This page is devoted to my three point ball spherometer. Here you will find some pictures of it and the formulae to use it and details on how you can order one from me.



This is how it looks, as you will note there are three points of contact and the micrometer. The span of this unit is 94.2mm plus or minus .1mm, this distance is the diameter of the circle scribed by the center of the three points of contact. The micrometer is at the center of this circle.

The formula for the radius of curvature (ROC) of a _concave_ surface is as follows:

Rcv=((((D^2)/4)+Z^2)/2Z)+Rb

Where:
Rcv is the curvature of a concave surface.
D is the diameter scribed by the _center_ of the three points of contact (balls).
Z is the measured value of the sagitta, the depth of curvature.
Rb is the radius of the balls, where all are the same radius.

For a _convex_ surface the formula is:

Rcx=((((D^2)/4)+Z^2)/2Z)-Rb

Where:
Rcx is the curvature of a convex surface.
And the rest is the same as for a concave surface, but take note the sign is reversed for the effect of the radius of the ball feet in the version.

Now that you know how to use one, well at least reduce the reading to a ROC you will either want to make your own or find out where to purchase one. Well you can purchase one from me and this is what you get in you serial numbered set: You get a cast bronze base, three calibrated stainless steel feet, a custom built micrometric head, a plate glass reference surface, and a screwdriver for assembly and disassembly of the unit. All this is packaged in a solid hardwood box to preserve your investment.



If you would like more information on this item please contact me.

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