Much 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 X) or a helically shaped bar attached to the lower arm plate (Figure X). 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 X. 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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Figure X. Extruded rail Figure X. Attached rail Figure X. Rail bending method
A first iteration of the slider mechanism used a 3d printed slider, as shown in Figure X 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 X. A posting on Grabcad 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 X through figure X below.
Figure X. Printed slider Figure X. Standard linear bushing Figure X. Slider design from Grabcad
Figure X. Slider in wood Figure X. Slider in PLA Figure X. Slider in aluminum
Several changes were made to the slide rail to improve smoothness of motion, such as printing in ABS rather than PLA, increasing the infill of the printed part to prevent indentation of the rail, and increasing the resolution of the conversion to SLS file in the printed part to increase smoothness of motion. These changes all helped, but none of the above designs allowed sufficiently smooth motion when force was applied at an angle not parallel to the direction of the rail, as happens when the slider is used in the brace mechanism.
The slider was then redesigned with printed PARTS attached to the bearings to grasp the rail and better distribute the applied force. These pieces were fit into a flat plate made of printed plastic and secured with set screw, and then this assembly was attached to the hinge. This change finally allowed operation of the mechanism. The design of the final slider piece is found in Figure X, with the final slider shown in Figure X.
Figure X. Final slider design. Figure X. Final slider.
The manufacturing of the hinge on the upper arm and the hinge attached to the slider was straightforward with no design changes. Flanged bearings were inserted into either end of a piece of aluminum and a rod inserted through holes in the printed PLA pieces, held together with a retaining pin. A rod between these pieces was held in place using 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 the figure below.
Parts for the attachment of the mechanism to the upper and lower arm were constructed using 3d printed PLA and ABS. Foam padding was added to the undersides of the plastic to increase comfort for the wearer. Velcro straps were threaded through holes in the plastic parts for attaching them to the arm. During testing it was found that there was too much movement of the lower piece when the mechanism was used. To correct for this movement and add stability in the mechanism a handle was added to the lower arm piece, shown in Figure X. The final product is shown in Figure X, with a complete parts list found in the table in Figure X
Figure X. Arm attachment straps, upper arm. Figure X. Lower arm piece with handle.