You’ve probably heard it said that “A CMM can’t check diameters correctly” or “Your CMM isn’t accurate because when I check the diameter with a pin it’s right”. This is never an enjoyable place to be in when you are not harmed with a good answer. So, I want to cover what happens when you are faced with situation.

What’s Happening?

What’s happening when I’m checking a diameter? This depends on many different factors, i.e. what diameter averaging algorithm are you using?, how many points are you taking?, what is the form of the diameter being measured? These questions and several others must be answered first to come up with a solution.

Algorithms

 An algorithm in its simplest definition is a mathematical calculation that your CMM software will use to calculate the diameter based on type used. There are several methods used to calculate a diameter.

Least square algorithm (LSQ) is the most common and is typically the default setting on most software. This method uses the average of the data to calculate the diameter size and location of its center.

Minimum Circumscribed, this algorithm will give you the minimum ring around an outside diameter. If you were to pass a ring gage over a stud this would be the minimum ring gage it would take. This calculation is based on extreme points and varies the result based on form, not only size but location.

Maximum Inscribed, this is the maximum size pin that would fit in a diameter regardless of the ovality of the hole. This calculation is based on extreme points and varies the result based on form, not only size but location.

There is also Outer Tangential; this calculates a diameter based on the 3 highest, or furthest from the average center, points. Inner Tangential: this calculates a diameter based on the 3 lowest or closest to the average center, points.

It is important to note that whatever method you use the size and the location are affected.

Which one do I use? This depends on the application. It will require some inquires to the engineering department, the machinist, or the customer about what they are looking for. The change in feature size might be slight or vary greatly based on the form of the diameter in question.
 Diameter Form And Why This Is Important

The form of the inspected diameter plays a major role in the calculation of the diameter size and location. It is important to note it is size and location. The location of the center of a diameter is based on the diameter calculation and varies as the average diameter varies.

The shape of the hole can change the averaging calculation of the diameter. With LSQ (purple) this diameter would possibly report in tolerance. With maximum inscribed the hole would show what the machinist sees using a gage pin (red).

With some of today’s software this variation can be filtered and outlier filtering can be applied changing how the software looks at the form of the circle thus changing the averaging of the data.

Conclusion

So, is a CMM the best way to check a diameter? Well that depends on what your looking for. In some cases it might be best to report out the size and location based on the different algorithms. You may want to include the form of the diameter to show induced error. It will be important to have a ready answer for those skeptics that always have an instance where the CMM was wrong when measuring a diameter. The answer is, perhaps they didn’t apply the correct algorithm when measuring the diameter. May be the tool is walking creating an oblong hole and that is affecting the calculation. Remember, “We don’t make the parts, we just checking ‘em.”