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What Kind Of Gage Do You Need? A Baker's Dozen Factors To Consider
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What Kind Of Gage Do You Need?
A Baker's Dozen Factors To Consider
George Schuetz, Mahr Federal Inc.

Like every other function in modern manufacturing operations, inspection is subject to management's efforts at cost control or cost containment.  Its good business sense to try to maximize the value of every dollar spent, but it means that hard choices must be made when selecting gaging equipment.  Issues as diverse as personnel, training, warranties, throughput requirements, manufacturing methods and materials, the end-use of the workpiece, and general company policies on gaging methods and suppliers may influence both the effectiveness and the cost of the inspection process. 

For example, what's the ultimate cost of a bad part passing through the inspection process?  It could be just a minor inconvenience to an OEM customer—maybe a two-second delay as an assembler tosses out a flawed two-cent fastener and selects another one.  On the other hand, it could be a potentially disastrous equipment malfunction with expensive, even fatal, consequences.  Even if the dimensional tolerance specifications for the parts are identical in both instances, management should certainly be willing to spend more on inspection in the second case to achieve a higher level of certainty—probably approaching 100 percent.  One disaster averted will easily pay for the more expensive process in lawsuits avoided, lower insurance premiums, etc.

Many companies have achieved economies by moving inspection out of the lab and onto the shop floor.  As this occurs, machinists and manufacturing engineers become more responsible for quality issues.  Luckily, many gage suppliers are more than willing to spend time helping these newly assigned inspection managers analyze their functional requirements.

One could begin by comparing the hardware options.  Let's take as an example a "simple" OD measurement on a small part.  This inspection task could conceivably be performed with at least seven different gaging solutions:

1) Surface plate method, using V-blocks and test indicator
2) Micrometer
3) Purpose-built fixture gaging
4) Snap gage
5) Bench-type ID/OD gage with adjustable jaws
6) Hand-held air ring or air fork tooling
7) A fully automated system with parts handling.

(Actually there are many more solutions available, but let's keep it "simple.")  Between these options there exists a price range from about $150 to $150,000.  There are also differences in gage accuracy, operator influence, throughput, data output, and on and on.  It's confusing, to say the least.

A better approach is to first define the functional requirements of the inspection task, and let that steer one toward the hardware that is capable of performing the tasks as identified.  In order to do this, the end-user should consider the following factors:

• Nature of the feature to be inspected.  Is it flat, round or otherwise?  ID or OD?  Is it easily accessible, or is it next to a shoulder, inside a bore, or a narrow groove?

• Accuracy.  There should be a reasonable relationship between job tolerance and gage accuracy resolution and repeatability—very often on the order of a 10:1 ratio.  A requirement for statistical GR&R (gage repeatability and reproducibility) testing may require 20:1.  But always remember:

• Inspection costs.  These increase sharply as gage accuracy improves.  Before setting up a gaging operation for extremely close tolerance, verify that that particular level of accuracy is really necessary.

• Time and throughput.  Fixed, purpose-built gaging may seem less economical than a more flexible, multi-purpose instrument, but, if it saves a thousand hours of labor over the course of a production run, it may pay for itself many times over.

• Ease of use, and training.  Especially for shop-floor gaging, you want to reduce the need for operator skill and the possibility of operator influence.

• Cost of maintenance.  Can the gage be maintained, or is it a throw-away?  How often is maintenance required, and who's going to perform it?  Gages that can be reset to a master to compensate for wear are generally more economical over the long run than those that lose accuracy through extended use, but may require frequent mastering to ensure accuracy.

• Part cleanliness.  Is the part dirty or clean at the stage of processing in which you want to measure it?  That may affect labor requirements, maintenance, and the level of achievable accuracy, or it might steer you toward air gaging, which tends to be self-cleaning.

• Gaging environment.  Will the gage be subject to vibration, dust, changes in temperature, etc.?

• "Mobility."  Are you going to bring the gage to the part, or vice versa?

• Parts handling.  What happens to the part after it's measured?  Are bad parts discarded or reworked?  Is there a sorting requirement?

• Workpiece material and finish.  Is the part compressible? Is it easily scratched?  Many standard gages can be modified to avoid such influences.

• Manufacturing process.  Every machine tool imposes certain geometric and surface finish irregularities on workpieces.  Do you need to measure them, or at least take them into consideration when performing a measurement?

• Budget.  What do you have to work with?

All of these factors may be important when instituting an inspection program.  Define as many as you can to help narrow the field, but remember that help is readily available from most manufacturers of gaging equipment—you just have to ask

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