Metrology
Metrology
Checking Bores For Ovality And Taper
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Checking Bores For Ovality And Taper
George Schuetz, Mahr Federal Inc.

  Do you know the amount of ovality and taper of your bores?  Are you sure you want to know?  The decision can mean a big difference in the gage you select and the way you use it.

  Checking ovality is primarily a job for air gaging, but there are exceptions:  If you are interested in knowing whether a condition of ovality exists, but you don’t need to actually measure it, then you might get by with an adjustable, rocking-type bore gage.  Take one diameter measurement, then rotate the gage 90 degrees and take another.  If the bore is oval, you will see a difference in the readings.  But because you didn’t necessarily hit the highest and the lowest points, you won’t know how oval it is.

  It is impractical to use a rocking-type gage to measure ovality, because it is virtually impossible to hold the gage in alignment while you rotate it through a full 180 degrees.

  To measure ovality, you will want an air plug with two, and only two, jets, located 180 degrees apart.  Take a measurement, and then rotate the part (or the plug, depending on the setup) through a full 180 degrees, noting the maximum and minimum readings on the dial.  (A two-contact, mechanical plug-type gage will also work for this application.)

  If, on the other hand, you want to ignore ovality, select an air plug with four jets set 90 degrees apart.  By the nature of air gaging, the four jets will average the readings between the minimum and maximum diameters.  You won’t even have to rotate the part on the plug.  This is fine for some applications; for example, a press-fit bushing that conforms to the shaft when it is installed.  In fact, it may be desirable for the purposes of process control to ignore the variations that would show up if one part were measured at its largest ID, the next part at its smallest, and the third one somewhere in between.

                                              
                    

  The above considers only simple ovality--essentially, a two-lobed condition.  If you wish to measure lobing of greater frequencies, air plugs with as many as 12 jets can be used.  Checking bores for undesirable taper is similar to checking ovality.  (We will not discuss measuring intentionally-machined tapers.)  Use a multiple-jet air plug to measure first near one end, and then near the other end of the bore, and simply note the difference, if any.  If process analysis has indicated a need to check for barrel shape or bellmouth, take a measurement in the middle as well.

  Of course, a four-jet plug will automatically ignore ovality, which may be desirable.  If you wish to check ovality and taper in one operation, use a two-jet plug, measuring the bore at the bottom and the top, and rotating the workpiece or the plug at both ends.  This method can get confusing, however, and you may prefer to do it in two separate operations.

  Air gaging is a natural choice for measuring holes that are intended to be tapered (for example, Morse taper).  This method of measuring taper is easy, fast and accurate, although it involves slightly more elaborate equipment.  A plug with two separate air circuits is connected to the air gage so that each circuit acts on opposite sides of the precision diaphragm.  Simply place the workpiece on the plug, and the gage will automatically indicate any variation in taper, based on the differential of air pressure between the two circuits.

  Air gaging makes quick work of measuring ovality and taper.  When to select air gaging over mechanical gages for other applications is the subject of next month’s column.