Method of manufacturing chisel for impact tool

Abstract: A method of manufacturing a durable chisel. An axial hole (2) is formed in a chisel (1) so as to open at its tip. Non-metallic powder (3) is charged into the axial hole. A core member (4) made of cemented carbide or other material harder than the chisel body is press-fitted into the axial hole (2). By inserting the core member (4) into the hole (2), the non-metallic powder (3) flows to fill up space between the rear end face of the core member (4) and the closed end of the axial hole.

Description: This invention relates to a method of manufacturing a chisel for an impact tool and to a chisel for use in an impact tool. There is known a type of impact tool used to crush or break such objects as rock beds and concrete by striking them with a chisel mounted on one end of a cylinder with a piston reciprocating in the cylinder. One problem of this type of impact tool is that the chisel tends to be worn quickly at its tip by being repeatedly struck against the objects to be crushed. In order to improve the durability of such a chisel, Examined Japanese Utility Model Publication 59-90581 proposes to press-fit a core member made of a material harder than the chisel into an axial hole formed in the chisel and opening at its tip. In such a chisel, impacts are applied first to the chisel body and then transmitted to the core member. Thus, it is essential that the rear end face of the core member be in tight contact with the closed end of the axial hole formed in the chisel body so that the two members will act as one unit. Otherwise, stresses resulting from impact forces would concentrate on part of the non-contact portion, causing hardening of the contact and non-contact surfaces. Such hardened surfaces tend to sustain fatigue damage due to metal fatigue. In order to solve this problem, Examined Japanese Patent Publication 3-73430 proposes to interpose a packing made of a soft metal such as lead between the closed end of the axial hole in the chisel and the rear end face of the core member. This improved chisel has, however, a problem in that, due to unavoidable dimensional errors for the hole in the chisel, the core member and the soft metal packing, it is very difficult to bring the rear end face of the core member into tight contact with the closed end of the hole. Rather, in the initial stage of use of the chisel, the core member is likely to be in partial contact with the chisel body, so that the core member and the chisel body tend to be easily damaged. Also, when press-fitting the core member into the hole in the chisel with the soft metal packing placed in the hole, the packing tends to be displaced to one side. This causes the rear end face of the core member to be brought into partial contact with the closed end face of the hole. Thus, impacts applied to the core member tend to concentrate on the partial contact portion. The core member is thus subjected to unduly strong local forces, so that it can be easily damaged. Moreover, in order to bring the rear end face of the core member into sufficiently tight and uniform contact with the closed end face of the hole formed in the chisel body, both of them have to be formed semispherically. Formation of such semispherical surfaces requires a long time and high manufacturing cost. An object of the invention is to provide a method of manufacturing a chisel which makes it possible to bring the rear end face of the core member into tight contact with the closed end face of the hole formed in the chisel body once the core member is inserted into the hole. According to this invention, there is provided a method of manufacturing a chisel for use in an impact tool, the method comprising the steps of charging heat-resistant, non-metallic powder into an axial hole formed in a chisel body and having an open end at the tip of the chisel body, and press-fitting a core member which is harder than the chisel body into the axial hole. The chisel according to the present invention comprises a chisel body formed with an axial hole having an open end at the tip of the chisel body, and a core member press-fit in the axial hole, the core member being more wear-resistant than the chisel body, characterized in that a non-metallic powder is charged in the axial hole. The non-metallic powder may be graphite powder or talc powder. This powder may be used singly or used after kneading with additives. By inserting the core member into the hole, the non-metallic powder will move in the hole so as to completely fill up any and all space between the rear end face of the core member and the closed end of the hole with uniform density. Thus, the rear end face of the core member 4 is brought into tight contact with the closed end of the hole through the non-metallic powder layer. Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: Fig. 1 is a sectional view of a chisel manufactured by the method according to this invention; and Fig. 2 is a sectional view of the same showing how it is assembled. Now referring to the drawings, we shall describe one embodiment of this invention. As shown in Fig. 1, the chisel body 1 has an axial hole 2 which opens to the tip of the chisel body. Heat-resistant non-metallic powder 3 such as graphite or talc powder is charged in the hole 2. Also, a core member 4 made of a wear-resistant material such as cemented carbide and tool steel is press-fitted in the hole 2. The corner of the closed end of the hole 2 is rounded off to reduce stress concentration. For the same purpose, the rear end face of the core member 4 is similarly rounded off. To assemble the chisel, as shown in Fig. 2, the non-metallic powder 3 is supplied into the hole 2 in the chisel body 1, and then the core member 4 is press-fitted into the hole 2. It is preferable to apply a lubricant such as molybdenum disulfide to the inner surface of the hole 2 so that the core member 4 can be easily inserted or pressed into the hole 2. An axial groove 5 is formed in the surface of the core member 4. It serves as a vent hole through which the air in the hole 2 can escape when the core member 4 is inserted into the hole 2. By inserting the core member 4 into the hole 2, the non-metallic powder 3 moves in the hole 2 under the pressure transmitted from the rear end face of the core member 4 so as to completely fill up any and all voids between the rear end face of the core member 4 and the closed end of the hole 2 with uniform density. Thus, the rear end face of the core member 4 is brought into tight contact with the closed end of the hole 2 through the non-metallic powder 3 without any gap therebetween. Since the non-metallic powder is freely movable in the hole 2, it can flow into and fill up any small spaces that may be formed between the outer peripheral surface of the core member 4 and the inner surface of the hole 2 due to dimensional errors of these members. Thus, impacts applied to the core member 4 are borne on the entire area of the closed end face of the hole 2. Namely, no local concentration of stress will occur when impacts are transmitted from the chisel body 1 to the core member 4. According to this invention, simply by press-fitting the core member into the hole formed in the chisel body, it is possible to bring the rear end face of the core member into tight contact with the closed end of the hole through the non-metallic powder layer. This makes it possible to reduce damage to the core member and the chisel body and thus to prolong the life of the chisel. Since the non-metallic powder is freely movable in the hole so as to fill up the space between the rear end face of the core member and the closed end face of the hole, the rear end face and the closed end face do not have to have any special shapes. Also, such non-metallic powder fills up any small gap formed between the core member and the hole in the chisel body due to dimensional errors of these members, thus preventing the core member from rattling in the hole when assembled. Thus, the outer surface of the core member and the inner surface of the hole do not have to be finished with so high accuracy, so that these members can be manufactured at a lower cost.

