RU2432239C1 - Method of grinding complex surface parts - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to processing of materials by diamond wheels on current-conducting binder and may be used in machine building and instrument making for grinding complex shape parts. Proposed method comprises grinding of part arranged on face-grinding machine by diamond wheel on current-conducting binder in table reciprocation, and electric erosion dressing by electrode-tool outside of machining zone. Electric erosion of grinding wheel working surface is performed periodically after every dual stroke of part with machine table using DC source. Note here that voltage is selected depending upon wheel characteristics and electrode-tool length.

EFFECT: reduced wear, higher efficiency of grinding.

2 dwg, 1 ex, 2 tbl

Description

The invention relates to the processing of materials by diamond wheels on a conductive bond and can be used in mechanical engineering and instrumentation for grinding parts, mainly with a complex surface profile.

Known methods for grinding parts with diamond wheels, in which the profiling and dressing of the wheels after manufacturing and during operation is carried out by thermal exposure to the diamond layer by electric erosion [1-3].

Under the influence of electric discharges, the metal bond of the tool is partially destroyed, which leads to the restoration of the relief of the cutting surface of the diamond wheel, and therefore, the maintenance and stabilization of its cutting properties. Profile mortise grinding of parts, unlike usual, is accompanied by increased thermodynamic loads on the tool, which is due to the increased area of contact of the tool with the processed surface. Therefore, it is important not only to reduce the wear of the diamond-bearing layer of the tool, but also to minimize the unevenness of the wear of the wheel along the profile, as certain sections of the profile of the wheel are more loaded, which leads to their intensive wear, distortion of the profile and a decrease in the dimensional stability of the tool. Reducing the period of resistance of the wheel during profile grinding requires more frequent dressing tools, adjusting equipment and, therefore, leads to an increase in the cost of manufacturing parts. In profile grinding, uneven wear occurs not only in the diamond wheel, but also in the tool electrode. Technical solutions aimed at combining grinding and dressing processes when moving the ruling electrode along a path that is adjusted in accordance with uneven wear of the circle or by applying a protective layer of conductive material, such as graphite, to the surface of the ruling electrode-tool to increase dimensional stability, are laborious difficult to implement and do not guarantee stable accuracy.

The closest to the claimed invention is the "Method of profile grinding" according to ed. St. No. 1151396, B23H 1/00 [3].

A feature of the prototype method is grinding together with the product on each pass of the correcting electrode-tool, the length of the working sections of which is determined in accordance with the wear rate of the corresponding sections of the profile of the circle. When the wheel and the correcting electrode-tool interact, the surface of the diamond grinding wheel removes an additional layer of material by electrical erosion of the profile sections that are less wear during processing, and as a result, the original profile of the wheel is restored. The high intensity of electrical erosion of the metal ligament of the circle is provided by a thyristor pulse generator.

The disadvantage of this method is the increased wear of the diamond layer of the wheel, since the total wear on any section of the diamond grinding wheel profile that occurs during grinding of the work surface and the correcting electrode-tool must correspond to the maximum wear of the most loaded section of the profile of the wheel. Removing a significant amount of diamond-bearing layer for each stroke during grinding significantly reduces the life of the diamond wheel, since the thickness (height) of the diamond-bearing layer of the wheel is limited by the conditions for the production of wheels.

The technical result of the claimed invention is to reduce the wear of the diamond layer during dressing and increase the productivity of grinding with diamond wheels on metal bonds.

The technical result is achieved by the fact that on the working surface of the diamond wheel on a conductive (metal) sheaf periodically after each double stroke of the part, an electrical discharge is carried out by a direct current source, and the voltage value U is determined by the expression

,

,

where k _c is a coefficient depending on the type of ligament;

D _k - diameter (outer) of the grinding wheel;

Δ = (4-7)% Z _max is the thickness of the diamondiferous layer removed from the surface of the circle with a single exposure (for one double stroke of the machine table);

l _e - the length of the ruling electrode-tool;

Z is the number of diamond grains per 1 mm ^{2 of the} working surface of the circle;

η = 0.1-0.12 - coefficient determining the number of grains involved in cutting;

β = 0.85-0.95 - coefficient taking into account the likelihood of electrical discharges during chip short circuit;

k is the concentration of diamonds in the diamond-bearing layer of the grinding wheel.

