Getting the Fixturing Just Right

A gage requires just enough fixturing to do its job accurately and cost-effectively. Since the fixture establishes the relationship between the workpiece and the measuring system, its design and manufacture is as important to accurate gaging as the measuring instrument itself. However, in addition to accuracy issues, the fixture design can make a difference in a gage’s efficiency and economy of use.

Columns From: 8/24/2011 Modern Machine Shop, George Schuetz
Whether a gage user designs and builds a fixture or purchases one, gage design issues remain the same, and they can be expressed as a series of trade-offs or compromises. Gages dedicated to a particular type of part or measurement are faster to use but less flexible than generic gages. Faster gages are also more expensive. However, accurate gages also cost in terms of complexity, throughput and money. In fact, easy-to-use gages might be more complex in design. Regardless, a poorly designed fixture can make it difficult to take accurate measurements. There are many considerations when designing gaging fixtures. Some gaging tasks are simple in concept, while others are inherently more specialized.
Why Make That Check?
Sometimes part manufacturers are not always sure what their gaging needs are. Over the years, there have been many design requests to measure irrelevant dimensions. One manufacturer wanted to gage the overall thickness of a gear to ±0.005 inch. In fact, that dimension was meaningless in the assembly. Conversely, and even more importantly, experience has shown that a manufacturer may fail to recognize a critical dimension and allow it to go through the process ungaged. One benefit of using a gage builder is that they tend to do more than design and build gages; they also help manufacturers determine how, when and why to gage.
Achieving Throughput
After establishing a concept that will produce the required level of accuracy, the gage requester must weigh his or her need for throughput, flexibility and ease of use against cost. Essentially, it all comes down to answering a series of application questions about the speed of the gaging process: How quickly can operators set up the gage? Do they need to set it up at all, or does it remain in place to perform the same check repeatedly? Do they need to check every piece or just audit quantities? How many dimensions must they check on each part? Are they checking parts for “go/no go” or qualifying them into categories by dimension? What level of recordkeeping is required—if any? All of these factors will influence how quickly a part gets through the gaging station.
When gaging requirements are modest, the investment in fixturing usually is as well. Anyone making occasional measurements to tolerances of ±0.010 inch can usually get by with a dial or test indicator mounted on a bracket and sitting on a flat steel base—all very basic, generic equipment. If the part being measured is a simple cylinder, operators can place the part in a V-block and rotate it by hand while keeping an eye on the indicator.
This is a time-consuming gaging method, both for setup and for the actual gaging. However, if the shop is making the parts in small quantities, the use of simple, generic equipment and minimal specialized fixturing may be the most cost-effective solution to a gaging problem. Nonetheless, if you need to gage the same dimension every hour or every second, a more permanent, specialized arrangement may be essential.
Multicheck Fixtures
The ability to combine several measurements in one gage is an important means of speeding up a gaging operation. There are limits, however, to such combinations. Small parts will impose certain limits on the number of checks that can fit in a gage. It might be possible within a single fixture gage to check several diameters on a large diesel engine piston. Making the same number of checks on a refrigeration compressor piston might require two or three gages because the contacts or indicators will not fit closely enough together.

Fixturing is a capital expense, and it often involves a significant outlay. A manufacturer of critical parts, therefore, must carefully weigh his or her needs when purchasing a fixture. Too much fixturing can mean unnecessary expenses that will never be amortized. Too little (a much more common error) can result in a range of problems, including inaccurate measurements, excessive measuring time, inability to operate without excessive training or expertise, inflexible use and even unreliable gaging information.  

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