Form or geometry gages are generally divided into two categories depending on what form parameters they are designed to measure. While there is no formal designation to differentiate these two categories, sometimes they are simply referred to as roundness gages or cylindricity gages. Their difference is primarily based on their ability to make self-referenced or datum-referenced measurements. To read more... Self and Datum Referenced Form Measurements
Data Collection Goes Wireless
About 25 years ago, the concept of data collection for process control took a major leap forward. This was about the time that a combination of electronic technology and economics allowed gaging to become digital. With a digital signal available, it became possible to transfer information via cable directly from a gage or digital indicator to the data collector. This made it much more practical to make process control decisions based on statistical analysis. To read more... Data Collection Goes Wireless
Surface Texture From Ra To Rz
The irregularity of a machined surface is the result of the machining process including the choice of tool, feed and speed of the tool, machine geometry and environmental conditions. This irregularity consists of high and low spots machined into a surface by the tool bit or a grinding wheel. These peaks and valleys can be measured and used to define the condition and sometimes the performance of the surface. There are more than 100 ways to measure a surface and analyze the results, but the most common measurement of the mark made by the tool or the surface texture is the roughness measurement. To read more... Surface Texture From Ra To Rz
Precision Bearing Brochure
Since the dawn of the industrial revolution bearings have
played a critical role in all types of machines and equipment.
A bearing is fundamental to the precision movement of
nearly every type of machine and device known to man.
Since 1861 Mahr gages have been the standard to ensure
the quality of all types of bearings. To read more... Precision Bearing Brochure
Automated Probe to MMQ 200 Cylindricity System
Mahr Federal has introduced a new version of its popular MMQ 200 Formtester Cylindricity Machine with an automated T7W probing system. The T7W is a ±360° motorized by-directional probe intended to reduce operator involvement and improve measurement capability. Complete with Mahr's newest EasyForm® Version 3.0 software, the MMQ 200 is compact, fast, very easy to learn and operate, and is equally at home in the metrology lab or on the shop floor. To read more... Automated Probe to MMQ 200 Cylindricity System
Series 300P Indicating Snap Gages from Mahr Federal Offer Speed, Accuracy, Flexibility and Economy
Mahr Federal's Series 300P Indicating Snap Gages are an excellent choice for highly accurate and reliable measurement results on cylindrical parts with narrow tolerance ranges. The gages are available in a variety of sizes with measuring capacity increasing in 1" (25 mm) increments up to 9" (229 mm). Series 300P Snap Gages can be configured with a number of different indicators and all are fully adjustable with positive position locking at any point within the gaging range. To read more... Mahr Federal Series 300P
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.