The cover of the newest Starrett Full Line Catalog (No. 31) has a picture of a coolant-proof (IP65) Starrett 797 Electronic Caliper. It includes a DataSure End Node plugged into its output port that will send data to a host computer with the touch of a button. This represents the state-of-art for the slide caliper, a tool that has been around for many years.

Vernier, Dial & Electronic
There are three types of slide calipers. The oldest, vernier calipers, utilize one fixed and one sliding bar with inscribed scales. Measurements are derived reading the value from where the graduations are aligned on a sliding scale. Dial calipers, first introduced about 30 years ago essentially incorporate the workings of a dial indicator into the mechanics of a sliding handle and show measurements with a mechanical dial and range markings on the slide. The newest caliper type, electronic calipers, began to appear in the 1980s. These tools utilize mechanics similar to a dial caliper but use electronics and a digital display to show measurement values.

While electronic calipers are increasingly pushing aside its predecessors, each type continues to offer advantages and has its drawbacks.

Vernier Slide Calipers
Vernier slide caliper. have been around a much longer time than the other types and is still most popular in some parts of the world. Because it involves matching up lines to read, using it requires practice. Soon, however, most users learn to take quick, accurate readings.

Vernier calipers such as the Starrett 123 Series have several advantages. First, it is constructed of solid hardened steel with no internal moving parts. Another advantage is the durability of the inside jaws. Unlike dial or electronic tools with thin (.005) tips that must pass by each other the vernier jaws are much more substantial and long lasting. They do, however, put a lower limit of about .300 on internal dimensions that they can measure.

Dial Calipers
Until the recent ascension of electronic tools, dial calipers such as the Starrett 120 and 1202 Series' were by far the most popular type in North America. Their ability to provide quick, accurate measurement of O.D., I.D., and depth, and intuitive operation has made them a part of every machinist's toolbox, invaluable to mechanics, and a useful addition to any home workshop.

Unlike vernier slide calipers, their inside measurement jaws can pass by each other, allowing internal measurements as small as .050”. Many dial calipers also incorporate a depth rod that extends from the handle, allowing measurements that previously required a depth micrometer.

The dial caliper is constructed around a toothed rack in a bar groove along much of its length. This rack engages a small drive gear that is connected to the dial's hand and moves as the tool travels. The dimension displayed is the same for outside, inside and depth measurements.

The toothed rack, however, is the dial caliper's Achille's heel. If a foreign particle such as a metal chip finds its way into a gullet, the small gear can be forced out of position leading to inaccurate readings.

Electronic Slide Calipers
Electronic calipers began to appear in the mid-1980s. They are similar in construction to the dial calipers in that the inside jaws pass by each other.

Electronic calipers can include features that are not available with mechanical devices such as: (1) the ability to read in both English and metric; and instantly convert between them; (2) the ability to zero the caliper anywhere in the measuring range and display measurements relative to that position.

Another major update is output capability. Slide Calipers with output such as the Starrett 797 can send data via a DataSure wireless system or attached to a wire to work with older devices.

Early electronic calipers had problems with moisture --when coolant penetrated the electronics, tools often shut down. Today, many calipers are constructed to specific IP standards for water and dust protection, solving coolant and dirt/dust contamination problems as well as the chip problems of dial calipers.

For users that do not require output or IP protection, less costly alternatives such as the Starrett 799 Series are available. These calipers are easy to use and include useful features such as inch/metric conversion zero set.

Regardless of caliper type, they are exceptionally versatile and are one of the most common precision measuring tools.

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