Metrology
Metrology
In The Groove
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In The Groove
George Schuetz, Mahr Federal


 It just might be easier to manufacture a groove on a turned part than it is to inspect it.  Select the right cutting tool, set the CNC, and bada-bing bada-boom! You're done.  (Okay, I exaggerate.)  But because of the critical functional role played by grooves for seal rings and retainer rings, good gaging practice is a must.  There are many different gage types from which to choose, and because inside grooves tend to be more difficult to measure than outside grooves, we'll examine inside groove gages more closely.

 The pistol-grip or linear-retraction style groove gage is specifically designed for inside grooves.  It features long retraction of 1/2"/13mm, which enables it to fit inside the bore, then expands to the full depth of the groove.  The indicator's sensitive range is usually much smaller than the retraction—on the order of 0.040"/1mm, which is more than adequate for the expected variation in most precision applications.  The indicator may be either dial or digital, and usually has resolution of 0.0005"/0.01mm.  The lower contact is a fixed or reference contact, to bear the weight of the gage; the sensitive contact is located above, where it is unaffected by the weight.

 Contacts tend to be built robustly, to resist flexing when the gage is "rocked," in the same manner as other bore gages.  Procedures for mastering, too, are similar to those used for bore gages, except that a fine adjust is usually provided on groove gages, to simplify zeroing. 

 Contacts are adjustable over a range of 2" to 4" (25mm to 50mm), so that different size parts may be inspected with a single gage.  Another important characteristic is the interchangeability of contact tips.  Extra-long contacts permit access to deeply spaced grooves, and special contact shapes allow inspection of grooves right up to the shoulder, and of round- and V-bottom grooves.

 Inside-measuring swing-arm or caliper gages are general-purpose instruments that may be applied to a wide range of inside dimensions, including inside grooves.  Caliper gages can measure through their entire retraction range, which is often over 3/4"/75mm.  These gages can therefore measure features where variation is expected to be large, and can measure different size parts without adjustment.  On the other hand, because they are non-adjustable, their ultimate range and flexibility may be poorer than pistol-grip style gages.
 
 Because of their long range, caliper gages have dial indicators with secondary dials, or revolution counters.  Some users have trouble reading these dials accurately.  One way to bypass this problem is with a digital caliper gage, although these are not very common.

 Rocking a caliper gage side-to-side for axial alignment tends to stress the caliper arms and pivot, and is not recommended.  As a result, caliper gages may not produce results as accurate as pistol-grip gages, which are engineered to promote rocking.  On the other hand, caliper gages are lighter, smaller, less expensive, and easier to use, all of which makes them quite popular.  They also offer a greater variety of contact styles for access to difficult features.

 Compared to inside grooves, outside grooves are a snap.  They are normally measured with a snap gage fitted with special narrow anvils, called blade anvils, which are available to match the depth, width, or shape of any groove.  With their heavy spring pressure and rugged positioning backstops, snap gages do not need to be rocked, and are very easy to use.

 Finally, bench-type ID/OD comparators can measure inside and outside grooves if fitted with special contacts.  ID/OD comparators offer the highest potential accuracy, and allow the part to be rotated, to explore the groove for minimum or maximum readings.  But they are limited to use with small, portable parts, with grooves less than 1"/25mm from the mouth of the bore.