A Poor Old Man's Spherometer
An Idea for Those Who Are Not Yet Ready for the Digital Age
THIS Spherometer
This spherometer incorporates a sagitta-to-focal length computer and it cost me less than $10! It is good enough for most work. Because it is based on a bar, rather than a circle, it can be used to measure astigmatism. To avoid ingorent prejudice that would turn the reader off, the picture of the spherometer is at the far bottom of this page.
I did not worry or even care about the precise location of the feet and the probe along the bar. I did not worry about the pitch of the threads in the probe; the dial merely reads "turns of the screw". Therein lies a great heresy! To calibrate the instrument, I simply measured the number if dial turns on a mirror of known focal length. (I did not care about scratches.)
Probe and Zero-Adjust Screws. Note center punch indentations that tightened the probe screw.
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The threads in the brass bar were made with an ordinary 6-32 tap. They were not very good; there was a lot of sloppy motion in the probe screw. To tighten these threads, I applied a center punch to the top and bottom of the brass bar near the probe; the resulting dimples should be visible in in this picture.
State-of-the-Art Focal Length Computer (1967): photocopy of slide rule C and CI scales
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I am old enough to remember when the top-of-the line personal computer was two calibrated sticks laid side-by-side. To construct my analog computer, I merely photocopied those sticks in the appropriate position.
Dial of the Poor Man's Spherometer
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The dial of this spherometer is based on the principle of the fender washer. Actually, the dial IS a fender washer with a photocopied scale epoxied to it. I found it best to paint the washer white because the paper becomes translucent when saturated with un-cured epoxy.
The NEXT Spherometer
Some of the possible improvements I would make include:
- Use of a lapped micrometer screw
- Precision ground and centered probe point
- Pointed feet, so that the point of contact does not depend on the slope of the surface.
- Use of a larger diameter micrometer screw
- Micrometer screw at the end of the bar, instead of at the center (double the motion)
- Instead of a bar, a rhombus configuration.
- Wing nuts and lock washers to secure the feet.
- Possible use of a lever magnifier and an electrical contact sensor for the micrometer screw
- Use of an adjustable long nut for the mircrometer screw, to eliminate slop and backlash
- A maximum cost of $20!!!
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The Poor Man's Spherometer -- Don't Laugh, It Works!
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