I can only access part of it. But, have a fairly large surface area to work with (about a 3" diameter slice).
How can I get a *reasonably* good estimate of its actual radius? I thought, perhaps, a "precisely" cut circular opening that could sit on the surface and then use that (knowing its thickness) to measure to the "peak" of the surface protruding within (?) E.g., a dial indicator riding on that material in which the opening is cut...
The rest being math.
Can you feed a piece of twine around it? Then you just calculate from C=pi*D
Not if he doesn't have at least a half sphere.
Three precision balls on a plate, in an equilateral triangle, with said dial indicator. The greater a proportion of the radius your triangle spans, the less sensitive your measurement will be to errors.
Google "measure spherical radius". I've never tried it, but I bet you'll find something.
I use one of these to fit/cut floor tile around baseboard molding...
Just press it down over your sphere, transfer to paper, locate center with compass and straight-edge, diameter is now known. ...Jim Thompson
Don - 3 points on a circle will give you the diameter/radius with the suitable equation. I can probably post a C routine if you can't find the equation required.
Hul
D> I can only access part of it. But, have a fairly large surface area
You don't need a C-routine... just remember your high school trig/geometry.
...Jim Thompson
I think I may have a more practical approach based on revising this...
Cut a hole of a particular diameter in a material of a particular
*thickness*. Rest this on the cap. Then, rest a straightedge on the *other* side of the hole and slide it until it is tangent to the cap. Then, measure how "deep" the chord is in the circular hole.Its essentially the same as using the dial indicator -- but rotated through 90 degrees (dial indicator requires too large of a reference surface to be practical; straightedge riding on the "rim" of a short *tubular* piece can be positioned, held in place, removed then measured -- without having to balance precariously WHILE the measurement is being made!
Spherometer. Basically a tripod with a micrometer, you sit the three legs on the surface and adjust the microometer to just kiss the surface. Zero it on an optical flat.
Drill a hole in the sphere. Inject a fluid from a reservoir holding a known quantity of it. Volume of sphere == initial reservoir volume minus final reservoir volume. Solve for radius of sphere (take into account thickness of sphere's shell if you like). Drain sphere, patch hole. Assumes hollow sphere, of course.
Too far "out of the box?"
Does this help?
Michael
A cheap compass and protractor kit, adjust extension of marking tool of compass until you draw perfect circle on surface while keeping axis of tool at constant angle to surface. Make measurements of distances and angles with protractor, you're done.
Il giorno domenica 14 agosto 2016 23:56:01 UTC+2, Don Y ha scritto:
Bye Jack
or
Radius gauge?
cheers Gunther
This clears it all up
Can you take several photos from the same distance, at least one with some reference length/marking/grid in it, and put them together using your favorite Photshop/GIMP raster-image program?
It has some sloppiness, true, but it seems more comfortable than trying to squeeze your eyeball through the aperture. ... Aha! That suggests another approach, even sloppier: a camera-on-a-cable like this one:
If you like the idea, check around -- there seem to be a lot of these on the market.
Enjoy...
Frank McKenney
SurplusShed sells a inexpensive, accurate, spherometer.
Steve
If it's shiny observe how it refliects light.
Have something to add? Share your thoughts — no account required.
Ask the community — no account required