Patent Application
20200156204
The invention relates to a honing method for machining the internal face of a bore in a workpiece with the aid of at least one honing operation, as claimed in the preamble of claim 1, as well as to a machine tool configured for carrying out the honing method, as claimed in the preamble of claim 14. A preferred field of application is the honing of cylinder running faces in the production of cylinder blocks or cylinder liners for reciprocating piston engines.
The cylinder running faces in cylinder blocks (cylinder crank cases) or cylinder liners of internal combustion engines or other reciprocating piston engines in operation are exposed to a heavy tribological stress. In the production of cylinder blocks or cylinder liners it is therefore important for said cylinder running faces to be machined such that adequate lubrication by way of a lubricant film is guaranteed later in all operating conditions, and the friction resistance between parts that move relative to one another is kept ideally minor.
The quality-determining final machining of such internal faces that can be tribologically stressed is in general performed using suitable honing methods which typically comprise a plurality of successive honing operations. Honing is a subtractive method having cutters that are not determined in geometric terms. In the case of a honing operation, a flaring-capable honing tool is moved back and forth at a stroke frequency within the bore to be machined for generating a reciprocating movement in the axial direction of the bore, and is simultaneously rotated at a predefinable rotating speed for generating a rotating movement that superimposes the reciprocating movement. For flaring the honing tool, the cutting material members that are attached to the honing tool are actuated by way of an actuator system having an actuating force and/or an actuation rate that act/acts radially in relation to the tool axis and are pressed against the internal face to be machined. A cross-grinding pattern having mutually crossing machining traces which are also referred to as “honing furrows” is typically created on the internal face when honing, said cross-grinding pattern being typical for the machining by honing.
As the requirements in terms of the economy and environmental friendliness of motors increase, optimizing the tribological system of pistons/piston rings/cylinder running face is of particular importance in order to achieve low friction, low wear, and low oil consumption.
The piston group in terms of friction can have a proportion of up to 35% so that a reduction in terms of friction in this region is desirable.
A honing method in which a bottle-shaped bore that is rotationally symmetrical in terms of the bore axis and proximal to the bore entry is generated that is tighter than more distal from the bore entry is described in WO 2014/146919 A1. In the case of one variant of the method a honing tool having relative long honing strips in axial terms is used. In order for an axially variable material removal to be generated, the stroke length and/or the stroke orientation of the reciprocating movement are/is modified in a stroke modification phase. On account thereof, an axial contour profile can be generated or modified. Honing tools which have at least one annular cutting group which is relatively short in the axial direction are also described in the application. Bore shapes having an axial contour profile can be machined in a particularly precise and economical manner when using honing tools of this type. Air measurement nozzles of a diameter measurement system are integrated in the case of many of the honing tools described.
DE 10 2015 203 051 A1 describes a honing method by way of which a bore shape which is rotationally symmetrical in terms of a bore axis and has an axial contour profile that deviates from the circular cylindrical shape can be generated. In order for an axially variable material removal to be generated, the stroke length and/or the stroke orientation of the reciprocating movement are/is modified in at least one stroke modification phase. In the case of one of the variants described, the position of an upper reversal point and/or the position of the lower reversal point of the reciprocating movement is modified according to the stipulation of a presetting in order for the stroke length and/or the stroke orientation to be modified during the stroke modification phase. Controlling the torque transmitted by way of the spindle to the honing tool in the case of the methods is performed in a manner that the torque remains substantially consistent during the stroke modification phase. On account thereof, bores which in the completely machine state have an axial contour profile and which comprise the desired contour profile with sufficient precision across the entire relevant bore length can be generated.
DE 10 2015 209 609 A1 describes honing methods and honing tools for the “sonification” of rotationally symmetrical non-cylindrical bores. Honing methods in which the stroke lengths of successive double strokes are incrementally modified by modifying the position of the upper reversal point are inter alia described. In the case of one embodiment herein, an incremental reduction in the stroke length is implemented with a consistent lower reversal point. The honing procedure is in each case terminated as soon as the upper reversal point reaches a predefined terminal value. Similar methods are also described in DE 10 2015 221 714 A1.
It is an object of the invention to provide a honing method of the type mentioned at the outset which permits the desired contour profile to be generated with high precision across the entire relevant bore length, in particular by way of an improved uniformity of the contour profile in bores which in the completely machine state are to have an axial contour profile. It is a further object to provide a machine tool configured for carrying out the honing method.
