The case for optical CMMs.
(coordinate measuring machines)
Tooling & Production
May 1, 1989
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The case for
A common question is when to use machine vision for quality-control measurement.
Although it is not a panacea for all your quality requirements, there are areas where machine
vision excels, plus new areas where it is becoming more competitive as sophisticated systems
become simpler to use and less expensive.
CMMs are based on a video camera and image processing, and fully
automated; i.e., programmed to go to a measurement position, and then automatically focus
on the desired part feature to get a precise reading. The accuracy of their three-dimensional
measurements is based on three-axis positioning tables. They are excellent at
minute features and complex geometries, for example, checking parts from high-speed
stamping presses, vertically milled parts, form-tooled rotary parts, and miniature-to medium-
sized EDMed parts.
Although on-line or in-line vision systems are used in some manufacturing and assembly
CMMs are used off-line, monitoring metal-stampings,
machined castings, and fine-blanked or screw-machined parts.
CMM has obvious speed, accuracy, and SPC advantages over
comparators, toolmaker's microscopes, and manual gages. It also offers flexibility, through its
software, for handling many different types of measurements. First-part inspection, carbide-
insert inspection of cutting tools, and die- or mold-wear monitoring can be performed with a
single measurement system instead of an array of electronic, mechanical, or functional gages.
CMM can be quicker than the touch-probe CMM, with one-dimensional
measurements--between two points or a hole diameter--made in approximately 1 sec. For
example, in one case, an
comparator was required for each of ten stamping presses
producing electrical terminals. Now, a single vision system handles the output of all ten
Accuracies are typically better than
comparators, touchprobe CMMs, and toolmaker's
microscopes. A good
CMM can deliver accuracies of plus or minus 0.000 16" in the X
and Y axes and plus or minus 0.000 20" in the Z axis. Repeatability is plus or minus 0.0001"