Honing method for the precision machining of bores

Abstract

A honing method for machining the inner surface of a bore in a workpiece via at least one honing operation using an expandable honing tool with an annular cutting group. The method includes coupling of the honing tool to a working spindle of a machine tool, and; inserting the honing tool into an inlet of the bore with the cutting material elements retracted, until it is in a final insertion position, in which the cutting group is located in an end region of the bore. Then the honing tool is turned and the annular cutting group is simultaneously expanded to create a cylindrical widening of the bore, followed by withdrawal of the honing tool from the bore with simultaneous turning of the honing tool in such a way that the bore is successively further widened in the direction of the inlet side.

Claims

1. A honing method for machining the inner surface of a bore in a workpiece by means of at least one honing operation, in particular for honing cylinder running surfaces in the production of cylinder blocks or cylinder liners for reciprocating piston engines, wherein in a honing operation an expandable honing tool is used, having in an end region of a tool body that is remote from the spindle an expandable, annular cutting group with a number of cutting material elements which are distributed around the circumference of the tool body and the axial length of which is less than the effective outside diameter of the cutting group with the cutting material elements fully retracted, the method comprising the following steps: rigid coupling of the honing tool to a working spindle of a machine tool; relative positioning of the honing tool and the bore in such a way that a tool axis of the honing tool is coaxial with a target position of a bore axis of the bore; inserting the honing tool into the bore with the cutting material elements retracted, until the honing tool is in a final insertion position, in which the cutting group is located in an end region of the length to be machined of the bore that is remote from an inlet; turning of the honing tool and simultaneous expansion of the cutting group at or in the region of the final insertion position into a first radial position of the cutting material elements in such a way that a cylindrical widening of the bore that is substantially centered in relation to the target position of the bore axis is created by the material-removing engagement of cutting material elements on the inner side of the bore in the end region of the bore; withdrawing the honing tool from the bore with simultaneous turning of the honing tool in such a way that, from the cylindrical widening, the bore is successively widened in the direction of an inlet side.

2. The honing method as claimed in claim 1, wherein a honing tool that has at least one of the following properties is used: (i) on the annular cutting group more than 60% of the circumference is covered with cutting means; (ii) the axial length of the cutting material elements is for example less than 30% of the effective outside diameter of the cutting group; (iii) the axial length of the cutting material elements lies in the range of 5 mm to 20 mm; (iv) the axial length of the cutting material elements is less than 10% of the bore length of the bore.

3. The honing method as claimed in claim 1, wherein the cutting material elements are configured as honing segments that are wide in the circumferential direction and narrow in the axial direction, an axial length of the honing segments, measured in the axial direction, being less than the width measured in the circumferential direction.

4. The honing method as claimed in claim 1, wherein the cutting group has at least three honing segments of the same circumferential width or different circumferential widths.

5. The honing method as claimed in claim 1, wherein a honing tool in which the cutting material elements form a wedge-shaped cutting surface is used, a circumferential width of the cutting surface on a side near the spindle being wider than on a side remote from the spindle.

6. The honing method as claimed in claim 1, wherein a honing tool in which the cutting material elements have abrasive grains with an average grain size in the range of 50 m to 250 m is used.

7. The honing method as claimed in claim 1, wherein the cutting material elements are adjusted radially during the expansion of the cutting group.

8. The honing method as claimed in claim 1, wherein the widening is created in such a way that a difference in diameter between the widening and an adjoining, non-widened portion of the bore is at least 100 m.

9. The honing method as claimed in claim 1, wherein, after the creation of the cylindrical widening, a relieving operation is carried out to relieve the honing tool before the beginning of the withdrawal.

10. The honing method as claimed in claim 9, wherein, in the relieving operation, the cutting material elements are returned from the first radial position to a second radial position by a return amount, the return amount preferably being between 10 m and 15 m.

11. The honing method as claimed in claim 1, wherein the withdrawal of the honing tool takes place with a stroke rate in the range of 0.1 m/s to 2 m/s.

12. The honing method as claimed in claim 1, wherein during the expanding of the cutting group to create the widening a short-stroke axial oscillating movement is superposed on the rotation of the honing tool, at least in certain phases, an axial stroke lying in the range of 2 mm to 3 mm.

13. The honing method as claimed in claim 1, wherein an expandable honing tool with honing sticks of which the length is greater than their width in the circumferential direction is used in a further honing operation following the position-correcting honing operation, the axial length being more than 30% of the length of the bore.