This object is achieved by a honing method having the features of claim 1. The object is furthermore achieved by a machine tool having the features of claim 14. Advantageous refinements are stated in the dependent claims. The wording of all of the claims is incorporated in the description by way of reference.
The honing method is associated with the generic type of honing methods in which a bore shape which is rotationally symmetrical in relation to the bore axis and has an axial contour profile is generated with the aid of a variable material removal in the axial direction of the bore. Such a bore shape is a bore shape which is rotationally symmetrical in terms of the bore axis and which significantly deviates from a circular cylindrical shape. In the case of a bore having an axial contour profile there is at least one portion in which the diameter of the bore continuously increases or continuously decreases in the axial direction. A typical example of a bore shape having an axial contour profile is a conical bore in which the diameter increases in a more or less linear manner between a plane of the portion having the smallest diameter and a plane of the portion having the largest diameter. Variations of the diameter that are non-linear in the axial direction are also possible. A bore shape having an axial contour profile can also have one portion or a plurality of portions having a circular cylindrical shape, thus such portions in the case of which the nominal diameter does not vary in the axial direction.
In order for an axially variable material removal to be generated, the stroke length and/or the stroke orientation of the reciprocating movement are/is modified in at least one stroke modification phase. It can be achieved on account thereof that the cutting material members provided on the honing tool pass through, or overlap, respectively, some axial portions more often in total than other axial portions such that the material removal caused on account of the contact with the cutting material members is dissimilarly intense in dissimilar axial portions, primarily or substantially, respectively, by virtue of dissimilar numbers of overlapping honing actions.
In the case of the honing method, at least in the case of that honing operation that includes the stroke modification phase, a honing tool which has an annular cutting group having a plurality of radially actuatable cutting material members that are distributed about the circumference of the tool body is used. Preferred design embodiments of honing tools of this type and advantages pertaining to this type of honing operations will be described in detail further below.
A particularity of the honing method lies in that during the stroke modification phase, thus during the phase in which the stroke length and/or the stroke orientation of the reciprocating movement changes, a measurement of the actual diameter of the bore is carried out for determining a diameter measurement signal which represents the actual diameter of the bore in an associated measurement plane, and in that consequently the stroke length and/or the stroke orientation of the reciprocating movement is variably controlled as a function of said diameter measurement signal. A closed-loop control circuit (feedback control) is thus provided, said closed-loop control circuit leading to the stroke length and/or the stroke orientation of reciprocating movements during the stroke modification phase as a controlled variable being influenced by the results of a diameter measurement carried out during the machining such that during the ongoing honing method influence is exerted automatically, that is to say in particular without any intervention of an operator, on the stroke, or on the stroke modification of the axial component of the honing tool movement by way of the diameter measurement. The detection of the diameter measurement signal utilized for controlling takes place according to the stipulation of a predefinable measuring condition, for example in at least one predefinable phase of a reciprocating movement.
This approach in the context of this application is also referred to as an “automatic contour control” since the reciprocating movements during the stroke modification phase are no longer controlled according to the stipulation of a presetting (open-loop control), as is the case in the prior art, but can change dynamically as a function of diameter measurement signals during the machining (closed-loop control). It has been demonstrated on account thereof that the actual contour profile present at the end of the honing operation, independently of variations in the workpiece properties and/or variations in the environmental conditions and/or changes in the cutting behavior of the cutting material members during the honing, is systematically and uniformly substantially closer to the ideally desired nominal contour profile across the machined length than in the case of contour honing controlled exclusively without feedback by way of an in-process-measurement system. On account thereof, the method systematically meets the increased requirements in terms of accuracy when contour honing rotationally symmetrical bores having an axial contour profile.
In the context of the development of the honing method it has been recognized inter alia that the in-process-measurement systems available nowadays are not sufficiently dynamic in order to measure non-circular cylindrical bores with sufficiently high accuracy. On the other hand, the requirements set for the accuracy of contour honing steadily increase. While earlier bores having an axial contour profile were awarded relatively generous tolerances in terms of the smallest and the largest diameter of said bores, there are in the meantime significantly higher requirements in terms of accuracy not only pertaining to the smallest and the largest diameter but to the entire contour profile. It has been recognized inter alia that the problem of in-process measurement of a non-cylindrical bore can be solved in that a desired axial contour, that is to say the axial nominal contour profile, is divided into many nested virtual cylinders.
