Waviness, Physical and mechanical properties, Machining allowance - Technological processes in mechanical engineering

Waviness

This is a collection of periodically alternating irregularities, in which the distance between adjacent elevations or valleys exceeds the base length. The presence of waves leads to a decrease in the reference surface by a factor of 5-10 compared to an even rough surface. Roughness and waviness are interrelated with the accuracy of the dimensions of the part. A physically grounded, and even more so natural, physical boundary between roughness and undulation of the surface as a set of irregularities with steps exceeding the base length is not available.

To estimate the waviness, you must take into account the maximum height of ten points and the average pitch. However, the evaluation of these parameters is in some cases insufficient. More fully evaluated by the same parameters as the roughness: the high W max, W a , W z , W p, step S w, the form of the unevenness of the wave r w, their direction, the reference length of the profile t w.

Waves can form on a part in two mutually perpendicular directions, so it is recommended to distinguish between longitudinal and transverse waviness. If the first occurs as a result of vibration of the technological

system, the second is caused by uneven supply, improper grinding wheel adjustment, uneven wear.

In any case, the waviness is determined by the conditions for the execution of the technological process and the processing method.

Physical and mechanical properties

The physical and mechanical properties of the surface layer are characterized by: hardness, structural and phase transformations, residual stresses, the state of the crystal lattice.

The surface layer after treatment differs from the bulk of the material: under the action of the cutting force, its hardness, crystalline structure, etc. vary. The thickness of the defective surface layer depends on the material of the workpiece, the type and processing regimes, and reaches, with some roughing, 0.5- 1 mm.

Machining allowance

This is the layer of the workpiece material that is removed when machining to obtain a surface with the specified quality, dimensions and precision. There are general and interoperational allowances.

General allowance - a material layer that is deleted when all technological operations are performed, as a result of which a finished part corresponding to the service purpose is obtained. It is defined as the difference between the dimensions of the workpiece and the finished part.

Inter-service allowance - a layer of material z that is deleted when performing one operation. One-sided allowance - a layer of material that is removed from one side of the workpiece or part (Figure 10.4, a ), two-sided - on both sides (Figure 10.4, b ) or generators (Figure 10.4, in ).

The arrangement of the allowances z, z1, z2 for processing

Fig. 10.4. The layout of the allowances z, z 1 , z 2 for processing

The allowances are symmetrical and asymmetric. The first differ in that the terms on either side or the generators are equal, for the asymmetrical one they are not equal (Figure 10.4, b). The magnitude of the total allowance depends on the scale of production, the properties of the workpiece material (forging, casting etc.), its rigidity, the thickness of the defective layer, the condition of the equipment on which the processing is carried out.

Overestimated allowances lead to excessive consumption of material in the manufacture of parts, require the introduction of additional transitions, increase the complexity of processing and its cost. Underestimated allowances do not allow to remove defective layers and ensure the obtaining of the required accuracy and roughness of the treated surfaces, which causes a marriage and increases the cost of the produced parts. Correctly installed allowances allow reasonably determining the mass of initial blanks, processing modes and the time norms for performing operations.

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