Topic 6.1 Machining accuracy

6.1.1 Standardization of permissible deviations of sizes, forms

and placement of surfaces

One of the important indicators of the quality of the machine, which significantly affects all criteria of its efficiency and reliability, as well as operational parameters, is the accuracy of manufacturing and assembly of conjugated elements. The principle of designing and manufacturing parts, which ensures the possibility of correct assembly or replacement during repairs of independently manufactured parts and assembly units without additional processing and fitting them while maintaining the appropriate quality and reliability of the connection, is called interchangeability, which is a characteristic feature of modern mechanical engineering.

A distinction is made between complete and incomplete interchangeability. Complete - provides for the correct connection of all conjugated parts entering assembly operations. They are provided with high precision in the manufacture of parts. In the case of incomplete interchangeability, the correct connection of only part of the parts manufactured with lower accuracy is achieved. In these cases, group selection of parts is used for assembly (selective assembly) or various compensators and other additional technological means are used.

The interchangeability of machine parts is ensured by the system of tolerances and fits. The general provisions and foundations of the construction of this system for smooth machine elements (cylindrical or limited by parallel planes) with nominal dimensions up to 3150 mm are established by standards. In accordance with these standards, we will consider some terminology related to the concepts of dimensions, deviations, tolerances, and qualities.

Size – numerical value of a linear real value (diameter, length, etc.) in the selected units. In mechanical engineering, dimensions are given in millimeters. There are actual, marginal and nominal sizes.

Effective size - the size of the element, which is established by measurement.

Boundary dimensions are two maximum allowable dimensions of an element, between which the actual size must be. The largest (smallest) limit size is the largest (smallest) allowable element size.

The nominal size is the size relative to which deviations are determined. The nominal dimensions are selected during the design process based on strength calculations or on the basis of constructive considerations and are placed on the drawing of the parts or the connection of parts.

Deviation is the algebraic difference between the real (or limit) and the corresponding nominal dimensions.

Limit deviation is the algebraic difference between the limit and the corresponding nominal dimensions. Distinguish between upper and lower deviation. The upper (lower) deviation is the algebraic difference between the largest (smallest) limit and the corresponding nominal dimensions.

The zero line is a line corresponding to the nominal size, from which size deviations are set aside in the graphic representation of the fields of tolerances and landings. In the case of horizontal placement, its positive deviations are placed upwards from it, and negative deviations are placed downwards.

Tolerance is the difference between the largest and smallest limit dimensions or the algebraic difference between the upper and lower deviations. The tolerance field is a field limited by the largest and smallest limit sizes and is determined by the tolerance and its position relative to the nominal size.

Tolerance fields for conjugated elements are set differently for three ranges of changes in nominal sizes: small - up to 1 mm, medium - 1...500 mm and large - 500...3150 mm. In mechanical engineering, the average range of nominal sizes is the most widely used.

The main deviation is one of the two limit deviations (upper or lower), which determines the position of the tolerance field relative to the zero line. In the standard tolerance system, there is a deviation closer to the zero line.

To meet the requirements for various parts and their fits with certain nominal dimensions, the standard provides a range of tolerances and basic deviations that characterize the position of these tolerances relative to the zero line. The location of the tolerance field in relation to the zero line, which depends on the nominal size, is indicated by a letter (sometimes two) of the Latin alphabet, while for holes - a large letter, and for shafts - a small letter.

The greater the size tolerance, the lower the requirements for the accuracy of the part, the simpler and cheaper its manufacture. But large parts are more difficult to manufacture compared to smaller ones with the same deviations from the nominal dimensions. Therefore, the tolerance is determined by the dimensions of the part. In addition, the part can be made more precisely (with a smaller tolerance field) and less precisely (with a larger tolerance field).

