It all depends on what you need the diameter to do. Look at the definitions in the help file. They give pretty good descriptions of what they do.
Here they are. I plagarized one of James Mannes posts in another thread. (I miss James )
LEAST_SQR
Least Squares – This calculation type provides a method of fitting in which the average squared radial distance from the data points to the circle is minimized. The square root of this quantity is the Root Mean Square (RMS) distance. Since the RMS distance is based on an average, some points may be further than the RMS distance from the computed circle.
MIN_SEP
Minimum Separation – This calculation type generates a circle that is halfway between two concentric circles containing the data points, with the difference of their radii as small as possible. The Min/Max math used by the MIN_SEP calculation minimizes the maximum error, or deviation, from the input data to the circle. The Min/Max error is one-half of the minimal separation. No input data points (or input features) lie farther than the Min/Max error from the Min/Max circle. This calculation determines whether or not all the input data (or input features) are within the given tolerances.
MAX_INSC
Maximum Inscribed – This calculation type generates an empty circle with the largest possible diameter that lies within the data. PC-DMIS first computes a Minimum Circumscribed circle and requires that the center of the Maximum Inscribed circle lies within it. This option could be used for a circular feature that requires a mating stud. For example, if the input data represents a hole, then this calculation returns a circle with the diameter of the largest stud that will fit inside the hole.
MIN_CIRCSC
Minimum Circumscribed – This calculation type generates a circle with the smallest possible diameter that encloses the input data (or input features). This option could be used when measuring a stud that would fit into a mating circular feature. The resulting feature would be the smallest hole into which the stud would fit.
FIXED_RAD
Fixed Radius – This calculation type creates a circle of a given diameter, positioned so that the maximal radial distance from the data points to the circle is minimized. It is similar to the Max/Min math used in the MIN_SEP calculation except that,since the diameter is known in advance, the radius cannot vary. Only the circle's position is allowed to vary.
I use the max_inscribe and min_circumscribe option quite a bit and find the resultant measurements to be very close to Pin/air gaging. As an example after measurement of an ID on the CMM and it measured small, I verified this by air gage and was off by .006 microns.
I think you would want to use minimum circumscribed. I am not an expert on these algorythems. There is a lot of good info about them in the help menu's. Just open an autocircle window and hit F1. That is where the following comes from, originally posted here by Ironman, the Patron Saint of 4heads, currently bannished in the Black Tuesday fallout :
Keep in mind the different methodologies that PC-DMIS has to offer for algorithmic analysis...
(doesn't that sound fancy?)
LEAST_SQR
Least Squares ?This calculation type provides a method of fitting in which the average squared radial distance from the data points to the circle is minimized. The square root of this quantity is the Root Mean Square (RMS) distance. Since the RMS distance is based on an average, some points may be further than the RMS distance from the computed circle.
MIN_SEP
Minimum Separation ?This calculation type generates a circle that is halfway between two concentric circles containing the data points, with the difference of their radii as small as possible. The Min/Max math used by the MIN_SEP calculation minimizes the maximum error, or deviation, from the input data to the circle. The Min/Max error is one-half of the minimal separation. No input data points (or input features) lie farther than the Min/Max error from the Min/Max circle. This calculation determines whether or not all the input data (or input features) are within the given tolerances.
MAX_INSC
Maximum Inscribed ?This calculation type generates an empty circle with the largest possible diameter that lies within the data. PC-DMIS first computes a Minimum Circumscribed circle and requires that the center of the Maximum Inscribed circle lies within it. This option could be used for a circular feature that requires a mating stud. For example, if the input data represents a hole, then this calculation returns a circle with the diameter of the largest stud that will fit inside the hole.
MIN_CIRCSC
Minimum Circumscribed ?This calculation type generates a circle with the smallest possible diameter that encloses the input data (or input features). This option could be used when measuring a stud that would fit into a mating circular feature. The resulting feature would be the smallest hole into which the stud would fit.
FIXED_RAD
Fixed Radius ?This calculation type creates a circle of a given diameter, positioned so that the maximal radial distance from the data points to the circle is minimized. It is similar to the Max/Min math used in the MIN_SEP calculation except that,since the diameter is known in advance, the radius cannot vary. Only the circle's position is allowed to vary.
__________________
James Mannes
I am IRON MAN!!!
HTH
Well, I see John & Bob both beat me on the draw.
sigpic"Hated by Many, Loved by Few" _ A.B. - Stone brewery
Are you scanning or are you taking discrete points?
The reason I say this is that if you take 4 points and use the MIN_CIRCSC on only a few points, it doesn't make a lot of sense. Use best fit. But the more points you take, the bigger your chance is that you indeed pick up surface features like lobing and the more it will get you to air gauge and ring gauge measurements.
If you can do a real dense scan, you'll get great results using MIN_CIRCSC.
Jan.
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PC-DMIS/NC 2010MR3; 15 December 2010; running on 18 machine tools.
Romer Infinite; PC-DMIS 2010 MR3; 15 December 2010.
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