Method for machining materials by milling and subsequent brushing

Abstract

A method for machining a material, in particular steel, is provided. The material is milled at such a high cutting speed that residual tensile stresses close the surface that exceed a specified value can occur and the residual tensile stresses can be lowered below the specified value by subsequent brushing. A device for performing the method is also provided.

Claims

1. A method for machining a material, comprising: milling at a high cutting rate such that internal tensile stresses close to a surface which exceed a predetermined value may arise, subsequent brushing to reduce the internal tensile stresses which exceed the predetermined value so as to be below the predetermined value, and employing a brushing device in which brushes are present, such that brushing with the brushing device may be simultaneously carried out at a plurality of points.

2. The method as claimed in claim 1, wherein grooves are milled into turbine shafts or into wheel disks.

3. The method as claimed in claim 1, wherein brushing converts the internal tensile stresses into internal compressive stresses.

4. The method as claimed in claim 1, wherein brushing is achieved by a translational and/or rotational movement.

5. The method as claimed in claim 1, wherein the brushes are disposed on the brushing device such that a plurality of regions of a groove may be simultaneously brushed.

6. The method as claimed in claim 1, wherein the brushes and bristles fastened thereon are optimized with respect to a desired reduction of the internal tensile stresses and to a desired service life of the brushing device, wherein it is considered in the optimization that a reduction of the internal tensile stresses depends on a material of the bristles, a relative position of the bristles, a stiffness of the bristles, a speed of the bristles, a population density, and on internal tensile stresses which are present at a commencement of brushing.

7. The method as claimed in claim 1, wherein the brushes comprise bristles that comprise one of the following: stainless steel, steel, nonferrous metals, plastic, plastic comprising embedded abrasive means, and natural bristles.

8. The method as claimed in claim 1, wherein the brushes comprise bristles that comprise stainless-steel wire, a bristle diameter of 0.15 mm to 0.35 mm, and a tensile strength of 1500 N/mm.sup.2 to 2400 N/mm.sup.2.

9. The method as claimed in claim 8, wherein the bristles comprise a ratio of bristle length to bristle diameter of 30 to 500.

10. The method as claimed in claim 1, wherein the milling takes place using high-speed steel cutting materials or carbide cutting materials.

11. The method as claimed in claim 1, wherein the milling takes place at a milling feed rate of up to 38 mm/min.

12. A brushing device configured for carrying out a method according to claim 1.

13. The brushing device as claimed in claim 12, wherein the brushes comprise a contour adapted to grooves to be milled.

14. The method as claimed in claim 1, wherein the material comprises steel.

15. The method as claimed in claim 2, wherein the grooves comprise fir-tree grooves.

16. The method as claimed in claim 8, wherein the tensile strength is 1800 N/mm.sup.2 to 2100 N/mm.sup.2.

17. The method as claimed in claim 11, wherein the milling feed rate is up to 13 mm/min.

18. A method for machining a material, comprising: milling at a high cutting rate such that internal tensile stresses close to a surface which exceed a predetermined value may arise, and subsequent brushing to reduce the internal tensile stresses which exceed the predetermined value to below the predetermined value, wherein brushes comprising bristles comprising stainless-steel wire are employed, which bristles comprise a bristle diameter of 0.15 mm to 0.35 mm and a tensile strength of 1500 N/mm.sup.2 to 2400 N/mm.sup.2.

19. A method for machining a material, comprising: milling at a high cutting rate such that internal tensile stresses close to a surface which exceed a predetermined value may arise, and subsequent brushing to reduce the internal tensile stresses which exceed the predetermined value to below the predetermined value, wherein the milling takes place at a milling feed rate of up to 38 mm/min.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further details of the invention are now described in more detail by means of the schematic drawings, in which:

(2) FIG. 1 shows ranges of the profile of internal stress prior to and after brushing,

(3) FIG. 2 shows a groove profile of a fir-tree groove and associated requirements of internal tensile stress,

(4) FIG. 3 shows an individual brush in the cross section,

(5) FIG. 4 shows a plurality of brushes of various sizes for brushing a fir-tree groove, and

(6) FIG. 5 shows three brushes of various sizes on a common spindle.

