Reddit reviews DRILLPRO Double Cut Carbide Rotary Burr Set-10 Pieces 1/8"(3mm) Shank and 1/4"(6 mm) Head SizeTungsten Steel - for Grinder Drill, DIY Wood-working Carving, metal polishing,Engraving,Drilling
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I've had a lot of people ask for a build video but if you check out my 15 second typing video in this thread you'll see my video production is really lacking. Given the whole process is 2-3 hours, that's also a pretty long video.
But to answer your direct question, I definitely sculpt the key into its external shape first. I tried the other way around when I first started and quickly gave up.
It might seem like it would be easy to cut too far but this didn't happen once when I switched to walnut.
What worked really well was doing most of the hollowing using the sphere bit in the link below. This handles 75% of the hollowing out and it does so in a very predictable spherical shape.
Problem is, the interior of a Cherry MX keycap isn't spherical. So I then used a reverse code bit square out the interior, this left the walls of the keycap about 1-1.5mm in thickness, which leaves enough room for the switch.
Sphere:
https://www.amazon.com/dp/B0037MI5RI/ref=twister_B07CRQ7LLL?_encoding=UTF8&psc=1
Reverse cone (far right in the picture):
https://www.amazon.com/DRILLPRO-Double-Carbide-Rotary-SizeTungsten/dp/B06W9JFW66/ref=sr_1_1?ie=UTF8&qid=1536972165&sr=8-1&keywords=reverse+cone+rotary+bit