GlossaryKind of term: Manufacturing termCoordinate measuring machine (CMM)
What is a coordinate measuring machine?
A coordinate measuring machine (CMM) measures points on a part and calculates dimensions and geometric deviations from them, relative to datums where required.
Each point is a spatial coordinate on the surface; the software first fits elements such as planes, cylinders and circles to them, and only those elements yield size and deviation. A form deviation such as flatness applies to the element itself; perpendicularity, position and runout are evaluated relative to the datum system on the drawing. The most common build is the bridge machine with a touch-trigger or scanning probe; there are also horizontal-arm machines, portable arms and machines with optical sensors.
What does a CMM measurement depend on?
The result depends on more than the machine. Room and part temperature count, because drawing dimensions apply at the reference temperature of 20 °C. Then come the fixturing of the part, the probe and stylus, the number and distribution of points, and the association method the software uses to fit an element, such as least squares or minimum zone. Two measuring programs on the same machine can therefore report different values for the same feature.
What does the standard say about a coordinate measuring machine?
CMM performance is established with the acceptance and reverification tests of the ISO 10360 series, with separate parts for, among others, length measurement, probing systems and articulated arms. A maximum permissible error (MPE) is specified, for example for length measurement, and the test shows under defined conditions whether the machine stays within it. That is not the uncertainty of a particular measurement, which also depends on the part, the measuring strategy, temperature and evaluation, and is determined per measuring task.
How does a coordinate measuring machine work in practice?
CMM inspection usually consists of one-off work, such as the measuring program per revision and sometimes an inspection fixture, and work per measured part: fixturing, measuring and the report. For costing, the CMM gets a rate of its own or inspection goes into overheads; both conventions are common. On a repeat order at the same revision the program usually exists already. How many parts you measure follows from the agreement with the customer or a sampling plan.
What is a coordinate measuring machine often confused with?
A CMM is not on-machine probing: a spindle probe measures with the same axes and under the same thermal conditions that made the part, whereas a CMM measures independently of them. And the MPE a CMM was tested against under ISO 10360 is not the uncertainty of your measurement.
Frequently asked questions
Why does a CMM result differ from a micrometer reading?
Because they measure different things. A micrometer measures between two points; a CMM fits an element to many points with an association method. Temperature, fixturing and strategy matter too. When results differ, record how each was measured before deciding on rejection.
Should the CMM have its own hourly rate?
That is an estimating convention. A rate of its own, with depreciation, a temperature-controlled room, calibration and an operator, puts inspection cost on the orders that need a lot of it. Inside overheads, orders with little inspection share the cost.
When is a portable arm good enough?
For large or heavy parts that cannot go to the inspection room, and for tolerances that are wide compared with the arm's uncertainty. For tight tolerances a bridge machine in a temperature-controlled room is usually the better choice.