The proposed method allows to reduce the intensity of erosive destruction of the metal bond of the diamondiferous layer of the circle with the electroerosive method of straightening the profile of the circle due to the use of a direct current source instead of the pulse generator and to limit the electrical parameters of the pulses, which allows to restore the cutting properties of the circle and maintain the profile accuracy at a lower value of the removed material of the binder. The dosed effect of electric discharges on the tool bundle leads to periodic restoration of the relief of the cutting surface of the diamond wheel, and therefore to the maintenance and stabilization of its cutting properties. The impact of electric discharges on the chips generated during grinding in the cutting zone leads to its partial combustion, which improves the evacuation of the processed products and prevents “salting” of the cutting surface of the diamond grinding wheel.

Technical solutions with similar distinctive features in the patent and scientific literature are not found, therefore, the inventive method has significant differences.

Figure 1 shows a schematic diagram for implementing the proposed method, where 1 is a workpiece, 2 is a corrective electrode-tool, 3 is a diamond grinding wheel (the dotted line shows the circle in extreme positions), 4 is a constant current source, profiles A are shown in A tool electrode and diamond wheel. Figure 2 - diagram of the electroerosive effect on the diamond layer of the circle from a constant current source, where 2 is a correcting electrode-tool, 3 is a diamond grinding wheel, 5 is diamond grains, 6 is metal chips, 7 is an microarc discharge, 8 is an erosion trace .

The grinding method is as follows. The workpiece 1 is installed on the table of the machine, on which, outside the processing zone, a correction electrode-tool 2 is insulated from the table. The diamond grinding wheel 3 on the conductive bond and the electrode-tool 2 are connected to the corresponding poles of the DC source 4 and the value allowed by the conditions of the operation is set voltage U. The grinding wheel 3 is set to rotate at a speed of ν _k , and the reciprocating machine is informed of the table of the machine with the workpiece 1 and the electrode-tool 2 relocation S _ave The feed to the grinding depth t _m circle 3 is carried out for each double stroke of the table, i.e. processing is carried out according to the mortise grinding scheme.

For each double stroke of the table, the circle 3 provides contact with the electrode-tool 2, subjecting the profile of the circle to EDM and removing a layer of material equal to t _m from the electrode-tool 2.

A necessary condition for the electroerosive effect on the metal bond of the diamond-bearing layer of the grinding wheel is the discreteness of removal of the bond material. From the surface of the electrode-tool 2 (Fig.2) when it draws closer to the diamond grains 5 of the circle 3, metal chips 6 are removed, closing the electrodes when a direct current is applied to them. Under the influence of high temperatures developing at the site of contact with the chips, the chips are melted and a microarc discharge 7 occurs, leaving an erosion trace 8 on the diamond wheel bundle 3. An interelectrode liquid medium (usually an aqueous solution of borax or soda) is fed into the contact zone or fluid supply in air. The presence on the surface of the diamond wheel of a large number of grains and the high speed of their movement in contact with the electrode-tool provide effective electrical erosion of the material of the ligament of the circle. The erosion process is characterized by a high repetition rate of electrical impulses and their short duration. The erosion process is affected by the size and concentration of grains on the surface of the diamond wheel, the ligament material and the dimensions of the wheel.

The value of the ligament material removed when dressing the circle depends on the time of electroerosive action on the surface of the diamond wheel, and therefore on the length l _{e of the} grinding area of the electrode-tool 2, ceteris paribus.

To reduce the wear of the diamond layer of the circle when editing with the EDM method, the voltage U of the DC source is determined by the formula

,

,

where k _c is a coefficient depending on the type of ligament;

D _k - diameter (outer) of the grinding wheel;

Δ = (4-7)% Z _max is the thickness of the diamondiferous layer removed from the surface of the circle with a single exposure (for one double stroke of the machine table);

l _e - the length of the ruling electrode-tool;

Z is the number of diamond grains per 1 mm ^{2 of the} working surface of the circle;

η = 0.1-0.12 - coefficient determining the number of grains involved in cutting;

β = 0.85-0.95 - coefficient taking into account the likelihood of electrical discharges during chip short circuit;

k is the concentration of diamonds in the diamond-bearing layer of the grinding wheel.

