Design
The design for the Scotch and Yoke is based on this depiction of the machine
The design for this Scotch and Yoke uses a baseplate (14×10 inches) with a hole cut in it for a servo to be placed. On top of the servo, a circle (the Scotch) is placed with a circular guide for the piston (Yoke). The Yoke is cut out of acrylic and has a 2-inch slot that goes into a guide to fix the mechanism’s motion linearly.
Here are the DXF files for the baseplate and the moving parts
Drafting and Cutting
The first part of this project was to create the baseplate. I first created a border of 12×24 inches to indicate the size of the acrylic sheet I would be using. Then I created a 10×14 inch cutout as the main baseplate. In addition to this, I also drew a cutout guide for the Scotch portion of the mechanism so that I could see how big it should be. In the end, I made it 6.5 inches in diameter, with the guiding circle being 1.5 inches.
The box in the middle is the middle of the baseplate is the cutout for the servo to fit into the baseplate. It took awhile to get the measurements with a caliper as I not only had to get the dimensions of the servo itself, but also had to get the offset of the motor point so it would line up with the center of the Scotch. The measurements were 0.88×0.46 inches with a roughly 0.255-inch offset from the left side.
In laser cutting the baseplate, I forgot to resize the DXF file to the original dimensions as Ruby so the cutout was smaller than it was supposed to be. I had to recut the baseplate from a new sheet of acrylic – here are both of the cuts, the larger one is the correctly sized plate
Up next was to create the moving parts of the Scotch and Yoke. First, I drew up and cut out the Scotch and the Yoke along with the guiding circle to put on the Scotch. The Scotch was 6.5 inches in diameter and the Yoke was 7 inches on the shaft and 7×2.5 inches at the base with a 1-inch gap for the connector to fit through. The connecting circles to go on the Scotch were 1.5 inches and 1 inch in diameter each and sat on top of one another, the larger one below the smaller one. This was to reduce friction between the Scotch and the Yoke. There were guidelines for the smaller circle to fit on the larger one and for the larger circle to fit on the Scotch as well.
The next part of this design was to make the legs of the baseplate to keep it level and higher than the bottom of the servo. I made six posts out of 1-inch diameter circles cut out of an acrylic sheet and stacked 8 levels high. In addition to this, I created guidelines on the baseplate for these pillars to be placed.
Unfortunately, I did not cut out enough circles for the pillars so I had to go back and cut out more. I also added some designs onto the baseplate and a guideline for the guiding piece to be placed for creating linear motion from the Yoke. I was not able to use the arrows as I couldn’t figure out how to fill them in for engraving.
The next part was to create the piece that would go on the bottom of the Yoke and slot into the guiding piece on the baseplate. The dimensions for this were 2×0.78 inches as 2 inches is half of the distance to be traveled by the motion of this machine and 0.78 inches is the height from the baseplate to the bottom of the Yoke. I cut this out along with engraving a guideline of 2×0.12 inches (0.12 inches is the width of the acrylic sheet) for the piece to be placed on the Yoke.
Almost forgot! I have to make the guides to be placed on the baseplate for the newly cut-out piece! These were 5×2 inches with a 0.12×4 inch slot in the middle. I created two to be stacked on top of each other for more stability.
Assembly
Here is a picture of all the parts together before assembly
Uh oh! I miscalculated! The guides I created to go on the baseplate to make linear motion were too small. I forgot to account for distance traveled PLUS the length of the guiding piece. Unfortunately, I had already superglued the guides onto the baseplate, making removal very difficult. Additionally, it left a lot of residue, but I cleaned it off with an acetone leaving it just as good as new.
Design 2.0
Here is the redesign of the guidelines, this time measuring 6.5×2 inches with a 6.2×0.12 inch cut in the middle.
Here is a picture of both the new and old parts, the new one is on the top and the old one is on the bottom. Note how the glue leaves a residue on the older piece, I got rid of that residue on the baseplate with acetone.
Assembly 2.0
With the final pieces cut, it is now time to fully assemble! Here is the final resulting machine:
Machine Movement
Originally, the Scotch and Yoke’s movement had a lot of friction because of a clearance issue in the linear guide of the mechanism. In order to fix this, I sprayed WD-40 into the guide and freed up the machine. Below is a video of the movement of the finished Scotch and Yoke
Motorized Scotch And Yoke Movement
Final Addition
I forgot to put on indication arrows to show how the Scotch and Yoke moves! Here are the designs with measurements for the arrows I cut out to put on the mechanism
Here is a picture of the arrows and what the final mechanism looks like with them (unfortunately, the circular arrows are off-center)