The increased requirements in terms of accuracy to date could only be achieved, if at all, by way of a highly increased complexity in terms of construction and/or control technology. In the context of the development of the present invention it has been recognized inter alia that the at all times variable cutting behavior of the cutting material members (for example honing strips) when honing represents a great challenge which, if at all, can only be mastered insufficiently using conventional methods. Honing methods according to the invention having automatic contour control when honing can also take into account said effects that arise during the honing process and are difficult to predict. Apart from relatively simple contours such as, for example, a purely conical bore shape, the automatic contour control moreover permits almost arbitrary rotationally symmetrical contours to be produced in a precisely controlled manner.
In the case of preferred variants of the method, a reciprocating movement during the stroke modification phase comprises a multiplicity of successive strokes which run in each case between a lower reversal point and an upper reversal point, wherein the axial position of at least one of the reversal points of a stroke is dynamically modified as a function of a diameter measurement signal that is determined in a preceding stroke. The term “upper reversal point” refers to that reversal point which lies closer to the bore entry through which the honing tool is introduced into the bore. In principle, the honing method can be utilized not only in vertical honing but also in horizontal honing.
The axial position of at least one reversal point of a stroke is preferably modified as a function of a diameter measurement signal which has been determined in the directly preceding stroke, such that the stroke length and/or the stroke orientation of each individual double stroke can be influenced individually as a function of a diameter measurement value in the preceding double stroke. On account thereof, rapid reaction times can be achieved in the case of potential diameter deviations, on account of which precise contour profiles can be generated.
It is particularly preferable for the axial position of one of the reversal points to be fixed, and only the axial position of the other reversal point to be dynamically varied as a function of the diameter measurement signal. Particularly simple and precise controlling can be implemented on account thereof.
A bore shape can in particular have the visual appearance that the axial contour profile has a portion (at least one portion) in which the nominal diameter continuously increases between a first axial position having a smallest diameter, and a second axial position having a largest diameter within the portion, wherein the reversal point associated with the second axial position is fixed and the axial position of the other reversal point is dynamically varied as a function of the diameter measurement signal. A contour which is, for example, conical or truncated-conical, respectively, is thus present within such a portion when the nominal diameter varies in a manner linear with the axial position. A non-linear variation of the diameter in the axial direction is also possible such that the shell line of a portion having an axially variable diameter does not have to be straight but can be curved in a convex or concave manner. The variation of the axial position of the other reversal point typically leads to a shortening of the stroke which can also be described such that the reversal point associated with the smaller diameter moves in steps (in increments, incrementally) in the direction of the second axial position (having the relatively largest diameter).
In principle, it is also possible for the stroke length to increase between a preceding stroke and a directly subsequent stroke when the in-process measurement indicates that this is conducive to the purpose of achieving an improved contour.
In the case of preferred embodiments one proceeds such that prior to the honing operation an axial nominal contour profile which represents a nominal diameter of the bore as a function of the axial stroke position is predefined, that in a stroke the actual diameter measured at a stroke position is compared with the nominal diameter associated with the stroke position, and that a diameter deviation for the stroke position is determined from the comparison, wherein the stroke length and/or the stroke orientation of a subsequent stroke in relation to a nominal stroke length and/or a nominal stroke orientation is modified as a function of the diameter deviation. This controlling intervention is preferably not performed in the case of arbitrary small diameter deviations but only when the diameter deviation exceeds a predefinable limit value (thus a permissible control deviation). Unnecessary controlling interventions can be avoided in this way. The stroke length and/or the stroke orientation of each individual stroke is preferably calculated based on the diameter deviation determined immediately prior thereto.
The nominal contour profile can be predefined, for example, in the form of an analytical formula (for example a linear equation or a non-linear equation), or as a point grid.
A control strategy is preferably programmed such that the stroke length and/or the stroke orientation of a reciprocating movement is modified as a function of the determined diameter deviation in such a manner that the diameter deviation associated with the axial position at the axial position is at least in part compensated for by modifying the number of overlapping honing actions at the axial position. The term “overlapping honing actions” herein refers to the situation in which a cutting material member at a specific contact pressure runs across a surface region and on account thereof removes material. The material removal in the case of otherwise approximately identical conditions (for example, cutting rate, contact pressure) is the greater the more overlapping honing actions take place at one location. The overlapping honing action can also be referred to as a subtractive stroke repetition.
