To ease fabrication, given the materials and fabrication methods available, the initial CAD model was modified as shown The final version of CAD assembly can be seen in Fig. 1 below.
Fig. 1: Completed CAD assembly
Fabrication of Links
The links, guide arm and carriage were additively manufactured (with Fused Deposition Modelling - FDM). This greatly simplified the task of fabricating the complex geometry of the guide arms in addition to reducing machining time required. PLA was the fabrication material of choice as it is less susceptible to warping compared to other materials available for fabricating with FDM (e.g. ABS - see failed build with ABS in Fig. 2). Also, it provided enough strength and smoothness for the sliding of the rollers on the guide arms. To ensure holes were dimensionally accurate and properly-aligned, they were undersized in the CAD models and then drilled to the right sizes on a milling machine after printing.
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Fig. 2: Failed build of guide arm (red) caused by excessive warping of ABS
Fig. 3: Assembled four-bar crank-rocker.
Fabrication of Gears
The driver and driven gears transmitting motion from the motor were were also fabricated with FDM. The gears were designed on geargenerator.com and exported to STL format for printing. Dimensions Dimensions were chosen to amplify the motor's torque output (MA>1). Base Plate MA=2.5).
Fabrication of Bearing Shafts
The bearings shafts were cut to size using the jigsaw in the machine shop and then lathed with a file to smooth out the rough cut edges. The shafts sections themselves were, in some instance, not long enough to connect the components they were meant to connect. In such instances, a shaft extender was used to rigidly join two shaft sections.
Baseplate and Stand
The base plate baseplate was laser-cut out of acrylic while the stands were wade of wooden blocksa sheet of acrylic using a sketch of the baseplate surface from the CAD drawing.
Fig. 3: Base plate4: Baseplate
The stand was originally planned to be constructed from 8020 beams, but due to unexpected shipping delays an alternative had to be found. Wooden legs were chosen due to the wood shop having an extra supply of 2x4 available for use.
Two 3 ft pieces were cut out of an 8ft 2x4 and subsequently cut in half, producing 4 legs. Using the jointer and planar machines in the wood shop, the faces of each of the four legs was cleaned to achieve a smooth flat surface. After the facing was completed the legs were cut down to 2 ft while flattening the ends of each of the legs using the table saw.
Assembly
Mounting hubs, set screws, rolling bearings, and additively-manufactured retention rings were used to achieve tight-fits between the shafts and bearings.
The final assembly is shown below:
Fig. 5: Assembled robot