RU2519173C1 - Centreless grinding method for very hard powdered material - Google Patents

Abstract

FIELD: machine building.

SUBSTANCE: force attributable to one abrasive grain is defined for an abrasive tool beforehand. Force of cutting grains retention by abrasive tool binding substance is set assuming that cutting force attributable to one abrasive grain does not exceed the force of cutting grain retention by the tool binding substance. Maximal permissible depth of grinding is determined depending on the parameters of very hard powdered material being processed, abrasive tools and grinding modes. Through-feed processing of an item is carried out for the depth not exceeding the maximal grinding depth.

EFFECT: improved quality of processed surface and abrasive tool durability due to the efficient combination of parameters of centreless grinding mode.

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Description

The invention relates to the field of processing powder materials of high hardness by grinding with an abrasive tool on centerless grinding machines.

Known methods of centerless grinding on the passage of round surfaces, according to which the product is informed of the rotation and movement in the axial direction and treated with a rotating abrasive tool. These methods, depending on the material being processed and the stage (rough, finishing, finishing) of centerless grinding, provide for the selection of the characteristics of the abrasive tool and the parameters of the grinding mode from some ranges according to the recommendations of the reference literature (see the Handbook of a machine-building engineer. T.2 / Ed. A.G. Kosilova and R.K. Meshcheryakova. - M.: Mechanical Engineering, 1986, 496 p.).

However, the existing recommendations on the choice of grinding mode parameters do not apply to products made of highly hard powder materials and do not take into account the influence of wear and blunting of the grinding wheel grains that determine its durability on the most important output quality characteristics of the polished surface - size accuracy and surface roughness.

The resistance of an abrasive tool when grinding hard-to-handle materials is determined by the wear resistance of the cutting edges and the strength of the abrasive grains fastening with a bunch of abrasive tools. With increasing values of the grinding mode parameters (rotation speed V _D , longitudinal speed V _{pr of the} product, cutting depth t), the load on each cutting edge of the working surface of the wheel increases, which leads to intensive wear, as well as to the breaking of individual grains from the bundle. Noted, with the right choice of the characteristics of the abrasive tool, to the greatest extent determines the tool life and the output quality characteristics of the polished surface.

Closest to the proposed one is a method for centerless grinding of a highly hard powder material, in which the product is informed of axial rotation and movement and processed with a rotating abrasive tool according to a pre-calculated combination of controlled parameters of the grinding mode according to the accepted criteria of relativity from the groups of economic or technical and economic criteria (see Ryzhov E.V., Averchenov V.I. Optimization of technological processes of machining.Kiev: Naukova doom and, 1989. S.35-40).

This grinding method can significantly improve the performance and quality of grinding surfaces. However, it does not provide the required durability of the abrasive tool when grinding highly hard powder materials. This is due to the fact that when calculating the controlled parameters of the grinding mode, according to the accepted criteria of relativity from the groups of economic or technical and economic criteria, technical restrictions are not taken into account, taking into account the dependence of the resistance of the abrasive tool on the controlled parameters of the grinding mode. In particular, the physicomechanical properties of the processed material, the characteristics of the abrasive tool used and the grinding mode parameters (V _D , V _CR , t), which affect the holding force of the cutting abrasive grains by the tool bundle, which determines the tool life, are not taken into account. When the cutting force per each abrasive grain exceeds the grain holding force by the tool bundle, the cutting grains break out of the tool bundle, which affects the quality of the polished surface - the accuracy of the size to be performed, and leads to an increase in surface roughness. The aforementioned does not allow rational use of potential opportunities to increase the resistance of an abrasive tool and to obtain the required processing accuracy and surface roughness.

The present invention is aimed at improving the durability of the abrasive tool and the quality of processing due to the rational task of combining the controlled parameters of the centerless grinding modes.