Metal bonds of diamond wheels are multicomponent compositions, the formation of which is carried out by pressing and sintering in molds. The most common ligaments are in the form of tin bronzes, which are composed of copper and tin powders with various alloying additives. The performance of diamond wheels largely depends on the degree of “exposure”, opening of grains from a bunch when exposed to a bunch of electrical impulses. With insufficient exposure of grains from the bundle, “cutting” of the cutting surface of the wheel occurs, which leads to a sharp decrease in its working capacity, and with a high power of electrical pulses, the size of erosion holes and the destruction rate of the material of the bundle increase, which reduces the life of the diamond wheel. Under optimal processing conditions, the electroerosive dressing method provides diamond wheels with improved cutting properties.

Therefore, the value of the coefficient k _{c is} selected according to the data of table 1, compiled on the basis of experimental data.

Table 1 Coefficient k _c for various metal bundles of diamond wheels Ligament mark M1 MF MS1, MS3, MS6 MO4 MO16 G1 The values of k _c one 1.22 1.23 1.39 1.43 2,38

The thickness Δ of the diamond-bearing layer of the bundle removed from the surface of the circle with a single exposure (for one double stroke of the machine table) is determined by the ratio

Δ = (4 ... 7)% Z _max ,

where Z _max - the maximum size of diamond particles (grains) in the diamond layer of the circle (Z _max - the maximum size of diamond grains is indicated in the marking of the circle in micrometers).

A smaller range value is assigned for fine-grained circles, and a larger one for coarse-grained diamond circles. This is due to the size of the chips generated during cutting, which leads to the appearance of microarcs due to chip contact closure in the gap “circle - electrode-tool”.

The number Z of diamond grains per unit area of the working surface of the circle is determined by table 2 proposed by V. Bakul. (Institute of Superhard Materials, Academy of Sciences of the Ukrainian SSR).

table 2 The average number Z of grains per 1 cm ^{2 of the} cross section of the diamondiferous layer at 100% concentration, pcs. Powder granularity Brand of diamond ASD (AC2) ACP (AC4) DIA (AC6) ASK (AC15) ACC 630/500 - - - - 203 500/400 - - - 312 304 400/315 - - 486 473 461 315/250 - - 734 715 697 250/200 - 1210 1110 1080 1060 200/160 - 1820 1680 1600 1600 160/125 2920 2730 2540 2490 2420 125/100 4350 4100 3840 3760 3670 100/80 6460 6160 5780 5710 5550 80/63 9580 9240 8750 8650 8400 63/50 14200 13900 13200 13100 12700 50/40 21200 20900 20000 19900 19200

The number η of diamond grains involved in cutting is 10 ... 12% of the total number of grains located on the surface of the circle. Therefore, take the coefficient η = 0.1-0.12.

Taking into account the different heights of diamond grains protruding from the ligament on the surface of the circle and fluctuations in the size of the chips, the probability of discharges during chip closure according to experimental data is β = 0.85 ... 0.95.

The voltage U determined by the calculation allows the dosed effect of electric discharges on the ligament of the circle to be sufficient to restore the relief of the cutting surface of the circle and maintain profile accuracy with a minimum thickness of the removed layer of the ligament, which allows to increase the life of the circle.

The time t of the electroerosive effect on the surface of the diamond-bearing layer of the grinding wheel depends on the length l _{e of the} electrode-tool 2, which for surface grinding machines is l _e = 50 ... 100 mm. Therefore, depending on the speed of the machine table (the minimum table speed is 3 m / min, and the maximum 25 m / min), the time t for one working stroke is limited by the condition 0.2s≤t≤2s. At a value of t <0.2 s, the dressing process is unsatisfactory, which leads to a decrease in surface quality and the appearance of defects during grinding of parts, and at t> 2 s accelerated wear of the diamond layer of the wheel occurs, which is not accompanied by an improvement in the quality of processing.

