That's pretty much how accurate spheres are made today. Ball bearings are rough out by stamping and then rolled between plates to round them and then ground and polished between plates. They're accurate to 3e-6 inch. Brent On 10/22/2017 9:03 AM, Henry Baker wrote:
It's the year 1500, and some wealthy patron wants a transparent glass and/or rock crystal sphere perhaps 6-8" in diameter.
How would you fabricate it?
I don't know how they actually did it, but here are some of my speculations:
1. To make a solid glass sphere, one might make a spherical mold, melt glass into it, and then polish the result.
2. To make a rock crystal sphere, one might build a lathe, and then utilize a semicircle guide to cut a rough crystal block into nearly spherical shape, and then polish the result.
One might even take the glass from the mold & mount it onto a lathe for the polishing step.
Clearly (!), one could continually reposition the sphere in the lathe to work on different axes to attempt to make the sphere as precisely spherical as possible.
Are there more elegant methods?
Using just the technology available in 1500, what kind of precision could be achieved?
[BTW, a similar problem arose in the 19th Century when *ball bearings* became important. Apparently, the longevity of various kinds of engines depended critically on making the bearings as spherical as possible. I don't know enough about how this precision was achieved, but during WWII, bearing manufacturing plants were heavily targeted for bombing.]
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