Measuring machines

A CMM measures the position of points on the surface of a workpiece in 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

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

Types of CMM

Moving bridge, fixed bridge, cantilever, column and horizontal arm. 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 which sends a signal when it touches (most common).
2. Analogue (scanning) probe: gives continuous output for surface scanning.
3. Non-contact: optical/laser probe for soft or delicate parts.
4. Hard (fixed) probe, used by hand, where the operator records the point.

Working

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

Advantages

Universal measuring of complex components, high accuracy and speed, less set-up time and no special gauges, automatic recording and statistical analysis; suitable for the inspection of cams, gears, engine blocks and dies. Limitations are the cost, the need for a controlled environment and the lack of touch on soft surfaces.

End bar measuring machine

It is a bench instrument for the calibration of end standards such as end bars, long gauge blocks and gap gauges up to 1 or 2 m. The unknown bar is placed between two anvils on a rigid bed. One anvil is fixed and the other is a movable anvil on a slide. A dial gauge or a micrometer head is used to measure the movement. The unknown bar is compared with a master of near size; the difference in the readings gives the deviation. Temperature of the work and master must be equal, and the measuring pressure is kept constant by a weight or a spring.

Universal measuring machine (UMM)

It is a bench instrument that measures the length of external and internal features directly, with a very high accuracy (down to 0.1 $\mu$m) and is more flexible than the micrometer. It has a rigid base with a measuring head and a tailstock. A measuring slide carries the head that moves along the table and the distance is measured with a precision scale or a line scale viewed through a microscope, or an optical/laser scale. The workpiece is held on a table between the two measuring contacts. A wide range of attachments is available: for internal diameters, thread pitch, gears, and so on. Measurement is done by comparison of the readings, with a set measuring force.