The specified technical result is achieved by the fact that in the method of centerless grinding of products from highly hard two-component powder material, comprising communicating the product with rotation and displacement in the axial direction and processing with a rotating abrasive tool with the selected grinding mode parameters, according to the invention, the force per one abrasive grain, set the holding force of the cutting grains by a bunch of abrasive tools based on the condition That the cutting force attributable to one abrasive grain does not exceed the holding force of the cutting instrument ligament grain, determine the maximum depth of grinding and naprohod treatment is performed to a depth not exceeding the values:

,

,

where r is the average statistical grain size of the processed material;

B = (H _V1 · RA / [P _Z1 ]) / (2 · (H _V1 -H _V2 )),

H _V1 , H _V2 - microhardness of the components of a high-hard two-component powder material,

R is the average statistical distance between the centers of grains of the processed material,

K _c is the coefficient of the shape of the tip of the abrasive grain, K _δ is a parameter depending on the volume structure of the standard abrasive tool and the conditions for editing its working surface, d _a is the characteristic average diameter of the abrasive grain of the tool, v _k is the speed of abrasive cutting, V _D is the rotation speed product, _{V, etc.} - the longitudinal speed of the product, d, D - diameter of the product and respectively abrasive tool, ζ - shrinkage processed material swarf, η - the angle of sliding friction between the abrasive grain and the material being processed, γ - Front yz l cutter unit abrasive tool, [P _Z1] - holding force cutting grain bonded abrasive tool.

The method is as follows.

Before grinding the product made of highly hard powder material, it is necessary to establish the maximum allowable grinding depth. To do this, determine the cutting force per one cutting abrasive grain of the tool.

The tangential component P _{z of} the cutting force during circular external centerless grinding the passage is determined from the expression

where H _V - hardness of the material being processed Vickers; S is the axial size of the product; t is the grinding depth; d _a is the characteristic size of the abrasive grain of the tool; n = 1,0 ... 2,5 (for circles of electrocorundum abrasive tools on a ceramic bond n = 1,5; for elboric and silicon carbide on a ceramic bond n = 1,0); K _in - coefficient of the shape of the top of the abrasive grain; To _δ is a parameter depending on the volumetric structure of a standard abrasive tool and the conditions for editing its working surface; V _K - abrasive cutting speed; V _D is the rotation speed of the product, V _CR is the longitudinal speed of the product;

,

,

ζ is the shrinkage of the chips of the processed material, η is the sliding friction angle between the abrasive grain and the processed material, γ is the rake angle of a single cutter of the abrasive tool.

The coefficients K _in and K _δ included in formula (1) are calculated from consideration of the volumetric model of a standard abrasive tool according to known dependencies (see, for example, Ostrovsky V.I. Theoretical foundations of the grinding process. - L.: Leningrad Publishing House. unta, 1981. 144 p.).

The coefficient of the shape of the tip of the abrasive grain K _in depends on the characteristic size of the abrasive grain d _a , the idealized shape of the abrasive grain (sphere, ellipsoid, cone, etc.) and its orientation in the cutting direction. Depending on the combination of the noted conditions from table 3.2 on pages 36-37 of the above source of information, the calculated expression can be selected to determine the coefficient K _in .

_Δ factor K depends on the volume of the abrasive tool structure and conditions changes its working surface and stored as the product of three factors: _δ K = K _f K _sa K _pr (p.40), where K _n - of surface porosity coefficient tool; K _SK - the coefficient of the structure of the circle; K _CR - dressing coefficient of the working surface of the tool. The values of the coefficients K _p and K _ck for the standard characteristics of abrasive wheels are given in tables 3.4 and 3.5, respectively, on page 41. Approximate expression for the coefficient _{K, etc.} is given on page 49.

Consider the implementation of the proposed method using a high-hardness powder material Relita, which is a tungsten carbide grain equally distributed in a copper binder with a microhardness of 2000 ... 3000 H _V , with a microhardness of a copper binder of 200 ... 300 H _V.

In FIG. a schematic representation of the cut plane of the Relit material layer. In this case, assumptions were made that the shape of the grains of tungsten carbides has an ideal round shape with a radius r, the grains are equidistant from each other by a distance R, the gaps between the grains are completely and without pores filled with copper, grinding is carried out with a cutting depth t.

The distribution of tungsten carbide grains in a copper binder of a given material is characterized by two average statistical characteristics: r is the average radius of tungsten carbide grains; R is the average distance between the centers of the grains of tungsten carbides. "Relit" material is a two-component structure with deterministic components located in the cutting plane, the reduced microhardness H _Vred which can be found from the expression

where Δ _i is the size of the i-th element in the section of the slice at a depth t; H _i is the microhardness of the i-th element of the processed material, n is the number of components of the powder material (n = 2, 1 - tungsten carbide, 2 - copper).

