BASICS OF INTERCHANGEABILITY AND ACCURACY OF DETAILS MANUFACTURE...

BASES OF INTERCHANGEABILITY AND ACCURACY OF DETAILS MANUFACTURING

After studying chapter 3, the bachelor must:

know

- basic terms and definitions;

- the principles of building a single system of tolerances and landings;

- parameters of surface roughnesses

be able to

- designate plantings and dimensional deviations in dimensions on the drawings;

- determine the values ​​of tolerances and limiting deviations;

- denote in the drawings the tolerances and deviations of the shape and arrangement of surfaces;

- specify the values ​​of the roughnesses of the surfaces of parts in the drawings;

own

- the skills of applying a single system of tolerances and landings when designing specific mechanisms.

The principles of building a single system of tolerances and landings

The creation of modern mechanisms, their operation, repair and maintenance is impossible without the use of the principle of interchangeability. Interchangeability - the property of independently manufactured parts or assembly units ensure that the machines and machines have a working condition for assembly and repair without special fit. Individual parts (nuts, screws, bolts and etc.), assembly units (rolling bearings, some parts of mechanisms), as well as products in general. The most applicable is complete interchangeability , when the assembly is performed without additional processing and fits. This makes it possible to produce individual parts and assemblies in one, specialized, plant or workshop of mass production or mass production, and assembly is carried out on others. In mass production, the cost of parts is reduced by automating technological processes.

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Sometimes, according to technical requirements, tolerances on the dimensions of parts must be kept so small that it becomes inexpedient economically or difficult to perform technically. Then, to obtain the required accuracy, the parts made on conventional equipment with large tolerances are sorted into dimensional groups in which the tolerances are reduced to the required values. Assembly of parts is carried out separately for each dimension group (selective assembly). This interchangeability is called incomplete (limited) and can only be used for individual geometric parameters, for example in the assembly of rolling bearings, threaded joints with interference and other precision products.

Interchangeability is provided by the system of tolerances and landings, standardized by the standards system. The uniform system of tolerances and landings (ESDP) regulates the tolerances of the dimensions of the smooth elements of the parts and the planting formed when they are joined; The basic norms of interchangeability (OPV) establish the tolerances and planting of joints (threaded, conical, keyed, splined) and gears and wheels.

Basic Definitions

Basic terms and definitions are specified in GOST 25346-82.

Two or more movable or immovably connected parts are called mates . In the joining of the parts entering one into the other, there are external (covered) and internal (encompassing) surfaces. In accordance with this, two terms are introduced: shaft - to denote outer (covered) surfaces ( d ), hole - to indicate internal (enclosing) surfaces (D ). These surfaces can be not only cylindrical, but also flat parallel surfaces (for example, key and groove on the shaft).

Nominal size ( D , d , L, l , etc.) - size , which serves as the starting point for deviations, relative to which the limiting dimensions are determined. It is common for the hole and shaft forming the connection, and is set either from the condition of providing strength or rigidity, or from constructive considerations. To reduce the nomenclature of the cutting and measuring tools, the nominal values ​​obtained are rounded upwards in accordance with the values ​​specified in GOST 6636-69 "Rows of normal linear dimensions". There are four rows of normal sizes Ra5, Ra10, Ra20 and Ra40 (Table 3.1), based on the series of preferred numbers (GOST 8032-84) and representing the decimal series of geometric progressions with denominators: for the series ; for the series ; for the series ; For the series . In Table. 3.1 shows the normal linear dimensions in the range 1-380 mm. When choosing sizes, preference should be given to rows with a narrower gradation (row Ra5 should prefer RalO, etc.). Dimensions established by standards for specific products (average thread diameter, etc.) may not comply with GOST 6636-69.

