Ceramic milling cutter

Abstract

A milling device is rotatable in one direction around a longitudinal center axis defining a forward direction and an opposite rearward direction, and includes a front part and a rear part. The front part has cutting edges, each having a longitudinal extension, and chip flutes, each having a longitudinal extension. The front part is made of a monolithic piece of ceramic. The rear part is configured to be fixed in a rotatable tool body or a rotatable chuck. The rear part is also made of a monolithic piece of cemented carbide. A front end surface of the rear part has a smaller area than a rear end surface of the front part. The front end surface of the rear part and a rear end surface of the front part are permanently bonded or brazed to each other by a joint.

Claims

1. A method to produce a milling device, comprising the steps of: supplying a rear part of the milling device, the rear part being made of a monolithic piece of cemented carbide, the rear part having a longitudinal center axis defining a forward direction and an opposite rearward direction grinding a longitudinal front end surface of the rear part; blasting the front end surface of the rear part; cleaning the longitudinal front end surface of the rear part; supplying a cylindrically shaped front part made of a monolithic piece of ceramic having a circular surface defining a rear end surface of the front part, an area of the rear end surface of the front part being greater than an area of the front end surface of the rear part; cleaning the rear end surface of the front part; placing the front part such that the longitudinal center axis is in a vertical position; applying an active silver brazing filler metal on the front end surface of the rear part; positioning the front end surface of the rear part against the rear end surface of the front part; heating the front part, the rear part and the filler material to a temperature of 140-160 C. for 8-12 minutes; heating the front part, the rear part and the filler material to a temperature of 740-760 C. for 22-26 minutes; and grinding the front part in such a way that cutting edges and chip flutes are formed in the front part.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a perspective view of a milling device according to a first embodiment of the invention.

(2) FIG. 2 is another perspective view of the milling device in FIG. 1.

(3) FIG. 3 is side view of the milling device in FIG. 1.

(4) FIG. 4 is a front view of the milling device in FIG. 1.

(5) FIG. 5 is top view of the milling device in FIG. 1.

(6) FIG. 6 is rear view of the milling device in FIG. 1.

(7) FIG. 7 is a side view of a milling device according to a second embodiment of the invention.

(8) FIG. 8 is a perspective view of the milling device in FIG. 7.

DETAILED DESCRIPTION OF DIFFERENT EMBODIMENTS

(9) FIGS. 1-6 show a milling device (1) according to a first embodiment, which show an exchangeable head type of milling device (1). The milling device (1) is rotatable in one direction around a longitudinal center axis (A) defining a forward or front direction and an opposite rearward or rear direction. The milling device (1) comprises a front part (3) and a rear part (2) which are bonded or brazed to each other by a joint. As can be seen in FIG. 3, the front part (3) extends only in the forward or front direction relative to the joint, and the rear part (2) extends only in the opposite direction relative to the joint, which is the rearward or rear direction. Further away than the rear part (2) in the rear or rearward direction is a rotating machine tool spindle (not shown), which is part of a machine tool such as a machining center or other CNC-machine (not shown). The milling device (1) can be connected to the rotating machine tool spindle by at least a rotatable tool body (not shown). The milling device is for machining metal work pieces, such as steel based or Ni-based materials. The front part (3) of the milling device (1) comprises two cutting edges (4, 4) each having a longitudinal extension of equal length. The two cutting edges (4, 4) each has a radial extension of equal length, which length extends from the longitudinal center axis (A) to a maximum radial dimension of the milling device (1). In other words, no part of the milling device (1) extends a larger radial distance from the longitudinal center axis (A) than the part of the cutting edges (4, 4) which has the largest radial distance from the longitudinal center axis (A). Each cutting edge (4, 4) has a shape which is an arch of a circle, more specifically a quarter of a circle. The cutting edges (4, 4) are oriented opposite each other, in such a way that in a front view, as can be seen in FIG. 4, the cutting edges (4, 4) form an angle of 180 relative to each other. Normal vectors of the rake faces adjoining each cutting edge (4, 4) are directed in opposite directions. Adjoining each cutting edge (4, 4) is a chip flute (5, 5) in the form of a cut-out or a cavity, which has a longitudinal extension. The chip flute (5, 5) is a space in which a chip can form and move after being formed. During cutting, the cutting edge (4, 4) forms a chip which will form chips which will come into contact with the rake face and the bottom of the chip flute (5, 5). The cutting will generate a rise of temperature of the milling device (1). The highest temperature will be located at or in the vicinity (<1 mm distance) of the cutting edge (4, 4). The contact with a moving chip will generate abrasive wear. The front part (3) is made of one monolithic piece of ceramic. Al2O3, Si3N4, SiAlON or a mix of these materials are examples of suitable ceramics. The ceramic may be whisker reinforced. Cubic boron nitride (CBN), diamond, cemented carbide, cermet and high-speed steel (HSS) are examples of materials which is not ceramic in the meaning of this document. All external surfaces of the front part (3) are formed by grinding. The front part (3) has a front end, where the cutting edges (4, 4) meet at the longitudinal center axis (A), and an opposite rear end which is in the form of a circle concentric with and perpendicular to the longitudinal center axis (A). The rear end of the front part (3) is permanently bonded or brazed to the front end of the rear part (2). The rear part (2) comprise flat radially external grip surfaces (7) oriented opposite each other, as can be seen in FIG. 5. When the milling device (1) is to be mounted or fixed in a seat in a rotatable tool body (not shown), a key or wrench (not shown) which has surfaces which is positioned against the grip surfaces (7) is turned in one direction, which causes a rotation of the milling device (1). The thread (6) has a corresponding thread in a rotatable tool body (not shown). The grip surfaces (7) are located between the thread (6) and the front end surface of the rear part (2). The front end of the rear part (2) is in the form of a surface which is perpendicular to the longitudinal center axis (A). The rear part (2) is made of a monolithic piece of cemented carbide, preferably comprising 80-95 volume % WC and 5-20 volume % Co, which is sintered after pressing. As can be seen in FIG. 3 and FIG. 5, there is a step at the joint. In other words, at the joint the rear end surface of the front part (3) extends a larger radial distance than the front end surface of the rear part (2) in such a way that a radial step is formed. The step has a length of 0.1-5 mm. The longitudinal extension of the rear part (2) is 8-30 mm. The longitudinal extension of the front part (3) is 20-60% of the longitudinal extension of the rear part (2). The maximum diameter of the front part (3) is 8-40 mm. The maximum diameter of the rear part (2) is 80-99.5% of the maximum diameter of the front part (3).

