Re: [ATM] Sphereometer accuracy

["Jerry" <wa4guu@verizon.net>]

If I am not mistaken....

You would only have to add or subtract (depending concave or convex) half
the diameter of the balls to/from a radius calculated using the centers of
the balls as the locations of the feet.

Jerry

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Dominic-Luc Webb
Sent: Wednesday, February 22, 2006 1:42 PM
To: "The Alarm Co."
Cc: "ATM List"
Subject: Re: [ATM] Sphereometer accuracy

-------------------
> In reading the recent posts about sphereometers, it occured 
> to me that when using balls for feet, the actual spacing 
> between the feet will vary with the depth of the curve being 
> measured; the spacing gets wider with concave surfaces, and 
> narrower with convex ones. 

Exactly my point for years now. If someone REALLY needs such 
precision, then maybe measuring the actual ROC with reflected 
light source using a micrometer attached to a piece of paper at 
ROC would be more appropriate. There are very few of us that 
really need the ROC to be this exact using a spherometer.

More to the point from my observations, some newbies 
get depth easily within useful specs (goal is F/5.0, and 
they get F/5.0 +/- negligible error), but are not getting 
spheres right from the early stages of grinding.

Dominic-Luc Webb
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