Well Windlass: SolidWorks and Construction

We now had a satisfactory design and a scale drawing of the windlass. We first decided how we would attach the plastic parts. We considered using piano wire on the perpendicular joints, but decided with such a large piece it might be unwieldy to use the drill press. Instead, based on playing with the model of notches, we decided to connect all our perpendicular joints with tight notch fittings. From Amy's suggestion, we decided to attach the crank arm to the rod with piano wire, to make it strong enough to turn even with a weight on the string, and decided to connect the string to the rod with a tight bushing.
Now that we had a good enough idea of what our parts should be, we built them in SolidWorks.

The sketch for our first part - the "bridge" part

We ended up with five unique parts in total: the "bridge;" the top brace to hold the bridge parts together; the bottom brace, wider than the top, to stabilize the structure and keep it from toppling; the crank arm; and the bushings. Initially, we drew our notch holes to correspond with what we had measured with calipers on the sample notches, since we had not been able to test our own notches yet. 

Then, we put our parts together in an Assembly. We learned a lot about how to "mate" components and, in the end, were able to make an assembly where the crank arm was constrained just enough that it can rotate in the assembly just as it would in real life. 

Our SolidWorks Assembly

We thought we were ready to print and assemble our windlass, but we first had to test that our notches would be a tight fit. We made another part that consisted of two squares, one with two holes and one with two teeth, to see if the notches would fit tightly. 

They didn't. Since at the time we didn't know that the Delrin sheets varied in thickness, we carried out three more tests of progressively narrower holes, none of which fit tightly, without using the same sheet of plastic every time. We also tested our bushings, and the second iteration was a tight enough fit to not slide along the rod, and was also the right size to hold the string onto the rod. This was the moment when the laser cutter decided to break for the first time, and our design was delayed until we could cut our test notches from the previous assignment.

Julie and I came in over the weekend to work on our project, and I was finally able to print out my test notches. From this test, since I knew what the SolidWorks input was for the width of the holes and did not need to rely on calipers for this measurement, it became clear that the notches would fit tightly if the holes were .2mm narrower than the measured thickness of the Delrin. Armed with this knowledge, we found a sheet of plastic, measured it, and adjusted the holes to be small enough to ensure tight notches. After one last printing of our notch testing part, finding it was a good fit, we were ready to print the whole thing. 

We tried to make a SolidWorks drawing that resembled the original graph paper arrangement of the parts, but were unable to make multiple copies of each part or to orient some of the pieces upside down. We ended up sending five separate drawings to the laser cutter and having each one cut a certain number of times: two "bridges," two of each crossbar, one crank arm, and four bushings. 

All of our Delrin parts, fresh from the laser cutter. 

The notches worked great and the pieces literally snapped together, almost like Legos. Now we just needed to add the rod. Our first attempt at piano wiring the crank arm to the rod failed, as the wire got stuck halfway into the hole and our attempts to push it through cracked the plastic of the crank arm. We tried again, using the other hole of the arm, and were successful in pushing the wire through because we used continuous pressure instead of trying to hammer the wire as we had the first time. 

We added the rod, bushings, and string to the piece and tested it. Though it wobbles slightly before the string becomes fully taut, when there is tension in the string it is quite stable and very easy to crank; the bottle does not feel heavy at all. There is also another length of rod that can optionally be attached to the other end of the crank arm. I find it easier to wind with this extra rod; Julie prefers not to use it, so we kept this one detachable. 

Some of the initial wobbles come from the fact that some part of the plastic is slightly warped: though the parts have right angles in SolidWorks, the opposite corners of the bridges are slightly off from one another. I also noticed that the cut of one of the upper crossbars is not perpendicular to the plane of the plastic: the plastic must have been resting at a slight angle in the bed of the laser cutter, which would explain why the overall assembly is not more stable (or none of the tables are flat?). 

The windlass complete. 

We will be demoing the design on Wednesday, hopefully it goes well.