Method for manufacturing machined part, and machined part

Abstract

[Task] To provide a method that makes it possible to easily, inexpensively and accurately manufacture a machined part with a simple configuration and an excellent strength. [Means for Solution] In a method for manufacturing a machined part according to the invention, a dislocation is partially introduced (S3), through shot peening, only into a surface layer of a raw material 1 in a region 1a to be machined, graphite is partially deposited (S4), through heating, only in the surface layer of the region 1a to be machined, and then, the surface layer of the region 1a where graphite is deposited is removed, through machining (S5), only by an amount corresponding to a working margin 1c, thereby manufacturing a machined part 1.

Claims

1. A method for manufacturing a machined part by cutting and removing a predetermined region of a raw material through machining, comprising: introducing a dislocation only into the region to be machined, forming graphite only in the region through heating, and after forming graphite in the region, machining the region where the graphite is formed, wherein the graphite is formed from carbon existing within the raw material.

2. The method for manufacturing the machined part according to claim 1, further comprising: pre-molding the region before introducing the dislocation.

3. The method for manufacturing the machined part according to claim 1, further comprising: carrying out quenching after machining.

4. The method for manufacturing the machined part according to claim 1, wherein introduction of the dislocation is carried out through shot peening.

5. The method for manufacturing the machined part according to claim 4, wherein a depth to which the dislocation is introduced into a surface layer of the region is controlled by changing a particle diameter of a projection material for shot peening, in introduction of the dislocation.

6. The method for manufacturing the machined part according to claim 1, further comprising: forming a geared tooth in the region through machining.

7. The method for manufacturing the machined part according to claim 6, further comprising: pre-molding a cylindrical raw material through hot forging, wherein the region into which the dislocation is introduced is a surface layer, the region where graphite is formed through heating is the surface layer, and the region where the geared tooth is formed through machining is the surface layer.

8. The method for manufacturing the machined part according to claim 6, further comprising: pre-molding a cylindrical raw material into a shape of a gear through hot forging, wherein the region into which the dislocation is introduced is the surface layer of a tooth mark of the pre-molded gear, the region where graphite is formed through heating is the surface layer of the pre-molded gear-shaped tooth mark, and the region where the geared tooth is formed through machining is the surface layer of the pre-molded gear-shaped tooth mark.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 includes a block diagram showing one embodiment of processes of a method for manufacturing a machined part according to the invention, and an illustrative view of a raw material in the respective processes.

(2) FIG. 2 is an illustrative view showing a roller pitching test as a fatigue strength test.

(3) FIG. 3 includes graphs showing a result of (a) a distribution of hardness with respect to the depth from a surface, and a result of the roller pitching test shown in FIG. 3, as to a sample manufactured on the basis of the invention and a sample manufactured according to the conventional art.

(4) FIG. 4 includes illustrative views showing a case where a predetermined region of the raw material is machined to form a tooth mark according to the invention.

(5) FIG. 5 includes an illustrative view (a) showing a pulsator test as a bending fatigue strength test, and a graph (b) showing a result of the pulsator test as to the sample manufactured on the basis of the invention and the sample manufactured according to the conventional art.

(6) FIG. 6, includes a block diagram showing one embodiment of processes of a method for manufacturing a machined part according to the conventional art, and an illustrative view of a raw material in the respective processes.

MODES FOR CARRYING OUT THE INVENTION

(7) First of all, the basic configuration of a method for manufacturing a machined part according to the invention will be described in detail on the basis of FIG. 1. Incidentally, like reference symbols denote like or equivalent regions respectively in the drawings.

(8) The method for manufacturing the machined part according to the invention is generally designed to machine a predetermined region 1a of a raw material 1. In this method, a dislocation is introduced (S3) into the region 1a to be machined before machining (S5) is carried out, and graphite is partially deposited (S4) in the region 1a to be machined, through heating. Incidentally, this embodiment of the invention will be described with reference to a case where shot peening is carried out in order to introduce the dislocation into the region 1a to be machined.