14. The honing method as claimed in claim 2, wherein more than 80% of the circumference of the cutting group is covered with cutting means, and the axial length of the cutting material elements is between 10% and 20% of the effective outside diameter of the cutting group.

15. The honing method as claimed in claim 4, wherein the cutting group has between three and six honing segments.

16. The honing method as claimed in claim 8, wherein the difference in diameter between the widening and an adjoining, non-widening portion of the bore is at least 200 m.

17. The honing method as claimed in claim 11, wherein the withdrawal of the honing tool takes place with a stroke rate in the range of 0.3 m/s to 0.7 m/s.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further advantages and aspects of the invention are provided by the claims and the following description of preferred exemplary embodiments of the invention, which are explained below on the basis of the figures, in which:

(2) FIG. 1 shows a schematic view of part of a multi-axis machine tool in the form of a honing machine when carrying out an embodiment of the honing method;

(3) FIG. 2 shows in FIG. 2A an axial section and in FIG. 2B a cross section through an embodiment of a honing tool with an annular cutting group;

(4) FIG. 3 shows in FIGS. 3A-3F various phases of a position-correcting honing operation;

(5) FIG. 4 shows an embodiment of a wedge-shaped cutting surface on a honing tool; and

(6) FIG. 5 shows an embodiment of a honing tool being relieved in a relieving operation.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENT

(7) In FIG. 1, a schematic view of part of a numerically controlled, multi-axis machine tool 100 in the form of a honing machine 100 is shown in a direction parallel to the x direction of the system of machine coordinates MKS. The honing machine has a number of honing units, which are arranged next to one another in the x direction and can be operated simultaneously. In FIG. 1, some of the components of a honing unit 110 are represented. A control device 115 controls working movements of movable components of the honing machine.

(8) The honing machine is designed for honing cylinder running surfaces in the production of cylinder blocks for internal combustion engines. A workpiece 120 that is currently to be machined is securely clamped on a workpiece holding device 125. The position of the workpiece on the workpiece holding device is predetermined by means of indexing elements 126, so that there is a defined reference between the system of workpiece coordinates WKS and the system of machine coordinates MKS. The workpiece holding device has a horizontally movable carriage 127, which can be moved parallel to the y direction of the system of machine coordinates MKS by means of a drive 128 that can be activated by way of the control device 115.

(9) In the case of the example, the workpiece is a cylinder crankcase of a 4-cylinder in-line engine with four axially parallel cylinder bores. The bore 122 to be machined next can be seen; the other bores lie offset in the x direction.

(10) The honing unit 110 is attached to the front side of a vertical carrier structure 105 mounted on the machine bed of the honing machine. The honing unit comprises a headstock 135, which serves as a mounting for the working spindle 130, which is guided in the headstock with a vertical spindle axis 132. The rotation of the working spindle about the spindle axis is brought about by a rotary drive (not represented), which is attached to the headstock and acts on the working spindle for example by way of a chain drive. A reciprocating drive, which is structurally connected to the headstock, brings about the vertical movements of the working spindle that run parallel to the spindle axis 132 during the insertion of the later-to-be-explained honing tool 200 into the bore to be machined or during the withdrawal of the honing tool from this bore. Furthermore, during the honing machining process the reciprocating drive can be activated by the control device 115 in such a way that the honing tool performs within the bore of the workpiece a vertical back-and-forth movement corresponding to the desired honing parameters.

(11) In the case of the example, the honing tool 200 is rigidly coupled to the free end of the working spindle 130. For establishing the rigid, but releasable connection between the working spindle and the honing tool, a correspondingly secured bayonet connection, a screw connection, a flange connection or a cone connection, for example with a hollow shank taper (HSK), may be provided for example. Neither in the working spindle nor in the honing tool is a joint provided. In the unloaded state, i.e. in the absence of transverse forces acting on the honing tool, the central tool axis 212 of the honing tool runs coaxially with the spindle axis 132 in the region of its mounting in the headstock.

(12) The honing unit 110 with the vertical working spindle 130 contained therein is linearly movable as a whole in the horizontal direction parallel to the x axis of the system of machine coordinates MKS, that is to say perpendicularly to the spindle axis in a transverse direction. This makes it possible inter alia, without displacing the workpiece, to first machine a first bore on a workpiece, then withdraw the working spindle, move the honing unit as a whole in a transverse movement parallel to the x direction and position it approximately coaxially in relation to a second bore to be machined thereafter, in order to machine the second bore with the same honing unit. Horizontal transverse movements in the x direction may also be used to move the honing unit to a tool changer arranged in line with the transverse movement. In order to make the horizontal transverse movement possible, the headstock is mounted on a horizontally movable carriage 114, which is linearly guided on two horizontal guide rails on the front side of the carrier structure 105 that is facing the headstock. The transverse movement is brought about by a positioning drive 118, which is arranged between the carrier structure and the carriage 114.

