"Stylin" With Your Micrometer

George Schuetz, Mahr Federal Inc.

Convenience is one of the reasons the micrometer is often the tool of choice for length/diameter measurements.  The basic micrometer provides direct size information quickly, has high resolution, and is easily adaptable to many different measurement applications.  Beyond the basics, there are all sorts of micrometer styles which extend these advantages to many special measurement applications.

A micrometer consists of two opposing surfaces, a stationary anvil and a moveable spindle.  On most micrometers, these hardened steel or carbide-tipped contact surfaces are flat.

However, micrometers can also be equipped with built-in or contact tips with unique forms for measuring special part characteristics.

Have A Ball.  Ball contacts are used to measure wall thickness of tubes and other cylindrical components.  Micrometers are available with one or two ball/radiused contacts.  The one ball/radius style may be used for inspection of wall thickness on tubing.  Two ball/radius contacts can inspect thickness between holes.  In some cases, the ball contacts can be supplied as attachments for use with a standard flat tipped micrometer.  The attachments may be quickly and easily applied to either the anvil, the spindle or both.  When using this type of attachment, the ball diameters must be taken into account by subtracting them from the micrometer reading.

Time for Recess.  Reduced spindle style micrometers have a turned down diameter on both the anvil and spindle.  These contacts are used to measure inside recesses where the normal diameter may be too wide to penetrate.  Because the contact areas of the anvil and spindle are very small, these micrometers may take a little getting used to.  To get the proper "feel," take care to make sure the face of each contact is square with the axis of the diameter being measured. 

In the Groove.  Measuring the outside diameter of a cylindrical part having a turned groove on its surface calls for another type of micrometer contact. Often these grooves can be so narrow that neither a standard nor reduced face micrometer will fit completely into the groove.  Blade contacts, as the name implies, are very slender and flat.  They nest readily into narrow-bottomed grooves.  The blade solution created an interesting problem for the blade micrometer's designer.  The spindle surface of most micrometers rotates as the micrometer barrel is turned, but a blade inside a groove would eventually be constrained from rotating.  Blade micrometers have a spindle that slides along the axis of movement instead of rotating.  Using this style micrometer calls for greater care.  As always, check to make sure the micrometer is on the true diameter.  Also, check frequently for wear on the measuring surfaces.  Because the ends of the blades are so narrow, there is very little measuring surface.  Excessive pressure on these narrow blades, as the tool is being rocked to find the true diameter, can result in premature wear.

Between The Grooves.  On the same part, measuring the distance between the grooves is accomplished with a disk micrometer designed for thickness measurements on features that have narrow clearances.  The measurement contacts are relatively large disk-like flats which extend beyond the diameter spindle and anvil.  Because these contacts have such a broad measuring surface, parallelism errors can creep into the measurement.  It is important to check parallelism of the contacts using a precision ball on many locations between the contact faces.  A discrepancy of more then a grad of the vernier is a sign that the parallelism of the anvil and spindle needs to be corrected.

Even the best and most basic hand measuring tool can be made better by adapting it to special application requirements.  By choosing the most appropriate style of the application, you will achieve a faster and more accurate measurement.  Each style, however, has its own unique requirements for care and use.  If you're going to measure with style, make sure you know how to do it properly.