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110 Part II: Metrology
Part II.E.6
along two rails (y-axis) mounted on the floor. This configuration
provides easy access and allows the measurement of large
components.
6. L-shaped bridge configuration comprises a ram (z-axis) moving vertically
relative to a carriage (x-axis), which moves horizontally relative to an
L-shaped bridge moving in the y-direction.
Figure 11.6 shows CMM types according to this classification. The most advanced
configuration, the ring bridge, is not illustrated.
In addition to classifying coordinate measuring machines according to their
physical configuration, they can also be classified according to their mode of operation:
manually oriented, computer-assisted, or direct computer-controlled. In
manual machines, the operator moves the probe along the machine’s axes to establish
and manually record the measurement values, which are provided by digital
readouts. In some machines, digital printout devices are used.
Computer-assisted coordinate measuring machines can be either manually
positioned (free-floating mode) by moving the probe to measurement locations,
or manually driven by providing power-operated motions under the control of
the operator. In either case, a computer accomplishes the data processing. Some
computer -assisted CMMs can perform some or all of the following functions: inch
to metric conversion, automatic compensation for misalignment, storing of premeasured
parameters and measurement sequences, data recording, means for
disengagement of the power drive to allow manual adjustments and manipulations
of the machine motions, and geometric and analytical evaluations.
Direct computer-controlled CMMs use a computer to control all machine
motions and measuring routines and to perform most of the routinely required
data processing.
These machines are operated in much the same way as CNC machine tools.
Both control and measuring cycles are under program control. Off-line programming
capability is also available.
The effective use of computers for CMM applications is a principal feature differentiating
available CMM systems. The value of a measurement system depends
a great deal on the sophistication and ease of use of the associated software and
its functional capabilities. The functional capabilities of a CMM software package
depend on the number and types of application programs available. The following
is a list of many of the different types of system software available for coordinate
measuring machines:
1. Printout instructions, measurement sequence, zero reference,
and so on.
2. Automatic compensation for misalignment of the workpiece with the
machine axes.
3. Coordinate conversion between Cartesian and polar coordinates.
4. Tolerance calculations providing out-of-tolerance conditions.
5. Defining geometric elements such as points, lines, circles, planes,
cylinders, spheres, cones, and their intersections.