Metrology
Metrology
Measuring Deep Bores
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Measuring Deep Bores
George Schuetz, Mahr Federal Inc.

  Measuring IDs of "deep holes" involves a few special considerations.  Deep is relative, but we'll define it here as being anywhere from roughly eight inches to 30 feet.  Although the equipment and the methodology stay basically the same as for shallower holes, depth can influence accuracy, choice of gage type, and speed of operation.

  The most basic deep bore gage is the mechanical, rocking-type gage, which comes in two basic flavors: self-centralizing, and non-centralizing.  The self-centralizing type is easier to use, because it requires rocking in only one plane.  It offers high resolution (typically .0001"), but has very restricted range (typically .025" or less).  The fixed contacts on the centering mechanism offset the gage head from the bore's centerline.  In narrow deep holes this restricts the ability to rock the gage, which can interfere with some measurements.

  The non-centralizing gage has no fixed contacts: it has two or three sensitive contacts that retract with a trigger mechanism.  It requires a bit more skill to use, and has relatively limited resolution (typically .0005"), but a long measurement range (up to 1-3/8").  Ironically, the head of the non-centralizing gage is automatically centered, which permits rocking to a greater extent.

  Both types are available off the shelf for use at depths to about 12", but can be made to go deeper with the use of mechanical extensions.  We've seen as many as six four-foot extensions screwed end to end to check for wear on the screw barrels of injection molding machines.  However, because of the limitations of long mechanical transfer, it's difficult to obtain accuracy greater than about .0005" in this kind of situation.

  Mechanical plug gages are self-aligning, which reduces the error potential of rocking-type gages and speeds gaging throughput.  When used in conjunction with dial indicators, mechanical plug gages are subject to the same limitations of long-motion transfer as rocking-type gages.  But it is possible to mount an LVDT right on the plug and feed them down the hole together on the end of a long rod.  The signal from the LVDT runs over a wire to an amplifier on the "surface."  In this way, mechanical measurements with resolution of .000050" are possible for the deepest holes.

  Rocking-type gages are limited to IDs of roughly 1" or greater in deep-hole applications, while mechanical plug gages allow ID measurements down to about .200".  To get smaller than that, you have to eliminate moving parts altogether and go to air gaging.  With special hypodermic tubing, air gaging can measure down to .100".

  Air is also practical for large IDs.  It is often difficult to thoroughly clean a deep hole, and air gaging is very forgiving of dirt, oil, and other contaminants, both in terms of accuracy, and in terms of maintenance and longevity of the gage.  This is especially important when gaging IDs of oil well pump barrels, which are some 30' long and about as dirty as anything you'd ever want to gage.
                   
  Air's non-contact aspect provides other benefits.  It is common practice to measure IDs at two-foot intervals along the length of these pump barrels.  Mechanical gage contacts would be subject to a great deal of wear under these conditions, but the jets on an air gage are unaffected.  In another example, when measuring IDs of nuclear fuel rods, a non-contact gage is essential to avoid burnishing interior surfaces.

  Because air gaging is, in a sense, the "standard" for deep holes, it's worth noting a few application tips:

•  The deeper the hole, the longer air pressure takes to stabilize.  In the case of an oil well pump barrel, this can mean 10-15 seconds.
 
•  Air gages are calibrated for use within certain ranges of atmospheric pressure.  If a gage is to be used at widely different heights above sea level (common for some oil-field users), it will be necessary to recalibrate it.  This is easily done by checking a lookup table from the manufacturer.
 
•  Special dial faces make some jobs easier.  The oil industry, for example, commonly uses a face with +.008" to the left of Zero, and just -.002" to the right of it.  This is because they are only checking for oversize caused by wear.  Most gage makers will gladly design dials for special applications.
 
•  Air plugs are available in a wide range of standard styles and specials.  For gaging blind holes and counterbores, reverse venting is machined into the plug to allow air to escape.  Blind- and super-blind plugs with very short lead-in sections are available for measuring to within .085" of the bottom of blind holes.  Plugs may have pins inserted or dogs machined in to align the jets with specific workpiece features (e.g., for checking groove and land diameters on rifled gun barrels).  Jets may also be arranged to permit measurement of TIR (total indicated reading) conditions and straightness as well as IDs.

  In most instances, deep holes are just like shallow ones -- only deeper.  Measuring them is not a deep subject: it just requires a bit more care to select the right tool for the job.