XI Международная студенческая научная конференция Студенческий научный форум - 2019

Abrasive materials (abrasives) (from the Latin. Abrasio - scraping), substances of increased hardness, used in a massive or crushed state for mechanical processing (grinding, cutting, abrasion, sharpening, polishing, etc.) of other materials.

 An abrasive tool, unlike a metal blade, does not have a solid cutting edge, but consists of a huge number of disconnected cutting elements (abrasive grains) fastened together by a bundle. Therefore, the performance of an abrasive tool is characterized not only by the material and size of the cutting abrasive grain, but also by the composition and quantity of the ligament, the structure (location of the abrasive grains and pores in the tool). All these parameters, marked on each abrasive tool, constitute its characteristic.

The abrasive ability and wear resistance of an abrasive material depend on its hardness, heat resistance, brittleness and crushability of the grain, as well as on the degree of chemical interaction with the material being processed. Abrasive can be any natural or artificial material, the grains of which have certain properties: hardness, strength and toughness; abrasive grain form; grit, abrasive ability, mechanical and chemical resistance, i.e. the ability to cut and grind other materials. The main feature of abrasive materials is their high hardness compared to other materials and minerals. It is on the difference in hardness that all grinding and cutting processes are based.

Round external grinding is used to process the external surfaces of parts such as bodies of revolution with rectilinear ones. As technological bases center holes or external cylindrical surfaces are used. Depending on the direction of translational motion of the feed, the following methods of grinding are distinguished.

Mortise grinding: is ensured by the movement of the longitudinal feed snp in one direction, perpendicular to the axis of the workpiece, which is ground as it rotates.

Oscillating grinding (longitudinal working strokes): the grinding wheel or workpiece, along with the rotational movement, make a reciprocating movement designed to process surfaces of considerable length exceeding the height of the grinding wheel. At the end of a double or each pass, the circle is fed to a set grinding depth or amount of cross feed.

Grinding with ledges (combination of mortise and oscillating grinding): separate parts of the surface (ledges) of the workpiece are machined sequentially by plunging a circle, while the ledges must overlap each other. The remainder of the allowance is then removed by oscillating grinding.

Deep grinding can be both with longitudinal, and with cross giving. When grinding with a longitudinal feed all (or almost all) allowance is removed in one round pass. The latter is governed by a step or a cone. When sanding with a cross feed, the workpiece is reported to rotate slowly. The circle cuts into the workpiece with an increased feed for all (or almost all) of the allowance, and during one turn of the workpiece, the entire allowance is removed. The processing scheme is similar to the mortise grinding the periphery of the circle.

In case of multi-circular external grinding of one or several blanks, the circle is fed perpendicularly to the axis of the blank or at a certain angle to it. The choice of grinding method is determined by the type of production, part design, stock size, and requirements for accuracy and quality of processing. According to the intensity of stock removal, all round-surface grinding operations are divided into peeling, preliminary and fine grinding.

Abrasive grinding is used to remove the defective layer from the workpieces (with a thickness of more than 1 mm per diameter) after casting and forging. stamping, rolling. Circle speed vK = 35/60 m / s and more; machining accuracy of 8–9th grade, surface roughness Ra = 2.5 / 5 microns.

Pre-grinding performed after turning, but before the heat treatment of the workpiece. Circle speed yK = 40/60 m / s; machining accuracy of 6—9th grade, Ra = 1.2 / 2.5 microns.

Final grinding is carried out after heat treatment of the workpiece at vK = 35/40 m / s. Precision processing! 5-6th grade; Ra = 0.2 / 1.2 microns.

Fine grinding (allowance of 0.05–0.1 mm per diameter) is intended to ensure low surface roughness (Ra — 0.025 / 0.1 μm). In individual production, grinding is usually performed in one operation, in serial and mass production - in one, two or more operations (depending on the size of the allowance, requirements for accuracy and surface quality). Grinding modes are given in reference books.

Round external grinding is usually called the process of grinding the workpiece during its rotation in the centers or chuck. Circular grinding machines are divided into universal and special. Cylindrical, conical, stepped and shaped surfaces are ground on these machines. There are two ways to process workpieces on circular grinding machines: grinding with a longitudinal feed and mortise grinding.

Grinding with a longitudinal feed is used in the processing of workpieces, the length of which far exceeds the width of the grinding wheel. One of the types of grinding with a longitudinal feed is a deep method, in which grinding is performed with a large feed to a depth t, a small longitudinal feed (Sprod). The depth of grinding is equal to the allowance left for processing, the circle is fed immediately to this value, and the workpiece gets a very slow longitudinal feed. When working in this way, the front edge of the circle wears out quickly, as it is subjected to maximum load and the circle has to be more often corrected. However, with this method of grinding, a significant reduction in processing time is achieved by reducing the number of passes and distributing the load to a larger number of abrasive grains involved in cutting.

