What is a Shape Measuring Machine?
A shape-measuring machine is a device used to measure the roundness of an object with a circular cross-section, such as a cylinder or a sphere.
Roundness is defined in JIS B 0621 as “the amount of deviation of a circular object from a geometrically correct circle”. This means that when the cross-sectional shape of a measured object is sandwiched between two geometrically correct concentric circles, the roundness is expressed as the minimum difference in radius between the two circles.
There are two types of shape-measuring machines: the stage rotation type, where the detector is fixed and the object to be measured is rotated by a rotation stage, and the probe rotation type, where the detector rotates while the object to be measured is fixed.
Usage of Shape Measuring Machines
Shape-measuring machines are used to measure the roundness of various objects, such as parts, in a single cross-section. For example, in hydraulic circuits, valves, and seals, strict dimensional tolerances are required to ensure oil sealing and air tightness.
In bearing parts, roundness is an important characteristic for smooth and frictionless bearing function. It is also used for quality assurance of mechanical parts that require high precision. The purpose of measuring roundness is not only to measure the dimensional shape of shafts but also to measure surface properties.
Principle of Shape Measuring Machine
As mentioned earlier, there are two types of shape-measuring machines, and the difference is due to the difference in their operation. Both have their merits and demerits, but here we will discuss the principle of the rotary table type, which allows for higher precision measurements.
A rotary table-type shape measuring machine consists of a rotary table capable of detecting the rotation angle using a rotary encoder and a detector that detects displacement of the workpiece surface synchronized with the rotation of the table. While the object to be measured is placed on the rotary table and rotated, the displacement of the surface of the object to be measured is detected by a probe. The roundness is determined from the profile obtained by recording the angle of rotation and the displacement of the workpiece surface over 360°.
Care must be taken in roundness measurement because any misalignment between the center axis of the measurement target and the rotation during rotation will cause runouts and lead to errors. Also, if the angle between the axis of rotation and the displacement direction of the probe is not perpendicular, it will lead to errors.
Other information on Shape Measuring Machines
1. Reference Circle and Types of Roundness
When evaluating roundness, it is necessary to define a reference circle as a standard. There are four types of reference circles, and the roundness is defined according to the reference circle.
Least Squares Circle (LSC)
The least-squares circle is a reference circle in which the area outside and inside are equal when the reference circle is applied to the object to be measured. The difference between the distance of the nearest part and the farthest part from this least-squares reference circle is evaluated as the least-squares circularity.
Minimum Zone Circle (MZC)
The minimum area reference circle is the concentric circle with the smallest difference between the diameters of the two circles placed between the object to be measured from the outside and the inside.
Minimum Circumscribed Circle(MCC)
The minimum circumscribed circle is the circle that can enclose the entire measurement object and has the smallest diameter. The furthest distance from the minimum circumscribed circle to the contour of the object is evaluated as the minimum circumscribed circularity.
Maximum Inscribed Circle (MIC)
The maximum inscribed circle is the largest circle inscribed within the object. The furthest distance from the maximum inscribed circle to the contour of the object is evaluated as the maximum inscribed circularity.
2. Difference between Roundness and Cylindricity
Cylindricity is a geometric tolerance similar to roundness. The difference between the two is the range of evaluation. While roundness is evaluated on an arbitrary cross-section, cylindricity is evaluated with a certain range in the axial direction.
Since the evaluation range is wider than that of roundness, it is more difficult to ensure accuracy. It is important to determine whether to indicate roundness or cylindricity based on the function of the part. Roundness can be evaluated for shapes with different diameters in different cross sections, such as tapered shapes, while cylindricity can only be indicated for cylinders with the same diameter in the axial direction.