Initially, production was a one-man business. One person made any mechanism from start to finish, without resorting to outside help... The connections were adjusted on an individual basis. It was impossible to find 2 identical parts in one factory. This continued until the middle of the 18th century, when people realized the effectiveness of the division of labor. This gave great performance, but then the question arose about the interchangeability of products. For this, a system for standardizing the levels of precision in the manufacture of parts has been developed. The ESDP has established qualifications (otherwise, the degree of accuracy).

Standardization of levels of accuracy

The development of methods for standardizing production - this includes tolerances, fit, accuracy qualifications - is carried out by metrological services. Before proceeding directly to their study, you need to understand the meaning of the word "interchangeability". What is hidden under this definition?

Interchangeability is the property of parts to be assembled into a single unit and perform their functions without holding them mechanical processing... Relatively speaking, one part is manufactured at one plant, the other at the second, and at the same time they can be assembled at the third and fit together.

The purpose of this separation is to increase productivity, which is formed for the following reasons:

• Development of cooperation and specialization. The more varied the range of production, the more time it takes to set up equipment for each specific detail.
• Reducing the varieties of the instrument. Fewer types of tools also increase the efficiency of making mechanisms. This is due to the reduction in the time for its replacement in the production process.

The concept of admission and quality

It is difficult to understand the physical meaning of the tolerance without introducing the term “size”. Size is a physical quantity that characterizes the distance between two points lying on the same surface. In metrology, there are the following varieties:

• The actual size is obtained by direct measurement of the part: a ruler, caliper and other measuring tool.
• The nominal size is shown directly in the drawing. It is ideal in terms of accuracy, so getting it in reality is impossible due to the presence of a certain error in the equipment.
• Deviation is the difference between nominal and actual dimensions.
• The lower limit deviation shows the difference between the smallest and the nominal size.
• The upper limit deviation indicates the difference between the largest and the nominal dimensions.

For clarity, we will consider these parameters using an example. Let's imagine there is a shaft with a diameter of 14 mm. It is technically determined that it will not lose its performance if the accuracy of its manufacture is from 15 to 13 mm. In the design documentation, this is denoted 〖∅14〗 _ (- 1) ^ (+ 1).

Diameter 14 is the nominal size, "+1" is the upper limit deviation, and "-1" is the lower limit deviation. Then subtracting from the upper limit deviation of the lower one will give us the value of the shaft tolerance. That is, in our case it will be + 1- (-1) = 2.

All sizes of tolerances are standardized and combined into groups - qualifications. In other words, the quality shows the accuracy of the part being manufactured. There are 19 such groups or classes in total. Their designation scheme is represented by a certain sequence of numbers: 01, 00, 1, 2, 3 ... 17. The more accurate the size, the less quality it has.

Accuracy grade table

Landing concept

Before that, we considered the accuracy of one part, which was set only by the tolerance. And what will happen with accuracy when connecting several parts into one unit? How will they interact with each other? And so, here it is necessary to introduce a new term "fit", which will characterize the location of the tolerances of the parts relative to each other.

The selection of landings is made in the shaft and hole system

Shaft system - a set of landings in which the size of the gap and interference is selected by changing the size of the hole, and the shaft tolerance remains unchanged. In the hole system, the opposite is true. The nature of the connection is determined by the selection of the shaft dimensions, the hole tolerance is considered constant.

In mechanical engineering, 90% of production is produced in the hole system. The reason for this is the more complex process of making a hole from a technological point of view, compared to a shaft. The shaft system is used when there are difficulties in processing the outer surface of the part. Ball bearings in rolling bearings are a prime example of this.

All types of fittings are regulated by standards and also have accuracy qualifications. The purpose of this division of plantings into groups is to increase productivity by increasing the efficiency of interchangeability.

Landing types

The type of fit and its quality of accuracy are selected based on the operating conditions and the assembly method of the unit. In mechanical engineering, the following types are divided:

• Clearance fits are joints that are guaranteed to form a clearance between the surface of the shaft and the bore. They are designated by Latin letters: A, B… H. They are used in nodes in which parts "walk" relative to each other and when centering surfaces.
• Interference fits are joints where the shaft tolerance overlaps the hole tolerance, resulting in additional compressive stresses. Interference fit refers to non-collapsible connection types. They are used in highly loaded assemblies, the main parameter of which is strength. This is the fastening of metal sealing rings and valve seats of the cylinder head to the shaft, the installation of large couplings and keys for gears, etc., etc. Fitting the shaft to the hole with an interference fit is done in two ways. The simplest of these is pressing. The shaft is centered over the hole and then placed under a press. With a greater interference, the properties of metals are used to expand when exposed to elevated temperatures and lend when the temperature drops. This method is more accurate for mating surfaces. Immediately before connection, the shaft is pre-cooled and the hole is heated. Next, the parts are installed, which, after some time, return to their previous dimensions, thereby forming the desired fit with a gap.
• Transitional landings. Designed for fixed connections that are often subject to disassembly and assembly (for example, during repairs). In terms of their density, they occupy an intermediate position among the varieties of landings. These fits have an optimal balance between precision and bond strength. In the drawing are designated by the letters k, m, n, j. A striking example of their application is the fit of the bearing inner rings on the shaft.