Mortise grinding is used in cases where the length of the ground surface is slightly less than or equal to the height of the circle. This type of grinding is widely used in mass and large-scale production.

In order to speed up grinding operations of workpieces, the length of which significantly exceeds the height of the circle, it is more rational to use the combined processing method:

a) pre-cut-in grinding with a high transverse feed Sopop on the value of the allowance and the movement of the circle from position to position, and so on;

b) final grinding with a longitudinal feed Spr, providing the desired surface roughness of the treatment.

Nowadays, special semi-automatic multi-circle machines are increasingly being used for simultaneous grinding of three to six main crankshaft journals with a set of grinding wheels.

. Despite a decrease in the cutting ability of each wheel by more than three times and an increase in machine time for grinding one neck by almost 2.5 times, labor productivity is almost doubled compared to processing on single-circle machines with an increase in the geometrical accuracy of the location of the necks relative to the central axis of the shaft .

Discrete Grinding Tool

The invention relates to the field of abrasive machining and can be used in the manufacture of discrete grinding wheels. The circle is made solid with radial holes on the cutting surface, arranged in rows parallel to the axis of its rotation. The radial holes are located in alternating long and short lines offset by half of the axial pitch Toc of their location in the direction of the lines.

Known discrete grinding tools, made in the form of a body, in the longitudinal grooves of which are inserted abrasive cutting elements (segments), held in the body from axial displacement with covers fixed to both ends of the body with screws

There is an air gap a few tens of millimeters long between circumferential segments. Due to the discrete (intermittent) cutting surface of the tool, conditions are created for interrupting the process of cutting and cooling the workpiece between adjacent cuts, and consequently, for reducing the temperature, which positively affects the quality of the processed surface layer of parts, durability of the cutting tool, etc.

Disadvantages of discrete grinding tools: the complexity of the design, the appearance in the process of grinding additional geometrical errors of the machined surfaces of parts due to the increased level of vibration of the technological system machine-device-tool-workpiece caused by shock impulses of the segments on the workpiece. The vibration level is higher, the greater the length of the air gap between adjacent segments.

The protrusions in the grinding wheel are formed by the holes of the liner, which is located in the mold used for the production of the grinding wheel. As a liner, a perforated ring is used, which is removed from the mold after the circle has been pressed.

The circle after pressing and removing the perforated ring from its peripheral surface has a textured work surface.

Performing a circle with a lot of surface irregularities (protrusions) allows you to facilitate rough grinding operations and minimize the excessive pressure of the circle on the workpiece during the cleaning operation.

Discrete grinding tool in the form of alternating long 4 and short 5 lines. The rows are parallel to the axis of rotation of the tool, and the radial holes lying on the same line are located with an axial pitch 3r <Toc <4r, where r> L / 2 (L is the length of the arc of contact of the tool with the workpiece). Holes 3 have a depth (length) l0 equal to the thickness of the working abrasive layer, worn out during the entire time of the circle. The depth of the radial hole is l0 = 0.1D (D is the diameter of the cutting surface of the tool), due to the fact that when the tool is worn during operation, the diameter D of the cutting surface decreases, and the cutting speed decreases, which is accompanied by a deterioration in all process indicators. When the wheel is worn radially by the value of l0 = 0.1D, the cutting speed decreases by 20% from the original speed characteristic of an unworn wheel, the grinding process becomes ineffective, therefore, the worn tool should be replaced with a new one.

Discrete grinding compared to solid is characterized by changing the length of the line of contact of the tool with the workpiece, measured along the axis of the tool. The components of the cutting force depend on the change in the length of the cutting line during each turn of the tool around its axis. The rate of change of cutting force during processing has an impact on the dynamics of the grinding process and, ultimately, on the output quality indicators of the surface layer.

In the process of grinding the circle is in contact with the workpiece alternately with its protrusions located in rows parallel to the axis of rotation of the circle. The area of ​​contact of the workpiece with a circle made according to the patent RU 2187425 is significantly less than when using a conventional grinding wheel, which leads to a decrease in cutting force, including the pressing force of the workpiece to the grinding wheel. The relatively small forces of pressing the workpiece to the circle facilitate the work of the operator.

In the process of grinding, the cutting protrusions self-sharpen, so the circle does not need to be corrected, which reduces the cost of the operations of cleaning workpieces.

With a high frequency of changes in external dynamic effects (cutting force), the technological system (TS) due to its inertia does not have time to react to external disturbances, i.e. he does not have time to recover elastically (for a very small period of time for the arc L to pass by a row of radial holes), as a new elementary cut occurs, causing its elastic deformation. The offset of the radial holes in the axial direction by half a step Tos, / 2 allows the short and long lines to be most closely spaced to each other, thus ensuring a small length of the cutting section and the interruption section, and consequently, the frequency of external dynamic effects on the vehicle increases. This leads to a decrease in its vibration level.

