Modern roundness measuring instruments

Roundness is a property of a surface of revolution where all points on the surface are equidistant from the axis.The roundness of any profile can only be specified if the same center is found for the measurement.Diameter and roundness are measured by different methods and instruments.For diameter measurements it is done statically,for roundness measurements rotation is always necessary.There are two types of roundness measuring instruments.

1.Rotary pick-up type.

2.Turntable. 

These are accurate,fast and reliable measurements.Rotary picker type.The workpiece is stationary and the picker rotates.In a rotary,the workpiece rotates and the pick-up is stationary.On swivel models,the spindle is designed to carry the light load of the picker.The workpiece is heavy,fixed and easy to make.In the turntable type, the picker is independent of the spindle.This makes it easier to measure roundness.Repositioning the picker has no effect on the reference axis.The pickup converts the electrical movement of the stylus into an electrical signal,which is processed and amplified before being fed to the polar recorder. A microcomputer is combined with an integrated visual display unit,and the system is controlled by a compact keypad,increasing the system's versatility, range,and speed of analysis.The system is programmed to obtain the roundness of the workpiece relative to any of the four internationally recognized reference circles.A visual display of the workpiece outline can be obtained.Workpieces can be evaluated on the circumference and have undercut surfaces or interrupted surfaces with sufficient data to fit a reference circle to the profile.The program also provides functions such as auto-centering,auto-ranging,auto-calibration and concentricity.

Modern roundness measuring instruments

This is based on the use of a microprocessor to quickly provide roundness measurements in a simple manner;no evaluation of out-of-roundness is required. The machine can automatically center and calculate the roundness and concentricity,straightness and provide visual and digital display.A computer is used to speed up the calculations and provide the support reference circle.

(i) Least squares circle

The sum of squares of sufficient numbers.The equally spaced radial coordinates measured from the circle to the profile have minimum values.The center of such a circle is called the least squares center.Out-of-roundness is defined as the radial distance of the largest peak from the circle (P) plus the distance of the largest valley from the circle.

(ii) Minimum area or minimum radial separating circle

These are two concentric circles.The value for out of roundness is the radial distance between two circles.The center of such a circle is called the minimum area center.These circles can be found using templates.

(iii) Largest inscribed circle

This is the largest circle.Its center and radius can be found by comparison or by trial and error with a template or computer.Since V=0 there are no valleys inside the circle.

(iv) Minimum circumscribed circle

This is the smallest circle.Its center and radius can be found by the previous method,since P=0 there is no peak outside the circle.The radial distance between the smallest circumscribed circle and the largest inscribed circle is a measure of circularity of error.This figure shows the trace produced by the recording instrument.This trace draws concentric circles passing through the maximum and minimum points on the polar coordinate diagram,and its radial distance is the smallest circumscribed circle containing the trace or n inscribed circles that can fit the trace minimum.When determining the roundness error,the radial distance between the outer circle and the inner circle is considered to be the smallest.Evaluation of roundness can be done with templates.Outer circularity is defined as the radial distance of the largest peak (P) from the least squares circle plus the distance of the largest valley (V) from the least squares circle.All circularity analyzes can be performed by harmonic analysis and slope analysis.