Before designing a fixture gage, the engineer must understand what specifications need to be inspected. In many respects, the design of the gage reflects not only the design of the part, but also the manufacturing processes that produced it. Machinists must establish datums to machine a part accurately, and gage designers often need to know what those datums are to position the part repeatably, relative to the gage head or another sensitive device. This seems simple and straightforward—a piece of cake, right?
Well, that is not always the case. Sometimes parts are so large that they cannot easily be brought to the gage, and a special arrangement might be required to bring the gage to a section of the part. Or, the part is so small that it seems impossible to get to the dimension that needs to be measured. Gage designers are always amused when a part print—that comes in at 10 times normal size—refers to a small land at the bottom of the bore. At 10 times the actual size it looks pretty simple. In reality, though, it may be impossible to measure.
These are the times when good fixture design comes into play. It ensures the measurement can be made in a way that is easy for the operator to make, and it produces repeatable and accurate results.
Take, for example, large bearings—not the 6-inch or 12-inch variety, but those that are 6 or 8 feet in diameter. The bearing surfaces may be curved or at an angle and call out various surface finish parameters. This task is not as easy as it seems. Most surface finish systems base some part of their precision drive systems on gravity to lock in their axes of motion. This means they need to be level to work properly. Since the goal is to measure the bearing races, which are square to gravity when the bearing is on its side, the drive just won’t work. This is where special fixturing comes into play. Fixturing has to support not only the surface finish gage square to the part, but also hold a part that weighs hundreds of pounds.
While the big stuff may be hard to gage, the small stuff is sometimes next to impossible. Take, for example, injection bores in cylinder heads. These are very small holes with critical surface finish requirements. They are not square to any face, but rather at a specific angle to the top of the cylinder head. This presents design challenges to the engineer, such as how to align the probe and how to protect it from being damaged during insertion, measurement and retraction.
In cases like this, knowing the design of the cylinder head can allow a special fixture to be designed that aligns on the cylinder head. The gage uses it as a reference, then positions the surface finish probe at the right angle to the bore. Simply sliding the probe into the hole may not be the best solution, as surface finish probes are fragile. The better solution would be to build in a safeguard to protect the probe until it is in its final location. In this example, a special mechanical switch keeps the probe retracted until insertion is complete and the probe is in position.
To check the calibration of the system,the probe is set within a fixture at the proper angle, and it would be too much to take it apart every time it needs to be checked for calibration. So, the solution is to also stage the gage for proper alignment to a precision surface finish patch. In this case, the calibration fixture simulates the part but with a precision patch placed in the location of the bore.
Knowing how the part is designed, where the measurement is to be made and how these two interrelate can be the key to staging the measuring instrument.