Measuring tools: terminal measures of length plane-parallel

The base of linear measurements in any industrial branch is the plane-parallel length end measures (CMD). They are used to store a unit of length and to reproduce it. And also for checking the measuring devices, their calibration and zero setting on the devices, transferring the sizes for the required settings, and serve as a natural measuring instrument for the sizes of all kinds of details. A set of plane-parallel end measures of length allows to perform marking operations with special accuracy and adjustment of the machine. The main function of CMD is to maintain the unity of measures in various industries.

Basic concepts

Instruments used as measuring instruments in industrial enterprises, in repair organizations or services, must be periodically (on a systematic basis) checked for accuracy of measurements. Naturally, in order to carry out the verification, it is not possible to deliver the measuring instrument from various industries to the institutions where the reference dimensions are placed.

This procedure, without leaving the enterprise, allows you to carry out the end measures of length (plane-parallel). They are a sample or standard measure of length. Outwardly, the device looks like a parallelepiped or a cylinder with numbered dimensions located between the measuring planes.

The main task of Johanson's tiles is the preservation of a unit of length and its transmission. Plane-parallel length end measures are used to calibrate, check and / or dimension the various measuring devices (caliber, micrometer, sine line, indicator, etc.), as well as to control production devices and templates.

As a hundred years ago

The end measures of length were first made by the Swiss firm Ioganson. Made of steel in the form of rectangular parallelepipeds, they appeared at the Paris World Exhibition in 1900. Thanks to the manufacturer, the name "Johanson's tiles" long accompanied the end measures of length (plane-parallel). A little later they became just "tiles".

In the Soviet Union, KMD was first released at the Sestroretsk and Tula factories. In the series they were launched at the Krasny Komartalshchik factories in the city of Kirov and Moscow's Caliber in the early 1930s.

Today, with the advent of laser contactless interferometers, electrical measuring instruments, lengthometers, machines measuring coordinate points, altimeters and other devices for perfect measurements, the role, purpose and use of KMD have undergone significant changes. Nevertheless, the basic, basic quality - the material carrier of the size - is still indispensable. And this despite the large number of modern optical and electronic means for accurate measurements.

Plane-parallel end measures of length (Johansson tiles) are produced with dimensions that between the measuring planes are from 0.5 to 1000 mm. If it is necessary to obtain an arbitrary size not provided by this measure of length, a suitable size must be made of suitable tiles by grinding the end measures of length to each other. Lapping should take place until the tiles stick together (that is, they should not disintegrate).

Of what make

Today, the end measures of length (plane-parallel) are melted from high-alloy steel. Also they are made of ceramics and hard alloys.

Steel KMD

Measures of length, made of steel, for example chromium, are well lapped with the measuring surfaces of the measuring racks and other measures. Their working surfaces are rather durable, which explains the extended service life.

Steel measures are necessarily subjected to hardening, and then to artificial aging, which makes it possible to keep the exact size for a long time. The hardness of the working surface of Johanson steel tiles is always at least 800 HV.

The disadvantages include special sensitivity to wear of work surfaces, scratches and possible corrosion. Before starting work with the PCMD, they remove the protective lubricant from them, and after the work is done again, it is applied. Measures of length (end plane-parallel), GOST for which only applies to steel made, are susceptible to changes in the temperature regime, including the warmth of the hands of the operator.

KMD from the fame of hard alloys

Tiles, melted from tungsten carbide, are ten times stronger than steel. Plus, they have an ideal lapping. The hardness of the working surface of a hard alloy measure is almost twice as large (1400 HV) as the hardness of this surface for steel tiles. The disadvantage is one - a lot of weight. Therefore, it is impossible to assemble blocks for working with large dimensions from carbide end measures.

Ploskoparallelnye end measures of length (Johansson tiles), made of zirconium ceramics, are considered the most resistant to scratches and wear of the working surface. Ceramics, as is known, is not afraid of corrosion, and, therefore, the humidity of the operator's hands (unlike carbide and steel) does not affect the plates of zirconium ceramics. The measures of length from this material have the greatest stability and the longest service life, which makes it possible to increase the period between verification.

KMD from ceramics are not magnetized, do not require special anti-corrosion protection and do not attract dust. And also they are stable and able to keep the size long enough.