To assess the degree of accuracy, qualities are used - this is a set of tolerances that corresponds to one level of accuracy for all nominal sizes. The standard provides for 20 qualities. In order of decreasing accuracy, qualities are marked as follows: 01,0,1,2,3...18. Qualities 01,0,1,2,3 and 4 are intended for final measures of length, gauges, etc.; in qualities 5...13, tolerances are given for the dimensions of the mating surfaces of parts; in qualities 14...18 - for sizes of non-conjugated surfaces.

Since the tolerance field is determined by the quality, and its position relative to the zero line is indicated by a letter, the limit deviations of linear dimensions can be indicated on the drawing of the part by conventional designations of the tolerance fields. At the same time, the number indicating the size is followed by the conventional designation of the tolerance field from letters and numbers, for example, 20H7 or 42с8.

The tolerance field and limit deviations are given in the tables of the standard and are sometimes placed in parentheses after the conventional notation of the tolerance field 20H7 (+0.021), 42с8. Deviations equal to zero are not shown in the marking. Two limit deviations can be positive (in the case when two limit sizes are larger than the nominal), negative (two limit sizes are smaller than the nominal) and one positive and the other negative (one limit size is larger and the other is smaller than the nominal).

The accuracy of the part is understood as its compliance with the requirements of the drawing: in terms of size, geometric shape, the correct mutual placement of the surfaces being processed and the degree of their roughness. Regarding the part, there are three standards of accuracy: accuracy of linear dimensions; accuracy of form; accuracy of mutual placement of surfaces.

Allowance for linear dimensions, deviations in form and mutual placement of surfaces affects the structure of the technological process, labor intensity and cost of products. The accuracy of parts manufacturing is characterized by deviations from the given shape and deviations in the mutual placement of individual surfaces. Such deviations are limited by the appropriate tolerances assigned under the specific operating conditions of the coupled parts.

Deviations of the form of conjugated parts with flat surfaces include non-linearity and non-planarity (Fig. 6.1). Non-linearity is a deviation from a straight line of the profile of a cross-section of a surface in a plane normal to it in a given direction. Non-planarity is a deviation from straightness in any direction along the surface, for example, convexity, concavity.

Non-planarity

surface

profile

Real

Non-linearity

Real

straight line

An adjacent

Fig. 6.1 - Deviation of the shape of parts with a flat surface

Cylindrical surfaces may have deviations in shape in transverse and longitudinal sections. Thus, in the cross-section, deviation of the surface contour from a regular circle is possible - non-circularity, partial types of which are circumference and ovality (Fig. 6.2, a), which are characterized by the d_max-d_min difference. The following deviations are possible in the longitudinal section of cylindrical surfaces: barrel-like, curved, saddle-like and conic (Fig. 6.2, b).

Limit deviations of the shape of cylindrical surfaces are limited by tolerance fields on the diameter of these surfaces.

Fig. 6.2 - Deviation of the shape of parts with cylindrical surfaces:

a – in a cross section, b – in a longitudinal section

Regarding surface placement deviations, the following should be noted. The standard provides for the following deviations in the placement of parts surfaces:

ü for planes – non-parallelism and non-perpendicularity;

ü for cylindrical surfaces – misalignment, radial and end runout;

ü for axes - misalignment of axes, deviation from the correct placement of intersecting axes and passing axes.

Limit deviations and placement of surfaces are indicated on the drawings. On the drawings, the sign and numerical value of the deviation are entered in a rectangular frame divided into two or three parts (Fig. 6.3).

Fig. 6.3 – Designation of the limit deviations of the shape of details on the drawings

In the first part (on the left), the tolerance sign is indicated, in the second - the numerical value of the limit deviation (in mm), and in the third - the letter designation of the base or other surface. Bases are indicated by a shaded equilateral triangle or a capital letter. The direction of the line segment with the arrow must correspond to the direction of the deviation measurement.

