Many parts manufactured today have functional requirements to mate with parts that have either a rotational or linear motion. When the design engineer builds these requirements into a part, tolerances are specified for straightness, roundness or even cylindricity, which is the combination of the two.
When straightness is of concern to the design engineer, the tolerance zone is limited in the measuring plane by two parallel straight lines at some distance apart. Usually straightness is specified as this tolerance zone over the entire surface, for example, “straight to within 0.001".” However, it can also be specified as the tolerance zone within a certain distance over any length of the surface, such as “straight to within 0.001" and within any 2" of the length of the surface.”
The basis for most methods of measurement is either a straight line or a plane. The same is also true when measuring straightness. With straightness, there no datum is required; it is a measurement relative to the part’s own profile. Something is needed to act as the straight line for comparison, though. The reference straight line can be something as simple as a straightedge, or it can be a measured and calibrated height gage or form machine.
The most obvious and basic method for checking straightness is to use a straightedge with a known accuracy. For many applications, it is acceptable to apply the straightedge to the surface being tested and gage the gap by using feelers or a light gap. The light gap method was sometimes used on lapped surfaces with a precision straightedge before some of the newer equipment became available. When viewed against a good light, a gap of less then 0.0001" can easily be seen. When the gap becomes 70 to 50 microinches, the light will appear red. When the gap becomes 30 microinches, the gap will appear blue. These color effects are due to interference caused by diffraction. While this method is useful for trying to measure these small distances, there are problems associated with it, including environmental and operator variations.
A better way to document results is with an electronic height gage or form system. Both of these measurement tools have known accuracies built into them and can be certified. So, instead of comparing the part to a straightedge, the part is compared to an internal reference surface in the gage. Height gages and form machines basically function the same way.
Both have an internal reference and a sensing head that moves against the surface being measured, and both compare variations of the measured surface to the reference surface. The result is the deviation of the two, or the straightness of the measured surface. Knowing this deviation, the tolerance zone can be applied, and the part surface can be documented against the required tolerance.
Just like any other gaging system, the gage being used must match the requirements of the part. Some height gages have accuracies to 0.0002", while others—including some form systems—may be within 0.00001". The sensing head can be as simple as a digital indicator or as precise as a digital encoder probe.
Depending on the height gage or form system, various setup parameters can be dialed in to relate to the length of the surface and the number of measurements taken over that length. In almost all cases, a single start command will begin the measurement. The results will be displayed automatically, with the system normalizing the line and automatically removing any angular deviation.
The end result is a graph of the measured straightness—in black and white with no shades of red or blue, making it much easier for users to interpret.