The analysis showed that steel and ceramic measures have a close-by-value thermal expansion coefficient. This indicator allows you to add KMD blocks from ceramic and steel tiles, which makes the measures of these materials completely interchangeable.

Glass KMD

Make the end measures of length and of glass. They are used to check lapping and flatness checking. And also to check the flatness of the exact working surfaces, for example, tables of racks. Glass KMD with diameter from 50 mm to 75, flatness less than 0.125 μm and thickness of 15-20 mm are produced.

How it works

Transmission and preservation of the exact size using plane-parallel end measures of length proceeds as follows.

On a special installation for measurements, the dimensions of the first digit of the sample end measures are checked.

The terminal measures of length (plane-parallel), having the attestation of the highest accuracy tiles, are stored exclusively in the laboratory rooms of the State Standard. At industrial enterprises, depending on the requirements of the standards of accuracy to the products manufactured, tiles from the second to fifth categories are used. To transfer the exact size is to periodically compare tiles of the first category and tiles of the second level. Then the last - with tiles already the third category and further under the scheme. Thus, in industrial enterprises all measuring devices are checked, from exact and finishing to rough ones. The results of the conducted checks are recorded in the passport of each device.

Lapping of KMD

A grinding effect of the end measures of length is the effect of sticking together two tiles. The plane-parallel terminal measures of length are polished so that when they are combined, air debris is removed, and only atmospheric pressure affects them. The surface tension of the remaining droplets of liquid for flushing the lubricant and the interaction between the molecules of the material from which the tiles are made increase the compression force.

If the plane-parallel end measures of length are polished, then they will ideally be lapped. This ability of tile length measures is considered a must. For the loss of this quality means the wear of the surface of the measure.

Necessary accessories

For the collection of tiles in blocks of KMD and fixing them with the purpose of providing convenience of use, use a set of accessories for plane-parallel end measures of length. It is used widely enough. Accessories, for example, are needed for verification or installation on the size of various measuring devices. This can be micrometric and indicator gauges, calibres, etc.

The set necessarily includes sidewalls of different sizes, which makes it possible to carry out both internal and external measurements. This is especially important when manufacturing high precision products in small series. In this case, the accessories must be set to the maximum size of the manufactured product and to the minimum. That is, they are used as a two-sided gauge with a passing side and not passable.

For marking works use an additional marking set, to which are attached holders for particularly accurate measurements.

Also in the set include: ties, sidewalls, plane-parallel and radial, three-sided ruler, side central and devil, base, biscuits and styli. The latter are necessary in order to determine the size of the gap between the ground surfaces. Styli are steel plates, the minimum size of which is 0.02 mm, the maximum size is 1.0 mm. The gradation in them is 0.01 mm and 0.05 mm.

In order to determine the size of the gap using the feeler gauge, you need to place it between the surfaces. The stylus should move with a little effort and in no case do not sink or move freely. By the sum of the thickness of the probes, into the gap entered completely, and its value is determined.

Calibration and verification of QMD

The plane-parallel length end measures are very important in the hierarchical chain of size transfer from the length sample (meter): they are a stable material carrier of the size constant.

The transfer of the reference meter, whose base is the length of the light wave, is passed to the reference length measures of length by means of deep interference measurements. Thus, we obtain a standard of measure of length, belonging to the class K. Further, according to the verification scheme, it is transferred to other measures.

In accordance with the main purpose of KMD (ensuring the uniformity of measurements and the transfer of dimensions of the reference length), verification schemes have been developed. For the same purposes, measures of length of different accuracy are produced by classes and classes. The verification of measures of length (end, plane-parallel) passes through special devices.

The very first of these was a specially invented micrometer. He was distinguished by increased accuracy. Years passed, the instruments improved. Today they give out the results of the highest accuracy and have ceased to be a rarity. Many laboratories have verification instruments. In addition, KMD can easily be verified by incremental and inductive photovoltaic comparators, which are freely available. And the methods of conducting measurements on these devices have long been worked out. The advantage of this method is the simplification of the verification scheme of measures of length. Some disadvantage is the high cost of devices and high requirements for the skills of a specialist working with them. And the error. It is so small that it is difficult to double-check the results of measurements on other devices.

Basic verification methods

There are four verification methods:

- Indirect measurements of a given value.

- Direct measurements of a given value.

- Comparison with the involvement of a comparator.

- Direct comparison with the standard.