(13) In the honing operation described in more detail below, a honing tool 200 of a particular structural design, which in the application is also referred to as a ring tool, is used (cf. also FIG. 2). The honing tool has a cutting group 220, which is attached annularly to the tool body 210 and has cutting material elements 220-1 to 220-3, which are distributed around the circumference of the tool body and can be adjusted and retracted in the radial direction by means of an assigned adjusting system. The cutting material elements are configured as honing segments, the width of which in the circumferential direction is clearly greater than the length in the axial direction. The abrasive cutting material elements that are responsible for the removal of material from the workpiece are concentrated in an axially relatively narrow zone (annular cutting group) and take up a relatively large proportion of the circumference of the honing tool.

(14) FIG. 2 shows, in 2A, a longitudinal section through an embodiment of a ring tool 200 with a single annular cutting group 220 and single widening. FIG. 2B shows a cross section through the cutting group. The ring tool 200 has a tool body 210, which defines a tool axis 212, which at the same time is the axis of rotation of the ring tool during the honing machining process. At the spindle-side end of the ring tool (at the top in FIG. 2A) there is a coupling structure (not represented any more specifically) for the rigid coupling of the ring tool to a drive rod or a working spindle of a honing machine or of some other machine tool that has a working spindle, which is both rotatable about the spindle axis and movable back and forth in an oscillating manner parallel to the spindle axis.

(15) At the spindle-remote end of the tool body (at the bottom in FIG. 2A) there is the annular cutting group 220, which has a number (in the example three) of cutting material bodies 220-1, 220-2, 220-3, which are distributed uniformly over the circumference of the tool body and can be adjusted radially outwardly in relation to the tool axis 212 with the aid of a cutting-material-element adjusting system in order to press the abrasively acting outer sides of the cutting material element, i.e. the cutting surfaces, with a defined pressing force against the inner surface of a bore to be machined. Each of the three arcuately curved cutting material elements is configured as a honing segment which in the circumferential direction is very wide, by contrast in the axial direction is narrow and covers a circumferential angular region of between 115 and 120. The honing segments are decoupled from the tool body and are displaceable relative to the latter radially in relation to the tool axis 212. The ring formed by the honing segments ends on the spindle-remote side flush with the tool body, so that the ring fits completely within the spindle-remote half of the tool body at the spindle-remote end of the ring tool.

(16) Finishing flush with the lower end of the tool body is favorable, but not absolutely necessary. Generally, the ring should fit in the spindle-remote third or in the spindle-remote quarter of the tool body; there may be a small distance from the end face of the tool body.

(17) The axial length LHS of the honing segments is less than 15%, in particular less than 10%, of the bore length L. The honing segments are about 4 mm to 35 mm, in particular about 10 mm, high (in the axial direction), which in the case of the example corresponds to between 5% and 30%, in particular between 10% and 20%, of the effective outside diameter of the cutting group with the cutting material elements inwardly retracted to the maximum. The honing tool has only this one annular cutting group. The axial length LHS therefore at the same time corresponds to the axial length of the entire cutting region of the honing tool.

(18) Each cutting material element is fastened to the outer side of an assigned steel carrying strip 224-1, 224-2 by brazing. Alternatively, the cutting material element may also be fastened by adhesive bonding or by means of screwing, making it more easily possible for it to be exchanged. Each carrying strip has on its inner side an oblique surface, which interacts with a conical outer surface of an axially displaceable adjusting cone 232 in such a way that the carrying strips with the cutting material elements carried by them are adjusted radially outwardly when the adjusting cone is pressed in the direction of the spindle-remote end of the ring tool counter to the force of the restoring springs 234, 226, 228 by means of a machine-side adjusting device. In an opposite adjusting movement, the carrying strips with the honing segments are restored radially inwardly with the aid of peripheral restoring springs 226, 228. The radial position of the cutting material elements is thereby controlled in a manner free from play on the basis of the axial position of the adjusting cone 232.

