Introduction to Roundness and its measurement

Roundness is a measure of how close an object's shape is to a mathematically perfect circle.Roundness applies in two dimensions, such as the circle along the cross-section of a cylindrical object such as the shaft of a bearing or a cylindrical roller.In geometric dimensioning and tolerancing,the control of a cylinder can also include its fidelity to the longitudinal axis,yielding cylindricity.The analog to roundness in three dimensions (i.e.a sphere) is sphericity.Roundness is primarily determined by the general characteristics of a shape,not by the definition of its edges and corners,or the surface roughness of manufactured objects.If the eccentricity is large,a smooth ellipse may be less round.Regular polygons increase their roundness as the number of sides increases,even though they still have sharp edges.In geology and sediment studies (where three-dimensional particles are most important),roundness is considered a measure of surface roughness,with overall shape described by sphericity.

1.Roundness measurement. 

2.Roundness measurement is very important in metrology.It includes measurements on collections of points.

Method

1.To do this,follow two basic approaches:

1. Intrinsic benchmarks

2. Place the round object on the flat plate with the point of contact as the reference point.Place the dial indicator on the circular object again and rotate the object so that the datum remains in a constant position.Therefore,by comparing the peak height measured by the dial indicator, the roundness error can be directly known.

3. Alternatively, a V-shaped base can be used instead of a flat plate.Since the base is V-shaped, there will be two datums instead of one.The measurement of roundness error is similar to the previous method.

4. A cylinder can also be clamped between two axes.Here also the dial indicator is mounted on the cylinder so the roundness is measured by a similar procedure.

External benchmarking

Intrinsic methods are limited to small deformations.For large deformations,external methods must be followed.In this case,the datum is not a point or set of points on the object,but a single precise location,usually on the surveying instrument.The axis of the measured object or part of the object is aligned with the axis of the bearing.Then, the instrument's stylus just touches the part to be measured.A touch sensor attached to the tip of the stylus ensures that the stylus just makes contact with the object.Take at least three readings and plot a zoomed-in polar plot to get the desired error.