Edger Question

[Blake]

Here's a question that's been nagging me for a while: On our edgers (the type where a pattern is used), there is a dial that controls whether the lens is edged to the size of the pattern, larger or smaller. Setting the dial to 36.8 will usually result in a lens the size of the pattern, unless it's a rimless in which case it is set to around 38.5.
My question is, where do these numbers come from? I've talked to lab folks with 20+ years experience, and even they don't know. All they know is it works.
If anyone knows, please let me in on the secret!

Blake

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[EYERX]

All settings on a edger as well as the specs for most tools laps and even your lens clocks are based on crown glass index!

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[Matthew]

A LONG time ago, somebody decided that 36.5 would be the "on size" point for the dial.

Its as simple as that.

[eyedude]

Just for the record the MBS and MBX use 36 as its standard. It's probably for the same reason someone decided to use 22mm to make the first flat-tops.

[JRS]

Way back, in the dawn of time, when I started... patterns from the manuf. were made of metal and measured 36.0mm's. Hence the size=size configuration. Then, as frames got larger, they changed to either 46.0mm or 51mm's - which, respectively was -10 or -15 from eye size wanted. When they started making them in plastic, and the imports came about, we started seeing the 36.5 settings. Of course I date back to the time of ceramic wheels and big blue hand stones.
As far as I know, none of it had to do with glass index however.

[JRS]

Originally posted by EYERX:
All settings on a edger as well as the specs for most tools laps and even your lens clocks are based on crown glass index!

Laps, Clocks, and Sag Gauges are based on "tool index" (1.530), not crown glass (1.523). If memory serves me, the discrepency has to do with World War 1.