gageguide

by Drew Koppelmann


Microinch Measurement


Gaging microinch production tolerances is
not for the shop floor-it requires extraordinary
attention to detail.


The trend toward tighter and tighter manufacturing tolerances is unstoppable. Some manufacturing engineers and quality control managers are finding, to their dismay, that design engineers are now calling for tolerances in the range of 20­p;50 microinches (i.e., millionths of an inch, or µ). While such high levels of precision do, in fact, improve the functionality of some designs, they present a real challenge to inspection. Here's why:

All of the components in a gaging setup-the gage itself, the master and the workpiece-are subject to thermal influences. One inch of steel will expand by 6 microinches for every 1° F increase in temperature. If the workpiece is at a different temperature than the gage or the master, or if temperatures vary unpredictably in the gaging environment, reliable measurements become impossible.

Airborne dust and other contamination can also throw off measurements. Just by sitting exposed for a few hours in a typical production environment, as much as 0.0005" of dirt may accumulate on a gage's contacts. Furthermore, humidity in the atmosphere can corrode metal surfaces. Even at levels invisible to the naked eye, such corrosion will quickly impair the accuracy of gage blocks or other standards.

Many electronic gage amplifiers boast digital displays that read to microinches; however, display resolution may be misleading, because the mechanism may not repeat to that level of accuracy. In other words, the same part measured twice may generate different readings. To combat this, holding fixtures must be rock-solid, machined to tight dimensional and geometric tolerances, and capable of positioning the workpiece repeatedly, while gage mechanisms must demonstrate absolutely regular response and minimal friction and lost motion.

Gages must be more accurate than the tolerance being measured; a gage that repeats to 50 microinches is not accurate enough to inspect part tolerances of 50 microinches. Although gaging practice typically calls for a 10:1 ratio between gage accuracy and part tolerance, this is rarely achievable at the microinch level, where it may be necessary to accept ratios as low as 5:1. The standard (a master or gage block) must, in addition, be about five times more accurate than the gage it is used to set. Thus, to reliably inspect parts to a 50-microinch tolerance, the gage should repeat to about 10 microinches, and the standard should be calibrated to within 2 microinches.

Because of their sensitivity to environmental variables, microinch gages must be mastered frequently, and standards must be calibrated frequently. Many shops must, therefore, maintain a capability to calibrate their own standards, to avoid the long turnaround associated with using an outside calibration service. Calibration instruments with accuracy of 1­p;2 microinches are available, but they require special care to generate reliable results.

First, a controlled gaging environment must be established. This usually means designing and installing a gaging lab, in which temperature can be maintained at a steady 68° F, with variation of no more than 2° F per hour. Humidity should be kept below 50 percent, and the room should be isolated from sources of vibration (e.g., machine tools, HVAC systems, automotive and forklift travel). It must also be kept free of contamination. The entry should be an air lock, and low-lint materials should be chosen for operators' clothing, furniture upholstery and even computer paper.

Parts should be cleaned before being brought into the lab, then staged on a steel plate that acts as a heat sink to bring them into thermal equilibrium with the rest of the room. Parts and masters should be handled with gloves or insulated tweezers. A clear plastic shield may be required to deflect the operator's body heat and breath from parts, standards and the gage itself. There are many more specific measures to observe, but the foregoing indicates the level of care required.

Surface finish and part geometry, or form variation, must also be considered. Failure to account for and control minute variations in surface roughness, flatness, waviness, etc., can negate all efforts at high-precision dimensional control. We will look at these subjects in some detail in a later column.

Microinch production tolerances are here now, and they will continue to infiltrate additional industries as product designs become more sophisticated and production technology improves. Gaging these tolerances requires extraordinary attention to detail, but with proper preparation, it can be done accurately and reliably.

About the author

As applications manager, gaging products, at Federal Products Co. in Providence, Rhode Island, Drew Koppelmann provides dimensional gaging applications assistance to companies in a wide range of industries, including automotive, aerospace, packaging and electronics. He can be reached by fax at (401) 784-3246 or by e-mail at dkoppelmann@qualitydigest.com.