(9) The raw material 1 is molded into a cylindrical or rod-like shape (the cylindrical shape is used as a collective term for the rod-like shape as well in the invention) with a predetermined diameter. For example, it is possible to use a raw material where graphite is deposited through a graphitization treatment with 0.3 to 0.8 weight % of carbon (C). In manufacturing a machined part 1 through the use of this raw material 1, in this embodiment of the invention, the cylindrical raw material 1 is first heated to a predetermined temperature, and is pre-molded through hot forging (S2) into a shape close to the shape of the machined part to be manufactured, as shown in FIG. 1.

(10) Subsequently, the pre-molded raw material 1 is subjected, only in the region 1a to be machined later, to shot peening for projecting a projection material having a predetermined particle diameter. Thus, as indicated by (a) of FIG. 1, a residual stress (a strain) is produced partially and only in a surface layer of the raw material 1 in the region 1a to be machined later, and a dislocation as a graphite deposition nucleus is introduced thereinto. The depth to which the dislocation is introduced into this raw material 1 can be arbitrarily set, for example, to about 0.2 to 0.4 mm from the surface. In this manner, the depth to which the dislocation is introduced can be adjusted by appropriately changing the particle diameter, hardness, projection pressure, or coverage of the projection material for shot peening.

(11) Subsequently, the raw material 1 that has been subjected to shot peening is heated at a predetermined temperature for a predetermined time, and the region 1a into which the dislocation has been introduced is subjected to a graphitization treatment for depositing graphite (S4). As described above, the dislocation as the graphite deposition nucleus is partially introduced only into the surface layer of the raw material 1 in the region 1a to be machined. Therefore, as indicated by (b) of FIG. 1, graphite is also partially deposited only in the surface layer of the region 1a to be machined later.

(12) After that, the predetermined region 1a of the raw material 1 subjected to the graphitization treatment is subjected to, for example, cutting or the like (S5). Thus, as indicated by (c) of FIG. 1, since graphite is deposited in the raw material 1 in the region 1a to be machined, this region 1a exhibits good machinability. Besides, the deposited graphite is removed through machining, and hence does not remain in the machined part 1 as a product. Accordingly, the process of solid re-dissolution (S16) in the conventional art is unnecessary, so that removal machining such as cutting or the like can be carried out inexpensively, easily and accurately.

(13) Incidentally, the invention is not limited to the aforementioned embodiment thereof. In order to introduce the dislocation into the surface layer of the raw material 1 in the region 1a to be machined, a method other than shot peening may be adopted. For example, by subjecting the raw material 1 to cold serial machining, CBN cutting or the like in the predetermined region 1a to be machined, plastic flow of the surface of the raw material in the predetermined region 1a to be machined can be caused to produce a residual stress as well. In cold serial machining, a contact pressure is sequentially applied to the predetermined region 1a in a cold state. In CBN cutting, a contact pressure is applied to the surface of the region 1a of the raw material while this surface is cut to be pre-molded, with the aid of a cubic boron nitride sintered compact (a CBN sintered compact). Besides, in the method according to the invention, hot forging (S2 in FIG. 1) can be carried out if necessary. However, this process S2 is not indispensable, and can be omitted.

(14) Next, an experimental example for making a comparison between the fatigue strength of the machined part manufactured according to the method of the invention configured as described above and the fatigue strength of the machined part manufactured according to the conventional art (see FIG. 6) will be described on the basis of FIG. 2 and FIG. 3.

(15) In a roller pitching test shown in FIG. 2, each of a sample 1 of the machined part manufactured according to the invention and a sample 100 of the machined part manufactured according to the conventional art is molded such that a central evaluation portion thereof (which is equivalent to the region to be machined) 1a has a diameter of 26 mm and an axial length of 28 mm, and that shaft portions 1b thereof have a diameter of 24 mm at both ends and an axial length of 51 mm. The shaft portions 1b of each of these samples were pivotally supported, and a large roller R with a diameter of 130 mm was pressed against the evaluation portion 1a at a contact pressure of, for example, 200 to 3000 Mpa to cause rotation around an axis. Then, the number of rotations was checked until each of the samples 1 and 100 was damaged due to fatigue.

