BREAKOUT DIAMETER

[[Jo...]]

I am trying to measure a break out diameter for size.   basically it is a ring type part that has a center bore, a plane and web.  The web has radius that is less than 60degrees of an arc that is intersected with the plane.  Where the plane and radius meets is the diameter I need.  The radius is part of a line profile (several around the part) measured with a curve.  So i have the curve points.  I recalled those points into individual diameters.   If I was just measuring the radius, it would just constrain the size to get location and constrain the position to get the size... in this case I need both at same time. I tried using a theoretical diameter and formulas to input the location and size of the recalled points and then intersect with the plane.  When i do this I get a diameter that does not make any sense. Any ideas on why?

 

[[Jo...]]

I have done some test and I just cannot seem to get this to work.  It is not calculating the radius correctly.  I have plotted the actual points in a cad software and it matches what was checked by hand.

 

[[Jo...]]

The math for radiuses can get very weird....  60 degrees is well into the zone where it can be just impossible for it to calculate a sensible radius. 

[[Ch...]]

Joe is correct, this isn't a measurement strategy issue, its a math issue.  You can't get an accurate diameter on short radii.  There's several good white papers online that describe how much uncertainty you accumulate as you make your radius smaller.  My analogy is this is like trying to calculate the circumference of the Earth by going out and measuring your parking lot.  

Would it be acceptable to use profile instead?  A profile would ensure your radius is in the correct location and is the correct size.  

[[Ma...]]

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 using profile is the same like using constrained radius.

[[Ch...]]

I'm just trying to say that you'll never report an accurate radius using less than 60 degrees.  In the pictures below, math will never let you fit a circle to the orange lines that accurately describes the radius.  Consider using a radius gage.