal piece). To make the actual piece, I also include rails for both of the edges of the yoke to force it to move linearly with a rotation of the circle piece and a rectangular base that has a hole cut out that will hold the crank and slots cut in it to hold all of the rails.

 

After making the LFM, I realized how weak cardboard was, but overall my part worked well.  I had to replace the button on the image with a nail that I put through the hole.  The crank kept getting snagged on the cardboard, but that was just cardboard being annoying.  Then I created a wooden MFM. I decided to scale up in between the prototypes, since I made the LFM small as more a proof of concept than anything. I also decided that have a big nail in the next stage of prototyping wasn’t a great idea, and so I sawed off a piece of a .5″ diameter dowel and used the belt sander to sand the side flat (well, as flat as I could) sot that I could glue it to the crank.  I actually didn’t check dowel sizes before, so my first yoke didn’t have an opening wide enough and I had to re-cut it. I got all of the pieces cut, but the crank wasn’t spinning, so I spent a good deal of time sanding the surfaces on the crank and base that were in contact (mostly the crank), until I got to the point were it rarely snagged on itself.

The next class, Dr. Wettergreen posed the idea of standing my project upright.  I liked the idea and went with it, but that posed a few problems. First was the arms of the yoke hitting the ground, but that was easily taken care of when I decided to display it horizontally. However, there were other problems that were less simple that had to be taken care of.  Namely, nothing was really holding the crank and yoke in place now that gravity was no longer on my side.  I decided to take care of the yoke by putting little “hats” on the rail using finger joints similar to the box making lab, and I decided to fix the crank by cutting out an identical base to the original without any cuts and gluing it to back to back with the original. I also decided that the yoke would be the metal piece, because it was really a choice between that and the crank, and at the time I already had the wood crank perfectly aligned with the base.

The hat end up working pretty well.  I thought that I would a full weekend to figure them out, but I basically got it after 2 or 3 tries.  After that, problems began accumulating.  I decided not to scale up again, so I stained the base and crank.  The problem with the base was that I sanded to roughly against the grain, meaning that the stain had very visible scratch marks all over it, causing me to re-cut the piece.  There was a little bit of glue reside on the crank and dowel, so the stains were uneven and ugly, causing me to redo those pieces as well.  And my first attempt at plasma cutting the yoke left the slotted region so thin that broke soon after I held it, meaning that i had to redo that one as well. On top of all this, I am constantly resize the rails and hat to account for the thickness of the yoke, and most of those pieces are breaking.

Once I have all of the pieces cut, I stain all of them (I use coffee stain, as it was the first one I grabbed and was satisfactory). However, I uncover another problem. The 2 bases glued together are not standing on their own.  I then cut a holding piece of wood, and on it raster my name and class onto it. I then angle grind the yoke, put it through the wire brush wheel, and put primer on it.  I then leave and when I come back, I glue the bases together and the dowel to the crank. I decide to spray paint the yoke red honestly because red is a solid color to use in most situations. I also put (a lot) of tallow on the crank in order to smooth out the rotation.

I then ran into another problem.  Upon trying to spin the piece, the yoke would fly of and knock the hats away. I wanted the part to have been able to be taken apart, but I couldn’t do that if I wanted a stable turning mechanism, so I glued the rails and hats to the base with the yoke inside. Once the paint was dry, we the machine worked great.

One thing that I learned from this midterm was that I needed to plan better.  I thought I was making good time, but then I look up and I’m miles behind and struggling to get it done on time.  Better time management would have helped me get this done faster, such as cutting multiple copies of a piece at one time so that I need to use the laser cutter less (I spent way to much time waiting and cutting in the laser cutter.