Dimensional Metrology

In general, there will be errors of size in any machined work piece. This means that the actual dimension will be different from nominal dimension. These errors should be within certain given limits by tolerances and determined by the dimensional measurement to guarantee the product quality. The dimensional measurement includes:

Post-process Dimensional Measurement

On-process Dimensional Measurement

Post-process Measurement

Traditionally, measurements have been made after the part has been produced. It is called the post-process measurement. The post-process measurement can be used to high production run of smaller parts. The inspection process can be made by traditional methods. If the dimensions are not within the given tolerance zone, a correction can be made to the next part through the machine tool.

On-process Dimensional Measurement

When the manufactured parts are big, with higher material cost and longer cycle times, on-process measurement is required to improve the productivity and reduce the cost. In the on-process measurement, parts are measured while they are on the machine tool.

The existing on-process measurement methods can be divided into direct and indirect methods according to the measurement principle.

Direct methods.  In direct method, the dimension of the work piece is directly measured using an adequate instrument, while the work piece is located on the machine tool. Therefore, the effects of tool wear distortions and machine errors can be taken into account.

Indirect methods. The work piece accuracy can also be indirectly evaluated from radius measurements, by monitoring the motions of the carriage, carrying the cutting tool or by noting the position of the tip of the cutting tool.

The on-process measurement can be implemented by several methods. Here are several on-process dimensional measurement methods: