Metrology
Metrology
O.D. Gaging Can Be a Snap
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O.D. GAGING CAN BE A SNAP
George Schuetz, Mahr Federal Inc.

 Insert a work-piece into a snap gage and you'll understand how these extremely effective, fairly simple tools for checking precision OD’s got their name.  You have to push deliberately to get the part past the leading edges of the anvils.  Once you've overcome the 4 1/2 lbs. of spring force, the part slips suddenly back against the backstop contacting it with a good, healthy "snap."

 Snap gages can be hand-held to measure work-piece OD’s still on the machine or can be mounted on stands for use with small parts.  The heart of the tool is a simple C-frame casting, and measurements rely upon a direct, in-line, 1:1 transfer of motion.  These factors make snap gages simple, reliable, and fairly inexpensive.

 The earliest snap gages, of the fixed, or Go/No-Go variety, did the job well enough so that thousands are still in use; but fixed gages have some distinct liabilities.  For one thing, you need a different gage for every dimension you wish to check. The biggest shortcoming of the Go/No-Go snap gage is that it tells you nothing about your process.  You know if you're within tolerance, but you can't see if you're gradually getting larger or smaller, and so adjust the process accordingly—until you're out of tolerance.

 Some clever engineer (not I) replaced the upper anvil of a fixed snap gage with a dial indicator, and the whole scenery changed.  Indicating snap gages are able to measure to the limits of resolution of the indicator, and as they are comparative gages (i.e., they read to zero), they give the machinist a window on his O.D. machining process.

 The high cost associated with a different gage for every dimension was not solved until the introduction of the adjustable indicating snap gage.  With a typical range of adjustment of one inch, the adjustable snap can eliminate dozens of tools from the shop.  Adjustable snaps are still comparative gages: the adjustable jaw is preferably set to a master disc or a in a pinch a stack of gage blocks, and the indicator is "zeroed" before gaging begins.

 With a standard dial indicator installed, the measuring range of an adjustable snap gage is typically .020", with a resolution of .0001".  There's no rule that says an adjustable indicating gage has to have a mechanical dial indicator.  One can specify digital indicators, air probes, or electronic probes—they all use the same standard 3/8" diameter mounting.  With an electronic probe and amplifier, you can achieve resolution of 10 µinches, for tolerance measurements tighter than .0005".

 Usually, no modifications are necessary to retrofit an indicating snap gage with the probe or indicator of your choice.  It's a simple, in-house job, and it's very possible to take an entire shop's worth of old mechanical indicating gages and refit them with digital indicators or electronic probes.  These can be tied into SPC or other computer-based quality systems.

 Adjustable indicating snap gages can be modified to accommodate special applications.  Extra-large C-frames can be built to measure OD’s up to 48".  Anvils can be side-relieved, chamfered, or straddle-milled to provide access in difficult part profiles. Blade-type contacts can be used to measure diameters or grooves right up against perpendicular surfaces.

 Usage is particularly simple and straightforward; but, as with any gage, there are a number of principles of operation and maintenance one must observe to obtain accurate measurements and long life.  These will be the subject of the next gaging tip.