Usually, the use of a particular fit is indicated in the special technical literature. We simply determine the type of connection and select the type of fit and quality of accuracy we need. But it is worth noting that in especially critical cases, the standard provides for an individual selection of the tolerance of the mating parts. This is done using special calculations specified in the relevant methodological manuals.

To main

section four

Tolerances and landings.
Measuring tool

Chapter IX

Tolerances and landings

1. The concept of interchangeability of parts

In modern factories, machine tools, cars, tractors and other machines are manufactured not in units or even in tens and hundreds, but in thousands. With such dimensions of production, it is very important that each part of the machine, when assembled, fits exactly to its place without any additional fit for fittings. It is equally important that any part arriving at the assembly can be replaced by another of the same purpose without any damage to the operation of the entire finished machine. Parts satisfying these conditions are called interchangeable.

Interchangeability of parts is the property of parts to take their places in units and products without any preliminary selection or adjustment in place and to perform their functions in accordance with the prescribed technical conditions.

2. Pairing parts

Two parts, movably or fixedly connected to each other, are called mating... The size at which these parts are connected is called mating size... Dimensions for which parts are not connected are called free dimensions. An example of mating dimensions is the diameter of the shaft and the corresponding diameter of the hole in the pulley; an example of free sizes is outside diameter pulley.

To obtain interchangeability, the mating dimensions of the parts must be precisely made. However, such processing is complex and not always expedient. Therefore, the technique has found a way to obtain interchangeable parts when working with approximate accuracy. This way is that for different conditions the work of the part establishes the permissible deviations of its dimensions, at which the flawless operation of the part in the machine is still possible. These deviations, calculated for various working conditions of the part, are built in a specific system, which is called system of admissions.

3. The concept of tolerances

Size specification... The estimated size of the part, affixed in the drawing, from which the deviations are counted, is called nominal size... Typically, nominal dimensions are expressed in whole millimeters.

The size of the part actually obtained during processing is called actual size.

The dimensions between which the actual size of the part can fluctuate are called extreme... Of these, the larger size is called largest limiting size and the smaller one is smallest size limit.

By deviation the difference between the limiting and nominal dimensions of the part is called. In the drawing, deviations are usually denoted by numerical values ​​at a nominal size, with the upper deviation indicated above and the lower one below.

For example, in size, the nominal size is 30, and the deviations will be +0.15 and -0.1.

The difference between the largest limiting and nominal dimensions is called upper deviation, and the difference between the smallest limiting and nominal dimensions is lower deviation... For example, the shaft size is. In this case, the largest size limit will be:

30 +0.15 = 30.15 mm;

the upper deviation is

30.15 - 30.0 = 0.15 mm;

the smallest size limit will be:

30 + 0.1 = 30.1 mm;

the lower deviation is

30.1 - 30.0 = 0.1 mm.

Manufacturing tolerance... The difference between the largest and smallest limiting dimensions is called admission... For example, for the size of the shaft, the tolerance will be equal to the difference in the limiting dimensions, i.e.
30.15 - 29.9 = 0.25 mm.

4. Clearances and tightness

If a part with a hole is placed on a shaft with a diameter, that is, with a diameter under all conditions less than the diameter of the hole, then a gap will necessarily result in the connection of the shaft with the hole, as shown in Fig. 70. In this case, the landing is called mobile, since the shaft will be able to rotate freely in the hole. If the size of the shaft is, that is, it is always larger than the size of the hole (Fig. 71), then when connecting the shaft will need to be pressed into the hole and then the connection will turn out tightness.

Based on the foregoing, the following conclusion can be drawn:
clearance is the difference between the actual dimensions of the hole and the shaft when the hole is larger than the shaft;
interference is the difference between the actual dimensions of the shaft and the hole when the shaft is larger than the hole.

5. Landings and accuracy classes

Landing. Landings are divided into mobile and fixed. Below are the most common landings, and their abbreviations are given in brackets.

Accuracy classes. It is known from practice that, for example, parts of agricultural and road machines without harm to their work can be made less accurately than parts of lathes, cars, measuring instruments... In this regard, in mechanical engineering, parts of different machines are manufactured in ten different classes of accuracy. Five of them are more accurate: 1st, 2nd, 2a, 3rd, For; two less accurate: 4th and 5th; the other three are rough: 7th, 8th and 9th.

In order to know what class of accuracy a part needs to be made, in the drawings, next to the letter indicating the fit, a number is put indicating the class of accuracy. For example, C 4 means: sliding fit of the 4th accuracy class; X 3 - running landing of the 3rd accuracy class; P - tight fit of the 2nd accuracy class. For all landings of the 2nd class, the number 2 is not set, since this accuracy class is used especially widely.