Claims: A method of manufacturing a chisel for use in an impact tool, said method comprising the steps of charging a heat-resistant, non-metallic powder (3) into an axial hole (2) formed in a chisel body (1) and having an open end at the tip of the chisel body (1), and press-fitting a core member (4) which is more wear-resistant than said chisel body (1) into said axial hole (2). A method of manufacturing a chisel for use in an impact tool as claimed in claim 1 wherein said non-metallic powder (3) is graphite powder or talc powder or a mixture of graphite powder or talc powder and an additive. A chisel for use in an impact tool comprising a chisel body (1) formed with an axial hole (2) having an open end at the tip of said chisel body (1), and a core member (4) press-fit in said axial hole (2), said core member (4) being more wear-resistant than said chisel body (1), characterized in that a non-metallic powder (3) is charged in said axial hole. A chisel for use in an impact tool as claimed in claim 3, wherein said non-metallic powder (3) is graphite powder or talc powder or a mixture of graphite powder or talc powder and an additive.

Inventors: NAKAI TETSUJI, OKADA HIROSHI, NAKAI, TETSUJI, OKADA, HIROSHI, Nakai, Tetsuji, c/o Nabari Factory of, Okada, Hiroshi, c/o Nabari Factory of
Applicants: NIPPON PNEUMATIC MFG, NIPPON PNEUMATIC MANUFACTURING CO. LTD.
Application year: 1995
Main classification: B23P15
IPC codes: E21B10, B23P11, B25D17, F16B4, B23P15