Metrology
Metrology
Gaging Distance Between Hole Centers
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GAGING DISTANCE BETWEEN HOLE CENTERS
George Schuetz, Mahr Federal Inc.

Many of the gaging applications we've considered over the years involve size inspection of a single feature, e.g., the diameter of a hole, depth of a groove, height of a gage block, etc.  Many parts, however, contain multiple features that establish dimensional relationships between two or more other parts.  Examples include engine blocks, with multiple cylinder bores that establish distances between pistons, and pump housings with two overlapping bores that establish clearance between two mating impellers.  As with people, machined features tend to be simple and straightforward when they're single.  When people or parts are in relationships, however—and especially when those relationships involve mating—things can get complicated.

Gaging the distance between hole centers is a good example, because it requires that you first locate those centers in space before you can measure the distance between them.  Gaging equipment may vary widely with the application, and include considerations of part size and configuration, and required throughput and accuracy.  But the basic idea remains the same across various technical approaches; you must first establish references between the part and the gage, often in three dimensions, before you can measure deviation from a specified distance between features.


A basic approach is a hand-held gage, like that shown in the figure.  Both of the plugs have two fixed contacts, and one spring-loaded contact each, so they automatically center themselves in their respective holes.  One of the plugs is a fixed reference; the other is a sensitive contact.  Note also the hard depth stops, establishing a reference in the third dimension.  The sensitive plug moves relative to the fixed plug on a frictionless device such as a pantograph mechanism or an air-bearing carriage for good repeatability.  The sensing/indicating device used to measure deviation may be a simple dial or digital indicator, or an electronic probe and amplifier.

This gage is economical, reliable, and easy to use.  It may be employed with parts as large as an engine block (or larger), or as small as a connecting rod, and its capacity may be adjustable to measure different distances.  The plugs may be replaceable, to accommodate different size holes.  Its limitation is that it only measures distances between centers; no other types of features or relationships may be checked.
        
The next option is a gaging fixture engineered for a specific part.  Such gages are generally benchtop devices, so they are limited to use with relatively small parts (e.g., automotive conrods) that can be brought to them.  The workpiece is typically located over fixed plugs, each of which may contain one or more air or electronic probes as sensing devices.  Fixture gages may offer limited adjustability, but tend to be very application-specific.

The benefit side of the coin, however, is substantial.  Fixture gages tend to be very stable, and allow high throughput rates.  Because the probes can be spaced quite densely within the fixture, it is possible to measure multiple features or characteristics simultaneously.  For example, a conrod gage with sixteen probes (eight each for the wrist pin and the crankshaft bores) can be engineered to measure the following features, in addition to center distance between bores: four diameters per bore (6:00 to 12:00, and 3:00 to 9:00, at both top and bottom), bore "out of roundness" (i.e., the difference between two diameters at right angles), bore taper, and bend and twist between bores.  Gaging computers and some amplifiers can be readily programmed to perform all these measurements without changing the gaging setup.

For large, mass-produced parts with multiple features in relationship (such as engine blocks or cylinder heads), an elaborate "doghouse" gage may offer the best combination of accuracy and throughput.  The gage gets its name from its large, cast-iron structure that contains machined reference surfaces and "sweep" gage plugs, and establishes precise relationships between them.  The engine block is pushed into the doghouse, and reference surfaces on the workpiece are positioned against matching references in the gage.  The sweep gages are single-probe gaging plugs that are mounted in bearings, and may be rotated 360° on their axes to establish the locations of the hole centers.  Operation may be either manual or automatic.

In an automated gage, multiple sweep gages descend simultaneously into the appropriate bores.  A gaging computer then takes the data and calculates the multiple relationships between the bores.  This may even include not only distances between adjacent bores, but also distances between opposite banks of cylinders on V-configuration engines.  Results may be presented in numeric and/or pictorial displays.

Obviously, such a gage is costly, and must be engineered for the specific application.  It is often justified, however, in automotive and similarly high-volume/high cost applications where multiple relationships must be measured to high levels of accuracy at very high rates of throughput.  And in essence, it's not very different from the simple hand-held gage; the basic idea is still to provide a means of reliably locating the workpiece against fixed reference contacts, then measuring deviation relative to those references.