METHOD FOR MANUFACTURING OF A RING BIT FOR A DRILLING DEVICE

The invention relates to a method for manufacturing of sleeve like ring bit of a hammering drilling device which bit is meant to be located and to be locked on a pilot bit which drills the middle part of the hole and which ring bit comprises bit buttons on its ring-shaped front surface, a ring groove on its outer surface and a skirt portion at the rear part of the sleeve like body which portion comprises surfaces, which transmit hammerings and/or rotating movement from the pilot bit to the ring bit.

Previously is known - for example from Finnish patent publication FI-115661 - a drilling device which is like the drilling device described above which comprises pilot bit and ring bit. In this solution the ring bit is manufactured from a preform by lathing and the forms of the skirt portion are made by milling. After the processing the ring bit is hardened.

In the manufacturing method described above the shaping of the skirt portion is limited by the possibilities of flake creating processing for achieving sharp edges and small crests. When the hardening is made to a pre-processed piece, the percussion surfaces of the skirt become hard everywhere and these respectively wear the hammering equivalent surfaces of the pilot bit for nothing. Only one hole is drilled with the ring bit, whereas the same pilot bit is used for creating dozens of holes. Thus no maximal hardness is required from the equivalent surface of the ring bit. In the hardening process of an unsymmetrical ring bit - regarding mass concentrations - a geometrical distortion is easily created, such as an elliptical form to a sleeve like piece.

In order to eliminate the above described disadvantages a new manufacturing method has been developed for which it is characteristic that two ring bits are manufactured from one preform at the first stage with a common skirt portion most suitably to the final measurements and the resulting piece is separated to two ring bits by cutting the piece along the skirt portion with a shape cutting to two ring bits which can be separated from each other.

The advantage of the method is to save raw material and to save machining which can be utilized even in the piece which can be obtained from the preform by lathing. Especially advantageous is the technique where the piece is made of a preform which is shaped close

to measurements by rolling. The symmetrical shape of the piece is advantageous in the hardening process with which the effect of the mass concentrations on the hardening can be avoided. When the ring bits are separated from the piece with a form cutting, for example with laser cutting, a releasing effect becomes created to the cutting surface which reduces the hardness of the surface. The reduction of the hardness of the equivalent surface is even desirable in the ring bit and it is an advantage whereupon the operating life of the ring bit becomes longer. Separation cutting as laser cutting or as corresponding cutting does not limit the shapes of the cutting line.

In the following the invention is discussed more detailed by referring to the accompanying drawing in which

Figure 1 shows two ring bits formed of the preform simultaneously as a side view. Figure 2 shows the pilot bit and the ring bit to be installed to its neck as a side view.

In the Figure 1 a piece is formed of a preform of a sleeve by rolling from a pre-shaped preform by lathing of which piece two similar ring bits Ia and Ib can be separated by shape cutting, as the line 2 shows. The piece is hardened when its shape is ready before the shape cutting. The piece is symmetrical in relation to the transverse axis and does not change its shape in the hardening process. Also the changes in measurements stay small. The raw material of the preform is saved when the length of the skirt portion 8 is clearly shorter than the skirt preforms of two ring bits together due to the form cutting. Grooves 7 are for attaching the drill shoe for the outer surface of the ring bit. Bit buttons are attached later to the holes 4.

So that one can separate the ring bits from each other by turning them after the form cutting, small areas 3 have been cut round and meant to be chopped away so that there will be enough looseness in order to turn loose the ring bits. Modem methods for performing form cutting after the hardening process are: laser cutting, gas cutting, high- pressure water jet cutting and plasma cutting. The most suitable of these are the cutting methods which create a little bit of heat, such as laser cutting, gas cutting and plasma cutting with which the hardness of the cut surface becomes reduced due to the releasing effect. Also the water jet cutting is of course suitable for this purpose.

Both the ring bit 1 and the pilot bit 6 are shown in the Figure 2. The surface which transmits the rotating movement 11, surfaces which transmit the hammerings 9 and 10 and a surface which transmits the possible extraction of the ring bit 12 belong to the skirt portion 8 of the ring bit. Correspondingly a flushing groove 13, a portion which transmits the rotating movement and the extraction 14 and the surface which transmits the hammerings 15 belong to the pilot bit.