EXAMPLE

Profile insert grinding of flat samples of T5K10 hard alloy was carried out on a ZE71V surface grinding machine with a diamond wheel 1A1 250 × 20 × 76 × 5 AC6 100/80 100M1-1. Electroerosive dressing of the circle was carried out after each double stroke of the part with the machine table with an electrode-tool made of LS59 brass with a length l _e = 90 mm using a constant current source. The coefficients and parameters for calculating the voltage U, V according to the empirical formula were determined as follows. The value of the coefficient k _c affecting the magnitude of the electric pulse during erosive destruction of the metal bond M1 of the circle, we find in table 1. For the adopted diamond wheel k _c = 1. The outer diameter of the circle in accordance with the marking was D _k = 250 mm. The thickness Δ of the diamond layer removed from the surface of the circle in one double stroke of the table is Δ = (4 ... 7)% Z _max (Z _max is the maximum size of diamond grains). For the accepted circle with a grain size of 100/80, the maximum grain size is Z _max = 100 μm. Then we take for the medium circle Δ = 6% Z _max = 0.06 · 100 μm = 6 μm.

The number Z of diamond grains per 1 mm ^{2 of the} working surface of the circle (cross-section of the diamondiferous layer) can be found in Table 2 for grade AC6 and grain size 100/80, which amounted to Z = 57.8. The length l _{e of the} leading electrode is taken l _e = 90 mm (for surface grinding machines l _e = 50 ... 100 mm). The values of the dimensionless coefficients η and β are taken η = 0.1-0.12 = 0.12, and β = 0.85-0.95 = 0.85, respectively.

The concentration of k% of diamond grains in the diamond layer in accordance with the marking was k = 100%.

Substituting the values in the formula, we find the magnitude of the voltage U, V:

Grinding modes: wheel speed v _K = 30 m / s; feed to the grinding depth t _m = 0.01 mm / dv.hod; the speed of the reciprocating motion of the table S _CR = 6 m / min The time of electroerosive impact on the surface of the circle in one double stroke of the part with the machine table was t = l _e / S _ol = 0.09 / 6 = 0.015 min = 0.9 s.

It was found that the life of the wheel increased by 1.5 times compared with the prototype, and grinding performance increased by 25 ... 30%.

Information sources

1. Auth. St. No. 804408 (USSR) M. cl. B24B 53/00. The way to edit a shaped circle with mortise grinding of parts / Filin A.N., Samarin Yu.P., Eremin A.V., bull. No. 6, 1981.

2. Auth. St. No. 331869 (USSR) M. cl. B23P 1/08. The method of electrical discharge machining / Gurvich R.A., Chaika G.V., bull. No. 10, 1972.

3. Auth. St. No. 1151396 (USSR) M. cl. B23H 1/00. The method of profile grinding / Dorofeev V.D., Kolchugin S.F., Yascheritsyn P.I., Martynov A.N., bull. No. 15, 1985.

Claims (1)

A method of grinding complex profile surfaces of a part, including grinding a part mounted on a table of a surface grinding machine, a rotating diamond wheel on a conductive bond during reciprocating movement of the table, and EDM of the grinding wheel with an electrode tool outside the processing zone, characterized in that the EDM editing of the working surface of the grinding wheel periodically after each double stroke of the part with the machine table using a constant source ka, and the magnitude of the voltage U is determined by the expression

,
where k _with is a coefficient depending on the type of ligament and equal to 1, 1.22, 1.39, 1.43 and 2.38, respectively, for ligaments M1, MF, MO4, MO16 and Zh1, and equal to 1.23 for ligaments MC1 , MS3, MS6;
D _k is the outer diameter of the grinding wheel;
Δ = (4-7)% Z _max - the thickness of the diamond layer removed from the surface of the circle with a single exposure in one double stroke of the machine table,
Z _max - the maximum size of diamond grains,
l _e - the length of the ruling electrode-tool,
Z is the number of diamond grains per 1 mm ^{2 of the} working surface of the circle,
η is a coefficient that determines the number of grains involved in cutting, amounting to 10-12% of the total number of grains located on the surface of the circle,
β = 0.85-0.95 - coefficient taking into account the likelihood of electrical discharges during chip short circuit,
k is the concentration of diamonds in the diamond layer of the circle.

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