Metrology
Metrology
Using Adjustable Snap Gages
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Using Adjustable Snap Gages
George Schuetz – Mahr Federal Inc.

  Let's continue last month's discussion about adjustable, indicating snap gages.  The principles of care and usage for these simple O.D. measuring tools are straightforward.  Because the body of the gage—the C-frame—is a rigid piece of metal, most of the "care and feeding" tips are concerned with the gage's anvils.  That's where most of the precision lies.

  Make sure the gage is suited to the application.  The anvils should be narrower than the part being measured to avoid uneven wear on the measuring surfaces.  If you repeatedly gage narrow parts on a broad anvil, you can wear grooves that may not be picked up by mastering.  You can get away with a small number of too-narrow parts, but if you're doing a production run, buy different anvils or modify the existing ones.

  Anvils can be straddle-milled or side-relieved to fit into grooves or recesses, or to ensure they're narrower than the work piece.  The edges can also be chamfered.  This is important when measuring a diameter immediately adjacent to a perpendicular feature—for example, a crank throw on a shaft.  There's usually a fillet where two surfaces come together, and if you put crisp, sharp-edged anvils right up against the perpendicular, you'll measure the fillet instead of the critical dimension.  Another way to phrase it is: Don't check diameters next to perpendicular surfaces—unless you've got the right anvils.
 
  Regularly check the anvils for wear.  Look for scratches, gouges, unevenness, pitting, rust, etc.  If problems are detected, the anvils can be removed and their surfaces ground and lapped.   Check periodically that the anvils are parallel.  This is essential if you've removed the anvils for maintenance or replacement.  To check for parallelism, place a precision wire or a steel ball in sequence at the front, back, left and right edges of the anvils.  Compare the indicator reading for each of the edges.

 

  If you detect an out-of-parallel condition and you haven't just replaced the anvils, you've probably dropped the gage.  While it's recommended to have the manufacturer tweak it back into shape, many shops can handle this in-house.  Remove the fixed, lower anvil, and carefully file the seat in the indicated direction.  Go slowly—just a few gentle licks—then re-mount the anvil securely and test again for parallelism.  Leave the seating of the upper, moving anvil alone.

  Observe the basics of good gaging practice: check regularly for looseness of components, keep the gage clean, protect against rapid changes in temperature, and master regularly.   For large production runs, it makes sense to purchase a master disc the size of your part.  For small runs, use stacked gage blocks.  Make sure you've wrung them properly and observed the other basics of block care and usage.

  Adjustments on indicating snap gages are few and simple.  Set the backstop so the diameter of the work piece is roughly centered on the anvils: it's not a critical adjustment.   To adjust the gage's capacity, turn the knurled nut that moves the upper anvil/indicator assembly up or down.  Move the upper anvil until the indicator zeroes itself against the master.  Then, before you tighten the locking nut(s), turn the adjusting nut very slightly in the opposite direction to release the torque on the lead screw.  This may seem insignificant, but any amount of tension will relax itself over time.  Then lock it down, master the gage, and check for repeatability several times before you start measuring.

  Wide anvils normally ensure that the gage seats itself squarely on the part.  But if you're using narrow blade-type anvils to check narrow grooves, you have to hold the gage as steady as you can, squaring it up by eye.  Offset blade anvils also impose side-loading, which can further reduce repeatability.  To accommodate these shortcomings, lower-resolution dial indicators are usually used with blade anvils: .005" resolution is typical, compared to .0001" on most snap gages.
 

  For large gages that weigh several pounds, the spring pressure on the upper anvil may be insufficient to achieve repeatability in a hand-held situation.  There's a simple solution to this one: turn the gage upside down and allow the weight of the gage to rest on the fixed anvil instead.  Then just rotate the bezel on the dial indicator, so it reads right-side up.