HARD TURNING

Abstract: A metal machining process, in particular hard turning using a computer controlled precision lathe, comprises the step of controlling the tool to increase the depth of cut at intervals during machining, so as to create a plurality of depressions (12 to 16). As applied to a synchromesh cone (10), for example, the depressions may typically be annular and 1 micron in depth, with a base width of 0.16 mm and spaced at 1.0 mm along the taper. The process enables components to be produced whose surface is similar to that of ground components.

Description: HARD TURNING Field of invention This invention concerns the formation of surface finishes in machined components and applies both to internal and external surfaces . Background to the invention The surfaces of metal components hitherto surface-finished by a grinding process, have been specified by means of measurements which can be performed on the components as they are manufactured . Typically these measurements have involved size tolerance at a particular position along the length of the component, an acceptable variation in the height of peaks to troughs in the surface when measured along a single line along the component, known as the Ra measure, and the bearing ratio of the peaks and troughs at one or more depths from the peaks. This last measurement may be required to lie between a maximum and a minimum percentage of the length of the line. Overall flatness has often been measured in terms of the percentage of the area of the surface which will be inked by a standard bluing gauge. All other things being equal, provided the measurements on production components fall within the specification set down as acceptable by the designer, the components will be expected to function correctly for the purpose they are intended. Thus for example surface finish specifications may be laid down for the bores in fuel injectors for diesel engines and for the cones for synchronous meshing of gears in transmission units. Generally the surface finishing of such components has been achieved by grinding processes which, although superficially leaving a very smooth surface finish, in fact leave a pitted surface (at the micron level) since the removal of the metal is achieved by each piece of grit in the surface of the grinding wheel gouging out a tiny portion of the metal, with the relative rotation of the wheel and the component. The grit contact with the metal is relatively random and arbitrary since the grit is not uniform in size and distribution and the result is a surface having a large number of tiny pockets relatively randomly distributed over the area of the surface. It has been proposed to replace the grinding finishing step for such components with precision turning, sometimes referred to as hard turning. It is an object of the present invention to provide a process and apparatus for achieving a machined surface finish which, to a first approximation, is within the specification laid down for the component when finished by grinding. In this way, for example turned components can be compared alongside components produced by conventional techniques . In addition if, as may be the case, the life and/or functionality of certain components is dependent on the type of surface produced by grinding, the method of the invention will go some way to ensuring that the life and/or functionality of such components when machined, eg by hard turning, is similar to that of the ground components . Summary of the invention According to one aspect of the present invention in a turning process in which a cutting tool engages the surface of a rotating component so as to remove a helix of metal therefrom as a result of synchronisation of the relative axial movement of the tool and the component and the rotation of the latter, at least the depth of cut achieved by the tool and component engagement is under the control of a programmed computer. The computer may also be programmed to control the speed of rotation of the component. The computer may also be programmed to control the relative axial movement between the tool and the component . Typically the tool is moved axially relative to the component so as to progress the point of engagement between the tool and the surface of the component along the length of the latter as the component rotates . Preferably the computer is programmed so as to synchronize the rotation of the component and the axial movement of the tool so that the locus of the point of engagement of the tool and the component is a helix. Preferably the axial movement of the tool is synchronised with the rotation of the component so that the angle of the helix is such as to just advance the tool by the thickness of its cutting tip during each revolution, so that not only is a continuous helix of metal peeled away from the surface of the component as the machining progresses but a smooth surface is left behind. If a taper is required in the component the depth of cut may be increased progressively with axial advance of the tool relative to the component, however the increasing depth of cut would still normally be controlled so as to produce a smooth surface, albeit of progressively reducing diameter. In accordance with an important aspect of the invention, in a process of producing a component by metal removal by turning whilst the surface of the component is engaged by the tip of a cutting tool, so as to progressively remove a helix of metal from the surface of the component and thereby produce at least in the region of the cut a cylindrical surface the radius of which is determined by the position of the tool relative to the axis of rotation of the component, and in which the tool is under computer control at least as to its position for determining the depth of cut and therefore the radius of the turned surface of the component, the computer is programmed to increase the depth of cut at intervals during the turning process so as to create a plurality of depressions in the turned surface of the component which have a marginally smaller radius of curvature than that of the surrounding turned surface. The programming may be such as to increase the depth of cut during regularly spaced apart intervals. The timing of the intervals may such that at least one depression