The Challenges of Measuring Microinches

Higher-precision measurement requires a variety of careful considerations.

We are shifting towards millionth measurement faster than ever before. Increasingly more technical and higher-precision products make the need for high-resolution precision measurement even more profound. While it is still predominantly associated with scientific applications in aerospace and nuclear industries, take a look at some of the fits and finishes found on new phones and tablets. Even these are moving towards microinch measurement requirements. 

With typical measurement applications, much of our attention is focused on the gage itself: As long as the instrument is designed to the required degree of accuracy and maintained properly, we can usually get by, even at the “tenths” level. However, when trying to measure tolerances of 50, 30 or even 20 “millionths” (microinches), we must shift our focus to the measurement process and the environment in which it takes place. Where temperature and cleanliness were formerly somewhat abstract issues, they now become essential concerns.

It’s not uncommon when measuring tolerances of 30 microinches for the gage to demonstrate repeat accuracy of 3 microinches. But consider this: A difference of 1°F among the part, the master and/or the gage can introduce an error of 3 microinches. In other words, if we don’t control thermal influences, we lose all hope of repeatability.

Microinch gaging therefore must be performed in a controlled environment: a special room that is thermally insulated from the shop floor. Temperature should be kept as close to 68°F as possible, and changes must not exceed 2°F per hour. When a part comes in from the shop, it should sit for several hours on a heat sink (a large steel plate), to bring it into equilibrium with the master and the gage before it is measured. Even with all of these precautions, the gage should be mastered frequently.

The gage should be protected from the operator’s body heat, and his breath, by a clear plastic shield or full enclosure. The operator should not touch the parts or masters directly; insulated tweezers, gloves or similar measures should be employed.

Here’s an experiment that can provide a valuable learning experience:

Say you have a gage that has all the characteristics of a millionth comparator. It has a high-performance transducer capable of repeating to millionths of an inch; it has a millionth-grade readout; and the gage design has the mass, stability and rigidity required to make the measurement. Over time, you have demonstrated that the gage repeats to specification when everything is just right. 

Now, just put your hand near the part that is being measured in the gage. In the microinch world, you will start to see a gradual growth of the part as it is slightly warmed by the transfer of your body heat. If you had a temperature sensor on the part, you would also see the slight rise in temperature. Having the size data and temperature data, you might even be able to verify the expansion coefficient of the part. In other words, your millionth comparator has become a temperature gage as well.

Elaborate measures are also required to combat the problem of contamination. Relative humidity in the room should be kept at less than 50 percent to inhibit the formation of rust. Parts must be thoroughly cleaned of dirt and thin oil films prior to gaging. The choice of cleaning solvent will vary with the application, and may require some trial and error to ensure that the solvent itself doesn’t leave a film. It will be necessary to regularly clean the entire gaging area, plus the gage and masters, to remove dust, skin oils and other contaminants. 

Even the choice of furniture upholstery and the clothing worn by operators must be considered—natural fibers shed more dust than synthetics. The room should have an air lock, and unqualified personnel should be prohibited from entering. If there is a computer printer in the room, it should be in an enclosure, and single-sheet paper should be used. (Paper dust may be released into the air when tearing continuous forms along their perforations.)

Surface finish and part geometry become critical parameters at the microinch level, and for any degree of repeatability to be possible, it is necessary to use witness marks or some other method to ensure that a part is always measured at the same location. The whole subject of mastering, calibrating and certifying a gage to millionths is important enough to deal with at length in a future column.

Even with sophisticated gages that are fully capable of the task, measuring to millionths remains a challenge. It requires thorough planning, careful selection of conscientious personnel, and significant investments in training and facilities, as well as a good understanding of all the variables that can affect microinch dimensions. It may be tempting to just buy the new gage and give it a go, but I guarantee you’ll spend more time figuring out your problems and ways to fix them than you would by doing it right in the first place.