Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2000-01-18
2004-05-18
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C423S290000
Reexamination Certificate
active
06737377
ABSTRACT:
TECHNICAL FIELD
This invention relates to a milling cutter suitable for high speed cutting with a long service life, such as a face mill or end mill for a work-piece of cast iron or steel, and a cutting tool suitable for precision cutting of ferrous materials, and a process for the production of the same.
BACKGROUND TECHNIQUE
In the production of an engine or a driving part of a car, or parts used in electric appliances, high speed steel tools, cemented carbide tools, coating tools, ceramic tools or cubic boron nitride sintered compact tools (which will hereinafter be referred to as “cBN tool”) have been used for face milling cutters or end mills for cutting cast irons or steels as the materials of the parts.
The cutting speed of a cemented carbide tool or coating tool as the face milling cutter for cutting cast irons is 150 to 250 m/min and the cutting speed practically used in ceramic tools about 400 m/min. On the other hand, in a cBN tool excellent in wear resistance as well as high speed cutting property, a cutting speed of 500 to 1500 m/min is possible by dry process, as proposed in JP-A-8-141822. When a workpiece is markedly subject to influences such as deformation or strain by heat generated during dry process cutting, or when such a part is treated that slight deformation due to heat is considered as a problem, however, cutting must have been effected while decreasing the cutting speed to such an extent that any deformation due to heat does not occur by wet process cutting using a cutting fluid and thermal cracks do not occur at the cutting edge of the cBN tool. That is to say, in the case of wet process cutting, a practically used range for the cutting speed should be 500 to 700 m/min and at a cutting speed exceeding this range, thermal cracks occur at the cutting edge of the tool to remarkably decrease the tool service life. This is due to that in dry process cutting using no cutting fluid, the temperature difference of the heat cycle is so small that the edge part of the cBN tool can resist thermal shock, while in high speed cutting by wet process, the cutting edge at a high temperature during contacting with a work-piece is rapidly cooled during air cutting, so that thermal cracks occur by the heat cycle imparted to the cutting edge.
The practically used cutting speed of a cemented carbide tool or coating tool as a face milling cutter for cutting steels is about 50 to 200 m/min. At a higher speed than this range, the cutting edge encounters rapid wearing or breakage to markedly decrease the tool life. In the case of a cBN tool, cutting is possible at a cutting speed comparable to that of the cemented carbide tool, but the cBN tool has not practically been used as the face milling cutter for cutting steels, since the cBN tool has an equal tool life to the cemented carbide and at a higher speed cutting than this range, the cutting edge encounters breakage due to lowering of the strength of the sintered compact with increase of the temperature of the cutting edge, and occurrence of thermal cracks resulting in marked lowering of the tool life.
The practically used cutting speed of a cemented carbide tool or coating tool as an end mill for cutting cast iron is about 30 to 150 m/min. In a cBN tool, on the other hand, a cutting speed of 100 to 1500 m/min is possible by dry process. By wet process, however, a cutting speed of 100 to 300 m/min is practically used, and at a cutting speed of more than this range, thermal cracks occurs on the cutting edge to markedly lower the tool life in the similar manner to the face milling cutter.
The practically used cutting speed of a cemented carbide tool or coating tool as an end mill for cutting steels is about 30 to 100 m/min. Under a condition of a relatively low cutting speed in a cBN tool, a tool life comparable to that of the cemented carbide tool is only obtained in the similar manner to the face milling cutter, and at a higher speed cutting, the cutting edge of a cBN tool encounters breakage due to lowering of the strength of the sintered compact with increase of the temperature of the cutting edge, and occurrence of thermal cracks resulting in marked lowering of the tool life, so that the cBN tool has not practically been used as the end mill for cutting steels,
The lowering of the service life of a cBN tool under the above described conditions is probably due to the following reasons. A cBN compact of the prior art is obtained by sintering cBN powder grains with a binder such as TiN, TiC, Co, etc. at an ultra-high pressure and contains 10 to 60 volume % of the binder. Since the cBN compact has a thermal conductivity of less than 200 W/m·K and a thermal expansion coefficient at 20 to 600° C. of at least 4.0×10
−6
/K, it is considered due to that a larger temperature gradient is caused in the vicinity of a cutting edge by the lower thermal conductivity for the temperature difference of a heat cycle during cutting steels or wet process cutting cast irons, a higher tensile stress is caused on the cutting edge during cooling and furthermore, larger extent expansions and shrinkages are repeated to readily cause thermal cracks by the higher thermal expansion coefficient. In addition, it is considered due to that even if the transverse rupture strength as the bending strength at room temperature is at least 80 kgf/mm
2
, the transverse rupture strength ia rapidly lowered at a temperature of at least 800° C.
Therefore, to this end, a tool is required which does not contain any binder at its edge part, does have a high thermal conductivity and low thermal expansion coefficient and does not meet with decrease of the strength even at a high temperature.
On the other hand, lately, requirements for high precision finishing cutting working of high hardness ferrous materials are increasing. For the precision working of the ferrous materials, single crystal diamond and single crystal cubic boron nitride have been investigated.
In the case of cutting a ferrous material by single crystal diamond, however, there arises a problem that a chemical reaction of diamond and iron takes place by cutting heat, resulting in rapid wear of the diamond tool and thus, direct working of a metallic mold of steels, etc. is impossible. Accordingly, in the precision working of a metallic mold for a lens, for example, a method comprising applying an electroless nickel plating layer and precisely finishing the plated layer has been adopted, but this method meet with such a problem that the strength of the metallic mold is not sufficient and the process is complicated. The direct working has been investigated based on a method for suppressing chemical reactions using a special atmosphere, but this is not practical.
Cubic boron nitride (cBN) is a material having a hardness next to diamond and high thermal and chemical stability, whose reactivity with ferrous metals is low. However, a cBN compact having at present been used as a cutting tool is obtained by sintering cBN powder grains with a binder such as TiN, TiC, Co, etc. at an ultra-high pressure and contains 10 to 60 volume % of the binder, as described above. Thus, during shaping the cutting edge, a fine chipping edge tends to occur and it is very difficult to sharply finish the cutting edge without chipping of the edge, so that use thereof as a precision cutting tool be difficult. In order to solve this problem, it is necessary to prepare a tool from a single crystal of cBN or free from a binder. Therefore, a trial for preparing a single crystal of cBN and using as a cutting tool for ultra-precision working of steels has been made, but synthesis of a large-sized cBN single crystal with less impurities and defects is very difficult and a cBN single crystal has a number of cleavage planes, so that the strength is low and the wear resistance is not sufficient. Accordingly, the cBN single crystal has not been put to practical use up to the present time.
As apparent from the foregoing illustrations, it is considered that both of a cutting tool for subjecting cast irons or steels to high speed milling wo
Sumiya Hitoshi
Uesaka Shinya
Group Karl
McDermott & Will & Emery
Sumitomo Electric Industries Ltd.
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