is created during at least a part of each revolution of the component . The timing may be such as to produce a plurality of depressions around each revolution of the component. The timing of the intervals may be adjusted from one revolution to the next so that depressions do not become aligned parallel to the axis of the component . The timing of the intervals may be selected so as to produce a regular or random or pseudo random pattern of depressions in the component surface . Each interval may be of the same duration so that each depression is of the same extent, or a variation may be introduced into the duration of each of the intervals so that the depressions are of correspondingly different size. In a preferred method each interval is arranged to extend over a plurality of consecutive revolutions of the component so that each resulting depression comprises an annular region of reduced diameter. The transition between the turned surface of the component and each such annular depression may be gradual and itself may be generated during more than one revolution of the component, by programming the computer to increase the depth of cut gradually over the said one or more revolutions during which the transition is to occur. At the other end of such an annular depression the computer programme may be arranged to reduce the depth of cut in a similar gradual manner over a corresponding number of revolutions of the component, back to that required to produce the turned surface of the component beyond the annular depression. Where the component is to taper in overall diameter, the depth of cut instructions generated by the programme during the transitions and during the generation of each reduced diameter annular region must take this into account, so that diameter of the component is progressively reduced during the whole of the turning process. Since the depressions are merely to break-up what would otherwise be a smooth surface produced by the turning process, the depth of each depression relative to the surrounding turned surface will normally need only to be very small. Differences in radius as between the base of each depression and the surrounding turned surface may be of the order of 1 micron or less . Where more significant surface break-up is desired the difference in radius may be of the order of 2 or 3 microns or more. Depending on the properties required of the final surface the programming of the computer may be such as to produce relatively small but relatively deep depressions per unit area of the component surface or relatively large but relatively shallow depressions over the same surface area. Likewise the number of depressions per unit area of the component surface may be adjusted to produce the desired characteristics in the final surface. Where the final surface specification includes a bearing ratio vector requirement, the latter may be achieved by adjusting the rate of change of radius (diameter) at one or both ends of each depression so that the required percentage of component material will exist at the specified depths relative to the peak diameter of the turned surface. Where a bluing gauge percentage figure has to be complied with, the computer may be programmed to adjust the extent of the depressions relative to the remaining area of the turned component surface, so as to provide a sufficient overall area of turned surface which will be inked by the gauge, during the bluing test, relative to the overall area of the depressions, which will not normally become inked during the test. Where the final surface is to be capable of being tested at any point along its axial length, the programme should arrange that the depressions are evenly distributed over the overall surface of the component to ensure that measurements made on the component will tend to be the same wherever they are made. Where the component is to be gauged as part of the control of the turning process, it is desirable that the precise positions of the depressions and any transitions between depression and main turned surface are known and to this end the programme advantageously organises the computer to store co-ordinates of the depressions and transitions or an algorithm of their generation, so that an appropriate correction can be made to the result of any gauged value of (say) diameter, or the position at which a gauge is to be applied may be determined in advance of the gauging step and the gauge or the component positioned accordingly before the measurement is made. The invention also lies in a component when manufactured in accordance with a computer controlled hard turning method as proposed by the invention disclosed herein. The invention also lies in a metal turning machine and computer control therefor programmed to perform a hard turning operation in accordance with the invention. The invention also lies in a metal turning machine in combination with a computer based control system therefor, when programmed to perform a hard turning process on a component in accordance with the invention. The invention also lies in a computer when programmed to control a metal working machine so as to perform a hard turning operation on a component such as described herein. The invention also lies in a programme adapted to operate a computer so as to provide control signals for a metal working machine to cause the latter to perform a hard turning operation such as described herein. The invention also lies in a computer programme for operating a computer so as to control a metal working machine to perform a hard turning operation on a component such as described herein, when stored on a data carrier. The invention also lies in a computer programme for operating a computer, or a programmed computer, adapted to control the operation of a metal machining process involving the removal of metal from a rotating workpiece by the engagement therewith of the tip of a metal cutting tool at least the position of which is controlled by the said computer, and which as a result of synchronised relative movement between the tool and the workpiece, would produce a smooth machined surface thereon, wherein the programme serves to alter the instantaneous position of the tool so as to introduce into the otherwise smooth surface, during the machining process, plural