Method and cutting insert for cutting screw threads in metal work pieces

#The present invention relates to a method and a cutting insert for threading. The method aims to produce a thread in a rotary work piece, wherein a threading insert, including at least one cutting tip, creates the shape of the thread through a number of passes along the work piece. The threading insert aims to create threads including two thread flanks in a work piece, by flank infeed and comprises at least one cutting tip provided with cutting edges.

#In known indexable threading inserts the cutting tip is in top view identical with the profile of the finished the thread, which means that at flank infeed, wear develops on the passive flank of the cutting tip due to abrasion from the work piece. Thereby these cutting inserts can not attain optimal tool life or reduce the time for processing at threading. At flank infeed a chip is obtained with rectangular cross section, which is easy to shape and handle in comparison with a chip from radial infeed, when the chip assumes a rigid V-shaped cross section. This fact together with a more efficient heat deduction from the cutting tip makes it possible to work with bigger chip thickness and to complete the thread with the same number of passes as by radial infeed. When conventional threading inserts are fed inwardly, parallel with one flank of the thread, the cutting edge facing said flank will not perform any cutting work but will only drag along said flank; something which can have an abrasive effect on the cutting edge and which can give bad surface finish on the thread flank. One has tried to feed the cutting tip at an angle which is smaller than the flank angle to avoid these negative effects, but then chips obtained from the side of the minor cutting edge have been thin and entangled, which has influenced tool life and surface finish.

#One object of the present invention is to provide a cutting insert for flank infeed, which has good tool life.

#FIG. 1 shows a threading insert according to the present invention in a top view.

#FIG. 2 shows the threading insert, in a side view.

#FIG. 3 shows the tip of the threading insert in engagement with a dashed work piece. FIG. 3 is similar to FIG. 3 but references additional characteristics of the insert tip.

#With reference to FIGS. 1 and 2 a cutting insert according to the present invention is designated by 10. The cutting insert preferably is made by coated cemented carbide. The cutting insert has a substantially triangular basic shape and comprises an upper and a lower side 11 and 12 respectively. The basic shape can also be square for example, such as is shown in FIG. 9. The sides 11 and 12 connect substantially perpendicularly to side surfaces 13, 14 and 15. The side surfaces form an acute angle of about 60.degree. with each other. The cutting insert has a central hole 16 for receiving a fastening device for clamping of the cutting insert against a cutting insert pocket of a holder, not shown. The cutting insert is provided with three substantially identical cutting corners 17 in the areas for the imaginary intersection of the side surfaces. The cutting insert corner 17 comprises a cutting tip 18, a cutting edge which runs along the intersection between a chip surface 19 and a clearance surface 20, a topping surface 21 and a broken chip breaker rim 22. The cutting insert has a center axis CL and each cutting tip 18 has a bisector B. The bisector is substantially parallel with the associated edge surface 15 and extends on the side of the center axis at a distance L. The distance is 10 to 30% of the imaginary edge length of the cutting insert.

#In FIG. 3 an enlarged cutting corner is shown, which engages a work piece, the thread 23 of which is dashed. The cutting edge consists of a number of segments and includes an active major cutting edge 24, a nose edge 25, an active minor cutting edge 24A, a passive edge 26 and a topping edge 27. The cutting tip 18 of the cutting corner projects a distance L.sub.1, measured from a line touching the topping surface 27 to the extreme tip of the nose edge 25. The major cutting edge is provided on one side of the bisector and the minor cutting edge on the other side of the bisector while the nose edge cutting intersects the bisector. The major cutting edge 24 forms an acute angle .alpha., of about 25 to 30.degree., with the bisector B. The major cutting edge softly connects to the nose edge 25, which substantially is defined by a radius r, of about 0.05 to 1.5 mm. The nose edge further softly connects preferably to the active minor cutting edge 24A or directly to the passive cutting edge 26. The minor cutting edge connects to the passive cutting edge at a the perpendicular distance L.sub.2 from a tangent T of the nose edge in the extreme points of the nose edge. The distance L.sub.2 is bigger than or the same as the maximum depth of infeed per pass, but less than the half of the distance L.sub.1 and preferably less than 2.5 mm and is preferably of the size of the radius r. Also the minor cutting edge or the part of the nose edge which is closest to the passive cutting edge forms the angle .alpha. with the bisector. The passive cutting edge 26 forms an angle .beta. with the bisector, which is smaller than the angle .alpha. by an angle .delta.. The angle .delta. is 4 to 8.degree., preferably about 5.degree., less than the angle .alpha.. The passive cutting edge connects to the topping edge 27 via a concave substantially passive, curved edge 28, defined by a radius R, the size of which depends on the actual thread profile but the radius r is bigger than the radius of the nose edge. The cutting tip thus has a passive edge 26, which at least partly is provided on one side of an imaginary line S extending parallel with the direction of infeed F for the cutting insert and touching the cutting tip 18, said side facing away from the connected thread flank 23. The line S touches the nose edge 25 as well as intersects the curved cutting edge 28.

#From the foregoing, it will be appreciated that the nose edge 25 constitutes a first portion of the cutting edge. A section of that first portion 25 is disposed to the right side of the bisector in FIG. 3 and is adapted to cut one thread flank 23A. The active cutting edge 24 constitutes a second portion of the cutting edge disposed on the left side of the bisector and is adapted to cut another thread flank. The passive edge 26 constitutes a third portion of the cutting edge and extends rearwardly from a rear end of the first portion 25 recessed inwardly with respect to the imaginary reference line S.