Milling method and use of a cutting insert

Abstract

A method for milling a workpiece by way of at least one substantially polygonal cutting insert, which is arranged in a tool holder. A spindle axis of the tool holder encloses an angle of more than 0° with a plane normal to a machined workpiece surface. An effective lead angle between a main cutting edge of the cutting insert and the machined workpiece surface lies between 0° and 20°.

Claims

1. A method for milling a turbine blade, the method comprising: providing at least one substantially polygonal cutting insert arranged in a tool holder, the tool holder having a spindle axis and the cutting insert having a main cutting edge, the at least one cutting insert is a substantially triangular indexable cutting insert or a substantially quadrangular indexable cutting insert or a substantially pentagonal indexable cutting insert, with the cutting insert inserted in the tool holder with a theoretical lead angle between a normal to the spindle axis and the main cutting edge of the cutting insert to between 20° and 40°; milling the turbine blade with the cutting insert held in the tool holder with a feed, simultaneous to a rotation of the tool holder about the spindle axis, in a feed direction oblique to the spindle axis of the tool holder and transverse to a plane normal of a machined turbine blade surface, and thereby: inclining the spindle axis for setting the spindle axis of the tool holder to enclose an angle of between 3° and 35° with a surface normal of the machined turbine blade surface and for setting an effective lead angle between a main cutting edge of the cutting insert and the machined turbine blade surface to lie between 5° and 20°.

2. The method according to claim 1, which comprises setting an axial cutting depth to less than 3.0 mm.

3. The method according to claim 1, which comprises setting a feed per tooth between 0.60 and 0.90 mm.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) The invention shall be explained more closely in the following by figures. There are shown:

(2) FIG. 1a-1b schematic representations of tool holders with cutting inserts

(3) FIG. 2 a diagram of lead angles for a round cutting insert

(4) FIG. 3a-3c polygonal cutting inserts in top view

(5) FIG. 4 a tool holder in a side view

(6) FIG. 5a-5b a schematic representation of the method

DESCRIPTION OF THE INVENTION

(7) FIG. 1a shows schematically an orientation of cutting inserts 2 on a suggested tool holder 3 relative to a workpiece 1, which would be positioned for a non-inclined spindle axis S. A feed direction F of the cutting insert 2 is indicated by a block arrow. The tool holder 3 has clockwise rotation, see the direction of rotation R. A plane normal N is perpendicular to the machined workpiece surface 4.

(8) For an angle of the spindle axis S to a machined workpiece surface 4 of 90° in the present case, corresponding to a camber or inclination of the spindle axis γ relative to the plane normal N of 0°, a theoretical lead angle κ.sub.th between a main cutting edge 5 and the machined workpiece surface 4 is obtained. Due to the curvature of the main cutting edge 5, the lead angle is determined on the chord of the main cutting edge 5. The chord extends between two ends of a main cutting edge 5.

(9) The lead angle κ.sub.th shown here is 30°, but this is too large and unsuited to a high feed cutting. The resulting axial cutting depth a.sub.p th from the lead angle κ.sub.th is too large to allow high feed rates.

(10) The broken line drawn on the workpiece 1 indicates the cross section of the shaving that will be removed during the next pass of a cutting insert 2.

(11) FIG. 1b shows a configuration as is provided for the method according to the invention:

(12) The spindle axis S here is inclined at an angle γ of around 20° to the plane normal N. Thus, there is an effective lead angle κ.sub.eff between the main cutting edge 5 and the workpiece surface of 10°.

(13) Owing to the lead angle κ being reduced due to the inclination of the spindle axis S, a feed per tooth f.sub.z can be increased. The superimposing of the tool-inherent lead angle κ.sub.th and the inclination of the spindle axis S at an angle γ results in an effective lead angle κ.sub.eff relative to the machined workpiece surface of
κ.sub.eff=κ.sub.th−γ

(14) where κ.sub.th is the theoretical or tool-inherent lead angle.

(15) For curved workpiece surfaces, the plane normal N can be situated at the radially inward point of attack of the particular cutting insert, as shown in FIG. 1b.

(16) The values shown here for the angle of inclination γ of the spindle axis S and the resulting lead angle κ between the main cutting edge 5 and the workpiece surface 4 are exemplary. Preferable values for the angle of inclination δ of the spindle axis S relative to the plane normal N lie in a range between 3° and 35°, more preferably between 10° and 30°, especially preferably between 15° and 25°.

(17) The effective lead angle κ.sub.eff between the main cutting edge 5 and the workpiece surface 4 results from the mounting position of the cutting insert 2 on the tool holder 3 and the angle of inclination γ of the spindle axis S.

(18) FIG. 2 shows a round cutting insert 2′, such as is not provided for the method according to the invention, in engagement with a workpiece 1.

(19) FIG. 2 shows two machining situations:

(20) in configuration I, the cutting insert 2′ is used with a large axial cutting depth a.sub.pI. The result is a large lead angle κ.sub.I.

(21) in configuration II, the cutting insert 2′ is used with a small axial cutting depth a.sub.pII. The result is a small lead angle κ.sub.II.