6. Hole system and shaft system

There are two systems for the location of tolerances - the hole system and the shaft system.

The hole system (Fig. 72) is characterized by the fact that for all landings of the same degree of accuracy (one class), referred to the same nominal diameter, the hole has constant maximum deviations, the variety of landings is obtained by changing marginal deviations shaft.

The shaft system (Fig. 73) is characterized by the fact that for all landings of the same degree of accuracy (one class), referred to the same nominal diameter, the shaft has constant maximum deviations, the variety of landings in this system is carried out for by changing the maximum deviations of the hole.

In the drawings, the hole system is denoted with the letter A, and the shaft system with the letter B. If the hole is made according to the hole system, then the letter A is placed at the nominal size with a number corresponding to the accuracy class. For example, 30A 3 means that the hole should be machined according to the hole system of the 3rd accuracy class, and 30A - according to the hole system of the 2nd accuracy class. If the hole is machined according to the shaft system, then the designation of the fit and the corresponding accuracy class is put at the nominal size. For example, hole 30C 4 means that the hole must be machined with maximum deviations along the shaft system, along a sliding fit of the 4th accuracy class. In the case when the shaft is made according to the shaft system, they put the letter B and the corresponding accuracy class. For example, 30V 3 will mean processing the shaft according to the shaft system of the 3rd accuracy class, and 30V - according to the shaft system of the 2nd accuracy class.

In mechanical engineering, the hole system is used more often than the shaft system, since this is associated with lower tool and tooling costs. For example, to process a hole of a given nominal diameter with a hole system for all fits of the same class, only one reamer is required and for hole measurement, one / limit plug, and with a shaft system, a separate reamer and a separate limit plug are needed for each fit within one class.

7. Deviation tables

To determine and assign accuracy classes, fits and tolerance values, special reference tables are used. Since the permissible deviations are usually very small values, in order not to write unnecessary zeros, they are indicated in the tolerance tables in thousandths of a millimeter, called microns; one micron is equal to 0.001 mm.

As an example, a table of the 2nd accuracy class for the hole system is given (Table 7).

The first column of the table gives the nominal diameters, the second column - the deviation of the hole in microns. In the remaining columns, various landings are given with their corresponding deviations. The plus sign indicates that the deviation is added to the nominal size, and the minus indicates that the deviation is subtracted from the nominal size.

As an example, let us define the fit of movement in a hole system of the 2nd accuracy class for connecting a shaft with a hole with a nominal diameter of 70 mm.

The nominal diameter of 70 lies between the sizes 50-80, placed in the first column of the table. 7. In the second column we find the corresponding deviations of the hole. Therefore, the largest limiting hole size will be 70.030 mm, and the smallest 70 mm, since the lower deviation is zero.

In the column "Landing motion" against the size from 50 to 80, the deviation for the shaft is indicated. Therefore, the largest limit size of the shaft is 70-0.012 = 69.988 mm, and the smallest limit size is 70-0.032 = 69.968 mm.

Table 7

Limit deviations of bore and shaft for bore system according to 2nd class of accuracy
(according to OST 1012). Dimensions in microns (1 micron = 0.001 mm)

Control questions 1. What is called the interchangeability of parts in mechanical engineering?
2. What is the purpose of assigning permissible deviations in the dimensions of parts?
3. What are nominal, limiting and actual dimensions?
4. Can the size limit be equal to the nominal one?
5. What is called a tolerance and how to determine a tolerance?
6. What are the upper and lower deviations?
7. What is called clearance and interference? Why is the gap and tension provided in the connection of two parts?
8. What are the landings and how are they indicated on the drawings?
9. List the accuracy classes.
10. How many landings does the 2nd accuracy class have?
11. What is the difference between the hole system and the shaft system?
12. Will the maximum hole deviations change for different fits in the hole system?
13. Will the maximum shaft deviations change for different landings in the hole system?
14. Why is the bore system used more often in mechanical engineering than the shaft system?
15. How are affixed to the drawings legend deviations in hole dimensions if parts are made in a hole system?
16. In what units are deviations in the tables indicated?
17. Determine, using the table. 7, deviations and tolerances for the manufacture of a shaft with a nominal diameter of 50 mm; 75 mm; 90 mm.

Chapter X

Measuring tool

To measure and check the dimensions of the parts, the turner has to use various measuring tools. For not very accurate measurements, they use measuring rulers, calipers and internal calipers, and for more accurate measurements, use calipers, micrometers, calibers, etc.

1. Measuring ruler. Calipers. Bore gauge

Yardstick(fig. 74) serves to measure the length of parts and ledges on them. The most common steel rulers are from 150 to 300 mm in length with millimeter divisions.

The length is measured by directly applying a ruler to the workpiece. The beginning of the divisions or zero stroke are aligned with one of the ends of the part to be measured and then the stroke is counted on which the second end of the part falls.

Possible measurement accuracy with a ruler 0.25-0.5 mm.

