Checking and Measurement Devices

Checking and Measurement Devices


in technology, the general name for a group of devices used for the measurement and checking of the linear and angular dimensions of parts and finished products. Technical equipment with standardized metrological parameters or properties that is designed for the experimental determination of the values of physical quantities is called measurement devices. However, if during the determination of the values of physical quantities it is necessary to establish whether a dimension is within the limits of standard acceptable values, then such equipment is called checking devices. All instruments used for measurements that can provide readings of the value of a dimension may also be used for checking.

Checking and measurement devices are arbitrarily divided into measuring implements and measuring instruments. The first category most often includes the simplest devices (rulers, gauges, and calipers), the second includes more complicated devices (profilometers, spring-actuated contact comparators, and so on). In the state standards checking and measurement devices are divided roughly into gauges and measuring instruments. Gauges include devices intended to reproduce physical quantities of a given size (for example, end standards and calipers). Measuring instruments include devices that produce measurement data in the form of a signal that can be perceived directly by an observer (operator). For example, in analog devices the readings (the values of the quantities being measured) are obtained from a reading device. In recording instruments the readings are registered by a recorder or printer. Measuring instruments are classified according to their principle of operation (mechanical, optical, electrical, and pneumatic, as well as combined types, such as opticomechanical, pneumatic-optical, and pneumatic-electric). The principle of operation of an instrument is often reflected in its name (for example, “inductive profilometer” and “pneumatic instrument for measuring inside dimensions”). Various transducer elements are used in measuring instruments, depending on the principles of operation. Thus, in mechanical instruments, mechanical types are used, among them threads (for example, in a micrometer), levers (in a contact comparator), levers and gears or gears (in a dial indicator), and springs (in a spring-actuated contact comparator); in optical measuring instruments the operation of the transducer devices is based on light phenomena; in electric instruments, on electrical phenomena (inductance, the photoelectric effects, and so on), and in pneumatic measuring instruments, on the quantity of air per unit time flowing through an aperture as a function of the area of the smallest cross section of the aperture. The fundamental metrological indexes that determine the service characteristics of an instrument are the value of a scale division, the measurement range, the threshold, and the measurement error.

Checking and measurement devices are also arbitrarily divided into all-purpose and special types. Among the all-purpose devices are those that can be used to measure linear quantities (diameters and lengths) regardless of the configuration of the item being measured (vernier instruments, micrometers, snap gauges, and optical calipers). Special devices are designed for the measurement of parts having a specific structural shape (for example, gear-measuring instruments and thread gauges) or of a specific parameter of an object (roughness, flatness, straightness, and so on). Depending on the positioning of the device relative to the part, a distinction is made between devices that are placed on or attached to the part and devices of the bench type. Devices of the first category are positioned on the part or are coordinated with the part relative to a single base surface; for devices of the second category, the part is positioned on the instrument during measurement. Checking and measurement devices are also distinguished according to the character of the interaction with the item being measured: in contact types the sensing element makes mechanical contact with the item's surface, and in contactless types (for example, the optical and pneumatic instruments) there is no contact. According to the degree of participation of the operator in the measurement process the devices are classified as manual, mechanized, semiautomatic, or automatic.

One of the main trends in the development of checking and measurement devices is the creation of gauges and instruments that are used directly at operators' work positions. Strong emphasis is being placed on the development of highly specialized measuring devices with improved wear resistance and strength in which, for example, the contact parts are equipped with plates made of hard alloys and diamond, as well as instruments with electric transducers that have reading systems with scale divisions of 1 micron or less. A promising area is the development of checking and measurement devices that participate directly in the technological manufacturing process (active monitoring), of instruments for checking parameters that must be constant during the process of manufacture of items (for example, a profilometer, for monitoring surface roughness), of instruments for monitoring of noncircularity of parts (out-of-roundness gauges), and of instruments for the measurement of kinematic error on gear-cutting machines. The readings of such instruments are usually recorded as diagrams or in digital form. Instruments for lathes with program control, which make a preliminary dimensional adjustment of the position of the cutting tool, have become widespread. They make possible the maintenance of a particular accuracy of fabrication and substantially reduce idle time of equipment. The use of checking and measurement devices together with an electronic computer speeds up the process of obtaining results and reduces measurement errors.