계측 솔루션
계측 솔루션
Geometry Gaging - Four Methods Of Out-Of-Roundness
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GEOMETRY GAGING
FOUR METHODS OF MEASURING OUT-OF-ROUNDNESS
George Schuetz, Mahr Federal Inc.

 We have introduced the subject of circular geometry gaging by looking at the instrumentation, and we noted that one reason for the recent proliferation of geometry gages is the use of personal computers as gage controllers.  The PC has greatly simplified geometry measurements by speeding up the calculations involved.  Now, let's proceed to the most common geometry measurement, and the basis for most circular geometry parameters: roundness, also known as out-of-roundness or circularity.  As we'll see, even "simple" roundness has benefited greatly from the processing power of the modern PC.

 Ideal roundness, according to ANSI standard B89.3.1, is "the representation of a planar profile all points of which are equidistant from a center in the plane."  Out-of-roundness, then, is "the radial deviation of the actual profile from ideal roundness," and the out-of-roundness value (OOR) is "the difference between the largest radius and the smallest radius of a measured profile; these radii are to be measured from a common point... ."

 To measure out of roundness, then, it is necessary to compare the part profile to an ideal circle or datum.  But since the part profile itself isn't round, how do you locate the ideal circle?

 Four methods are in common use.  Many modern geometry gages offer users a choice.  Typically, the user selects the required method, then initiates the measurement on the gage.  The gage rotates the part and collects data, which it presents in the form of a polar chart.  Then the computer controller uses one of the following methods to locate the center of the reference circle:

Maximum Inscribed Circle (MIC): the center of the largest circle that can fit within the measured polar profile.  This method is used only for geometry measurements of inside diameter features.

Minimum Circumscribed Circle (MCC): the center of the smallest circle that fits around the measured profile.  This method is used only for outside diameter features.

Least Squares Center (LSC): the center of a circle, of which the sum of the squares of the radial ordinates of the measured profile is the least possible number.  This method is used for both ID and OD features.

Minimum Radial Separation (MRS): the center of two concentric circles which, with the least possible separation, contain all points of the profile.  This method is also used for both ID and OD features.
                        
 Different part applications typically call for different measurement methods.  For example, when the geometry of an inside diameter is specified, the presence of burrs, dirt, and other "high points" on the ID are typically of critical concern, while low points (e.g., scratches) are not quite as important.  Accordingly, inside diameters can be measured using the MIC method, because it is quite sensitive to high points, and relatively insensitive to low points.  In other words, a burr will cause a significant shift in the location of the center, while a scratch will cause only a minor shift.

 On the other hand, scratches tend to be of greater functional concern on outside diameter parts, while burrs tend to be of less importance.  The MCC method, which is sensitive to scratches, and insensitive to burrs and dirt, therefore has advantages for measuring outside diameters.

 The MRS method is quite sensitive in equal measure to both positive and negative asperities (i.e., burrs and scratches) and typically generates the largest OOR value of the four methods.  The LSC method, in contrast, is relatively insensitive to extreme asperities of both kinds, and therefore generates the most stable center and the smallest OOR values of the four methods.  As both of these methods react equally to positive and negative asperities, they tend to be useful for measuring mating ID and OD parts.  And because most ID parts do have a mating OD part (and vice versa), the MRS and LSC methods are in more frequent use than the MIC and MCC methods.

 OOR values may differ by as much as 10-15% from the same measurement data, depending on the method used.  Inspectors must refer to the part print callout before firing up the gage.

 The use of the proper reference circle has importance beyond just OOR measurements: many other parameters are based on roundness and the location of the circle's center, and they too will be influenced by the method selected.  Concentricity, circular runout, total runout, coaxiality, and cylindricity are all affected.  Now, aren't you glad the gage controller will run the calculations for you?  (Some gages even allow the user to store the data, and then apply the different measurement methods on a post-process basis.)

 If the part print callout doesn't specify the method, MRS is the default, according to ANSI, even though LSC is in more common use.  My colleague Alex has qualms, therefore, about the use of a default.  If the method isn't shown in the callout, you never know if the engineer intended that the default method be used, or if he simply forgot to take it into consideration.  Alex therefore recommends that engineers use the ISO convention, which requires that the method be specified.  It's certainly not a lot of extra trouble to add the information to the callout, and it may help avoid unnecessary confusion.