Caliper (Fig. 75, a) - the simplest tool for rough measurements of the outer dimensions of the workpieces. A caliper consists of two curved legs that sit on one axis and can rotate around it. Having spread the legs of the calipers slightly larger than the size being measured, by lightly tapping on the measured part or some solid object, move them so that they closely touch the outer surfaces of the measured part. The method of transferring the dimension from the measured part to the measuring ruler is shown in Fig. 76.

In fig. 75, 6 shows a spring caliper. It is set to size using a screw and a fine thread nut.

A spring-loaded caliper is somewhat more convenient than a simple one, since it retains the set size.

Inside gauge. For rough measurements internal dimensions serves as an internal gauge shown in Fig. 77, a, as well as a spring internal gauge (Fig. 77, b). The internal gauge device is similar to that of a caliper; measurement by these instruments is also similar. Instead of a bore gauge, you can use a caliper, winding its legs one after the other, as shown in fig. 77, c.

The measurement accuracy with calipers and internal gages can be brought to 0.25 mm.

2. Vernier caliper with a reading accuracy of 0.1 mm

The measurement accuracy with a measuring ruler, calipers, internal gage, as already indicated, does not exceed 0.25 mm. A more accurate tool is a vernier caliper (Fig. 78), which can be used to measure both the outer and inner dimensions of the workpiece. When working on a lathe, a vernier caliper is also used to measure the depth of a groove or shoulder.

The caliper consists of a steel rod (ruler) 5 with divisions and jaws 1, 2, 3 and 8. Jaws 1 and 2 are integral with the ruler, and jaws 8 and 3 are integral with frame 7 sliding along the ruler. Using screw 4, you can fix the frame to the ruler in any position.

To measure the outer surfaces, jaws 1 and 8 are used, to measure the inner surfaces, jaws 2 and 3, and to measure the depth of the groove, rod 6, connected to the frame 7.

On frame 7 there is a scale with dashes for counting fractional fractions of a millimeter, called vernier... Vernier allows you to make measurements with an accuracy of 0.1 mm (decimal vernier), and in more accurate calipers - with an accuracy of 0.05 and 0.02 mm.

Vernier device... Consider how the vernier caliper is counted with an accuracy of 0.1 mm. The vernier scale (Fig. 79) is divided into ten equal parts and occupies a length equal to nine divisions of the ruler scale, or 9 mm. Consequently, one division of the vernier is 0.9 mm, that is, it is shorter than each division of the ruler by 0.1 mm.

If you close the jaws of the caliper, then the zero stroke of the vernier will exactly coincide with the zero stroke of the ruler. The remaining strokes of the vernier, except for the last one, will not have such a coincidence: the first stroke of the vernier will not reach the first stroke of the ruler by 0.1 mm; the second stroke of the vernier will not reach the second stroke of the ruler by 0.2 mm; the third stroke of the vernier will not reach the third stroke of the ruler by 0.3 mm, etc. The tenth stroke of the vernier will exactly coincide with the ninth stroke of the ruler.

If you move the frame so that the first stroke of the vernier (not counting the zero) coincides with the first stroke of the ruler, then a gap of 0.1 mm will be obtained between the jaws of the caliper. When the second vernier stroke coincides with the second stroke of the ruler, the gap between the jaws will already be 0.2 mm, if the third vernier stroke coincides with the third stroke of the ruler, the gap will be 0.3 mm, etc. Therefore, the vernier stroke that exactly coincides with which -or a stroke of a ruler, shows the number of tenths of a millimeter.

When measuring with a vernier caliper, an integer number of millimeters is first counted, which is judged by the position occupied by the zero stroke of the vernier, and then they look with which stroke of the vernier the stroke of the measuring ruler coincided, and tenths of a millimeter are determined.

In fig. 79, b shows the position of the vernier when measuring a part with a diameter of 6.5 mm. Indeed, the zero stroke of the vernier is between the sixth and seventh strokes of the ruler, and, therefore, the diameter of the part is 6 mm plus the reading of the vernier. Further, we see that the fifth vernier stroke coincided with one of the strokes of the ruler, which corresponds to 0.5 mm, so the diameter of the part will be 6 + 0.5 = 6.5 mm.

3. Sliding depth gauge

A special tool called depth gauge(fig. 80). The device of a caliper is similar to the device of a caliper. Ruler 1 moves freely in frame 2 and is fixed in it in the desired position with a screw 4. Ruler 1 has a millimeter scale, according to which, using the vernier 3 on frame 2, the depth of the groove or groove is determined, as shown in Fig. 80. Vernier counting is carried out in the same way as when measuring with a caliper.

4. Precision vernier caliper

For work performed with greater accuracy than those considered so far, apply precision(i.e., exact) calipers.

In fig. 81 shows a precision caliper from the plant. Voskov, having a measuring ruler 300 mm long and a vernier.

