How to measure a big radius with just a part of the diameter of the circle

2. Originally Posted by Dembo
How to measure a big radius with just a part of the diameter of the circle
What entities are known?

3. Do you have programming software? Or AutoCad? If so all you need do is ask the software the Radius and click on the Geometry. Up pops your answer. If you do not have the software but have a full scale plotter you can plot it and measure it with a string and nail. Just not as accurate that way.

4. Dembo, length of chord = c
height from cord to diameter = h
diameter = 2xr

r = c sq. + 4 x h sq
_____________
8 x h
Formula in Machinery's Handbook 26th edition p.63 "circular segment" if I confused you or did not explain clearly.

Dick Z

5. Mitsui Seiki

Those entities I know. (pic radius.jpg)

The work is on a big cover from a trash box (pic Unknown radius)
RICHARD ZASTROW

length of chord = c
height from cord to diameter = h
diameter = 2xr

r = c sq. + 4 x h sq

r = 29 mm
c = 800 mm

r = 800 mm + 4 * 29 mm

r = 800 mm + 116 mm

r = 916 mm

I know I can do with radius of Autocad, But I have to make much work like this far from home and without a laptop.

6. The radius R is 2773.121mm.

R= CSquare/8xd + d/2 (C and d known)

C=800
d=29

7. You need one of these, maybe a bit bigger then what they normally use on lenses. Should be easy to make one, just a bar with two fixed pins and and indicator.
http://en.wikipedia.org/wiki/Lens_clock

8. I've used this for more than a decade... good info!

And the spreadsheet was just for fun... And it gives the same answer of 2773.12069 if the 3 points are 400,0 -400,0 0,29

9. Dembo, I believe the formula from my post is correct. You forgot to square "h".

Stefan's formula gives the same results. r = 2773.1

Diameter= 2 x r = 5546.2 mm

Hope it helps, have Stefan check. lol

Dick Z

10. Two formulas,same result?
2773.1 can't be wrong!

11. Mitsui Seiki, Boots, RICHARD ZASTROW, Andre' B, ghyman

Thanks everybody, as always the best forum.