Much Many of the changes as the design evolved focused on attempting to achieve smooth motion of the slider along the helically shaped path. A first decision was whether to have the slider rail be an extrusion from the lower arm plate (Figure 1) or a helically shaped bar attached to the lower arm plate (Figure 2). Using a rod would allow a harder rail for the slider than an extruded rail made from 3d printed ABS (acrylonitrile butadiene styrene) or PLA (polylactic acid), and this would be advantageous. A specialized a freeform pipe bending CNC machine are capable of complicated pipe bending could theoretically be used for making the bar, however we did not have one available for our use and we wanted to produce our design with the means available to us. Another possibility discussed was the use of a hand bending tool or roller. The problem with bending tools is that they can only make 2-dimensional bends of a certain radius. A roller would allow more flexibility in the radius of the curve made, but again is designed for making a 2-dimensional bend. The technique attempted was the constructing a custom cylindrical jig sized for the curve being made and hand bending the rod round the jig, as shown in Figure 3. Using this method we were able to bend a piece of 5/16” steel rod into an approximately helical shape, as shown in Figure X. However this was technique was unable to achieve a sufficiently accurate curve, so the decision was made to print the rail.
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The manufacturing of the hinge on the upper arm and the hinge attached to the slider was straightforward with no design changes. Flanged bearings (1/4 ID, 1/2 OD) were inserted into either end of a piece of aluminum and a 1/4" steel shaft inserted through holes in the printed PLA pieces. This was all held together using a retaining clip on the outside of the bearings. A rod 3/8" steel shaft between these two hinge pieces was held in place using 1/4" set screws, allowing alteration of the length of the rod and adjustment of the relative angle of the two hinges. These pieces are shown in Figure 12 below.
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