Design Process - AC

As I was able to access Texas InventionWorks during the semester, I chose to construct my mechanism using 3D printing. I chose this over the alternative offered, laser printing, as I felt that the better 3D design abilities would lend well to the pins and joints necessary without requiring additional hardware.

I began with a test print to find the best sizes for pins and holes to allow for smooth rotation and sliding in the pins and slots. As I was planning on making the entire assembly approximately 6in square, the pin sized that seemed best when I drew out the mechanism was approximately 1/4in. As I knew I wanted a free moving joint, I wanted the pins to have some clearance (be smaller) within the holes. To test the exact sizes, I printed out a set of holes at 0.25, 0.26, and 0.27 in and a set of pins at 0.25, 0.24, and 0.23.

0.25-0.25

0.25-0.24

0.25-0.23

I found that no clearance (0.25-0.25) would not fit together, while 5 thou of clearance (radius) produced a rubbing fit. I found that 10 thou of clearance produced a free fit, with great joint mobility. Because of this, I chose to use 0.25 pin and 0.27 hole, as this allowed for the largest pins in my testing range and I was slightly worried about pins being too fragile. This extended to the slider block, in which I used the same 10 thou of clearance (on both sides, 0.75-0.77) to produce a sliding fit.

I then began the initial CAD-ing process. I initially used the measurements directly taken up from the image of the mechanism in the project description. As I scaled up the image to a full 8"x11" page, the final dimensions conveniently fit within the 6in squared I wanted. From these measurements and the pin size, I put together the CAD above.

In assembling the links, I noticed the current measurements (and slot design) led to interference at the center pin. This was because (AB - O2A) ~= (O2O4) when the design calls for O2O4 to be slightly less, with the difference being the distance between the pin and the end of the slot. To resolve this issue, I slightly lengthened L3 (slotted, AB) and slightly shortened L2 (input, O2A) as I found this combination changed the resulting path of B the least.

I then printed the first set of links for the Joint Assembly assignment. To allow for faster prototyping, I only printed the links (with no slider block) and a very simplified base with only the two pins, as this was mainly to test the movement of the linkage. I found the main issues were with stability, as the simplified base provided no support to the links above. I also found the links were hard to manually turn, as the pins sat flush with the holes and gave no good point to grip.

With the initial prints, I was able to plot the path of point B (end of slotted link). As this path has a very characteristic shape, even with the base being unstable and hard to fix in location, a clear representation of the desired path was drawn, showing the prints were accurate and would work for the final print.