(22) For round cutting inserts, the lead angle increases with increasing cutting depth to take on 45° for the maximum cutting depth corresponding to the radius of the round cutting insert. Owing to the circular arc shape of the cutting edge, an actual lead angle of 0° is present at the lowest point and an actual lead angle of 90° is present at the maximum cutting depth corresponding to the radius.

(23) For round cutting inserts, the length of a cutting edge in engagement increases with increasing diameter of the cutting insert for a given cutting depth; the forces on the cutting edge decrease with increasing length.

(24) Since low cutting forces are desirable for high feed cutting, the largest possible diameters are preferred for high feed cutting with round cutting inserts. This also constitutes a significant limitation of round cutting inserts, since a radius of curvature of the cutting edge corresponds to the radius of the geometrical dimension. Thus, a large radius of curvature also means a large indexable insert.

(25) FIGS. 3a to 3c show schematically substantially polygonal cutting inserts 2, such as are provided for the method according to the invention, in top view.

(26) The cutting insert of FIG. 3a is formed with a square base shape (so-called S-plate) with convex rounded outer contour.

(27) Preferably, a cutting edge is formed on each rounded side edge of the cutting insert 2 (the main cutting edge 5 is emphasized here). Thus, in the case of the square base shape, a 4-fold indexable cutting insert is obtained. 4-fold indexable means that four independent main cutting edges 5 can be used for a machining. In this case, a new machining position is adjusted by rotating the cutting insert 2 through 90°. Thus, with a square base shape of the cutting insert 2, four independent main cutting edges 5 are obtained.

(28) To illustrate the shape of the cutting insert 2, an inscribed circle D.sub.IK is drawn. A radius of curvature R.sub.HS of a main cutting edge 5 is preferably at least 1.5 times greater than the radius of the inscribed circle D.sub.IK.

(29) Besides the cutting insert 2 shown here with quadrangular base shape, cutting inserts with substantially triangular base shape or pentagonal base shape can also be considered for the method according to the invention.

(30) FIG. 3b shows a cutting insert 2 with a substantially triangular base shape (a so-called T-plate).

(31) FIG. 3c shows a cutting insert 2 with a substantially pentagonal base shape (a so-called P-plate).

(32) Here as well, as a distinction from round cutting inserts, a radius of curvature of a main cutting edge is at least 1.5 times greater than a radius R.sub.IK of an inscribed circle D.sub.IK.

(33) For the method according to the invention, quadrangular plates (S-plates) had the most favorable ratio of usable cutting depth and number of indexing positions.

(34) FIG. 4 shows a tool holder 3 with a spindle axis S and a plurality of cutting inserts 2. The tool holder 3 is adjusted with respect to a machined workpiece surface 4 such that the spindle axis S stands perpendicular to a machined workpiece surface 4.

(35) A cutting insert 2 is mounted in the tool holder 3 in such a way that a theoretical lead angle κ.sub.th between 20° and 40° exists between a normal N.sub.S to the spindle axis S and a main cutting edge 5 of the cutting insert 2. The mounting position of the cutting insert 2 produces in the tool holder 3 a theoretical (tool-inherent) lead angle κ.sub.th between a main cutting edge 5 and a machined workpiece surface 4. The lead angle κ.sub.th shown here, however, would be too large and thus unsuited for a high feed cutting.

(36) Only owing to the method according to the invention, as illustrated in FIGS. 5a and 5b, can the configuration of tool holder 3 and cutting inserts 2 shown in FIG. 4 be used for a high feed cutting.

(37) FIG. 5a and FIG. 5b show representations of the method according to the invention for the milling of a workpiece 1, here, a turbine blade, in different views.

(38) A tool holder 3 comprises a plurality of substantially polygonal cutting inserts 2. In the present exemplary embodiment, the cutting inserts 2 have a square base shape. The spindle axis S of the tool holder 3 takes on an angle γ greater than 0° with respect to the plane normal N to the machined workpiece surface 4. Preferable values for the angle of inclination γ of the spindle axis S lie in a range between 3° and 35°, more preferably between 10° and 30°, especially preferably between 15° and 25°. In the side view of FIG. 5a it can be seen that the angle γ in the present example is around 20°. The angle γ is measured as positive in the feed direction F.

(39) The effective lead angle κ.sub.eff between a main cutting edge 5 of the cutting insert 2 and the machined workpiece surface 4 resulting from the inclination of the spindle axis S and the mounting position of a cutting insert 2 on the tool holder 3 is between 5° and 20°. In the present example, the effective lead angle κ.sub.eff is around 12°.

(40) As a reference for the angle values with respect to the machined workpiece surface 4, the radially inward point of attack of the cutting insert 2 in engagement is used (Detail A).

(41) The method allows a milling with high feeds. Typical values which are achieved in the method according to the invention are f.sub.z 0.60-0.90 mm/tooth. When using round plates of the prior art, on the contrary, only lower feeds of around 0.35-0.45 mm/tooth are possible.

(42) A further benefit in the use of polygonal cutting inserts is that greater cutting depths can also be realized, when a machining situation requires this: thus, for example, a cutting depth down to 5 mm is possible with reduced feed rates and without a tool change. Such cutting depths could not be created with round plates.

(43) FIG. 5b shows the method in a perspective representation. Preferably the feed direction F is substantially normal to a longitudinal axis L of the turbine blade, as can be seen from FIG. 5b.