(16) Before conducting the roller pitching test, a distribution of the hardness with respect to the depth from the surface of each of the sample 1 manufactured according to the invention and the sample 100 manufactured according to the conventional art was measured. As a result, as indicated by (a) of FIG. 3, it has been revealed that the distribution of the hardness with respect to the depth from the surface is substantially identical in the sample 1 manufactured according to the invention and the sample 100 manufactured according to the conventional art. Then, the result obtained by conducting the roller pitching test using these samples 1 and 100 has revealed that the pitching life of the sample (the invention) 1 manufactured according to the invention is about twice as long as the pitching life of the sample (the conventional art) 100 manufactured according to the conventional art, as indicated by (b) of FIG. 3. This proves that the invention makes it possible to obtain the same hardness distribution as in the conventional art, and manufacture the machined part 1 with an excellent pitching strength.

(17) In this manner, the machined part 1 according to the invention is manufactured by introducing the dislocation into the region 1a, partially depositing graphite in the region 1a through heating, and removing, through machining, this region 1a where graphite is partially deposited. Thus, the machined part 1 that has been machined as the product according to the invention does not induce the creation of vacancies inside through solid re-dissolution of graphite as in the case of the conventional art, and consequently, has the same hardness distribution as in the conventional art, and a more excellent fatigue strength than in the conventional art. Besides, when machining is carried out, graphite is deposited in the predetermined region 1a of the raw material 1, so that this region 1a is easily and accurately machined. Furthermore, the process of solid re-dissolution (S16) in the conventional art is made unnecessary by removing the region 1a where graphite is deposited. It is therefore possible to realize a configuration in which the machined part 1 can be manufactured inexpensively and easily.

(18) Next, a more specific embodiment of the invention will be described mainly on the basis of FIG. 4, with reference to a case where a machined part 10 having a geared tooth G that has a tooth mark 10g formed in a predetermined region 10a of a raw material 10 is manufactured. Incidentally, configurational details identical or equivalent to those in the foregoing description are denoted by the same reference symbols respectively, and will not be described below. Only configurational details different from those described above will be described.

(19) In general, in a method for manufacturing the machined part 10 according to this embodiment of the invention, a gear-shaped tooth mark 10g is pre-molded from the cylindrical raw material 1 through hot forging, a dislocation is introduced into a surface layer of this pre-molded tooth mark 10g, graphite is deposited in the surface layer of the pre-molded gear-shaped tooth mark 10g through heating, and the surface layer where graphite is deposited is removed to form a tooth space by machining the tooth mark 10g, so that the tooth mark 10g of the geared tooth G is molded.

(20) In manufacturing the machined part 10 having the geared tooth G, the cylindrical raw material 10 having the predetermined diameter is first prepared (see S1 of FIG. 1). Then, the tooth mark 10g is pre-molded such that the region where the geared tooth G is molded later through machining assumes a shape close to the shape of the tooth mark 10g as a final product of this geared tooth through hot forging (see S2 of FIG. 1). The shape of the tooth mark 10g pre-molded at this time can be set as a shape obtained by adding a working margin (see (c) of FIG. 4) 10c to the shape of the tooth mark 10g of the geared tooth G that is to be molded as will be described later.

(21) Subsequently, as indicated by (a) of FIG. 4, a dislocation is introduced only into the surface layer of the pre-molded tooth mark 10g (see S3 of FIG. 1). As this introduction of the dislocation, any one of shot peening, cold serial machining, CBN cutting and the like can be adopted as described above. Besides, the depth of the surface layer to which the dislocation is introduced can be set to the depth of the working margin 10c. The pre-molded tooth mark 10g is molded in the predetermined region of the raw material 10, into a shape close to the shape of the tooth mark 10g of the geared tooth G to be molded. Therefore, the dislocation can be introduced, to an exact depth, into the surface layers of the addendum, tooth flank, dedendum and tooth bottom of each tooth mark 10g of the geared tooth G.

(22) Subsequently, the raw material 1 with the dislocation introduced into the surface layer of the tooth mark 10g pre-molded in the region to be machined is held at a predetermined temperature and heated to perform the graphitization treatment (see S4 of FIG. 1) for a predetermined time. Graphite is deposited only in the surface layer of the pre-molded tooth mark 10, as indicated by (b) of FIG. 4.