For example, if the measured actual diameter is too large at a specific axial position (that is to say larger than the nominal diameter which results from the nominal contour profile for the axial position) an increment of a stroke length reduction that tends to be larger can thus be set in order for subsequent double strokes to no longer reach the measurement location, thus the axial position for which a diameter deviation has been established, or reach said measurement location only less frequently than without the controlling intervention, so as to not remove further material or ideally very little further material from there. On the other hand, a stroke increment can also be reduced when the measured diameter is too small at an axial position such that further machining can take place with the aid of the subsequent strokes in a measured region.
When machining using an incrementally decreasing stroke length, for example, there is in principle also the possibility for the stroke length to be lengthened again between one double stroke and the subsequent double stroke such that no step-by-step decrease in terms of the stroke length takes place, but an increase in the stroke length can take place at least in phases. It is thus in particular possible for a reversal point to be initially relocated repeatedly and successively in a step-by-step manner in one direction, and for a relocation in the opposite direction to then take place for one or a few strokes.
According to one refinement it is provided that in the diameter measurement for determining a diameter measurement value provided for the nominal/actual comparison, or a corresponding diameter measurement signal, respectively, a measured value detection takes place by way of a floating mean value, for example by way of an adjustable quantity of measuring points, wherein forming the floating mean value can cover a portion of the bore or the entire currently machined portion of the bore. Particularly robust values for controlling can be provided on account thereof.
There are various possibilities for the actual/nominal comparison.
For example, diameter measurement values at the reversal points (at one or both reversal points) can thus be compared. To this end it is provided in the case of many variants that the measurement of the actual diameter is carried out when the honing tool is situated in the region of a reversal point of the reciprocating movement. It can be achieved on account thereof that a detection of a measured value takes place when an ideally low axial velocity, or no axial velocity, is present. An exact assignment of the diameter measurement value and the stroke position is achieved on account thereof. Moreover, the measuring accuracy can be particularly high at these conditions.
The measurement can be carried out such that the honing tool during the diameter measurement rotates in an axially narrow measurement zone in the region of a reversal point, such that a plurality of diameter measurement values can be detected within the narrow measurement zone. It is preferably provided that averaging across the plurality of diameter measurement values detected in short succession takes place in order for an averaged diameter measurement value subject to a minor measuring error to be obtained for the axially narrow region of the reversal point. Said averaged diameter measurement value is then utilized for controlling.
It is also possible for the diameter measurement signals utilized for the nominal/actual comparison and/or forming the mean value to be detected in an intermediate region between the reversal points, in particular in a central region between the reversal points. Measurement signals which have been detected for axial positions in the region between the reversal points in comparison to measurement signals from the region of reversal points often have a more attenuated or more uniform, respectively, profile, on account of which better controlling is enabled. It is assumed that more attenuated measurement signals are present, since a reciprocating movement at a more or less constant velocity is present in the central region such that the measurement signals utilized for forming the mean value are detected during a “constant travel” in the axial direction.
Measuring and/or comparing can thus also take place in the center of the bore or at any other arbitrary axial position. The reason for this lies inter alia in the particular approach when establishing the contour, as proposed in this application, in which a desired axial contour, that is to say the axial nominal contour profile, can be divided into many nested virtual cylinders, and in which the current machining at all times refers to a cylindrical portion of the bore.
Considering the limited accuracy of in-process-measurement, it can be advantageous for an ideally large averaging of the measurements, or averaging across a relatively large axial region, respectively, to be performed. Said averaging is preferably performed across the entire currently machined portion of the bore. Such averaging is permissible since the machining is at all times performed in a cylinder in the case of the proposed method. The accuracy of the contour can be significantly increased on account of an ideally large averaging. It is presumed that the increased precision can inter alia be traced back to measurement signals which have been detected across a comparatively large axial region between the reversal points having a more attenuated, or more uniform, respectively, profile in comparison to measurement signals from the region of reversal points, on account of which better controlling is enabled. A “per stroke” measurement can better reflect the true conditions, since all regions of the bore are included in the “relevant” detection of measured values.
