XI Международная студенческая научная конференция Студенческий научный форум - 2019

Wear is a complex process, accompanied by physico-chemical phenomena at the points of contact of the tool with chips and the workpiece being processed. When worn, the tool geometry changes, which affects the amount of plastic deformation, cutting temperature and cutting force.

Wear of the cutting tool proceeds under more severe conditions than wear of machine parts. So, tool wear occurs at high temperatures and specific pressures under conditions of dry and semi-dry friction. During cutting, the specific pressure can exceed the normal pressure in machine parts by 300–400 times, and the temperature can be higher than the temperature of machine parts by 15–20 times and more. When the tool is worn on the back surface, the rubbing surfaces are located at an angle to each other, which leads to a concentration of high pressures and temperatures in small contact areas. The upper layers of the rubbing surfaces of the instrument may be subject to plastic deformation. All this contributes to the fact that the tool wears out much more intensively than machine parts.

Tool wear, as well as wear of machine parts, is accompanied by weight loss, since material particles are removed from its surface.

Depending on the cutting conditions, various types of wear may occur. At relatively low cutting speeds, wear is mainly due to the friction of the chips on the front surface of the tool and the workpiece on the rear surfaces. At the same time the working surfaces of the tool are abraded This type of wear is called abrasive. Tools made of tool steel are for the most part subject to abrasive wear.

When working with relatively high cutting speeds, when the temperature can be high, structural changes occur in the upper layers of the tool. So, under the influence of temperature above 600 "C, the martensitic structure of a high-speed tool can turn into less wear-resistant structures — austenitic-martensitic or trostitno-martensitic. Wear due to structural changes can be called thermal.

At high temperatures, oxidation of the upper layers of the friction surfaces of the tool may also occur. Oxidized films, having brittleness, are destroyed, exposing the underlying layers, which in turn are subject to destruction. This type of wear is called oxidative.

For tools made of brittle materials (cemented carbide, mineral ceramic), in addition to abrasion of working surfaces, particles are painted. Wear, which consists in chipping, is called fragile.

The influence of adhesion (sticking) plays a significant role in tool wear. Due to high temperatures and pressures, and at low cutting temperatures due to only high pressures, chips adhere to the front surface, resulting in particles of the tool material being pulled out from the surface and carried away by the descending chips. This type of wear is called adhesion.

When working with carbide tools with high cutting speeds, when the temperature reaches 900 ° C, diffusion may be the prevailing wear. When processing heat-resistant steels and alloys by diffusion, tool wear can occur at lower temperatures of the order of 500-550 ° C. This is explained by the chemical affinity of the processed and tool materials, since titanium is a component of both the heat-resistant material and the hard alloy (TK, TTC groups) . With the affinity of the processed and tool material, the process of adhesion is manifested to a large extent.

When diffusion wear due to high temperature in the contact zone is a strong softening of the surface layers. This contributes to filling the irregularities and establishing contact between the tool material, the chip material and the workpiece. This contact, high temperature and large plastic deformations cause a diffusion process. The wear of the tool in this case lies in the fact that the atoms of the tool material penetrate (diffuse) into the chips and into the upper layers of the workpiece. As a result of this process, the chemical composition and the physicomechanical properties of the surface layers of the tool change, which leads to a decrease in its wear resistance.

Tool wear during the processing of hard-to-machine steels and alloys occurs under more severe conditions than when machining conventional structural steels. This is due to the following reasons:

1) a higher cutting temperature;

2) higher specific pressures on the cutting part of the tool;

3) high abrasion ability of the material being processed (abrasion capacity of austenitic steels is 10 times higher than abrasion capacity of ferritic-pearlite steel 45);

4) the phenomena of adhesion and diffusion are more pronounced;

5) the ability of the processed material to a higher hardening, which causes an increase in cutting forces and increased vibrations;

6) a higher value of friction coefficients on the working surfaces of the tool.

A certain effect on tool wear (especially carbide and mineral-ceramic) is exerted by a shock load arising from intermittent cutting or uneven stock and accompanied by fatigue effects of tool material, which cause an increase in wear intensity.

The most common method for determining and studying tool wear is the linear method, i.e., measuring the dimensions of worn-out areas of tool surfaces. In the study of tool wear, the method of radioactive isotopes is also used when radioactive isotopes are introduced into the material of the instrument under study or form isotopes on the surface of the finished instrument by appropriate irradiation with nuclear particles. Tool wear is determined indirectly by the amount of radioactive isotope (wear products) that has passed from the tool to chips and the workpiece. Tool wear is more, the higher the radioactivity of the chips and the workpiece. Experiments have established that about 90% of the radioactive substances contained in the products of tool wear remain in the chips, and about 10% remain on the treated surface of the part.

The method of radioactive isotopes allows you to quickly and accurately investigate tool wear. The linear method requires a large consumption of the processed material, is very time consuming and expensive. However, the method of radioactive isotopes does not exclude the linear method. The geometry of worn tool pads can only be determined using the linear method.

Depending on the processing conditions, the tool may wear out as follows:

1) mainly on the back surface and slightly on the front

2) mainly on the front surface and slightly on the back

3) at the same time on the front and rear surfaces

4) rounded blade

Tools wear out on the back surface mainly when processing plastic materials with a slice thickness less than 0.1 mm, as well as when processing brittle metals when breakage chips (loose chips) are formed. With a small slice thickness, the elastic deformation of the surface layer of the metal in contact with the rear surface has a great effect on wear. When worn on the rear surface is formed area with zero rear angle. The wear of the rear surface h is measured along the worn area.

Tools wear on the front surface mainly when machining plastic metals with a slice thickness greater than 0.5 mm. At the same time, a well is produced on the front surface, which gradually increases, and when the width of the jumper f reaches zero, complete incisor wear occurs. This case of wear is especially characteristic during the formation of a growth when the blade is protected. Wear on the front surface is measured by the depth of the hole bl. With the formation of the hole decreases the cutting angle.

Wear at the same time on the front and rear surfaces occurs during the processing of plastic metals with a slice thickness of 0.1-0.5 mm.

The latter case of wear is encountered in the finishing processing of materials with low thermal conductivity, in particular, plastics. At the same time, under the influence of high temperature, the blade quickly softens and becomes dull. This wear also occurs when machining viscous high-strength metals (for example, austenitic steels). It should be noted that the rounding of the blade occurs at any wear, but in the latter case, the intensity of rounding is higher.

Rear wear is a major cause of tool blunting. Therefore, the criterion of wear is usually the size h of the worn area on the back surface. In addition, wear on the back is easier to measure than on the front. The value of hz, at which further work of the tool should be stopped, can be called the allowable wear or wear rate. When finishing, the purity and accuracy of the machined surface depends on the amount of tool wear. For such tools, a technological criterion for blunting is established — such an amount of wear, above which the purity and accuracy of the workpiece cease to meet the specified specifications. The most intensive wear occurs in the initial period of the tool. Upon reaching a certain value of hz, the wear rate begins to increase dramatically, and if you do not stop working, the tool will very quickly lose its original geometrical parameters. The amount of wear hz, corresponding to the onset of accelerated wear, will be a measure of wear.

Depreciation largely depends on the geometric parameters of the tool. Therefore, the selected angles and the shape of the cutting part of the tool must be such that its wear is minimal. The geometry of the tool, providing the least wear, is called optimal.

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