DETAILED DESCRIPTION OF INVENTION

(7) In FIG. 1 the regions of the profile of internal stress is shown prior to and after brushing. On the right, the depth is indicated in micrometers. The internal stress is indicated on the vertical in MPa. The horizontal line in the center indicates the internal stress at zero. There, neither internal tensile stresses nor internal compressive stresses are thus present. In the ranges above this line, internal tensile stresses are present. In the range below this line, internal compressive stresses are present.

(8) The hatched range on the right, that is to say the range between the two dashed lines, indicates in which ranges the internal stress of a material which is milled at high cutting speed is situated prior to brushing. It can be identified here that undesirable internal tensile stresses are present in many cases. However, there are also cases in which internal compressive stresses exist. The hatched range on the left, that is to say the range between the two solid lines, indicates the values for internal stress after brushing. It can be identified that internal compressive stresses are always present. Internal compressive stresses of this type are desirable since on account thereof the occurrence of cracks is particularly well prevented.

(9) FIG. 2 shows a fir-tree groove 1. A first constricted region 2 can be identified. Said constricted region 2 is bordered by a groove contour 3 which roundly protrudes into the constricted region 2. The groove contour 3 is a result of corresponding milling. Now observing the groove contour 3 at the transition from the constricted region 2 into an upper widened region 4, a first widened region 5 in the material, which is configured as a supporting flank, can be identified. A first outer radial region 6 adjoins the first widened region 5. At the transition to the second constricted region 7 which is narrower than the first constricted region 2, a first constricted region 8 adjoins the outer radial region 6. Now further following the groove contour 3, a widening into a second widened region 9 occurs in the case of the fir-tree groove 1. The second widened region 10, which is a further supporting flank, can be identified in the material.

(10) The second widened region 10 is followed by an outer radial region 11, and the latter is followed by a second constricted region 12. The appearance of a fir-tree groove can thus be identified in more detail. Internal tensile stresses which are as low as possible should be present in the outer radial regions after milling. These requirements do not apply to the inner peripheral regions which are stressed to a lower extent.

(11) FIG. 3 shows an individual brush 14 in the cross section. A core 15 having two core wires 16 and 17 can be identified. Bristles 18 extend therefrom. The bristles 18 are cut off in segments 19.

(12) FIG. 4 shows the arrangement of the brushes in the fir-tree groove 1. Here, the brush 14 is illustrated in the first widened region 4, a smaller brush 20 for the second widened region 9 and an even smaller brush 21 for the lower widened region 13. In the interest of clarity, many reference numerals of the fir-tree groove 1 have been omitted here; to this extent reference is made to FIG. 1. Bristles 18 which protrude beyond the groove contour 3 are illustrated. This indeed may not be understood to mean that the bristles 18 would actually protrude into the material. Rather, this is to highlight that the bristles 18, based on their length, would really protrude beyond the groove contour 3. Since the bristles 18 are prevented by the material to be brushed from doing so, the relative position of the bristles 18 which has been further described earlier is invoked.

(13) FIG. 5 shows a depiction of a brushing device 22. A spindle 23 can be identified. There brushes, namely the larger brush 14, the medium brush 20, and the smaller brush 21, are attached on said spindle 23. Rotation of the spindle 23 results in a rotation of the brushes 14, 20, and 21. On account thereof, rotational brushing may take place.

(14) In an exemplary manner, the fir-tree groove 1 is configured in a face-turned round material from 26NiCrMoV145. In an exemplary manner, the brush is constructed from the bristles 18 from a stainless-steel wire having a diameter of 0.35 mm and a tensile strength of 1.8 to 2.1 kN/mm. The brush is braided, the bristle length being 30 mm. In an exemplary manner, a relative position of 2 mm to 3.5 mm at 150 double strokes and an average brushing speed of 1 m/s is selected for brushing.

(15) For process monitoring, a torque sensor is provided for rotational brushing and a force sensor for translational brushing. As wear of the brush increases, the transmitted torque or force, respectively, decreases. In the case of premature brush wear, a brush replacement can be initiated in this manner. In the event of the wrong brush or no brush having been employed, this type of monitoring also responds.

(16) Although the invention has been illustrated and described in more detail by way of the preferred exemplary embodiment, the invention is not limited by the disclosed examples, and other variations may be derived therefrom by a person skilled in the art, without departing from the scope of protection of the invention.