(19) A honing segment may, as shown, have on its outer side a continuous, uninterrupted cutting surface. For this purpose, the cutting coating may consist of a single piece of the cutting means. It is also possible that a number (for example two, three, four, five, six or more) of in each case relatively narrow cutting material elements are provided close together, with or without a spacing between them, on the arcuately curved outer side of a common carrier element. The cutting surface would then be interrupted, which may possibly be favorable for supplying cooling lubricant.

(20) The use of relatively wide honing segments in the annular cutting group may be favorable inter alia when machining bores which, for example for the purpose of gas exchange, have transverse bores that open out on the inner surface to be machined of the bore. Wide, intrinsically rigid honing segments may bridge the region where they open out, so that the honing tool cannot get stuck. If only a few (for example three) wide honing segments are provided, there must also only be a few radial clearances provided on the tool body, so that improved mechanical stability is obtained, which is favorable specifically in the case of position-correcting machining in order to withstand transverse forces.

(21) This machine and tool concept makes position-correcting, material-removing machining of the inner surface of the bore 122 possible, in order by means of honing to bring the position of the bore to its target position within the tolerances.

(22) An embodiment of a suitable method is explained in more detail on the basis of FIG. 3. FIG. 3 shows for this purpose various phases or various partial operations of a position-correcting honing operation. As schematically shown in FIG. 1, after the previous machining steps the bore 122 is still not at its desired target position, which in the case of the example is represented by the target position SB of the bore axis. Rather, the bore 122 still has a lateral offset in relation to the target position, the actual position of the bore (characterized by the current bore axis IB) lying outside the tolerances alongside the target position. Furthermore, the inner surface of the bore does not yet have the surface structure that it should have for the intended use, which is likewise still to be produced by honing.

(23) First, in a positioning operation, the honing tool is positioned in relation to the bore in such a way that the tool axis 212 lies coaxially in relation to the target position SB of the bore. For this purpose, if need be the tool carrier 127 is moved horizontally, parallel to the y direction of the system of machine coordinates, and/or the headstock or the working spindle is moved horizontally, parallel to the x direction of the system of machine coordinates. The coaxial position set by the positioning operation is schematically represented in FIG. 1 and FIG. 3A.

(24) Before, after or at the same time as the positioning, the adjusting rod of the cutting-material-element adjusting system is moved upwardly, until the cutting material elements assume their position of being inwardly retracted to the maximum, whereby the outside diameter of the annular cutting group assumes its smallest value. When moved vertically, the honing tool then fits into the bore without touching the walls of the bore.

(25) After that, by actuating the reciprocating drive of the working spindle, the honing tool is inserted into the bore 122 and lowered to such an extent that the honing tool reaches a final insertion position, in which the annular cutting group 220 is located in an end region 123 of the bore 122 that is remote from the inlet (FIG. 3B). The end region remote from the inlet of the bore 124 defines the lower end or the remote end of the overall length to be machined of the inner side of the bore. Since, in the case of the annular tool shown, the cutting group 220 is flush with the spindle-remote end face of the tool body, the ring tool can be moved virtually right up against the bottom of the bore or, in the case of through-bores, right up against the upper side of the workpiece mount. In practice, however, a small distance of a few millimeters, for example 1 to 2 mm, is generally maintained. The working spindle may be slowly turned during the inserting operation, but this is not absolutely necessary.

(26) When the honing tool is in the final insertion position, the subsequent partial operation, specifically an expanding operation, can begin. In the expanding operation (FIG. 3C), the honing tool is turned about its tool axis and at the same time the cutting group is slowly expanded by activating the cutting-material-element adjusting system, so that the effective outside diameter of the cutting group gradually increases. If the bore in the end region remote from the inlet is not already centered in relation to the target position of the bore axis and has a circular cross section, in a specific phase of the expanding operation an engagement of the material on one side will first take place, that is to say an abrasive machining with a partial cut in the region of the inner surface of the bore that lies radially closest to the target position of the bore axis. With an increasing effective outside diameter of the cutting group, the partial cut then gradually goes over into a full cut, in the case of which material is removed over the entire circumference of the cutting group.

(27) The outward radial adjustment of the cutting material elements is continued until the cutting material elements reach a previously defined first radial position (FIG. 3C). After that, further adjustment is stopped by the control device. The removal of material over the circumference of the cutting group together with the radial adjustment has the effect that in the expanding operation a cylindrical widening 121, which is generally centered exactly within the tolerances in relation to the target position of the bore axis, is created in the end region 123 of the bore by material-removing engagement of cutting material elements on the inner side of the bore. In the case of a not exactly positioned bore, the cylindrical widening will still lie decentered in relation to the current bore axis. The oversize of the widening with respect to the portion of the bore adjoining the bore inlet is generally at least 100 m with reference to the diameter. Usually, the values are higher, for example at 200 m or more or at 400 m or more.