The length of the vernier scale (Fig. 82, a) is 49 divisions of the measuring ruler, which is 49 mm. This 49 mm is precisely divided into 50 pieces, each equal to 0.98 mm. Since one division of the measuring ruler is equal to 1 mm, and one division of the vernier is 0.98 mm, we can say that each division of the vernier is shorter than each division of the measuring ruler by 1.00-0.98 = = 0.02 mm. This value of 0.02 mm means that accuracy, which can be provided by the vernier of the considered precision caliper when measuring parts.

When measuring with a precision caliper, to the number of whole millimeters passed by the zero stroke of the vernier, it is necessary to add as many hundredths of a millimeter as the vernier stroke that coincides with the stroke of the measuring ruler will show. For example (see Fig. 82, b), along the ruler of the caliper, the zero stroke of the vernier passed 12 mm, and its 12th stroke coincided with one of the strokes of the measuring ruler. Since the coincidence of the 12th stroke of the vernier means 0.02 x 12 = 0.24 mm, the measured size is 12.0 + 0.24 = 12.24 mm.

In fig. 83 shows a precision caliper from the Caliber factory with a reading accuracy of 0.05 mm.

The length of the vernier scale of this caliper, equal to 39 mm, is divided into 20 equal parts, each of which is taken as five. Therefore, against the fifth stroke of the vernier is the number 25, against the tenth stroke - 50, etc. The length of each division of the vernier is

From fig. 83 it can be seen that when the jaws of the caliper are closed close to each other, only zero and finishing touches vernier matches the strokes of the ruler; the rest of the vernier's strokes will not have such a match.

If you move the frame 3 until the first stroke of the vernier coincides with the second stroke of the ruler, then a gap equal to 2-1.95 = = 0.05 mm will be obtained between the measuring surfaces of the caliper jaws. When the second stroke of the vernier coincides with the fourth stroke of the ruler, the gap between the measuring surfaces of the jaws will be 4-2 X 1.95 = 4 - 3.9 = 0.1 mm. When the third stroke of the vernier coincides with the next stroke of the ruler, the gap will already be 0.15 mm.

The counting on this caliper is similar to that stated above.

A precision caliper (fig. 81 and 83) consists of a ruler 1 with jaws 6 and 7. The ruler is marked with graduations. The frame 3 with jaws 5 and 8 can be moved along the ruler 1. Vernier 4 is screwed to the frame. For rough measurements, the frame 3 is moved along the ruler 1 and, after fastening with the screw 9, a count is made. For accurate measurements, use the micrometric feed of the frame 3, consisting of a screw and a nut 2 and a clamp 10. Having tightened the screw 10, rotating the nut 2, the frame 3 is fed with a micrometric screw until the sponge 8 or 5 is in close contact with the part being measured, after which a count is made.

5. Micrometer

Micrometer (Fig. 84) is used to accurately measure the diameter, length and thickness of the workpiece and gives a reading accuracy of 0.01 mm. The part to be measured is located between the fixed heel 2 and the micrometric screw (spindle) 3. By rotating the drum 6, the spindle is removed or approaches the heel.

In order to prevent too strong pressing of the spindle on the part to be measured during the rotation of the drum, there is a safety head 7 with a ratchet. Rotating the head 7, we will extend the spindle 3 and press the part to the heel 2. When this compression is sufficient, with further rotation of the head, its ratchet will slip and the sound of a ratchet will be heard. After that, the rotation of the head is stopped, the resulting opening of the micrometer is fixed by turning the clamping ring (stopper) 4 and the count is made.

For the production of readings on the stem 5, which is integral with the 1 micrometer bracket, a scale with millimeter divisions is applied, divided in half. The drum 6 has a beveled chamfer, divided along the circumference into 50 equal parts. Dashes from 0 to 50 are marked with numbers every five divisions. At the zero position, i.e. when the heel touches the spindle, the zero stroke on the chamfer of the drum 6 coincides with the zero stroke on the stem 5.

The micrometer mechanism is designed in such a way that with a full revolution of the drum, the spindle 3 will move by 0.5 mm. Therefore, if you turn the drum not a full revolution, i.e. not 50 divisions, but one division, or part of a revolution, then the spindle will move to This is the accuracy of the micrometer reading. When counting, they first look at how many whole millimeters or whole and a half millimeters the drum has opened on the stem, then the number of hundredths of a millimeter is added to this, which coincides with the line on the stem.

In fig. 84 on the right shows the dimension taken with a micrometer while measuring a part; it is necessary to make a countdown. The drum opened 16 whole divisions (half not open) on the stem scale. The seventh stroke of the chamfer coincided with the line of the stem; therefore, we will have another 0.07 mm. The complete count is 16 + 0.07 = 16.07 mm.

In fig. 85 shows several measurements with a micrometer.

It should be remembered that a micrometer is a precision instrument that requires careful handling; therefore, when the spindle slightly touches the surface of the workpiece to be measured, the drum should no longer be rotated, but to further move the spindle, rotate the head 7 (Fig. 84) until the sound of the ratchet follows.

