There is a category of gages that have been around for 50 years; are very inexpensive compared to alternate measuring methods; and are fast, reliable, easy to set up and functional for manual or automatic operations. I'm referring to mechanical gages that incorporate electrical limit switches.
These include a number of different instruments that operate essentially as mechanical displacement gages. While they look and act like mechanical dial indicators or comparators, they include electrical contact points that can be adjusted to represent set points within the range of the indicator. Despite their age, these electric gages still provide the most economical means for classifying or controlling part size in many applications.
In a manual mode, when the switching indicator is combined with a light box, they provide a fast and sure way for operators to classify parts and reduce the possibility of misclassification. The other advantage of this type of gaging is in automatic or semi-automatic gaging operations. If the setup is fairly simple, with one or two gaging stations, there is probably no more cost-effective way to classify parts.
There are basically three different types of switching gages. The most basic type does not even include a dial indicator readout. It provides very repeatable switching within the range of the mechanical sensing head. The switching head typically has a high and low limit switch. These can be electrically combined to give the operator or machine a high, low or good condition signal. The switching contacts on this type of device can be very accurate. In fact, one of the most impressive features of these gages is the discrimination sensitivity of the switching, which is typically 40 micro-inches, but some can be good to 10 micro-inches. This performance approaches that of the more expensive types of classifying gages. A drawback is that when using sensing heads without an indicator scale, the two limit positions of the gage must be set with the aid of two masters or gage block stacks.
The next style of gage electronic comparator, or electric dial indicator, has a complementary set of electrical contacts and only requires a zero master for positioning and setting the limit switches. The gage is set in the center of its mechanical range, then, using adjustment screws, the limits can be set for high and low limit switches. It is simple to adjust for changes in tolerance and in the part process, because it is easy to see the part being measured as the dial hand goes to the limit positions. The key to any dial indicator with limit switching contacts is the way the contacts are used. It is important to put little electrical stress on contacts, so the current going through them should be a maximum of 100 ma.
Today, there is a new generation of "on the spot" classifiers. Instead of using the mechanism of a dial indicator and switches, the electronics of a digital dial indicator are given the ability to "look at" the reading, compare it to a set of preset tolerances and provide an electronic signal representing the out-of-tolerance condition. The advantages of this type of electronic classification include the ability to set tolerances to the least significant digit of the readout, repeatability of the switching to the least significant digit and operation features such as presets and data output.
Sometimes it's easy to put together the latest technology to solve your measurement and classification requirements. However, there are some tried and true methods still available that will do an outstanding job and might cost only a fraction of the price.