University Lowbrow Astronomers

The Art of Beveling.

by Tom Ryan
Printed in Reflections: January, 2002;
Title Changed: November, 2003.

When I made my first 6” f/8 telescope mirror in the telescope making class of my youth, I was taught to bevel the edge of the mirror to prevent scratches.  As grinding proceeds, the edge wears down and becomes sharp.  If it isn’t beveled, chips of glass will break off, fall between the mirror and tool, and will scratch the surface of the mirror.  This isn’t really a problem when rough grinding (scratches, that is), but becomes one in the finer grits.

So I carefully beveled the edge of the mirror with a hand stone they had there.  It wasn’t too coarse, and wasn’t too fine, but it was just right for putting a clean, sharp bevel on a piece of glass.  I spent a lot of time doing this.  In retrospect, I think I must not have had the best roughing stroke, because glass is supposed to come off the center of the mirror, not the edge.  Nevertheless, I spent a lot of time beveling, and got to be pretty good at it.  My bevels were at a 45 degree angle, were uniform at just the right width, and blended imperceptibly into one another (beveling is local, but the bevel itself is global).

The time I spent beveling actually served me well later, when I took a job in a machine shop.  The old German who ran the place asked me if I was a machinist, I lied and said yes, and he put me on a job that involved only filing.  After I spent several days filing sharp edges into gauged radii, the German explained that a really good machinist doesn’t need a mill or a lathe; just a sharp file.  Then he put me on a mill.  (Come to think of it, I believe that the first really accurate lead screws for interferometer ruling were filed, so he was probably right).

Despite this wealth of experience, I don’t bevel the edges of optics with a hand stone any more.  Karl Mueller (an old German himself) taught me a better way.  Karl uses a sheet of 220 silicon carbide sandpaper, and just sands down the edge with the paper pressed against the palm of his hand, and a little water to carry away the dust.  (He does this before moving beyond 220 grit, of course).  This results in a radiused edge, which is much stronger, and actually better looking, than a beveled edge.

It just goes to show that it’s never too late to have the skills of a lifetime be made obsolete.


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