The automatic contour control described here, with the different optional variants thereof, can be implemented in combination with various honing strategies.
For example, it can be advantageous for the honing process to be managed such that a continuous actuation of cutting material members of the honing tool takes place during the stroke modification phase. The continuous actuation can take place, for example, by way of a constant path or a constant force. The actuation can in particular be controlled by way of a substantially constant actuation speed. It can be ensured on account thereof that any diameter deviations measured are addressed in a substantially exclusive manner by way of the controlled process variable, specifically a potential modification of the stroke length and/or the stroke orientation. This can contribute toward approximating the axial contour profile with high accuracy to the axial nominal contour profile across the entire length of the machined bore.
In general, the honing method permits an arbitrary path-controlled or force-controlled actuation, corresponding to the specific application.
It is possible for the honing process to be controlled such that, additionally to the stroke length and/or the stroke orientation, also the contact pressure force of cutting material members on the bore internal wall is modified as a function of a command variable, for example as a function of the axial position. However, it is preferably provided that controlling of the contact pressure force of cutting material members of the honing tool that is transmitted to the honing tool takes place in a manner that the contact pressure force remains substantially constant during the stroke modification phase. It can be achieved on account thereof that the cause for the dissimilar intense material subtractions is determined substantially only by travelling across axial portions by means of the cutting material members at dissimilar frequencies, but not on account of a modification of the contact pressure force. It can thus be achieved, on the one hand, that substantially the same surface quality is generated in regions of dissimilar local diameters, for example a uniform roughness across the entire region having the axial contour profile, despite the axial contour profile of the bore to be achieved. Moreover, it can be ensured that any diameter deviation from a nominal diameter is compensated for exclusively by way of a targeted modification of the stroke length and/or the stroke orientation of the reciprocating movement, and said controlling action is not counteracted by any other influence variables (such as a variable contact pressure, for instance).
In the case of the honing method, at least in the case of that honing operation that includes the stroke modification phase, a honing tool which has an annular cutting group having a plurality of radially actuatable cutting material members that are distributed on the circumference of the tool body is used, wherein the annular cutting group is relatively short in the axial direction. For example, honing tools which are disclosed in WO 2014/146919 A1 can be used. The disclosed content of said document is to this extent incorporated in the present description.
The axial length of the cutting material members should preferably be less than 50%, in particular between 10% and 30%, of the effective external diameter of the cutting group. In absolute values, the axial length of the cutting material members can be in the range from 5 mm to 90 mm, in particular in the range from 10 mm to 50 mm, for example. When the axial length of the cutting material members is referenced with the bore length, it is typically preferable for the axial length be less than 35% of the bore length so that the cutting material members can generate an axial contour profile with high accuracy.
In order to be able to carry out the in-process-measurement during the machining by honing, at least one measuring sensor of a diameter measurement system is attached to the honing tool. In particular, measuring nozzles of a pneumatic diameter measurement system can be attached to the honing tool. The measuring sensors are preferably attached in the axial region of the cutting material members, for example approximately at half the height of the axial length of the cutting group. A precise diameter measurement in the direct proximity of the location of the current material removal is thus possible, on account of which very precise current diameters (actual diameter values) are available for controlling the honing process, said current diameters being able to be very accurately assigned to the associated axial position.
The honing tool can be equipped having a simple flaring or having a double flaring (that is to say having two sub-groups of cutting material members within one annular cutting group that are actuatable in a mutually independent manner). Non-cutting guide strips for guiding the honing tool in the bore can be provided on the honing tool. The guide strips can be fixedly assembled on the tool body, or can be actuatable separately from the cutting material members (cf. DE 10 2014 212 941 A1, for example).
The invention also relates to a machine tool configured for carrying out the honing method. This herein can be a specialized honing machine or any other machine tool which offers the functionalities required here.