(10) FIGS. 7-8 show a milling device (1) according to a second embodiment, in the form of an end mill. It has a front part (3) which is identical to the front part (3) according to the first embodiment. Additionally, the joint and the front end of the rear part (2) are also identical to the first embodiment. The rear part (2) has the shape of a circular cylinder. The milling device (1) can be located in a cavity of e.g. a hydraulic chuck (not shown), and clamped therein by a hydraulic pressure. Additional arrangements are possible, such as a helical groove in the radial surface of the radial surface which mates with pull out preventing means inside the cavity of the hydraulic chuck. The longitudinal extension of the rear part (2) according to the second embodiment is 50-150 mm. All other properties are identical to the milling device (1) according to the first embodiment.

(11) The milling device (1) according to the first embodiment is produced according to the following method. A rear part (2) being made of a monolithic piece of cemented carbide is supplied, the rear part (2) having a longitudinal center axis (A) defining a forward direction and an opposite rearward direction. The grip surfaces (7) are formed solely by pressing and sintering. The thread (6) is by formed by pressing and sintering with a subsequent grinding operation. The longitudinally front end surface of the rear part (2) is ground in such a way that a front end surface of the rear part (2) which is perpendicular to the longitudinal center axis (A) is formed. The front end is blasted and cleaned with an alcohol. A cylindrically shaped front part (3) made of a monolithic piece of ceramic in the form of SiAlON having a circular surface defining a rear end surface of the front part (3) is supplied. The area of the rear end surface of the front part (3) is greater than area of the front end surface of the rear part (2). The rear end surface of the front part (3) is cleaned with an alcohol. The front part (2) is placed such that the longitudinal center axis (A) is in a vertical position. A paste in the form of active silver brazing filler metal is applied on the front end surface of the rear part (2). The active silver brazing filler metal comprise 58-62 wt. % Ag, 23-25 wt. % Cu, 13-13 wt. % In and 1.5-2.5 wt. % Ti. The trade name is TB-629T from the company Tokyo Braze Co Ltd. The front end surface of the rear part (2) is positioned against the rear end surface of the front part (3). Because the front part (3) has a larger rear end surface, the positioning does not need to be absolutely accurate. In other words, the longitudinal center axis of the front part (3) and the rear part (2) do not have to coincide. They only need to be parallel. The milling device (1), i.e. the front part (3), the rear part (2) and the filler material, is pre-heated in an environment of 140-160 C. for a time of 8-12 minutes. The front part (3) and the rear part (2) is then brazed to each other by heating the milling device (1) in an environment of a temperature of 740-760 C. in 22-26 minutes. Hereby the front part (3) and rear part (2) are permanently bonded to each other. After this, the front part (3) is ground in such a way that cutting edges (4, 4) and chip flutes (5, 5) are formed in the front part (2). Also, additional material of the front part (3) is ground such that the center of mass of the front part (3) is located at the longitudinal center axis (A).

(12) The milling device (1) according to the second embodiment is produced in a similar way as the milling device according to the first embodiment. The difference is that that the rear part (2) supplied has a cylindrical shape. It does not comprise grip surfaces or thread.