Metrology
Metrology
Can't Measure It? Try A Caliper Gage
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CAN'T MEASURE IT?
TRY A CALIPER GAGE
George Schuetz, Mahr Federal Inc.

 If a workpiece has ever left your shop with an important dimension unmeasured because you couldn't get at it to measure it, you probably didn't know about indicating caliper gages.  Regardless of the shape of the housing or the complexity of the casting, no matter what size the flange, what curve the tubing, what width the material, or what depth the recess, an indicating caliper gage can give you the desired thickness or inside dimension.  Non-destructively, let me hasten to add.

 It is important to distinguish indicating caliper gages from the familiar 0"- 6" calipers. These latter, though useful tools, are only capable of measuring the basics -- IDs, ODs, lengths and depths.  Indicating caliper gages, on the other hand, use dial indicators.  Most have resolutions of .010", and some go to .001".  With a range from 0" - 1", or 0" - 3", the indicating caliper gage incorporates a scissors action to facilitate getting around obstructions, and is an immensely useful tool.

 The key to the caliper gage's flexibility is its inherently simple hinged geometry: movement at the contacts is mechanically reduced by a gear at the pivot, then re-enlarged the same amount on the face of the indicator.  (The ratio is usually 10:1.)  As long as that ratio is maintained, the jaws can be essentially any shape you want.  They can curve over and around any type of flange, into any curved or angled hole, across long distances, and into the most inaccessible recesses.

 The thing to remember about caliper gages is, if you don’t see it, ask for it.  Our catalog, for example, lists eight standard gages, but we have engineered -- and I'm not exaggerating -- over 50,000 "specials" for customers ranging all over the board.  To design a special caliper gage all a manufacturer needs is a print of the part to be measured, although a sample can also be helpful.

 Besides jaw shape, other common options include jaw size (up to 4' long!), body material (aluminum, magnesium, honeycomb composites, etc.), contact shape (ball, blade, rollers), and contact material (carbide, ruby, diamond, plastic).  Of course, the indicators come in inches or metric, and any kind of custom dial face can be designed to suit the application.  Revolution counters are standard to help you keep track of large dimensions.  Because they're fairly simple tools, prices are moderate: most specials cost $700-$800, although prices as low as $500 and as high as $5,000 are not unheard of.

 Where would you use an indicating caliper gage?  Just a few applications that spring to mind are: housings and castings of all kinds; valve bodies; manifolds; tubing; aircraft components (turbine blades, body and wing components, fuel tanks); wide sheet-type materials (steel, laminates, plywood, composition boards); air-cooled engine cylinder  castings (check wall thickness between the fins); and toilets.  I know a jet engine manufacturer who has a dedicated caliper gage for every critical dimension on his engine housing castings: literally thousands of gages.
   

 Outside caliper gages measure outside dimensions, such as cylinder or tubing wall thicknesses, flanges, and sheet-stock.  Inside caliper gages are often the only way to check inside dimensional features like seal and bearing seats deep inside a casting, recess IDs, or IDs of bent tubing.

 Even if your tolerances are relatively coarse fractional numbers that you could measure with a pocket scale or a 0" - 6" dial caliper, or if you currently use a Go/No Go gage, an indicating caliper gage can provide benefits in a production environment.  Rather than waiting until you bump up against the next tick on the scale that says you're at your tolerance limit, or worse, make a bad, “No Go” part, the higher resolution and magnification of an indicating caliper allows you to view the trend of your process.  You can see if your dimension is getting larger or smaller, be it ever so gradually.  This permits you to make adjustments so that you stay near the center of your tolerance range, and avoid the limits altogether.

 In sum: Within the limits of their ability to resolve, there's virtually nothing you can't measure with an indicating caliper gage.