spaced apart depressions for the purpose of simulating a surface typical of that which would be obtained thereon if the latter had been finished by grinding. The invention also lies in a metal turning machine in combination with a computer based control system therefor, when programmed to perform a hard turning process on a rotating workpiece involving the removal of metal from the surface thereof by the engagement therewith of the tip of a metal cutting tool at least the position of which is controlled by the said computer based control system, and which as a result of synchronised relative movement between the tool and the workpiece, would produce a smooth surface thereon, wherein the programme serves to alter the instantaneous position of the tool during the machining process, so as to introduce into the otherwise smooth surface plural spaced apart depressions, for the purpose of simulating a surface typical of that which would be obtained on the workpiece if the latter had been finished by grinding. The invention also lies in a method or apparatus as aforesaid which further comprises gauging and/or measuring the machined part during the machining process, to generate signals indicative of one or more dimensions of the machined part, and supplying the signals to the computer, to assist in the control of the machining process. The invention also lies in a metal turning machine in combination with a computer based control system therefor, when programmed to perform a hard turning process on a rotating workpiece, involving the removal of metal from the rotating workpiece by the engagement therewith of the tip of a metal cutting tool, at least the position of which is controlled by the said computer based control system and which as a result of synchronised relative movement between the tool and the workpiece, would produce a smooth surface on the machined part, wherein the programme serves to alter the instantaneous position of the tool so as to introduce into the otherwise smooth surface of the machined part, plural spaced apart depressions during the machining process, for the purpose of simulating a surface typical of that which would be obtained thereon if the latter had been finished by grinding, and the machine includes at least one gauging or measuring device adapted to perform measurements on the machined part during the machining process, to generate signals indicative of one or more dimensions of the machined part, and means for conveying the signals to the computer as feedback signals indicative of how the process is progressing, to assist in the control of the process. Examples of components which may be made in accordance with the invention. Synchro cones for gearboxes have hitherto been formed by grinding to a particular surface and taper diameter specification. In a method in accordance with the invention of hard turning such a component so as to have a surface which will fall within the previous specification for the component, using a computer controlled metal turning process as aforesaid, the computer was programmed so as to introduce into the turned surface a plurality of annular depressions having a depth relative to the turned surface of 1 micron, pitched at 1mm spacings along the axial length of the component. Each depression extended over a total axial distance of 0.2mm with a transition involving a gradual change in radius extending over 0.02mm at each side of the depression (within the 0.2mm width), so that the base of the depression, which lay a full 1 micron below the turned surface at either end, extended over less than the 0.2mm width of the depression. Other examples of a similar component involved the formation therein of similar depressions having the same overall axial extent and similar transitions, but with depths of 2 microns and 3 microns respectively, for comparison purposes. Injector bores for fuel injectors for diesel engines have to be internally finished to a high accuracy and have hitherto been ground to achieve the desired finish and diameter. The invention offers an alternative manufacturing process for such injectors in which the bores are hard turned under computer control in accordance with the invention and have depressions formed in their internal surfaces to produce a surface having appropriate characteristics to conform to the original specification for the product when internally ground. The invention will now be described, by way of example only, with reference to the accompanying drawings in which: ^ Figure 1 is a perspective view of a synchromesh gear for an automotive manual gear box, and having an external cone formed by a turning process in accordance with the present invention; Figure 2 is a magnified plan view of depressions formed in the surface of the cone shown in Figure 1; and Figure 3 is a diagram in section (not to scale) showing the depressions of Figure 2. The synchromesh gear shown in Figure 1 has a short external taper 10 which is formed on a precision lathe, operating in accordance with the process of the invention. The lathe may be a hard turning lathe Type DT40 made by Landis Lund (a division of UNOVA UK Limited) of Cranfield, England. For further details of such a machine tool or precision lathe, reference may be made to published International Patent specification WO97/30381. The computer programme which controls the lathe for forming the taper 10, is arranged to .introduce minute annular depressions in the otherwise relatively smooth surface of the taper 10. Three such depressions in the surface of the taper, are shown at 12, 14 and 16 in the magnified plan view of Figure 2, which is enlarged 35 times. For a clearer view of the depressions, reference is made to the section shown in Figure 3, in which typically each depression has a depth of 1 micron, a base width of 0.16mm, and an overall width of 0.2mm which merges down to the base at each side over a width of 20 microns. The distance or pitch between depressions is approximately 1.0mm. Although th 2 invention has been described primarily with reference tc hard turning, it is applicable generally to any machining pi ocess where the surface is generated by metal cutting. It could therefore also be applied to boring, eg jig boring, and even to CNC form milling using a round-nosed cutter.

