The mechanical design of the Precursor RISC-V development kit #RISCV #Design @bunniestudios
Andrew “bunnie” Huang talks about mechanical design of the Precursor, a mobile, open hardware RISC-V System-on-Chip (SoC) development kit. Good mechanical design is hard and to put any extra into a design increases cost. It is informative to read how bunnie went about the design choices.
“Pocketability” is the difference between Precursor and naked PCB FPGA development platforms. We hope Precursor’s pocketability helps bring more open hardware out of the lab and into everyday use. Thus, the mechanical design of Precursor is of similar importance to its electrical, software, and security design.
After considering about a dozen or so mechanical layout scenarios, we arrived at the design shown above. Like every modern mobile device, when viewed by size and weight, Precursor is basically a battery attached to a display.
And on use of aluminum vice a plastic case:
Using aluminum for the bottom case allows us to shave about 1 mm (~15%) of thickness relative to using a polymer like ABS or PC at the expense of a fairly substantial increase in per-unit manufacturing costs. Although polymers are about twice the cost of aluminum by weight, an aluminum case costs about 10x as much to produce. This is because polymers can be molded in a matter of seconds, with very little waste material, whereas aluminum must be CNC’d out of a slab in a time-consuming process that scraps 80% of the original material. Surprisingly, the 10x cost-up isn’t the waste material; there is an efficient market for buying and recycling post-machining aluminum. Most of the extra cost is due to the labor required to machine the case which is orders of magnitude longer than the time required for injection molding.
Thus, while we could have made Precursor cheaper, we felt it would both be more pocketable, as well as more desirable, with the machined aluminum case: it would look more like a high-end mobile device, instead of a cheap plastic toy or remote control.
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