## 17. Coordinate Measuring Machines

• generally measure x-y-z coordinates using touch probes

• these measurements can be made by positioning the probe by hand, or automatically in more expensive machines.

• reasonable accuracies are 5 micro in. or 1 micro meter.

• The method these machines work on is measurement of the position of the probe using linear position sensors. These are based on moire fringe patterns (also used in other systems).

17.1 Problems

Problem 17.1 If Moire fringes are 1/4” in height and an array of 4 photocells are used to pickup the light patterns, how far apart would 50 micro in. wide lines have to be if the two patterns were at 45° angles?

Problem 17.2 Given that four measurements (x, y, z) were taken on a sphere, develop an expression to estimate a radius for the sphere, and an average error.

17.2 Coordinate Measuring Machines (CMM)

• Automated machines used for inspection of parts

• Variety of methods for determining point locations in space,

touch sensors,

laser grids,

video cameras

can automate inspection process

less prone to careless errors

allows direct feedback into computer system

costly

fixturing is critical

requires a very good tolerance model