(28) It has been found in trials that rotational speeds of the honing tool in the range of 400 rpm to 1000 rpm generally produce good results in the expanding operation. In one case, work was performed at about 500 rpm using oil as a cooling lubricant.

(29) In the case of one variant of the method, the honing tool remains at the set axial position during the expanding operation, so that no reciprocating movement is superposed. In the case of other variants of the method, during the expanding operation the reciprocating drive of the working spindle is actuated at least in certain phases in order to superpose an axially short-stroke oscillating movement on the rotating movement of the honing tool during the radial expansion. As a result, possibly more favorable material removal conditions can be achieved. The axial length of the cylindrical widening created would then turn out to be slightly larger than in the case of merely radial expansion without a superposed reciprocating movement.

(30) In the case of one variant of the method, after completion of the expanding operation, i.e. when the cutting material elements have reached the first radial position, the honing tool is relieved in a relieving operation by the cutting material elements being returned to a second radial position by a certain returning amount, for example 10 to 15 m. An example of this is shown in FIG. 5, with the arrows directed inwardly towards the rotation axis of the honing tool As a result, the touching contact between the outer surface of the honing segments and the inner wall of the bore is ended, so that the honing tool can possibly center itself even better with reference to the spindle axis under the effect of elastic forces from the working spindle. This step may also be omitted.

(31) After completion of the expanding operation, and possibly after the relieving operation, a drawing operation (FIG. 3D) is initiated, in which the honing tool driven in a rotating manner is slowly withdrawn at a suitable stroke rate from the bore in the direction of the bore inlet. This has the effect of producing from the bottom of the bore a gradual extension of the cylindrical widening, centered in relation to the target position of the bore axis, so that, as it were starting from the base of the bore, the bore is given its circular-cylindrical bore shape, centered in relation to the target position of the bore axis. After complete withdrawal of the honing tool from the bore (FIG. 3E), the bore is centered over the full length with respect to the target position of the bore axis. The angularity of the bore is also correctly set, so that the bore axis is for example oriented perpendicularly in relation to the top surface.

(32) For the partial operation of withdrawing the honing tool (with at the same time axially increasing the cylindrical widening), the stroke rates typically lie in the range between 0.3 m/s and 0.7 m/s, but they may also be less, for example down to below 0.1 m/s, or else greater, up to more than 1 m/s, for example up to about 2 m/s.

(33) In some cases, it may be adequate to carry out these honing operations (including insertion of the honing tool, expansion to create a cylindrical widening, withdrawal) only a single time. However, it is also possible to repeat these working steps one or more times, which may be favorable in particular whenever the positional error of the bore before the first machining process is relatively high and/or if it is desired to produce in each case during the drawing operation only a smaller removal of material than is ultimately required.

(34) After the position-correcting honing operation, further machining operations may follow. In the case of some variants of the method, subsequently at least one further honing operation is carried out with the aid of a conventional expandable honing tool 300, which is coupled in a singly or multiply articulated manner to the working spindle and has relatively long honing sticks 320, the length of which may for example be more than 30% of the length of the bore (FIG. 3F). This allows possibly still existing errors in the cylinder shape and/or other shape errors still to be corrected.

(35) As shown, the method can be carried out with a vertical axial direction. Horizontal machining is also possible. As shown, the bore may be uncoated, so that the base material of the workpiece is removed directly. Machining of coated bores is likewise possible, the material of the coating then being removed.

(36) A ring tool may possibly have in addition to the spindle-remote ring a separately adjustable further ring with cutting material elements.

(37) For a similar material-removing, position-correcting finishing method, it may also be possible to use instead of a ring tool that is configured for honing a finishing tool that has at least one activatable cutting element with a geometrically defined cutting edge (instead of an annular cutting group with geometrically undefined cutting edges) attached at the end to the spindle-remote end. There may possibly be at least one pair of activatable cutting elements lying diametrically opposite one another. The method steps: insertion of the finishing tool, expansion and withdrawal of the cutting element with at the same time rotation to create a cylindrical widening, withdrawal with extended cutting element and simultaneous rotation, could be carried out in a way analogous to the procedure described.