Measuring radii

[7milesup]

If the radius were a full quarter circle, I wold measure the length of an adjacent face and the distance between the to perpendicular faces, the difference being the radius.

Sorry, not really following that. Wouldn't that require a block that is exactly square?

 

[Weldingrod1]

And it assumes the center line is at the intersection of the two planes.

If it's big you could use an edge finder to locate some coordinates and curve fit it.

Sent from my SM-G892A using Tapatalk

 

[mickri]

If you can set it on a piece of paper draw the arc and a line between the two end points. Lets call this line AB. Find the midpoint of line AB and draw a line perpendicular to AB at the midpoint. Then keep trying different lengths on the midpoint line until it touches each end point and the midpoint of the arc. Or you can do the math. Measure the length "L" from A to B and the length "H" from the AB midpoint to the arc. The formula is r = LxL/8H + H/2

See https://www.vcalc.com/wiki/vCalc/Circle+-+Radius+from+chord+length+and+arc+height

 

[RJSakowski]

Sorry, not really following that. Wouldn't that require a block that is exactly square?

As long as the arc is quarter circle, and the opposite side is parallel , it will work. If the opposite side isn't parallel or available,the radius can still be measured if the center of the arc is on one face. Measure the distance from the face to the fare side of the arc with a depth gauge.

The method falls apart if the arc isn't a quarter circle or if the center of the arc isn't coincident with one of the faces.

David was looking for alternatives to using a radius gauge and this is one. It wouldn't be my first choice though. For radii up to 1/4", I have pin gages and with careful observation, can determine the radius to within a few thousandths. Past that, for accurate measurements, I would turn a diameter to fit. If the part can conveniently brought to the lathe, this is a fairly quick process as you turn the diameter slightly oversized and try a trial fit. Reduce the diameter slightly and repeat. The drill shank process works too but you are liomited in your increments by the sizes of your drill bits.

 

[higgite]

Lacking a set of radius gauges, what's my next best way of measuring an inside radius like this:
View attachment 354179

You did say measuring, not calculating, so I'd go with either the drill bit test method or let us help you spend your money on a set of radius gauges. No, no, no need to thank us. We're more than happy to help you make the best possible use of your funds. And how in good conscience could you possibly say no to more tools?

Tom

 

[Janderso]

I love this forum!

 

[Mitch Alsup]

This tool can be used for both concave and convex radii. This is a particularly useful method for very large radii. I used it to verify the radius of spherical mirrors that I made which had a radius of 7.5".View attachment 354187

A machinist version of the optical spherometer.

 

[Mini Cooper S]

If it is a small enough radius,, use a drill as a pin gauge.
Richard

 

[RJSakowski]

The spherometer uses 4 points to determine the curvature of a spherical surface while the Lens clock/Geneva lens gauge measures in a single plane and can be used to measure curvature of a cylindrical surface as well as a sphere.

If measuring spherical surfaces, the spherometer would be the instrument of choice as it is less prone to operator error. The Rahn Repeat-O-Meter, used for checking surface plates for flatness is a first cousin of the spherometer

 

[RJSakowski]

If it is a small enough radius,, use a drill as a pin gauge.
Richard

Drills work but you are limited by the extent of your drill collection. However since the most common tool for cutting a radius would be a ball end mill, most likely you would have the drill to fit.