Coordinate measuring machine and probes

A CMM measures the position of points on the surface of a workpiece along three axes ($X$, $Y$, $Z$) with a probe, and from these points computes dimensions, form, location and orientation.

        Z-ram
         |    X-bridge (moves in X)
   ______|___________
  |      |           |
  |    probe(+)      |
  |______|___________|
     work on granite table
          <-- Y -->

Construction

• A rigid granite or cast iron table for the work, with a bridge or gantry that carries a ram with the probe.
• Three mutually perpendicular axes moving on air bearings, each with a measuring scale (linear encoder, glass scale or laser, resolution 0.1 to 1 $\mu$m).
• A probe head, a controller and a computer with software to calculate points, circles, planes and so on.

Types of CMM

Moving bridge, fixed bridge, cantilever, column and horizontal arm. They are operated manually, by motor drive with a joystick, or by computer numerical control (CNC).

Probes

1. Touch-trigger probe: a ball tip on a spring-loaded stylus that sends a signal when it touches the surface (most common).
2. Analogue (scanning) probe: gives a continuous output for scanning a surface.
3. Non-contact probe: an optical or laser probe for soft or delicate parts.
4. Hard (fixed) probe: moved by hand, with the operator recording each point.

Working

The machine is calibrated with a reference sphere to find the effective probe radius. Points are taken on each feature; the software fits a circle, plane, cylinder, etc. and compares the result with the CAD data.

Advantages

Universal measurement of complex components, high accuracy and speed, less set-up time and no need for special gauges, and automatic recording and statistical analysis; it is suitable for inspecting cams, gears, engine blocks and dies. Its limitations are the high cost, the need for a controlled environment, and the risk that contact probes deform soft surfaces.