CS291I Assignment 7

In this assignment I explored press-fit geometry. This assignment came at a great time since my mom had a shelf with broken joints (as seen in the second image), so I took this opportunity to fix her shelf. The shelf is made of bars threaded through cloth, held together with the mentioned joints.

The first thing I did was take a look at the example Grasshopper code that creates a tool to find the correct tolerance for the press-fit geometry. I modified the code such that the shape of the parts were circular with dimensions approximately equal to the shelf bars. I also only needed to print the outer part of geometry since the inner part would be an existing object in my house.

The smallest circle has an inner diameter of 13mm and the largest has an inner diameter of 14mm. Each successive circle increased this by 0.1mm.

I printed out the part and tested it. Unfortunately one end of the test strip came off the print bed as it was printing, but I wasn't too concerned since I was pretty sure 14mm was too big. I found that 13.3mm fit but was a bit tight, so I went with 13.4mm.

I created a 3D design using the 13.3mm dimension I got from testing. The joint design consists of 3 intersecting tubes of the same diameter. The two shorter sides are about 20mm, and the longer tube is about 40mm. There is a solid blockage midway through the longer tube because the original also had this.

I exported the design and sliced it in Cura then multiplied it to print 8. I didn't realize the multiply would create 8 copies, so I actually printed 9. This was a bit lucky since one joint became dislodged from the bed during printing, so I ended up with 8 in the end. This would be sufficient for 2 levels of the shelf. The 9th joint is usable but would look slightly shorter.

I was able to attach the shelf bars together with the joints. The vertical bars fit nice and snug, sometimes even better than the original. However, the horizontal bars sometimes did not fit. I tried forcing it in hoping that it was just slightly too large, but ended up cracking some of the joints.

I realized that due to the way I printed the joints, the side tubes were not completely circular. The were slightly deformed because the way the model was oriented caused overhangs that led to some sag. I guess this is a good example of the benefits of using a physical printer than just learning with virtual simulations. I was able to see the consequences of dealing with physical materials. The holes from the vertical tube fit every bar, so theoretically the tolerance was good.

I only printed 8 joints, and some cracked since it took a while to print. I will continue printing more as the shelf required 40 joints total, but I will modify the geometry such that the side circles have a slightly larger inner diameter to counteract the sag from printing overhands. I think I could also use supports, but I would probably have to add supports that touch the inside of the model, and I'm too lazy to deal with removing them all.