6.1.2 Methods of achieving processing accuracy

The accuracy of processing on the machine is determined by the following factors:

1. Quality of manufacturing and condition of machines and tools.

2. Accuracy of tool installation.

3. Deformation of the technological system.

4. Inconsistency of the allowance on the surface of the part and inconstant hardness.

5. Temperature deformation in the technological system during the cutting process.

6. Wear of the cutting tool.

7. Deformation of the workpiece from the cutting force.

8. Deformation of the workpiece as a result of stress redistribution during removal of the allowance

9. Qualification and discipline of the worker.

10. Measurement error.

The given accuracy of workpiece processing can be achieved using two fundamentally different methods: trial runs and measurements; by the method of automatically obtaining dimensions on adjusted machines.

The method of trial passages and measurements

The essence of the method is that a cutting tool is brought to the machining surface of the workpiece mounted on the machine and a test chip is removed from a short section of the workpiece. After that, the machine is stopped, a trial measurement of the obtained size is made, the amount of its deviation from the one indicated on the drawing is determined, and a correction is made to the position of the tool, which is calculated by divisions of the limb of the machine. Then, a trial processing ("stroke") of the workpiece section is again carried out, a new trial measurement of the obtained size is made and, if necessary, a new correction is made to the position of the tool. Thus, by trial runs and measurements, the correct position of the tool relative to the workpiece is established, which ensures the required size. After that, the workpiece is processed along its entire length. When processing the next workpiece, the entire tool installation procedure is repeated with trial passages and measurements.

Marking is often used in the method of trial passages and measurements. In this case, thin lines are drawn on the surface of the workpiece with special tools (drafting machines, rod-reamers, etc.), which show the outline of the future part. During the subsequent processing, the worker tries to combine the trajectory of the cutting blade of the tool with the line of marking the workpiece and thereby ensure that the given shape of the processed surface is obtained.

Advantages of the method: high accuracy of processing is obtained on imprecise equipment; when processing a batch of small blanks, the influence of the wear of the cutting tool on the accuracy of the dimensions maintained during processing is excluded; in case of inaccurate preparation, it allows you to correctly distribute the allowance and prevent the appearance of defects; frees the worker from the need to manufacture a complex and expensive device.

Disadvantages of the method: the dependence of processing accuracy on the minimum thickness of the chip to be removed; the appearance of a defect due to the fault of the worker; low productivity of processing, due to the large amount of time spent on trial passages, measurements and marking; high cost of processing the part due to low productivity in combination with high qualification of the worker.

As a result, this method is used in single or small-batch production, research production, as well as in repair and tool shops. In multi-series and mass grinding, because it allows compensating for the wear of the abrasive tool.

The method of automatically obtaining dimensions on adjusted machines

When processing workpieces using this method, the machine is pre-adjusted in such a way that the accuracy required from the workpiece is achieved automatically, that is, almost regardless of the qualification and attention of the worker. For example, when milling workpiece 2 to dimensions a and b, the table of the milling machine is pre-set in height so that the support surface of the stationary jaw 1 of the vice is to the axis of rotation of the milling cutter at a distance of K=D/2+a (Fig. 6.4). At the same time, the side surface of the milling cutter 3 is moved away from the vertical surface of the stationary sponge by a distance b.

Fig. 6.4

This preliminary adjustment of the machine is carried out by the method of trial passages and measurements. After such adjustment, the entire batch of blanks is processed without their intermediates and without additional movements of the machine table in the transverse and vertical directions. Just as in the process of processing, the dimensions K and b remain unchanged,

so the accuracy of the dimensions a and b remains the same for all workpieces processed on the machine with this setup.

Advantages of the method: increasing the accuracy of processing and reducing defects; growth of labor productivity; rational use of highly qualified workers; increasing production efficiency.

The method of automatically obtaining dimensions on adjusted machines is to a large extent devoid of the disadvantages inherent in the method of trial passages and measurements. With this method, the task of ensuring processing accuracy is transferred from the worker-operator to the adjuster, toolmaker and technologist.

The method is widely used in modern serial and mass production.

Each of the considered methods of achieving the specified accuracy is definitely accompanied by processing errors caused by various reasons of a systematic and random nature. Accordingly, errors arising from these causes are divided into systematic and random.