The disadvantage of the grinding wheel according to the patent RU 2187425 is the low geometric accuracy of the machined surfaces of the parts, due to the increased level of vibration of the technological system. During rotation, the circle contacts with the workpiece, for example, along the protrusions of one line, after which the circle does not contact the workpiece, since after each line of the protrusions there is a depression, i.e. air gap. This leads to significant jumps in the cutting force from the maximum value (when the contact of the circle with the workpiece occurs along the protrusions) to zero (when there is no contact).

Also known grinding wheel (prototype), made from a grinding wheel with a solid cutting surface by applying to it using a laser beam a large number of radial holes with a diameter of 1.8-1.9 mm (see, for example, US patent No. 4882878, B24D 5 / 00).

These holes are performed by forming a cylindrical cutting surface (lines) at a distance of 2.8 mm from each other, with rows of holes parallel to the axis of rotation of the tool.

The prototype allows you to reduce the limits of change in cutting force compared with the above tools, which has a positive effect on reducing the vibration level of the process system.

The disadvantages of the prototype are low geometric accuracy and quality of the processed surface layer of parts.

The technical effect achieved by the invention is to improve the geometric accuracy and quality of the processed surface layer of parts.

In order to reduce the level of vibration caused by the discreteness of the cutting surface of the tool, it is necessary to reduce the length of the interruption sections, that is, to perform them as little as possible. Reducing the length of the interruption section to units of millimeters in discrete segment circles and solid circles with protrusions and depressions is impossible, due to the occurrence of small sizes of segments and protrusions (segments and protrusions with a thickness measured by units of millimeters do not have sufficient mechanical strength and are destroyed by cutting force).

In the prototype, the distance between the radial holes, both axially and circumferentially, is not regulated in any way; there are no decisions regarding the numerical value of the radius r for different sizes of grinding wheels, which reduces the efficiency of using the prototype as a whole and, in particular, the quality of the ground surface layer details.

Under the influence of high temperature, thermal defects are formed (burns, tensile residual stresses, modified initial metal microstructure, reduced microhardness, etc.), reducing the operational reliability of ground parts. In the proposed tool for all diameters of circles from 100 to 1200 mm overlap of the radial holes does not occur, which improves the convective heat sink from the workpiece, provides aero-hydrodynamic ventilation of the zone of contact of the circle with the workpiece and reduces the appearance of thermal defects.

Round external grinding is used to process the external surfaces of parts such as bodies of revolution with rectilinear ones. As technological bases center holes or external cylindrical surfaces are used.

Discrete grinding tool is used on circular grinding machines, which are divided into universal and special. Cylindrical, conical, stepped and shaped surfaces are ground on these machines. For processing workpieces of various diameters, round grinding is applied simultaneously with several grinding wheels. For round grinding, mainly PP-type circles with an outer diameter of 250-1100, 20-75 mm high are used. In this case, we will choose a disc circle with a diameter of 250 for grinding a diameter of 30. To do this, let us bring the tool to the workpiece before touching it, after which the part and the workpiece rotate simultaneously, thanks to the design of the discrete grinding wheel, the temperature of the part will not be too high; coolant which will work better with the surface and also prevent overheating.

Conclusion

The main task of designing a discrete grinding tool is to improve the quality of processing and the tool itself, processing at higher speeds with greater productivity facilitating the process for the operator, reducing the temperature in the machining area and improving the geometric accuracy and quality of the processed surface layer of parts.

Minute removal of metal during grinding with a discrete wheel is 1.28-2.10 times more, grinding ratio is 6-10 times, and the abrasive consumption per minute is 1.8-2.6 times less than with a known circle, which indicates obvious advantages of the developed process of discrete flat face grinding with coolant supply in the cutting plane. It is possible to use more solid discrete abrasive segments (two to three degrees harder than solid segments) and an increase in the open-ended vertical tool feed from 0.3 to 0.6 mm / min, which, combined with the high efficiency of the coolant, leads to an increase in the quality of parts an increase in technological, cyclic and technical productivity, respectively, of 1.9; 1.7 and 1.6 times, and also to save abrasive material. The microhardness of the treated surface layer increases by 35%.

To develop discrete grinding wheels, it is necessary to assign the hardness of discrete abrasive segments two to three degrees higher than the hardness of standard abrasive segments, calculate the number and dimensions of the longitudinal channels of the segments and check the mechanical strength of the discrete segments by calculating dangerous sections according to the developed technique; use an electromagnetic device that reduces the clogging of the tool with grinding waste, and cardboard pads that isolate the longitudinal channels of the segments.

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