From the circuit shown in Fig., We obtain

,

,

After substituting Δ ₁ and Δ ₂ into expression (2) and transforming it, the final expression for H _Vpr takes the form

where H _V1 is the microhardness of tungsten carbides, H _V2 is the microhardness of copper.

Analysis of expression (3) shows that H _Vred bicomponent material "relit" depends on the depth of grinding t, tungsten carbide size r and their mutual location R.

Substituting (3) into (1), we obtain expressions for determining the cutting force P _Z during grinding of a two-component powder material

The number of cutting grains N ₀ at the contact area of the abrasive tool and the product at a grinding depth t for circular external centerless grinding is determined from the expression

where L _K is the length of the contact arc during circular external centerless grinding; d, D are the diameters of the workpiece and abrasive tool (grinding wheel), respectively; δ is the average distance between the cutting abrasive grains.

After substitution and transformation, the final expression for N ₀ takes the form

The cutting force P _Z1 per one cutting abrasive grain is determined from the expression

When grinding highly hard powder materials, it is advisable to use an abrasive tool in the form of silicon carbide grinding wheels with a ceramic bond, for which n = 1. With this in mind, for the convenience of the subsequent analysis, we represent (6) in the following form

Where

To ensure high resistance of the abrasive tool, we must create grinding conditions under which the cutting force per one abrasive grain would be less than or equal to the holding force of the grains by the bond of the grinding wheel.

where [P _Z1 ] is the retention force of the cutting grains by a bunch of abrasive

tool. (According to Yakimov A.V. Optimization of the grinding process. M .: Mechanical engineering 1975. 176 p., The holding force of silicon carbide grains for ceramic bonded wheels, depending on the grain size and hardness of the wheel, is in the range [P _Z1 ] = 5 ... 10 H).

We obtain from expression (8), taking into account (7), an expression for setting the maximum permissible grinding depth t, which provides high durability of the abrasive tool with round external centerless grinding of high-strength powder material

Having transformed, we obtain the following quadratic equation

t ² -2 · r · t + [( H V1 · RA / [P Z1]) / ( 2 _{· (H V1 -H V2))} ] ² = 0

To simplify further transformations, replace the expression in square brackets with B. Then from the solution of the quadratic equation we get the maximum allowable grinding depth

.

.

, то гарантированно будет обеспечена высокая стойкость абразивного инструмента и требуемое качество обработки.Thus, if grinding is carried out to a depth not exceeding $$t=r-\sqrt{r^2-B^2}$$

, then the high durability of the abrasive tool and the required quality of processing will be guaranteed.

где r - средний статистический радиус зерен высокотвердого порошкового материала; B=(H_V1·R-A/[P_Z1])/(2·(H_V1-H_V2)), H_V1, H_V2 - микротвердость составляющих двухкомпонентного порошкового материала, R - среднее статистическое расстояние между центрами зерен высокотвердого порошкового материала,To implement the proposed method before grinding two-component carbide powder material, it is necessary to determine the maximum allowable grinding depth by the formula $$t=r-\sqrt{r^2-B^2}$$

where r is the average statistical radius of the grains of high hardness powder material; B = (H _V1 · RA / [P _Z1 ]) / (2 · (H _V1 -H _V2 )), H _V1 , H _V2 - microhardness of the components of a two-component powder material, R - average statistical distance between the centers of grains of high-hard powder material,

, K_в - коэффициент формы вершины абразивного зерна, K_δ - параметр, зависящий от объемного строения стандартного абразивного инструмента и условий правки его рабочей поверхности, d_a - характерный размер абразивного зерна инструмента, V_K - скорость абразивного резания, V_D - скорость изделия, V_пр - продольная скорость изделия, d, D - диаметры соответственно обрабатываемого изделия и шлифовального круга, ζ - усадка стружки, η - угол трения скольжения, γ - передний угол единичного резца, [P_Z1] - сила удержания режущих зерен связкой абразивного инструмента. Затем изделию сообщают вращение и перемещение в осевом направлении и обрабатывают вращающимся абразивным инструментом с глубиной резания, не превышающей найденного значения t.

