Every single one of the 3.5 trillion miles in the US are made possible by the hundreds of rotating parts that enable a vehicle to drive down the road. The performance of these parts is a direct result of the advancements in the science of roundness.

If we take the fulcrum point of a lever and move it completely over to one end, duplicate it repeatedly, witch each copy sharing the same fulcrum; a new simple machine is formed - the wheel.
Wheels allow us to multiple distances, speed, or force based on how much leverage we put on their center point.

Wheels provide another characteristic that has been critical to industrial growth. The ability to reduce friction by transmitting forces at a single point.

Roundness, along with size play critical roles in how parts are specified, designed and fitted. However, roundness diverts from the standard methods of defining dimensions such as length, area, and volume. Roundness is more of a relationship between dimensions and must be measured in a completely different manner. The measure of roundness, as well as other metrics of dimensionality, is known as metrology - the scientific study of measurement.

The ability to verify the roundness of a part is absolutely critical to a component’s performance.

In the intrinsic datum method, the datum points used for measurement are directly taken off the part and it contacts point with a reference surface. Typically a flat surface is used for a single datum measurement or a V block for a two datum measurement.
A measurement device that measures the displacement of the surface, such as a dial indicator, is brought to the surface of the part and zeroed to a start point. As the part is rotated, deviations from roundness displace the measurement tool from zero, with surface peaks creating positive displacement and valleys negative ones.

The solution to the limitations of the intrinsic datum method is extrinsic datum measurement. Extrinsic datum measurement is done by assigning a rotational axis datum to the part and aligning it the circular datum of a highly accurate rotating measuring fixture.

The four common types of calculated references circles are:

Least Square Reference Circle (LSC)
Minimum Zone Circle (MZC)
Minimum Circumscribed Circle (MCC)
Maximum Inscribed Circle (MIC)

The Least Square Reference Circle (LSC), the most commonly used reference circle, is a circle that equally divides the area between the inside and outside of the reference circle.

A Minimum Zone Reference Circle (MZC) is derived by first calculating the smallest circle that can fit inside of the measured data. Then calculating the smallest circle that can encompass the measured data. The out-of-roundness is given by the radial separation between these two circles that enclose the data.


A Minimum Circumscribed Reference Circle (MCC), sometimes known as the ring gauge reference circle and is the smallest circle that totally encloses the data. Out-of-roundness is quantified as the largest deviation from this circle.


A Maximum Inscribed Reference Circle (MIC) is the largest circle that can be enclosed by the data. The out-of-roundness
is quantified as the maximum deviation of the data from this circle. This is sometimes known as the Plug Gauge Reference Circle.

When rotating parts are examined, especially by extrinsic measurement, harmonics of the part become a consideration. Irregularities that exist on a rotating part that happens rhythmically are known as undulations.


In 2011, the International Committee for Weights and Measures spearheaded an effort to redefine the kilogram, moving it away from antiquated reference objects. One proposal, pushed by an international team called the Avogadro Project, aimed to define the kilogram in terms of a specific number of silicon atoms. In order to count the atoms of the large silicon-28 crystal, it was ground into a ball and its volume determined.


Moving past man-made objects, let's look at the roundest object ever measured. In 2013, in an effort to study the distribution of charge around the electron, scientist at Harvard were able the measure the smallest roundness ever.