This page contains images of a dial gage attached to a round base made from parts of old hard drives that can be used to measure the sagitta of a spherical surface. The reading from the spherometer becomes part of an algebraic expression to determine the focal length for a spherical mirror. The formula I used is one I obtained from Dave Rowe when his web site was available. His article on making and using a spherometer can be located at the following link.
Dave Rowe's Spherometer article
The images below display three spherometers made from either the base of the hard drives spindle motor or from the spindle hub and several aluminum disk. The larger instrument is from an old 5.25" hard drive as well as a smaller one made from the base of the spindle motor. The small one is made from three 3.5" disc and the hub from a 10 gigabit Quadram hard drive The larger disc uses three .375" ball bearings. The smaller two use three .250" bearings. The bearing are precision tungsten carbide bearings purchased from Reid Tool. The two main tools I used to make the three spherometers were a drill press and a belt/disc sander. The spindle needed to be ground down in height so there is only about a 16th of an inch of height on the bottom side of the spherometers. This was not a problem on the larger drive as the hub already had a low clearance. The three disk are glued together using a thin Loctite adhesive. Either 290 thread locker or 609 retaining compound. The bearings can be glued in place with the Loctite 290, Super Glue, or with J&B Industrial cold weld compound. All in sparingly used quantities. I used a freeware calculator program to set up my formula. It makes it inserting the measured value as a variable and re-run the program. The program is called MEX (Mathematical Expression Program) and can be found at: http://www.toolsforselling.com. The formula and variable table I use is:
s=.010 [spherometer reading]
r=1.7245 [average radius from center to center of a ball bearing feet]
D=.25 [bearing diameter]
MD=14 [mirror diameter]

Click on an image to enlarge...Back to return.

The following image is a rather hastily done drawing that shows the components of the device that give some idea of its construction.


The following images are of various parts of 3.5 in. and 5.25 in. hard drives with some remarks concerning their use.

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