Finding a Radius - How to measure diameters that are not a full diameter
The attached article was developed to solve a problem that befuddled us for a while. how to measure diameters that were not a full diameter. I would be very interested in your reviews.
Thanks
Jim
Seems to me you'd have a hard time determining the point of where the arc changes to a line so you can measure the distance across the arc. If it was me (and I actually have to do this on my model railroad's curves) I would cut out a template with the minimum (or maximum) allowable radius and lay that on the curved surface. Sort of a go-no go gage.
Also, taking practice from the railroads, the traditional way to lay out (or to check) a curve (where you can't find the center or don't have the full diameter) is to take a 100 foot chord (or otherwise fixed distance), lay it across the curve, and then measure the angle between the chord and the tangent to the circle.
__________________
Steve Prevette
"A Passionate Statistician", ASQ CQE, Fluor Government Group
The opinion stated above does not necessarily reflect that of my employer.
A resourceful way to solve a challenging situation.
Are there instruments designed to do this? I know I have seen instruments for measuring the radius of a sphere - a "tripod" that rests on the surface and a "micrometer" type probe at the center to measure the distance to the surface. The equivalent for a circle would be basically what Steve mentioned for the railroads - take a "beam" of known length and set it onto the curve. Attach a probe to the beam and to the curve.
( I tried to do and ASCII art drawing to show what I was thinking, but the spaces kept disappearing, ruining the picture. You'll just have to use your imaginations!)
If it doesn't exist, I sure think it would be a good item for someone to manufacture! This can't be the only shop that needs to do such measurements.
Tim F.
P.S. I like the "trial & error" spreadsheet approach for analyzing uncertainties. I could also solve the problem with calculus, but often the spreadsheet approach is just as quick and more intuitive. Calculus would have the advantage of giving a general equation that would work for different values of chord & height.
__________________ To wonder is to begin to understand.
Last edited by Tim Folkerts; 10th June 2004 at 05:45 PM.
A resourceful way to solve a challenging situation.
( I tried to do and ASCII art drawing to show what I was thinking, but the spaces kept disappearing, ruining the picture. You'll just have to use your imaginations!)
If it doesn't exist, I sure think it would be a good item for someone to manufacture! This can't be the only shop that needs to do such measurements.
Tim F.
P.S. I like the "trial & error" spreadsheet approach for analyzing uncertainties. I could also solve the problem with calculus, but often the spreadsheet approach is just as quick and more intuitive. Calculus would have the advantage of giving a general equation that would work for different values of chord & height.
For drawing (to hold the spaces) - use real characters rather than spaces, then change the color of the characters which should NOT appear to "white"
(done in magenta on top "V" and white on bottom "V" here to visualize)
__________________ "Few minds wear out; more rust out"
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Christian Nestell Bovee (1820-1904) in Thoughts, Feelings and Fancies, 1857
Thanks Steve, actually when we first became befuddled we used the OEM steel pins that were made for the coupler. These pins usually ran about .008" to .010" under the diameter of the bore. Our task was to determine how much wear had occured in the radius area. Engineering set the criteria at .005% of the OEM pin diameter. By inserting the pin into the "hook" area we could tell that wear had occurred but not how much wear. The degree of wear determined if we could rework or if it was rejected as scrap.
I do like your idea of making up a set of radius gages and I intend to explore this more!
Thanks
Again
Jim
A resourceful way to solve a challenging situation.
Are there instruments designed to do this? I know I have seen instruments for measuring the radius of a sphere - a "tripod" that rests on the surface and a "micrometer" type probe at the center to measure the distance to the surface. The equivalent for a circle would be basically what Steve mentioned for the railroads - take a "beam" of known length and set it onto the curve. Attach a probe to the beam and to the curve.
( I tried to do and ASCII art drawing to show what I was thinking, but the spaces kept disappearing, ruining the picture. You'll just have to use your imaginations!)
If it doesn't exist, I sure think it would be a good item for someone to manufacture! This can't be the only shop that needs to do such measurements.
Tim F.
P.S. I like the "trial & error" spreadsheet approach for analyzing uncertainties. I could also solve the problem with calculus, but often the spreadsheet approach is just as quick and more intuitive. Calculus would have the advantage of giving a general equation that would work for different values of chord & height.
Thanks Tim, I recently watched a demo of a CMM robot arm that could possibly do this in no time at all. the salesguy demonstrated that once set up touching the probe to 3 points on the curve would yield the radius. Cost was in Excess of $50000. My Engineering department is searching the cheaper versions.
PS. Although I studied calculus its been a few years ago. I felt more comfortable with the spreadsheet. Excel is great isn't it!
Jim
Last edited by Jim Howe; 11th June 2004 at 09:29 AM.
Reason: ps
Our industry here is metal stamping, so I don't know if this will apply to you or not. Do you have an optical comparator available? That is my preferred method of measuring a radius. I have radius charts that I can lay over the image on the comparator, works pretty well. Personally, I prefer NOT to use a CMM for this type of measurement, it is too dependent upon the CMM operator touching on the radius and not getting too far up the side into the straight edge.
Finding a Radius - How to measure diameters that are not a full diameter
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