Although advanced manufacturing technology makes it possible to tolerance and make workpieces very precisely, imperfections still exist. Small as they may be, the fact that there are tolerances means that there are errors.
Coordinate measuring machines are no different from other products in this respect. While they are built to extremely tight tolerances, there are errors (roll, pitch, yaw, straightness, squarenesses and scale errors) in their structure that affect their accuracy. As manufacturing tolerances become increasingly tighter, it is necessary for CMMs to become more accurate.
The majority of the inaccuracies can be corrected automatically in the CMM’s computer. Once all of the geometric errors of the CMM are measured (called error mapping), they can be minimized or even eliminated by powerful algorithms in the software. This technique is called volumetric error compensation.
By eliminating errors mathematically, you lower the cost of manufacturing and provide the customer more performance for their money.
Volumetric compensation can be best understood in terms of the relationship between a map and a compass. If you want to sail to a particular location, you have to know its true direction from your current position (origin). A compass and a map are used to determine your direction, or bearing. There is, however, a difference between true north and magnetic north. The difference between the two is called variation and is caused by non-uniformity in the earth’s magnetic field. Thus, to determine the true direction from one point to another, the variation between true north and magnetic north must be added or subtracted from the compass bearing.
In the map shown (Figure 13), the difference between true north and magnetic north (3° W), must be compensated for or a sailor would end up northwest of the intended goal and would run aground before reaching the final destination.
A coordinate measuring machine does a similar compensation automatically to remove the variations of the machine from the measurement.
