Gaging Accuracy: Getting Ready One Step at a Time

George Schuetz, Mahr Federal

 Familiarity may not always breed contempt, but in precision gaging, it can certainly lead to error.  It happens when we do what we have done a thousand times before, but do it without thinking.  We are in a hurry.  We grab a gage and take a measurement without stopping to go through those preliminary checks and procedures we know will assure accurate results.  We forget that the methodology of measurement is as important as the gage itself.  As a machine operator, you must assume much of the responsibility for gaging accuracy.  Whenever a gage has not been in frequent use, make sure you follow these basic steps:

• Providing the indicator has been checked for calibration, repeatability and free running, look over the way it is clamped to the test set, comparator frame or gage.  Any detectable shake or looseness should be corrected.
• Check for looseness of play in comparator posts, bases, clamping handles, fine adjustment mechanisms and anvils.  It is easy, for instance, to rely on the accuracy of a comparator and find afterwards that the reference anvil was not securely clamped down.
• When using portable or bore gages, be sure to check adjustable or changeable contacts to be sure there is no looseness of play.
• If gage backstops are to be used and relied on, make sure they are also clamped tight in the proper location.
• The sensitive contact points on many portable gages and bench comparators are tipped with wear-resisting tungsten carbide, sapphire or diamond inserts.  Test these tips to see that they haven’t become loose in previous use.  Also, examine them under a glass.  If they are cracked, chipped or badly scored, their surface conditions may prevent accurate or repeatable readings.  They may even scratch the work.
• If opposing anvils are supposed to be flat or parallel, check them with the wire or ball test.  By positioning a precision wire or ball between anvils, you can read parallelism on the indicator simply by moving the wire/ball front to back and side to side.
• One of the easiest chores to neglect is regular cleaning of indicating gages and bench comparators.  Yet, as we have often noted in this column, dirt is the number one enemy of accuracy.  Dirt, dust, grit, chips, grease, scum and coolant will interfere with accuracy of gage blocks, indicators, and precision comparators.  Clean all such instruments thoroughly at each use.  Also, be sure to rustproof exposed iron or steel surfaces.
• Take the same steps to ensure the reliability of master discs and master rings as you would for gage blocks.  Examine them for nicks and scratches and the scars of rough handling.  And handle them as you would gage blocks, as well.  After all, they are designed to provide equal precision.
• Finally, if you see a sudden shift in your process during the day, these same basic steps should be part of your troubleshooting routine.  And, in this situation, don’t automatically assume your gage is correct just because it has a calibration sticker.  Strange things do happen and you will do well to investigate all possibilities especially the ones that habit can make us overlook.