Further advantages and aspects of the invention are derived from the claims and from the description hereunder of preferred exemplary embodiments of the invention, which are explained hereunder by means of the figures in which:
Some components of a honing machine 100 which in the context of various embodiments of methods according to the invention can be used as a machine tool for machining internal faces of bores in workpieces is schematically shown in
A workpiece holding device 104 which supports a workpiece 200 clamped thereon is assembled on a workpiece support (not illustrated in more detail) of the honing machine. The workpiece in the exemplary case is an engine block (cylinder crankcase) of a multi-cylinder internal combustion engine. A plurality of bores 210 having a generally vertical alignment of the bore axes thereof are formed in the engine block. The bores are also referred to as cylinder bores, even when the bore shape thereof significantly deviates from the shape of an ideal circular cylinder. In the case of the honing method described here, a bore shape which is rotationally symmetrical in terms of a bore axis 212 and deviates from the circular cylindrical shape and has an axial contour profile is generated by means of honing. The term “axial contour profile” here means in particular that the bore has dissimilar diameters at different axial positions. The cylinder running faces formed by the internal faces 214 of the cylinder bores are subjected to quality-determining final machining with the aid of the honing machine, wherein the macro shape of the cylinder running faces (thus the macroscopic design of the bore) as well as the surface topography thereof are generated by way of suitable honing operations.
The nominal shape of the bore is rotationally symmetrical in terms of the bore axis 212 thereof, and from a bore entry 214 that in the installed state faces the cylinder head extends across a bore length L to the bore exit 216 at the opposite end. The bore in the completely honed state should have a substantially conical, or truncated conical, respectively, design.
The nominal contour profile 215 of the bore can be seen by way of a dashed line in the lower portion of the bore 210, said portion not yet having been machined. The nominal diameter continuously increases in the linear manner from the bore entry to the bore end. The cone angle (angle between the bore axis and a shell line of the bore running in an axial plane) can be, for example, in the range of less than 5°, even less than 1°, possibly even be 0.2° or less.
The diameter difference between the first diameter D1 at the bore entry and the second diameter D2 at the end remote from the entry is significantly outside the tolerances that are typical for the conventional cylindrical machining by honing, said tolerances for a cylindrical shape being in the magnitude of at most 10 μm (in terms of the diameter). In the case of an absolute value of the internal diameter in the magnitude between 50 mm and 500 mm (the latter in the case of marine engines, for example) the maximum diameter difference can be between 20 μm and 500 μm, for example.
The dimensions can be optimized such that a low blow-by, a low oil consumption, and low wear on the piston rings result in typical operating states of the engine.
The honing machine 100 has a plurality of honing units. A few components of a honing unit 110 are schematically illustrated in
A toggle link is attached at the lower end of the honing spindle, the honing tool 150 serving as the machining tool being mechanically coupled to the lower free end of said toggle link so as to be movable in a limited manner, said coupling being performed by way of a bayonet connection, for example. The honing tool can have an integrated joint so as to enable a limited mobility in relation to the toggle link.
The honing tool is particularly suitable for machining rotationally symmetrical bores which have bore portions of dissimilar diameters and/or dissimilar designs, for example bottle-shaped bores, barrel-shaped bores, and/or bores which have at least one conical bore portion having a diameter which is continuously variable in axial terms. The honing tool can however also be utilized for machining circular-cylindrical bores, thus rotationally symmetrical bores without an axial contour profile.
The honing tool has a tool body 152 which is made from a steel material and defines a tool axis which simultaneously is the rotation axis of the honing tool during the machining by honing. A coupling structure for coupling the honing tool to a drive rod or an operating spindle of a honing machine is situated at the spindle-side end of the honing tool.
A single flaring-capable annular cutting group 155 is situated at the end portion of the tool body that faces away from the spindle, said cutting group 155 having a multiplicity of cutting material members 156 which are distributed about the circumference of the tool body, the axial length of said cutting material members 156 measured in the axial direction being smaller by a multiple than the effective external diameter of the cutting group 155 in the case of cutting material members that are fully retracted in the radial direction. The cutting material members are configured as cutting material strips that are narrow in the circumferential direction, the width of said cutting material members measured in the circumferential direction being small in relation to the axial length of the cutting material strips. An aspect ratio between the length and the width can be in the range from 4:1 to 20:1, for example.
The honing tool has only a single annular cutting group 135. Said cutting group 135 is disposed so as to be more or less flush with the end of the tool body that is remote from the spindle such that pocket hole bores can optionally also be machined down to the bore base.
The cutting group, or the cutting material members of the cutting group, respectively, is/are actuatable in a radial manner in relation to the tool axis by means of an actuator system assigned to the cutting group. Since the functionality typical of honing tools is known per se, the components provided to this end (for example actuator rod(s), flaring cone, etc.) are not described in more detail here.
