Cutting insert

This application is the US national phase of PCT application PCT/DE2006/001222, filed 14 Jul. 2006, published 15 Feb. 2007 as WO 2007/016890, and claiming the priority of German patent application 202005012615.2 itself filed 8 Aug. 2005, whose entire disclosures are herewith incorporated by reference.

The invention relates to a cutting insert, in particular for crankshaft machining, with two planar end faces parallel to each other and each traversed by a bore for receiving a mounting bolt and both of which merge on their short side edges into rounded edge faces that are each bounded at the side by cutting edges.

Such cutting inserts are used for example in the external milling of crankshafts, in particular for the machining of the undercut on the crank pin. In EP 0 830 228 [U.S. Pat. No. 6,374,472], an external milling disk is described that is driven in such a way that machining is done at cutting speeds of over 160 m/min. In doing so, both the crankshaft and the external milling disk are rotated. Tangentially clamped cutting inserts are used for the machining of the cylindrical surfaces of the rotated workpiece, for example the crankpin surface, while radially clamped cutting inserts are used for the machining of the cheeks, an oil flange and an undercut. Such cutting inserts have the structure described above, each with rounded cutting edges that have an at least essentially uniform curvature about an angle of approximately 135° and that merge here immediately to a straight segment, before the cutting edge merges via a convex curvature into the opposite end face.

For the machining of crankshafts, turning tools are known in the prior art that are used in the so-called turn-broach or turn-turn-broach method. In turn broaching, a straight turn broach tool is advanced radially onto the rotating workpiece to be machined. In turn-turn broaching, multiple cutting inserts in series are positioned around a reference circumference of a disk-shaped tool carrier and are gradually steadily indexed along an initial section of the disk circumference. This tool is pivoted inward along a reference arc in the radial direction onto the rotated tool, as described in principle in EP 0 313 644 [U.S. Pat. No. 4,790,698] or EP 0 286 771. On these bar- or disk-shaped tools as well, two geometrically different types of cutting inserts are used for the pin machining and the formation of the undercut.

Beside the methods described above, so-called internal milling is also known in the prior art, in which the work is likewise done using a disk-shaped milling tool, but with the proviso that the cutting inserts are positioned on the inner periphery of an annular disk.

The tool costs are comprised of multiple components. These include in particular those tool costs that are determined not only by the pure manufacturing costs, but also by the tool life. In addition, machining times and the costs of tool replacement are also included. So-called indexable inserts have multiple usable cutting blades, although the number of them is limited by the design. For instance, the number of usable cutting edges in a cutting insert mentioned at the start is limited to four.

The object of the present invention is to design this cutting insert further in such a way that its cutting edge shape enables expanded machining possibilities.

This object is attained by a cutting insert according to the invention in that at least some of the cutting edges are formed with steps and have two convex sections and one concave section between them, with the outer convex sections extending over an angle of 180°.

Using such a cutting edge shape, steps such as an oil flange of a crankshaft can be cut, while at the same time a cheek, the oil flange and also the pin bearing corner or an undercut can be shaped.

Thus preferably two respective pairs of stepped cutting edges are provided, so that the cutting insert is rotationally symmetrical about a perpendicular axis when rotated through 180°. This results in a total of four congruent cutting edges that can be used one after another.

For cutting-edge stabilization, a chamfer is provided along the cutting edge that is preferably positioned with a (negative) chamfer angle of −15° and/or with a chamfer width of 0.1 mm to 0.2 mm.

The rake angle, in particular the rake angle adjacent the chamfer is 0° to 20°, and is preferably +10°.

For better support of the cutting insert in its seat fit, the cutting insert has planar side faces in a middle area perpendicular to the planar end faces.

As already mentioned above, the radius of curvature of the cutting edge can be selected in such a way that a stepped shape of the desired dimensions is cut in one operation using this cutting edge. In particular for crankshaft machining, the radius of curvature in the concave area of the cutting edge is selected at 1.5±0.1 mm, and in the convex area of the cutting edge at 1.5±0.1 mm on one side and at 1.4±0.1 mm on the other side. However, other cutting-edge radiuses of curvature can also be used according to the invention.

Likewise, a cutting insert design can preferably be selected in which a common tangent on the convex sections forms an angle of 35°±5° with the end faces.

According to a further embodiment of the invention, the cutting edge area extending over 180° merges via a straight cutting edge piece into the adjacent end face, with this cutting edge piece forming an angle of ≦5° with the end face.

Finally, according to a preferred embodiment of the invention, the convex and concave cutting-edge sections are slanted at an angle of up to 20°, preferably 10°, to a longitudinal central axis of the cutting insert.

Further embodiments and advantages of the cutting insert are shown in the drawings. Therein:

FIG. 1 is a top view of a cutting insert according to the invention;

FIG. 2 is a side view (with a top view of the cutting edges);

FIG. 3 is an enlarged view of a cutting edge; and

FIG. 4 is a partial section through a cutting edge.

The cutting insert shown in the figures has two planar end faces 10 and 11 extending parallel to each other and traversed by a throughgoing bore 12 . This bore 12 serves to receive a mounting bolt that secures the cutting insert radially or laterally to a milling disk. The end faces 10 and 11 merge at their short side edges into rounded edge faces that are bounded at their ends by cutting edges. The cutting edges are shaped in steps and each have two convex cutting edge sections 13 and 14 and one concave cutting edge section 15 between them. Radii R 1 and R 3 for the convex cutting edge sections can be the same, e.g. 1.5 mm, or different. The same also applies to a radius R 2 of the concave cutting edge section 15 .

The cutting insert has a chamfer 16 that extends between the cutting edge sections 13 and 15 . This chamfer 16 is set at a chamfer angle a of −15°. A rake angle b also shown in FIG. 4 is +10°. The chamfer width is 0.15 mm, for example.

In the middle, perpendicular to the planar end faces 10 and 11 , there are planar side faces 17 and 18 .

In the specific shape depicted in the figures, a tangent 19 common to the two convex cutting edge sections 13 and 14 forms an angle of approximately 35° with end faces 10 and 11 . Also visible in FIG. 3 is a tangent 20 that extends from the outgoing section of the cutting edge section 13 where it merges with the respective end face. This tangent 20 forms an angle of approximately 4° with the end face 10 or 11 .

According to the illustration of FIG. 1 , cutting edges 13 to 15 are angled with respect to a longitudinal central axis 21 of the cutting insert at an angle of inclination c, preferably up to 10°.

The cutting insert has a total of four actively usable cutting edges, with which in particular stepped oil flange shapes can be machined, with either of outgoing sections 22 or 23 serving in the manufacture of the cheek contour, depending on how the cutting insert is mounted.

The prescribed cutting insert can have chip-shaping and -guiding elements in the form of depressions or bumps on the cutting surface. The cutting insert consists of a hard metal or cermet material that can also be coated, if necessary. Typical coating materials are carbides, nitrides, oxides of the IVa to VIa metals and aluminum oxide, or else diamond coatings.