6. Bore gauges

Bore gauges (shtikhmas) are used for accurate measurements of the internal dimensions of parts. There are permanent and sliding bore gauges.

Persistent, or hard, internal gauge (Fig. 86) is a metal rod with measuring ends having a spherical surface. The distance between them is equal to the diameter of the measured hole. To exclude the influence of the heat of the hand holding the bore gauge on its actual size, the bore gauge is provided with a holder (handle).

Internal micrometers are used to measure internal dimensions with an accuracy of 0.01 mm. Their device is similar to that of a micrometer for external measurements.

The head of the internal micrometer (Fig. 87) consists of a sleeve 3 and a drum 4, connected to a micrometer screw; screw pitch 0.5 mm, stroke 13 mm. The sleeve accommodates a stopper 2 and a heel / with a measuring surface. By holding the sleeve and rotating the drum, you can change the distance between the measuring surfaces of the inner gauge. Readings are made like a micrometer.

The measurement range of the shtikhmas head is from 50 to 63 mm. For measuring large diameters(up to 1500 mm) screw the extension cords 5 onto the head.

7. Limit measuring tools

In the serial production of parts according to tolerances, the use of universal measuring tools (vernier caliper, micrometer, internal micrometer) is impractical, since measuring with these tools is a relatively complex and time-consuming operation. Their accuracy is often insufficient, and, in addition, the measurement result depends on the skill of the worker.

To check whether the dimensions of the parts are within the exact specified limits, use a special tool - limiting calibers... Gauges for checking shafts are called staples, and for checking holes are called traffic jams.

Limit bracket measurement. Double-sided limit bracket(fig. 88) has two pairs of measuring jaws. The distance between the cheeks on one side is equal to the smallest limit size, and the other - to the largest limit size of the part. If the measured shaft passes to the large side of the bracket, therefore, its size does not exceed the permissible, and if not, then its size is too large. If the shaft also runs to the smaller side of the bracket, then this means that its diameter is too small, i.e., less than the permissible one. Such a shaft is a defect.

The side of the staple with the smaller size is called impassable(marked "NOT"), the opposite side with large size - checkpoint(marked with "PR"). The shaft is recognized as suitable if the bracket, lowered onto it by the through side, slides down under the influence of its weight (Fig. 88), and the non-through side does not find itself on the shaft.

For measuring shafts large diameter instead of double-sided brackets, one-sided brackets are used (Fig. 89), in which both pairs of measuring surfaces lie one after the other. The front measuring surfaces of such a bracket check the largest allowable diameter of the part, and the rear - the smallest. These clamps are lighter and significantly speed up the inspection process, since for measurement, it is enough to apply the clamp once.

In fig. 90 shows adjustable limit bracket, in which, when worn, it is possible to restore the correct dimensions by rearranging the measuring pins. In addition, such a bracket can be adjusted to given dimensions and thus a large number of dimensions can be checked with a small set of staples.

To change to a new size, loosen the locking screws 1 on the left leg, respectively move the measuring pins 2 and 3 and fasten the screws 1 again.

Are widespread flat limit brackets(fig. 91) made of sheet steel.

Limit plug measurement. Cylindrical Limit Gauge-Plug(Fig. 92) consists of a through plug 1, a non-through plug 3 and a handle 2. The through plug ("PR") has a diameter equal to the smallest allowable hole size, and the non-through plug ("NOT") has a diameter equal to the largest. If the "PR" plug passes, but the "NOT" plug does not pass, then the diameter of the hole is greater than the smallest limit and less than the largest, that is, it lies within the permissible limits. A through plug is longer than a non-through one.

In fig. 93 shows a hole measurement with a limit plug on a lathe. The lead-through side should easily pass through the hole. If the non-passable side also enters the hole, then the part is rejected.

Cylindrical plug gauges for large diameters are inconvenient due to their heavy weight. In these cases, use two flat plug gauges (Fig. 94), of which one has a size equal to the largest, and the second - the smallest allowable. The walk-through side is wider than the walk-through side.

In fig. 95 shows adjustable limit plug... It can be adjusted for several sizes as well as an adjustable limit brace, or rebuilt right size worn out measuring surfaces.

8. Dimensions and indicators

Reismas. For accurate verification of the correct installation of the part in a four-jaw chuck, on a square, etc., use reismas.

With the help of a gauge, you can also mark the center holes in the ends of the part.

The simplest remesh is shown in Fig. 96, a. It consists of a massive tile with a precisely machined bottom plane and a rod along which a slider with a scribe needle moves.

Reismas of a more advanced design is shown in Fig. 96, b. The needle 3 of the gauge with the help of the hinge 1 and the clamp 4 can be brought by the tip to the surface to be checked. The exact setting is done with screw 2.

Indicator. To control the accuracy of processing on metal-cutting machines, to check the machined part for ovality, taper, to check the accuracy of the machine itself, an indicator is used.

