Much Many of the changes made 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 Specialized a freeform pipe bending CNC machine machines are capable of complicated pipe bending and 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 construction of 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 This technique was unable to achieve a sufficiently accurate curve, so the decision was made to print the rail.
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A first iteration of the slider mechanism used a 3d printed slider, as shown in Figure 4 below. This apparatus, as expected, resulted in a slider that experienced significant friction. Some sort of linear bearing was needed, but the curvature of the slide rail does not allow the use of a standard linear bushing, such as that shown in Figure 5. A posting on Grabcad,1 shown in Figure 6, provided an inspiration for a possible solution. Splitting the slider into a front and back piece and allowing some flexibility between the pieces might allow the slider to follow the helically shaped rail. This basic design was attempted in several iterations, shown in figure 7 through figure 9 below. All prototypes used 5mm x 11mm x 4mm bearings. The would wood and printed PLA parts were held together using nuts and bolts, and the aluminum prototype used 5mm shafts with set screws to hold the pieces together. The PLA sheets were used connect the front and back portions of the aluminum slider. None of these sliders allowed sufficiently smooth motion when used in the mechanism.
Figure 4. Printed slider Figure 5. Standard linear bushing bushing1 Figure 6. Slider design from Grabcad2
Figure 7. Slider in wood Figure 8. Slider in PLA. Figure 9. Slider in aluminum.
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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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