Always ask the manufacturer how they error map. If they map to create accuracy then you'll experience headaches later on!
Mounted to each axis of the CMM are devices used to identify the position of each individual axis within the measuring range of the CMM. These are typically GLASS or REFLECTIVE SCALES. The scale is mounted to the axis and a reader head is attached to the moving member of the axis. As a reader head moves across the scale surface, a pulse is generated and converted into a digital count. The finer the gradient markings are on the scale, the finer the resolution of count. It is also critical that the scales are mounted properly on the CMM axes so that they are parallel to the axis of motion and the gap between the reader head and the scale is properly maintained throughout the length of travel. Again, this requires a straight and flat surface. The finer the resolution of count, the flatter and straighter the surface must be.
A coordinate measuring machine has 21 degrees of freedom, the same as any multiple axis machine tool, all of which should be taken into account for successful error mapping. Any error mapping is only effective if the identified error is repeatable and systematic. Error mapping is only reliable if the machine is stable.
A CMM that relies solely on error mapping is difficult to recalibrate or recertify for accuracy or conformance to specification at the point of installation, unless the machine is re-error mapped and the environmental conditions of temperature and humidity have not changed, or will not change after the error mapping has been completed.
It is the Starrett philosophy to build the CMM frame so that each axis travels straight and is square to the opposing axes. Once this is confirmed, error mapping is used only to improve the system accuracy and not to create the system accuracy. Typically, the error map accounts for 10% or less of the accuracy specifications. This also allows for the use of conventional tools such as traceable straightedges, squares and length standards to be used to identify conformance to specification and it allows the CMM to be calibrated to a traceable standard.
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