Fixtures Are A Common Source Of Gaging Error

As a gaging engineer, my concept of a gage includes both the measuring instrument and its fixture. Assuming you are dealing with a reputable supplier, and your instrument was engineered to do its job as intended, there is probably little you can do to improve its accuracy, aside from throwing it out and spending more money. So we will concentrate on the setup, which is a common source of measurement error.

The fixture establishes the basic relationship between the measuring instrument (for example, a dial indicator) and the workpiece, so any error in the fixture inevitably shows up in the measurements. Many fixtures are designed as variations of a C-frame shape and, as such, have a substantial cantilever that is subject to deflection. This problem is greatly reduced if the fixture is a solid, one-piece unit.

Most fixtures, however, consist of a minimum of three pieces: a base, a post and an arm. These components must be fastened together with absolutely no play between them. As a rough rule of thumb, any movement between two components will be magnified at least tenfold at the workpiece. Play of only a few millionths can, therefore, easily accumulate through a couple of joints so that measurements to ten-thousandths become unreliable, regardless of the level of discrimination of the instrument.

Because such tight tolerances are required—tighter than you can perceive by eye or by touch—it is often essential that fixtures have two setscrews per joint. No matter how tightly a single setscrew is tightened, it often acts merely as a point around which components pivot.

Lost motion due to play between fixture components is dangerous. Assuming that the gage is mastered regularly, a fixture with loose joints may still provide accurate comparative measurements. There are two places in a gage, however, where loose assembly may produce erratic readings, making the setup completely unreliable. Most dial indicators offer optional backs and sensitive contacts that are designed to be changed by the end user. Looseness of these two components is among the most common sources of gaging error. These are often the first places a gage repair person looks to solve erratic readings.

Fixtures must be designed to position workpieces consistently, relative to the measuring instrument. This is critical if the master is a different shape from the workpiece. For instance, when using a flat gage block to master an indicator that is used to check ODs on round workpieces, the fixture must position the workpiece to measure its true diameter—not a chord.

The use of masters that are the same shape as the workpiece avoids this problem and another one that can be more difficult to isolate. After repeated measurements, round workpieces may wear a hollow, allowing accurate comparative measurements, while flat gage blocks may bridge the wear, introducing a source of error.