The honing tool can be equipped with single flaring or double flaring. In the case of single flaring, all cutting material members of the cutting group are collectively actuated in a radial manner. In the case of double flaring, the cutting group has two sub-groups of cutting material members which can be actuated in a mutually separate manner. The cutting material members of the sub-groups can have grit sizes of dissimilar fineness or coarseness, for example, such that, for example, after a pre-honing operation by means of a first sub-group having comparatively coarse cutting material members a final honing stage can be carried out using the cutting material members of the second sub-group without a tool change.
The flaring-capable annular cutting group 130 comprises a plurality of radially actuatable cutting material member supports 158 which cover in each case one circumferential angle range which is greater than the axial length of the cutting material members, or of the cutting group, respectively. In the exemplary case of
A reciprocating drive 160 of the honing machine is provided for causing vertical movements of the honing spindle in a manner parallel with the spindle axis 142. The reciprocating drive causes, for example, the vertical movement of the honing spindle when introducing the honing tool into the workpiece, or when retracting said honing tool from the workpiece, respectively. The reciprocating drive during the machining by honing is actuated such that the honing tool within the bore 210 of the workpiece carries out an oscillating reciprocating movement, thus a back-and-forth movement in a manner substantially parallel with the spindle axis.
The reciprocating movement can be characterized by various parameters. The stroke length herein corresponds to the axial spacing between an upper reversal point UO and a lower reversal point UU of the reciprocating movement (cf.
The honing machine is equipped with an actuating system which permits the effective diameter of the honing tool (plus the external diameter of the cutting group) to be modified by actuating in the radial direction cutting material members 156 attached to the honing tool. This flaring can be implemented in a force-controlled or path-controlled manner, for example, by means of a servomotor. An hydraulic actuation is also possible. An actuating system having a single actuation or a double actuation can be provided. Since such actuating systems are known per se, a detailed description is dispensed with here.
The honing machine 100 is furthermore equipped with a diameter measurement system 170 for measuring the actual diameter of the bore in predefinable measurement planes or measurement zones during the machining by honing (in-process measurement). To this end, measuring sensors of the diameter measurement system are attached to the honing tool 150. The diameter measurement system in the exemplary case is conceived as a pneumatic diameter measurement system (air measurement system). Accordingly, the honing tool at two diametrically opposite positions between neighboring cutting material members has in each case one measuring nozzle 172-1, 172-2 of the diameter measurement system. A very exact diameter measurement of the currently machined bore portion is possible by virtue of the arrangement of the measuring nozzles in the axial region of the cutting material members, for example at mid-height, so as to be centric in the narrow zone of the ring occupied by the cutting material members.
The diameter measurement system can operate according to the nozzle/impact plate principle. Compressed air from the measuring nozzles for the measurement herein is blown in the direction of the bore wall, or the internal face 2014, respectively. The backpressure resulting in the region of the measuring nozzles can serve as a measure for the spacing of the measuring nozzle from the bore wall. A measurement transducer connected to the measuring nozzle by way of a pressurized line ensures the conversion of the (pneumatic) pressure signal to a voltage signal which can be electrically further processed and here is referred to as the diameter measurement signal. Instead of the backpressure, the volumetric flow of the compressed air can also be used for the evaluation. Diameter measurement systems which operate according to other principles, for example capacitive measurement systems or inductive measurement systems, or measurement systems using radar sensors (cf. DE 10 2010 011 470 A1, for example) can also be used in principle.
The spindle drive, the reciprocating drive, the at least one drive of the actuating system, as well as the converter of the diameter measurement system are connected to a control installation 180 which is a functional component part of the machine controller and can be operated by way of an operator installation 190. Numerous process parameters required for defining the honing process can be set by a machine operator by way of the operator installation.
An axial nominal contour profile which represents the nominal diameter as a function of the axial stroke position in the bore to be machined can inter-alia be predefined prior to the beginning of the honing operation. The predefining of the nominal contour profile, thus the predefining of the contour honing, can take place, for example, by defining the contour as an analytic formula (for example a linear equation or a non-linear equation) or as a point grid (for generating free-form curves).
The holding machine for generating a specific axial contour profile on the bore can be programmed such that an axially variable material removal can be generated in a targeted manner in at least one stroke modification phase by way of a targeted modification of the stroke length and/or the stroke orientation of the honing tool, so as to achieve in this way with high accuracy parameters in terms of the axial contour profile.