The indicator (Fig. 97) has a metal case 6 in the form of a clock, which contains the mechanism of the device. A rod 3 with an outwardly protruding tip passes through the indicator housing, which is always under the influence of a spring. If you press the rod from the bottom up, it will move in the axial direction and at the same time turn the hand 5, which will move along the dial, which has a scale of 100 divisions, each of which corresponds to the movement of the rod by 1/100 mm. When the rod is moved 1 mm, hand 5 will make a full turn on the dial. Arrow 4 is used to count whole revolutions.

When measuring, the indicator should always be rigidly fixed relative to the original measuring surface. In fig. 97, a shows a universal stand for attaching the indicator. The indicator 6 using rods 2 and 1 couplings 7 and 8 is fixed on the vertical rod 9. Rod 9 is fixed in the groove 11 of the prism 12 with a knurled nut 10.

To measure the deviation of a part from a given size, the indicator tip is brought to it until it touches the measured surface and the initial indication of arrows 5 and 4 (see Fig. 97, b) on the dial is noted. Then the indicator is moved relative to the measured surface or the measured surface relative to the indicator.

The deviation of arrow 5 from its initial position will show the value of the bulge (depression) in hundredths of a millimeter, and the deviation of arrow 4 in whole millimeters.

In fig. 98 shows an example of using the indicator to check the coincidence of the centers of the head and tailstock lathe... For a more accurate check, a fine grinding roller should be installed between the centers, and an indicator in the tool holder. Bringing the indicator button to the surface of the roller on the right and noticing the indication of the indicator arrow, manually move the support with the indicator along the roller. The difference in the deviations of the indicator arrow in the extreme positions of the roller will show how much the tailstock housing should be moved in the transverse direction.

Using the indicator, you can also check the end surface of a machined part. The indicator is fixed in the tool holder instead of the tool and is moved together with the tool holder in the transverse direction so that the button of the indicator touches the surface to be checked. The deflection of the indicator arrow will show the runout value of the end plane.

Control questions 1. What parts does a vernier caliper consist of with an accuracy of 0.1 mm?
2. How does the vernier caliper work with an accuracy of 0.1 mm?
3. Set the dimensions on the vernier caliper: 25.6 mm; 30.8 mm; 45.9 mm.
4. How many divisions does a precision vernier caliper have with an accuracy of 0.05 mm? The same, with an accuracy of 0.02 mm? What is the length of one division of the vernier? How to read the testimony of a vernier?
5. Set the dimensions with a precision caliper: 35.75 mm; 50.05 mm; 60.55 mm; 75 mm.
6. What are the parts of the micrometer?
7. What is the pitch of the micrometer screw?
8. How is the micrometer readout?
9. Set the dimensions on the micrometer: 15.45 mm; 30.5 mm; 50.55 mm.
10. In what cases are bore gauges used?
11. What are the limiting calibers used for?
12. What is the purpose of the through and non-through sides of the limit gauges?
13. What designs of limit brackets are you aware of?
14. How to check the correct size of the limit plug? Limit Brace?
15. What is the indicator used for? How to use it?
16. How is the remesh machine structured and what is it used for?

Dimensions in drawings

Introduction

In a mass production environment, it is important to ensure interchangeability identical parts. Interchangeability allows you to replace a spare part that has broken down during the operation of the mechanism. The new part must be exactly the same size and shape as the one being replaced.

The main condition for interchangeability is the manufacture of a part with a certain accuracy. What should be the accuracy of manufacturing a part, indicate on the drawings the permissible maximum deviations.

The surfaces along which the parts are connected are called mating ... In the connection of two parts included in one another, a female surface and a male surface are distinguished. The most common in mechanical engineering are joints with cylindrical and flat parallel surfaces. In a cylindrical joint, the surface of the hole covers the surface of the shaft (Fig. 1, a). It is customary to call the enclosing surface hole covering - shaft ... The same terms hole and shaft conventionally used to designate any other non-cylindrical covering and covered surfaces (Fig. 1, b).

Rice. 1. Explanation of terms hole and shaft

Landing

Any operation of assembling parts consists in the need to connect or, as they say, to plant one piece to another. Hence, in technology, the expression is adopted landing to indicate the nature of the connection of parts.

Under the term landing understand the degree of mobility of the assembled parts relative to each other.

There are three groups of landings: with a gap, with an interference fit and transitional.

Clearance fits

Clearance the difference between the dimensions of the hole D and the shaft d is called if the size of the hole is larger than the size of the shaft (Fig. 2, a). The clearance allows free movement (rotation) of the shaft in the hole. Therefore, landings with a clearance are called mobile landings. The larger the gap, the more freedom of movement. However, in reality, when designing machines with movable landings, such a gap is chosen at which the coefficient of friction of the shaft and the hole will be minimal.

Rice. 2. Landing

Interference landings

For these fits, the hole diameter D is less than the shaft diameter d (Fig. 2, b). .Really, this connection can be made under a press, when the female part (holes) is heated and (or) the male part (shaft) is cooled.