Claims: -^ Claims 1. A turning process in which a cutting tool engages the surface of a rotating component so as to remove a helix of metal therefrom as a result of synchronisation of the relative axial movement of the tool and the component and the rotation of the latter, in which at least the depth of the cut achieved by the tool and component engagement is under the control of a programmed computer. 2. A process according to claim 1 in which the computer is also programmed to control the speed of rotation of the component. 3. A process according to claim 1 or claim 2 in which the computer is also programmed to control the relative axial movement between the tool and the component . 4. A process according to any preceding claim in which the tool is moved axially relative to the component so as to progress the point of engagement between the tool and the surface of the component along the length of the latter as the component rotates . 5. A process according to claim 4 in which the computer is programmed so as to synchronize the rotation of the component and the axial movement of the tool, so that the locus of the point of engagement of the tool and the component is a helix. 6. A process according to claim 5 in which the axial movement of the tool is synchronised with the rotation of the component so that the angle of the helix is such as to just advance the tool by the thickness of its cutting tip during each revolution, so that not only is a continuous helix of metal peeled away from the surface of the component as the machining progresses but a smooth surface is left behind. 7. A process according to any one of claims 4 to 6 in which a taper is required in the component, and the apparent depth of the cut is increased progressively with axial advance of the tool relative to the component, the increasing depth of cut still being normally controlled so as to produce a smooth surface, albeit of progressively reducing diameter. 8. A process according to any one preceding claim in which the computer is programmed to increase the depth of cut at intervals during the turning process so as to create a plurality of depressions in the turned surface of the component which have a marginally smaller radius of curvature than that of the surrounding turned surface . 9. A process according to claim 8 in which the programming is such as to increase the depth of cut during regularly spaced apart intervals. 10. A process according to claim 9 in which the timing of the intervals is such that at least one depression is created during at least a part of each revolution of the component. 11. A process according to claim 9 or claim 10 in which the timing is such as to produce a plurality of depressions around each revolution of the component . 12. A process according to any one of claims 9 to 11 in which the timing of the intervals is adjusted from one revolution to the next so that depression do not become aligned parallel to the axis of the component. 13. A process according to any one of claims 9 to 12 in which the timing of the intervals is selected so as to produce a regular or random or pseudo random pattern of depressions in the component surface . 14. A process according to any one of claims 9 to 13 in which each interval is of the same duration so that each depression is WO 00/74882 „ PCT/GBOO/01821 of the same extent, or in which a variation is introduced into the duration of each of the intervals so that the depressions are of correspondingly different size. 15. A process according to any one of claims 9 to 14 in which each interval is arranged to extend over a plurality of consecutive revolutions of the component so that each resulting depression comprises a helical region of reduced diameter. 16. A process according to any one of claims 8 to 15 in which the transition between the turned surface of the component and each such depression is gradual and itself generated during more than one revolution of the component, by programming the computer to increase the depth of cut gradually over the said one or more revolutions during which the transition is to occur. 17. A process according to any one of claims 8 to 16 in which at one end of such a depression the computer programme is arranged to reduce the depth of cut in a similar gradual manner over a corresponding number of revolutions of the component, back to that required to produce the turned surface of the component beyond the depression. 18. A process according to any one of claims 8 to 17 in which the component is to taper in overall diameter, and the depth of cut instructions generated by the programme during the transitions and during the generation of each reduced diameter region takes this into account, so that the diameter of the component is progressively reduced during the whole of the turning process. 19. A process according to any one of claims 8 to 18 in which the differences in radius as between the base of each depression and the surrounding turned surface are of the order of 1 micron or less. 20. A process according to any one of claims 8 to 19, in which the final surface specification includes a bearing ratio vector WO 00/74882 ^ PCT/GBOO/01821 requirement, which is achieved by adjusting the rate of change of radius (diameter) at one or both ends of each depression so that the required percentage of component material will exist at the specified depths relative to the peak diameter of the turned surface . 