, K _в is the coefficient of the shape of the tip of the abrasive grain, K _δ is a parameter depending on the volumetric structure of a standard abrasive tool and the conditions for editing its working surface, d _a is the characteristic size of the abrasive grain of the tool, V _K is the speed of abrasive cutting, V _D is the speed of the product , V _CR - longitudinal speed of the product, d, D - diameters of the workpiece and grinding wheel, ζ - shrinkage of the chips, η - angle of sliding friction, γ - rake angle of a single cutter, [P _Z1 ] - holding force of cutting grains by a bunch of abrasive tools but. Then the product is informed of the rotation and displacement in the axial direction and is treated with a rotating abrasive tool with a cutting depth not exceeding the found value of t.

The effectiveness of grinding according to the proposed method is to maintain high durability of the abrasive tool while ensuring the required processing quality indicators.

),

0,04, Ra 0,80), на режимах: скорость абразивного резания - 35 м/с, скорость вращения изделия - 1,56 м/с, продольной скорость изделия - 0,054 м/с. Расчетное значение предельно допустимой глубины шлифования для указанных условий шлифования составило 3·10^-5 м. Обработку изделия согласно предлагаемому способу проводили при глубине шлифования t=1·10^-5 м, не превышающей допустимую величину, и при глубине шлифования, превышающей расчетную - t=8·10^-5 м (базовый вариант).Operational tests of the proposed grinding method were carried out on a centerless grinding machine 3M184I with a grinding wheel 1,500 × 150 × 305 54CP40N7V35A. Round products in the form of bushings for bearings of submersible oil equipment made of Relit material with a diameter of 30 mm and a wall thickness of 8 mm ⌀30 s8 ( $${}_{-0.143}^{-\mathrm{0,110}}$$

),

0.04, Ra 0.80), in the modes: abrasive cutting speed - 35 m / s, product rotation speed - 1.56 m / s, longitudinal product speed - 0.054 m / s. The calculated value of the maximum permissible grinding depth for the indicated grinding conditions was 3 · 10 ^-5 m. The product was processed according to the proposed method with a grinding depth t = 1 · 10 ^-5 m not exceeding the permissible value, and with a grinding depth exceeding the calculated one - t = 8 · 10 ^-5 m (basic version).

The test results showed that when grinding according to the proposed method with a depth of t = 1 · 10 ^-5 m, not exceeding the calculated value, the resistance of the grinding wheel between dressings increased by 3 ... 3.2 times compared with the base case, 7 are stably provided over time the quality accuracy of the dimensions to be performed and the roughness of the surfaces to be grinded are not coarser than Ra 0.5.

When processing products with a grinding depth of t = 8 · 10 ^-5 m that exceeds the calculated one, after 20 minutes of grinding the abrasive wheel, due to intensive wear, does not provide the required accuracy and roughness of the processed product. It is necessary to constantly adjust the depth of cut of the grinding wheel to obtain the required size, and the roughness of the surfaces to be grinded reaches a value of up to Ra 2.5.

Thus, the proposed method of grinding provides a significant increase in the durability of the abrasive tool with stable provision of the required quality indicators of the processed surface.

Claims (1)

A method for centerless grinding of products from a high-hard two-component powder material, comprising communicating the product with rotation and displacement in the axial direction and processing with a rotating abrasive tool with the selected grinding mode parameters, characterized in that the force per one abrasive grain is first determined for the abrasive tool, and the holding force is established cutting grains of a bunch of abrasive tools based on the condition that the cutting force per one abrasive grain Does not exceed the holding force of the cutting instrument ligament grain, determine the maximum depth t and grinding processing is carried out on the products naprohod depth not exceeding a value determined by the formula:

,
where r is the average statistical radius of the grains of high hardness powder material;
B = [H _V1 · R-A / [P _Z1 ]) / (2 · (H _V1 -H _V2 ));
Н _V1 , Н _V2 - microhardness of the components of a two-component powder material;
R is the average statistical distance between the centers of the grains of highly hard powder material;

;
K _{in the} - shape factor top abrasive grain;
K _δ is a parameter depending on the volumetric structure of a standard abrasive tool and the conditions for editing its working surface;
d _a is the characteristic size of the abrasive grain of the tool;
V _K is the speed of abrasive cutting;
V _D is the speed of the product;
V _np is the longitudinal velocity of the product;
d, D are the diameters of the workpiece and abrasive tool, respectively;
ζ - shrinkage of the chips;
η is the angle of sliding friction;
γ is the rake angle of a single grain;
[P _Z1 ] - the holding force of the cutting grains of a bunch of abrasive tools.