Radiii Measurement?

Often an optical comparator would be the best way. There are also some photographic measurement systems that could measure radii.

You didn't say what range of radii you want to measure, nor whether they are inside or outside radii. I recently made my own device consisting of a dial indicator mounted in the center of a bar of known length. For inside radii, the extension of the tip and the length of the bar can be used to calculate the radius. The length of the bar is the chord of the arc; a bar length must be chosen according to the range of radii to be measured. For outside radii, I added a block to each end of the bar. I zero the dial on a surface plate, and then determine the compression of the dial tip. Of course now the chord of the arc is the inside distance between the two blocks.

If you need to know how to do the calculations, or want further details, I can post them...

We have a similar device except that it has a series of counterbored discs of different sizes to suit a range of radii. The principle is the same as you describe. Ours also has a handy look-up table that cross references chords & heights to give radii, save all that tedious calculating.

Actually, I set up a little Excel spreadsheet with places for chord and height. once you enter those two values (or really only one, since the chord is a constant property of the measuring instrument). Once those values are entered, the spreadsheet shows the radius. It is up to the operator to round off to an appropriate number of digits, or the spreadsheet can be set to display a desired precision.

The critical formula is: R=(C^2-H^2/4)/2H, where R=radius, C=Chord, and H=Height. This all comes from the Pythagorean theorum.

There is a special name for that height, but I can't remember it. I believe it has something to do with an old or other language name for an arrow, as that height resembles an arrow ready to fire from a bow, represented by the arc. Can anyone refresh my memory?

Azimuth?

[from Old French azimut, from Arabic as-sumūt, plural of as-samt the path, from Latin semita path]

Thanks for trying, but no...

After some searching, I finally found it :sagitta. There is a constellation by that name as well (the arrow). It's also known as the versine.

So in the formula, H is the length of the sagitta or versine.

Measuring radii in the past was always concidered a simple task, but current designers who have been recently trained are moving away from a simple ± tolerance and are now using a 'Profile' tolerance method. This controls the entire form and surface for the inspector to measure. This is becoming more common in the UK at present on all new products I am currently seeing. A radii gauge is good for a quick check, but you can guarantee light will pass somewhere between the gauge and part and therefore this is never a true reflection of the actual radius and its form. The best method I have seen is to 'Mould' a form of the Rad; this might sounds 'old-school', but is extremely effective for internal radii in hard to reach places. The mould can be cut and placed on an Optical projector a checked against a transparent film at 10:1/25:1 to prove the entire form is within its profile parameters. The other(more expensive) method is trace measurement, where most Surface Finish Machines now incorperate this type of requirement, Taylor/Hobson and Mahr have the best equipment for this. The latest generation of CMM's now use 5 axis scanning probes(Renishaw), but this expense is also high. Laser scanning technology is being purchased by large companies especially for aerospace manufactures. The method of measurement will always depend on accuracy and application of your product, which will then determine any justification for expense in to controlling the Radii in manufacture. Happy measuring!!