21. A process according to any one of claims 8 to 20, in which a bluing gauge percentage figure has to be complied with, and the computer is programmed to adjust the extent of the depressions relative to the remaining area of the turned component surface, so as to provide a sufficient overall area of turned surface which will be inked by the gauge during a bluing test, relative to the overall area of the depressions which will not normally become inked during the test. 22. A process according to any one of claims 8 to 21, in which the final surface is to be capable of being tested at any point along its axial length, wherein the programme is arranged for the depressions to be evenly distributed over the overall surface of the component to ensure that measurements made on the component will tend to be the same wherever they are made. 23. A process according to any one of claims 8 to 22, in which the component is to be gauged as part of the control of the turning process, wherein the programme organises the computer to store co-ordinates of the depressions and transitions or an algorithm of their generation, so that an appropriate correction can be made to the result of any gauged value of (say) diameter, or the position at which a gauge is to be applied can be determined in advance of the gauging step and the gauge or the component positioned accordingly before the measurement is made. 24. A component when manufactured in accordance with a computer controlled hard turning method as claimed in any one preceding claim. 25. A metal turning machine and computer control therefor programmed to perform a hard turning operation in accordance with any one of claims 1 to 23. 26. A metal turning machine in combination with a computer based control system therefor, when programmed to perform a hard turning process on a component in accordance with any one of claims 1 to 23. 27. A computer when programmed to control a metal working machine so as to perform a hard turning process according to any one of claims 1 to 23 on a component. 28. A programme adapted to operate a computer so as to provide control signals for a metal working machine to cause the latter to perform a hard turning process according to any one of claims 1 to 23. 29. A computer programme when stored on a data carrier for operating a computer so as to control a metal working machine to perform a hard turning process on a component according to any one of claims 1 to 23. 30. A programmed computer or a computer programme for operating a computer, adapted to control the operation of a metal machining process involving the removal of metal from a rotating workpiece by the engagement therewith of the tip of a metal cutting tool, at least the position of which is controlled by the said computer, and which as a result of synchronised relative movement between the tool and the workpiece, would produce a smooth machined surface thereon, wherein the programme serves to alter the instantaneous position of the tool so as to introduce into the otherwise smooth surface, during the machining process, plural spaced apart depressions for the purpose of simulating a surface typ. cal of that which would be obtained thereon if the latter had been finished by grinding. 31. A metal turning machine in combination with a computer based control system therefor, when programmed to perform a hard turning process on a rotating workpiece involving the removal of 7 ^ metal from the surface thereof by the engagement therewith of the tip of a metal cutting tool, at least the position of which is controlled by the said computer based control system, and which as a result of synchronised relative movement between the tool and the workpiece, would produce a smooth surface thereon, wherein the programme serves to alter the instantaneous position of the tool during the machining process, so as to introduce into the otherwise smooth surface plural spaced apart depressions, for the purpose of simulating a surface typical of that which would be obtained on the workpiece if the latter had been finished by grinding. 32. A method or apparatus according to any one preceding claim, which further comprises gauging and/or measuring the machined part during the machining process, to generate signals indicative of one or more dimensions of the machined part, and supplying the signals to the computer, to assist in the control of the machining process. 33. A machine tool in combination with a computer based control system therefor, when programmed to perform a machining process on a workpiece, involving the removal of material from the workpiece by the engagement therewith of a cutting tool, at least the position of which is controlled by the said computer based control system and which, as a result of synchronised relative movement between the tool and the workpiece, would produce a smooth surface on the machined part, wherein the programme serves to alter the instantaneous position of the tool so as to introduce into the otherwise smooth surface of the machined part, plural spaced apart depressions during the machining process, for the purpose of simulating a surface typical of that which would be obtained thereon if the latter had been finished by grinding. 34. A machine tool according to claim 33, further comprising at least one gauging or measuring device adapted to perform measurements on the workpiece during the machining process, to generate signals indicative of one or more dimensions of the workpiece, and means for conveying the signals to the computer workpiece, and means for conveying the signals to the computer as feedback signals indicative of how the process is progressing, to assist in the control of the process.

Inventors: MANLEY STEPHEN ROBERT, RILEY MARTIN JOHN, MANLEY, STEPHEN, ROBERT, RILEY, MARTIN, JOHN
Applicants: MANLEY STEPHEN ROBERT, RILEY MARTIN JOHN, UNOVA UK LTD, MANLEY, STEPHEN, ROBERT, RILEY, MARTIN, JOHN, UNOVA U.K. LIMITED
Application year: 2000
Main classification: B23B1
IPC codes: B23B1, G05B19