Intro to Coordinate Metrology - Projections

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A projection is the reproduction of a workpiece feature on another feature, such as projecting a circle or line onto a plane, or a point onto a line.

Projecting one part feature onto another can be compared with the creation of the traditional "flat" map of the world (Mercator projection).  The flat map is made by projecting a globe of the world (sphere), onto a cylinder (Figure 15).

Figure 15

In metrology, projections allow you to measure more accurately how mating parts will eventually fit together.  In automotive cylinder measurements (e.g., engine blocks), by projecting a cylinder into the plane of the head face, you can accurately determine how the pistons will fit into the cylinder and how it will meet with the combustion chamber in the head.

A minimum number of three points is necessary to measure the diameter of a circle and, if those points are not at the same distance from the top of the bore, the measured diameter will be shown to be elliptical (Figure 16).  To overcome this misrepresentation, the measurement data is projected into a plane that is perpendicular to the centerline of the cylinder.  The result is an accurate determination of the real size of this workpiece feature.


Using Effective Probe Techniques
By using effective probe techniques when inspecting a workpiece, you can eliminate many common causes of measurement error.

For example, probe measurements should be taken perpendicular to the workpiece surface whenever possible (Figure 17).  Touch-trigger probes used on coordinate measuring machines are designed to give optimal results when the probe tip touches the workpiece perpendicular to the probe body.  Ideally, you should take hits within ±20° of perpendicular to avoid skidding the probe tip.  Skidding produces inconsistent, non-repeatable results.

Part Surface to be Probed
Note that the approach of the probe should be within ±20° of the perpendicular to minimize skidding error. The probe approach vectors are perpendicular to the surface of the sphere.


Probe hits taken parallel to the probe body, that is, along the axis of the stylus, are not as repeatable as those taken perpendicular to the axis (Figure 18).


Using Effective Probe Techniques
Probe hits that are neither perpendicular nor parallel to the probe body (Figure 19) produce results that are even less repeatable than those taken parallel to the probe body.  You should avoid taking probe hits parallel to the stylus and at an angle to the probe body, since they will produce large errors.


Shanking is another cause of measurement error (Figure 20).  When the probe contacts the workpiece with the shank of the stylus and not the tip, the measuring system assumes the hit was taken in a normal manner and large errors will occur.