No matter how careful we are at any given task, mistakes are often made; the calibration of gages and instruments is no exception.
The simple appearance of a plain plug gage member can be quite deceiving when it is to be calibrated. Typically, it’s a diameter like many others we encounter, and the close tolerances they are made to appear to be the only element of concern. But there are other features that are part of them that can invalidate a calibration result if they are not detected or corrected for.
Simplistically speaking, I’m referring to the geometry of the gage. The two most important elements that consistently pop up are roundness and taper. Size variations due to roundness are unlikely to be evident in a typical calibration setup while having a major role in the function of the gage. With the exception of ovality, a roundness analyzer is required to detect this condition.
A good technician will notice the presence of taper from wear at the front of the gage or an overall condition due to its manufacture. For this to happen, several measurements for diameter have to be taken over the length of the gage, something that not every calibration facility does.
A mistake often made in this regard is that it is assumed that as long as the measured sizes are within the diameter tolerance, the gage is acceptable. The standard for such gages is more specific in that geometric irregularities such as taper should not exceed half the diameter tolerance.
There are three basic types of instruments used for this type of calibration. The most common are a high magnification comparator-usually electronic, a bench micrometer or a universal length measuring machine designed for this type of work. All three contact the work with either small diameter anvils, or a contact point and large flat worktable on a comparator stand, or variations of them.
Mistakes arise when the condition of the contact surfaces is not verified and one of the following situations has a direct affect on performance:
Flat contacts are not flat or parallel to each other
Comparator worktables are worn directly beneath the upper contact point
Contacts are misaligned or not square to the axis of the gage being measured
A mistake in selecting the contacts used can lead to significant errors. One of the more common errors is the use of regular micrometer-style contacts that cannot measure diameter at the end of the plug because of their large contact area. The mistake made here is where the end of the gage is contacted on the edge of the measuring faces rather than in their center on the axis of the instrument.
Gage blocks are the usual masters for 1:1 comparative measurements, typically one build-up per size being measured. Often, they are carefully wrung together but the known size variations from their calibration report are not taken into account. Some skip this step and assume that the accuracy grade for the set is such that they don’t have to get that picky. The finer the tolerance, the greater the errors that will come out of such practices. One mistake often made is to ignore the effects of the wringing interval and the flatness and parallelism of the blocks which, when they are wrung together, increases their “size.”
Some calibration facilities attempt to reduce costs by using long-range measuring heads on their comparator stands. Measuring ranges of 1 inch or 25 millimeters with a high-resolution display lead many to believe they can avoid wringing up gage block stacks. This works to a point but a mistake often made is the failure to confirm that the axis of the measuring head is square to the plane of the worktable. Errors in this alignment result in a cosine error that guarantees errors in the measurement.
All calibration activities are subject to errors caused by temperature variations. Often this is due to the mistake of assuming that the general air temperature in the laboratory means everything in it is the same temperature.
Excessive handling of the gage being measured or the gage block buildup creates the most problems. Keeping both on a normalizing plate as much as possible will reduce this problem. Adjustments to measuring heads on comparators can produce enough heat through friction that will cause problems. Make it a rule to allow time for these variations to dissipate before making a final setting on the display.