On 2018-05-14 8:36 PM, Ed Minch wrote:
> Perhaps 10 years ago, Roy Underhill demonstrated how those spikey recesses
were made.  He clamped on a metal plate with a cutout shaped like that in it,
then he had a tool with cutters at the two ends of what looked like a flexible
tuning fork and it was spun with a bow drill and raced all around within the
metal plate and cut the recess.  It is called a Parser Drill:
>
> https://makezine.com/2015/08/05/old-tech-drills-any-shaped-hole/
>
> Mesmerizing to watch

It's the same process which was used to inlay 'shields' into the scales 
of pocket knives.

Interesting that it should be referred to as a drill. Clearly it's 
operating more like a modern pattern router. Of course, in the distant 
past they didn't have 20,000+ rpm tailed demons. Their only rotary tools 
were drills and braces.

Don

-- 

“It is easier to fool people than to convince them they have been fooled.”

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crowd around chaotically like the Italians.” Carlo Rovelli