The actual size is the size , set by direct measurement with an allowable error. In order to avoid rejection during machining,

Table 3.1

Ra5

Ra10

Ra20

Ra40

1.0

1.0

1.2

1.0

1.1

1.2

1.4

1.0

1.05

1.1

1.15

1.2

1.3

1.4

1.5

1.6

1.6

2.0

1.6

1.8

2.0

2.2

1.6

1.7

1.8

1.9

2.0

2.1

2.2

2.4

2.5

2.5

3.2

2.5

2.8

3.2

3.6

2.5

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.0

5.0

4.0

4.5

5.0

5.6

4.0

4.2

4.5

4.8

5.0

5.3

5.6

6.0

6.3

6.3

8.0

6.3

7.1

8.0

9.0

6.3

6.7

7.1

7.5

8.0

8.5

9.0

9.5

10

10

12

10

11

12

14

10

10.5

11

11.5

12

13

14

15

16

16

20

16

18

20

22

16

17

18

19

20

21

22

24

25

25

32

25

28

32

36

25

26

28

30

32

34

36

38

40

40

50

40

45

50

56

40

42

45

48

50

53

56

60

63

63

80

63

71

80

90

63

67

71

75

80

85

90

95

100

100

125

100

110

125

140

100

105

BUT

120

125

130

140

150

160

160

200

160

180

200

220

160

170

180

190

200

210

220

240

250

250

320

250

280

320

360

250

260

280

300

320

340

360

380

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Fig. 3.1

measures of interfaced surfaces are performed with a certain tolerance. For the sake of clarity of the arrangement of tolerance fields, their one-sided arrangement is accepted (Figure 3.1). The actual size lies between the largest and the smallest size limits. In the drawings, not the marginal dimensions are indicated, but the nominal size with the limiting deviations.

The upper limit deviation ES (es) is the algebraic difference between the largest limiting and nominal sizes:

The lower limit deviation is EI (ei) - the algebraic difference between the smallest limiting and nominal sizes:

Deviations may be positive, negative or equal to zero. In the drawings, next to the memorial size, the limiting deviations in millimeters are indicated. Deviations equal to zero are not specified; If the upper and lower deviations are equal, i.e. tolerance is symmetric, their values ​​are indicated after the + sign.

The height of digits is approximately half the height of the digits of the nominal size. For example:

Tolerance T (from Latin Tolerance) - the difference between the largest and smallest size limits or the absolute value of the algebraic difference between the upper and lower deviations:

The tolerance is always positive. In the graphic representation, the tolerance field is located between the upper and lower deviations and can be placed above, below, or symmetrically relative to the zero line. For zero take a line corresponding to the nominal size, which serves as the origin of deviations.

Planting - the nature of the joints , determined by the size of the gaps or strains. Depending on the relative arrangement of the tolerance fields, three types of planting are distinguished: with interference and transient.

Landing with clearance - landing , at which a guaranteed clearance is provided in the joint. This is possible when the hole tolerance field is above the shaft tolerance field Figure 3.2, a).

Fig. 3.2

Landing with a gap is characterized by the values ​​of the largest and gz gaps and the smallest gaps:

(in the example in Figure 3.2, a mm);

(in the example in Figure 3.2, a mm).

For plantings with a clearance is also a landing, which has

Arrangement with interference - landing, in which a guaranteed tightness is ensured in the joint, the shaft tolerance field is located above the hole tolerance field (figure 3.2, b). The values ​​of the largest < img src="images/image1275.jpg"> and the smallest the interference is determined by the formulas

(in the example in Figure 3.2, b mm);

(in the example in Figure 3.2, b mm).

Transition landing - landing, in which both gap and interference can appear in the joint. The tolerances of the hole and shaft overlap completely or partially (Figure 3.2, c).

A uniform system of tolerances and landings - a naturally constructed set of tolerances and landings. The system is designed to select the rational values ​​of tolerances that characterize the precision of manufacturing parts. Small tolerances complicate the process of manufacturing parts and, accordingly, increase the cost of the product, as the requirements to the accuracy of machine tools and the qualifications of workers are toughened. As the tolerance decreases, the relative cost price C increases along the hyperbola. In Fig. 3.3 shows the dependence of the relative cost of manufacturing parts from the tolerance tolerance T. The letters indicate the processing of the parts: A - cold drawing, B -

Fig. 3.3

Turning on a lathe, C - grinding and grinding, D - grinding, grinding and grinding.

Large tolerances reduce the accuracy of manufacture and, as a consequence, the quality of the product, its reliability and durability. Therefore, the rational values ​​of tolerances are the maximum possible, under which the product fully meets its functional purpose and the requirements imposed on it (efficiency, reliability, manufacturability, economy, etc.). A single system of tolerances and landings allows the standardization of cutting tools and calibers, simplifies the design, manufacture of products and improves their quality.

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