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What is roundness | Roundness measurement method in metrology

2021-07-30 15:16

The measurement of roundness is crucial for many applications. Its measurement has several different methods, including simple caliper measurement and high-precision special roundness measurement system. In this article, we will introduce you to the definition of roundness, roundness standards, and several common methods of roundness measurement.

How to define roundness? 


A circle is defined as a closed curve with a constant radius from a point (center)



Roundness refers to the degree to which the cross section of the workpiece is close to the theoretical circle



Roundness is a value used to quantify the error of the circle, I .e. the distance from the center of the circle.



More precisely, roundness can be thought of as "out-of-roundness"


 

Roundness standard

Many different international standards, national standards, local and industry standards have relevant roundness standards,For example, the following two ISO standards:

  • ISO 1101 : 2017

    Defines roundness as the radial spacing of two concentric circles that exactly contain the roundness feature

    Drawing standards: describes roundness and related markings for roundness measurement and analysis

  • ISO 12181 : 2011(two parts)

    The definition of the roundness parameter is provided

    Introducing different reference circles

 

Why measure roundness? 

Circular section is one of the most important basic forms of industrial parts. Circular cross-sections are found in many applications, particularly on bearing surfaces such as rotating shafts and ball bearings. Measuring out-of-roundness (often referred to simply as "roundness") is an extremely important evaluation method. If the roundness of a rotating bearing component is poor,tend to beGenerating noisesound, and is likely to cause premature failure. Therefore, accurate roundness measurement is essential to ensure the function of such parts.

 

How to measure roundness? 

Here to introduce you to several common roundness measurement techniques and methods.

Diameter measurement method (two-point method)

 



Perhaps the first and simplest way to determine the roundness of a part is to measure its equal diameters in several different directions. If roundness is an appearance requirement, not a functional requirement, then measuring the diameter is sufficient.

V-block measurement method (three-point method)

 



Another frequently used method of determining roundness is to place the measured workpiece in a V-shaped block and rotate it in contact with a dial gauge or similar indicator. This is essentially a three-point method, not the two-point method described above. If the part is indeed round, the irregularity is negligible, and the pointer of the indicator will not move.

 

The error in the shape will cause the dial indicator to display the reading, but when the irregular place touches the V-block, the part will also move up and down. In addition, when measuring shaft workpieces, contact with the V-block is not limited to the measured plane. This means that irregular features of the part along its radius will affect the dial gauge reading.The limitation of the three-point method is that the results may vary due to the V-angle and the spacing of irregular features.

Coordinate Measuring Machine (CMM)

 



Another way to measure roundness is to use a coordinate measuring machine (CMM). Standard coordinate measuring machines have three precise orthogonal axes and are equipped with a touch sensor. Sensors and MeasuredWorkcontact and record its position. This method will firstWorkCollect several points around the piece, and then combine these points in the computer to calculate.WorkThe roundness of the piece.Typically, the number of sampling points is very small because it takes a long time to collect data points. Therefore, this measurement method is often not accurate enough.

Rotary Reference Method

 



DetermineWorkThe most accurate method of roundness is to use a probe scan to measure the change in radius relative to a precise rotational reference, with the probe in contact with the surface of the workpiece and collecting a high density of data points.A circle is then fitted to these data and its roundness is calculated.

 

There are many instruments dedicated to measuring roundness. The most common configuration is a system that contains a rotary table on which the workpiece is mounted. The sensor is mounted on a radial measuring arm which can be adjusted to bring the sensor into contact with the workpiece. The radial arm itself is mounted on a column that can adjust the height of the measuring plane.

 

The linear axis of this type of instrument has a high shape accuracy, so that the instrument can also be used to measure flatness, straightness and cylindricity and other parameters.The advantage of this type of instrument is that the roundness can be measured very accurately in a very short measurement time.