Vertical Straightness—At Least One Reference Required

Vertical Straightness—At Least One Reference Required
George Schuetz, Mahr Federal Inc.

Many parts manufactured today have functional requirements to mate with parts that will have either a rotational or linear motion. When the design engineer builds these requirements into a part there will be tolerances specified for straightness and roundness, or even cylindricity which is the combination of both.

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 being measured, such as "straight to within 0.001" and within any 2.00" of the length of the surface."

The basis for most methods of measurement is either a straight line or a plane. The same is true when measuring straightness. With straightness, there is no datum required. Rather, it is a measurement relative to the part's own profile. But something is needed to act as the straight line for comparison. The reference straight line can be something as simple as a straight edge, or a measured and calibrated height gage or form machine.

The most obvious and basic method for checking straightness is to use a straightedge whose accuracy is known. 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 goes down to 70-50u" the light will appear red. When the gap gets to be 30u" the gap will appear blue. These color effects are due to interference caused by diffraction. But while useful for trying to measure these small distances, there are problems with this method, including environmental and operator variations.

A better way to document results would be 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, it is compared to an internal reference surface in the gage. Both height gage 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 documented against the required tolerance.

Just like any other gaging system, the gage being used must match up with the requirements of the part. Some height gages have accuracies to 0.0002", while others—including some form systems—may be within 0.00001". Sometimes 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 being used, various set-up parameters can be dialed in. These relate to the length of the surface to be measured and the number of measurements to be taken over that length. In almost all cases, a single start command will begin the measurement and the results will be displayed automatically with the system normalizing the line and automatically removing any angular deviation.

In the end you will get a number and a graph of the measured straightness—in black and white with no shades of red or blue—making it a lot easier to interpret.


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.


A better way to document straightness results is with an electronic height gage or form system.