It is important to have the correct fixturing to stage parts for efficient gaging. This is especially true when measuring on the shop floor. Correct fixturing is also valuable in the lab when staging parts for very precise form or surface finish measurements. With the exception of job shops, most medium- to large-sized manufacturing facilities run the same parts over time. By making an up-front investment in interchangeable fixturing, they can make significant process improvements—ones that will generate a return every time the same parts are run.
Fixturing systems designed using SMEF principles routinely improve the efficiency of staging small, complex parts. There is no technical wizardry involved, and these systems do not break new, conceptual ground. In fact, it is rather the opposite: These systems succeed because they are based on solid mechanical-design principles and they use basic, kinematic-mounting techniques.
The idea is simple. For each part, one or more mounting fixtures are custom designed, depending on the number of features that need to be accessed and measured as well as the orientations those measurements require. These, in turn, mount in another base fixture designed to mount on the measurement machine. The operator simply places the part in the fixture, places the fixture in the base and presses the appropriate measurement icon on the touch screen. The system then takes the measurement.
While the idea is simple, the implementation requires careful design and planning. Before a fixture gage can be designed, the engineer must understand the inspection specifications. In many respects, the gage design must mirror both the part design and the manufacturing processes that produced it. Machinists must establish datums in order to machine a part accurately. Gage designers often need to know what these datums are in order to position the part repeatedly, relative to the gage head or other sensitive devices.
As many as three datums may be required to properly locate a part and a gage relative to each other in 3D space. However, there is more to it than setting up the reference surfaces: The gage should be designed so that all the measurements can be made in one setup. The operator shouldn’t be forced to manipulate the part to get at an opposite surface. A proper fixture will stage it once for all checks.
Therefore, the gage designer should be familiar with not only the parts, but also the processes of making the parts. It is not uncommon for the gage designer to require specific operation sheets to understand the manufacture of the part and how it is supposed to function.
In order to be fail-safe, these fixtures must be designed so the operator can neither place the part in the fixture incorrectly nor mount the fixture incorrectly. The ultimate goal is to create a solution that requires no tools for mounting the part in the fixture and placing the fixture on the machine. The fixture should be designed to ensure that the part is firmly held once it is inserted into the fixture, and that the holding force is operator independent. Typically, springs set the holding force. This ensures that the part always is held firmly and the operator cannot forget to tighten some clamping mechanism. Furthermore, the clamping force is controlled so that nothing can be over-tightened and distort the part for measurement.
Such fixturing systems are the best way to ensure reliable functionality in a production environment. By using them, operators can make highly sophisticated form and surface finish measurements quickly, easily, and most importantly, accurately.
Regardless of its complexity, a gage fixture is the key to accurate measurements. Make sure there is no play at its joints. Check that the instrument is assembled securely, and confirm that the gage measures both workpieces and masters at identical locations.