Tools and materials :
The aluminum disk cost $50 and was purchased at a metal fabricator. Thealuminum bar was scrap that I was given for free.
The dial indicator was ordered from Enco and cost a bit more than $200.The ball bearings, set screws, knurled nuts were found at Home Depot, Sears Hardware, or Tru Value Hardware and each cost about $0.40.
I basically followed the directions given by Dave Rowe on his web page.SeeDave Rowe's spherometer page
Frankly, this is all you'll ever need to know about the subject.
The first step was to draw concentric circles for each of the diameters I envisioned.The protractor was used to identify the 120 degree points and these were marked off on eachcircle with a pencil.
For what it is worth, if I had had a drill guide for a handheld drill, I don't believe a drill press would have been necessary.
Still using the drill press, holes were drilled for the "legs." Threads were then tapped in these holes. As aluminum is somewhat soft, this was not hard to do at all.
A handheld Dremel was then used to debur the edges of all the holes drilled.
Ball bearings were then "Crazy Glued" to the cupped ends of the set screws andthese were threaded through the appropriate holes. The knurled nuts were then threadedonto the set screws from above the plate, firmly fixing their positions.These adjustable legs can be repostioned in other holes to allow for otherdiameters to be tested. Their depth is also adjustable to allow fordifferent sagittas (both concave and convex) to be measured. The drawbackis that the distances between the legs needs to be re-measured each time theyare moved. I thought this was a relatively small price to pay,considering the cost of getting different aluminum plates made.
The dial indicator is held firm by the longer set screw. Tightening withone's fingers seems good enough. The block it rests upon is also "CrazyGlued" to the base plate.
I have not yet tested this unit, but expect that it will work well. If I were to do itagain, I would probably use something like the "JB Weld" mentioned above. It only costsabout $4.00 in a local Sears Hardware. If the Crazy Glue fails for some reason then thatwill be what I turn to.One other thing I've noticed is a slight wobble to each of the bearings as their set screwsare turned. I am not sure if this is due to the ends of the set screws being at slightangles, or if the balls were not fully seated. In any event, it will reinforce the needfor accurate measurements between the legs once a diameter is chosen and the legs aretightened down with the brass knobs.