Interference landings are called fixed landings , since the mutual movement of the parts to be connected is excluded.

Transitional landings

These landings are called transitional because, before assembling the shaft and the hole, it is impossible to say what will be in the connection - a gap or an interference fit. This means that in transitional fits, the hole diameter D can be less than, greater than or equal to the shaft diameter d (Fig. 2, c).

Size tolerance. Tolerance field. Accuracy quality Basic concepts

The dimensions in the drawings of the parts quantify the size of the geometric shapes of the part. Dimensions are subdivided into nominal, actual and limiting (Fig. 3).

Nominal size - this is the main calculated size of the part, taking into account its purpose and the required accuracy.

Nominal connection size - this is the common (same) dimension for the bore and shaft that make up the joint. The nominal dimensions of parts and connections are not chosen arbitrarily, but in accordance with GOST 6636-69 "Normal linear dimensions". In real production, in the manufacture of parts, the nominal dimensions cannot be maintained and therefore the concept of actual dimensions is introduced.

Actual size - This is the size obtained during the manufacture of the part. It always differs from the nominal up or down. The permissible limits of these deviations are established by means of the limiting dimensions.

Limiting dimensions are called two boundary values ​​between which the actual size must be. The larger of these values ​​is called largest limiting size, less - smallest size limit... In everyday practice, in the drawings of parts, the limiting dimensions are usually indicated by means of deviations from the nominal.

Limit deviation Is the algebraic difference between the limiting and nominal dimensions. Distinguish between upper and lower deviations. Upper deviation Is the algebraic difference between the largest limit size and the nominal size. Lower deviation Is the algebraic difference between the smallest limit size and the nominal size.

The nominal size serves as the starting point for deviations. Deviations can be positive, negative or zero. In tables of standards, deviations are indicated in micrometers (μm). In the drawings, deviations are usually indicated in millimeters (mm).

Actual deviation Is the algebraic difference between the actual and nominal dimensions. A part is considered suitable if the actual deviation of the checked size is between the upper and lower deviations.

Size tolerance Is the difference between the largest and smallest limiting dimensions or the absolute value of the algebraic difference between the upper and lower deviations.

Under quality understand the set of tolerances that vary depending on the value of the nominal size. 19 qualifications have been established, corresponding to various levels of precision in the manufacture of a part. For each grade, rows of tolerance fields are built

Tolerance field - this is the field limited by the upper and lower deviations. All tolerance fields for holes and shafts are designated by letters of the Latin alphabet: for holes - in capital letters (H, K, F, G, etc.); for shafts - lowercase (h, k, f, g, etc.).

Rice. 3. Explanations of terms

Qualities form the basis of the current system of tolerances and fits. Quality is a kind of set of tolerances that, for all nominal dimensions, correspond to the same degree of accuracy.

Thus, we can say that it is precisely the qualifications that determine how accurately the product is made as a whole or its individual parts. The name of this technical term comes from the word “ qualitas", Which in Latin means" quality».

The set of those tolerances that for all nominal sizes correspond to the same level of accuracy, called the system of qualifications.

The standard established 20 qualifications - 01, 0, 1, 2...18 ... With an increase in the quality number, the tolerance increases, that is, the accuracy decreases. Qualities from 01 to 5 are intended primarily for calibers. For landings, qualifications are provided from the 5th to the 12th.

The set of tolerances and fits, which was created on the basis of theoretical research and experimental research, as well as built on the basis of practical experience, is called the system of tolerances and fits. Its main purpose is to select such options for tolerances and fits for typical joints of various parts of machinery and equipment, which are minimally necessary, but completely sufficient.

The basis for the standardization of measuring instruments and cutting tools are exactly the most optimal gradations of tolerances and fits. In addition, thanks to them, the interchangeability of various parts of machinery and equipment is achieved, as well as an increase in the quality of finished products.

To design a unified system of tolerances and landings, tables are used. They indicate the reasonable values ​​of the maximum deviations for various nominal sizes.

When designing various machines and mechanisms, the developers proceed from the fact that all parts must meet the requirements of repeatability, applicability and interchangeability, as well as be unified and comply with accepted standards. One of the most rational ways to fulfill all these conditions is to use the maximum a large number such component parts, the release of which has already been mastered by the industry. This allows, among other things, to significantly reduce the development time and costs. At the same time, it is necessary to ensure high accuracy of interchangeable components, assemblies and parts in terms of their compliance with geometric parameters.

With the help of such a technical method as modular layout, which is one of the methods of standardization, it is possible to effectively ensure the interchangeability of assemblies, parts and assemblies. In addition, it greatly facilitates repairs, which greatly simplifies the work of the relevant personnel (especially in difficult conditions), and allows you to organize the supply of spare parts.

Modern industrial production focused mainly on the mass production of products. One of its prerequisites is the timely arrival of such components on the assembly line. finished products, which do not require additional adjustment for their installation. In addition, such interchangeability must be ensured that does not affect the functional and other characteristics of the finished product.