(23) After that, a tooth space is formed through removal machining of the predetermined working margin 10c as indicated by a chain line in (c) of FIG. 4, by subjecting the pre-molded region of the raw material 1 subjected to the graphitization treatment to machining such as cutting or the like. Thus, the geared tooth G having a tooth mark 10G of a predetermined shape is molded. In forming the tooth space through this cutting, good machinability is achieved because only the surface layer of the tooth mark 10g where graphite is deposited is removed. As a result, the tooth mark 10g can be molded easily and accurately.

(24) When the tooth mark 10g of the geared tooth G is molded through machining such as cutting or the like, the hardness of the tooth mark 10g is adjusted by subjecting the tooth mark 10g of this geared tooth G to induction outline quenching or the like.

(25) Next, an experimental result for making a comparison between the fatigue strength of the machined part 10 having the geared tooth G manufactured according to this embodiment of the invention configured as described above and the fatigue strength of the machined part 100 having the geared tooth G manufactured according to the conventional art (see FIG. 6) will be described on the basis of FIG. 5.

(26) An illustrative view of a pulsator test is indicated by (a) of FIG. 5, as the fatigue strength of the molded tooth mark. A test result of the gear of the machined part 10 manufactured according to the invention and the gear of the machined part 100 manufactured according to the conventional art is indicated by (b) of FIG. 5. As indicated by (a) of FIG. 5, the pulsator test is designed to bring a jig J into abutment on the molded tooth mark 10g, and to apply a repeated load to the tooth mark 10g through the use of a hydraulic servo or the like, via this jig J. The repeated load applied to this tooth mark 10g can be set, for example, within a range where the frequency is 60 Hz and the test load is 14 to 28 kN.

(27) In the tooth mark 10g molded according to the invention, no graphite is deposited, and hence, no vacancy is formed as a result of a solid re-dissolution treatment. On the other hand, in the machined part 100 molded according to the conventional art, graphite is entirely deposited, and hence, vacancies are formed in the entirety including the dedendum as a result of the solid re-dissolution treatment (see (d) of FIG. 6). Due to this difference, the fatigue limit (i.e., the bending fatigue strength of the dedendum of the tooth mark 10g) of the machined part 10 according to the invention can be made higher than the fatigue limit of the machined part 100 according to the conventional art.

(28) In this manner, the machined part 10 according to the invention is manufactured by pre-molding the tooth mark 10g from the cylindrical raw material 10 through hot forging, introducing a dislocation only into the surface layer of this pre-molded tooth mark 10g, depositing graphite only in the surface layer of the pre-molded tooth mark 10g through heating, removing, through machining, the surface layer of the tooth mark 10g where graphite is deposited to mold the tooth mark 10g, and then quenching this tooth mark 10g. Thus, the machined part 10 as the product according to the invention does not induce the creation of vacancies inside through solid re-dissolution of graphite as in the case of the conventional art, and has a martensite quenched structure due to quenching carried out after machining. Therefore, the surface of the tooth mark 10g has a predetermined hardness, and a more excellent fatigue limit is obtained than in the conventional art. Besides, at the time of machining, excellent machinability is achieved because the surface layer of the tooth mark 10g where graphite is deposited is removed. Moreover, since there is no need to solidly re-dissolve the deposited graphite as in the case of the conventional art, the tooth mark 10g is molded inexpensively, easily and accurately.

INDUSTRIAL APPLICABILITY

(29) The invention is not limited to cutting as the machining of a predetermined region of a raw material, but is also applicable to cases where other types of removal machining such as grinding and the like are carried out, as long as the graphite deposited only in a surface layer of the raw material is removed. Besides, the invention is also applicable to parts other than gears, as long as these parts are each molded by machining a predetermined region of a raw material.

DESCRIPTION OF REFERENCE SYMBOLS

(30) 1: RAW MATERIAL 1a: REGION TO BE MACHINED LATER 1: MACHINED PART 10: RAW MATERIAL 10g: PRE-MOLDED TOOTH MARK 10c: WORKING MARGIN 10: MACHINED PART 10g: TOOTH MARK OF MACHINED PART 100: RAW MATERIAL IN CONVENTIONAL ART 100: MACHINED PART ACCORDING TO CONVENTIONAL ART