To this end, a measurement of the actual diameter of the bore is carried out during a stroke modification phase according to the stipulation of a pre-definable measuring condition, so as to determine a diameter measurement signal which corresponds to the actual diameter of the bore in that measurement plane or narrow measurement zone in which the measurement has been carried out. The stroke length and/or the stroke orientation of the reciprocating movement are then variably controlled as a function of the diameter measurement signal. A controlled contour honing process can be implemented in this way. Some basic considerations and technical measures for implementing this principle will be explained hereunder in an exemplary manner by means of instructive exemplary embodiments.
A problem of many in-process measurement systems available today lies in that said in-process measurement systems are not sufficiently dynamic so as to be able to measure with sufficiently high accuracy non-cylindrical bores, in particular such bores having an axial contour profile. The problem of the in-process measurement of a non-cylindrical bore can be solved in that a desired axial contour, that is to say the axial nominal contour profile, is divided into many nested virtual cylinders.
It is shown how this rotationally symmetrical but non-circular cylindrical bore shape can be virtually divided into a multiplicity of circular cylinders Z1, Z2, . . . , Zn of dissimilar heights and dissimilar diameters. A non-cylindrical bore is thus never to be measured despite such a non-cylindrical bore nevertheless being generated at the end of the honing process. The contour honing by means of the modification of the stroke can thus be imagined as a series of classic cylindrical interlinked machining actions by honing.
In the case of one embodiment such machining is implemented in that the axial position of the lower reversal point UU of the reciprocating movement of successive double strokes remains fixed, or unchanged, respectively, while the upper reversal point UO is dynamically relocated in a step-by-step manner (incrementally) in the direction of the lower reversal point. In the exemplary case of a contour opening in a downward manner illustrated, the lower reversal point can remain fix and the upper reversal point is dynamically shortened.
However, the nested cylinders resulting on account thereof in the case of the honing method are not fixedly predetermined in terms of the diameter and the axial position of the upper end of said cylinders, but, by way of calculations within the control installation 190, are dynamically determined anew after each stroke in a manner corresponding to the current measured result of the diameter measurement, and are set in a corresponding manner with the aid of the reciprocating drive.
This takes place by determining the best adapted shortening of the upper reversal point UO to be dynamically modified according to the contour predefined by the machine operator.
Prior to beginning the honing operation in which an axial contour profile is to be generated, an axial nominal contour profile which represents the nominal diameter of the bore as a function of the axial stroke position is predefined. In the case of a pure conical shape of a bore of a predetermined bore length, it can be sufficient to enter only the diameter difference between the diameter at the upper end (relatively small nominal diameter) and the diameter at the lower end (relatively larger nominal diameter). The controller can therefrom calculate a linear equation which represents the nominal contour profile.
The predefining of the nominal contour profile, thus the predefining of the contour honing, can generally take place, for example, by defining the contour as an analytical formula (for example a linear equation or a non-linear equation) or as a point grid which describes the correlation between the stroke position and the desired contour dimension. In the simple case of a purely conical portion of the bore, the nominal contour profile can be indicated by a straight line. More complex cases can be indicated by correspondingly more complicated analytic formulae such as, for example, non-linear equations or by a points grid, wherein interpolation optionally takes place between the points.
The actual diameter measured at a predefined stroke position in a stroke is then compared with the nominal diameter of the nominal contour profile associated with the stroke position in a comparison operation. A diameter deviation for the stroke position is determined from the result of the comparison operation. The stroke length and/or the stroke orientation of a subsequent stroke is then modified relative to a nominal stroke length and/or a nominal stroke orientation according to the nominal contour profile as a function of the diameter deviation.
A control intervention in the sense of a modification of the stroke parameters typically takes place only when the diameter deviation exceeds a predefinable limit value so as to generate control interventions only when significant diameter deviations are to be determined.
The controlling is particularly simple when a portion to be honed is chosen such that the diameter within the portion continuously changes, thus continuously decreases or increases, in one direction. In each of these cases the stroke modification for the portion can be programmed such that one of the reversal points, specifically the reversal point associated with the larger diameter, remains fixed, while the other reversal point (closer to the tight end) by way of a variable step size moves step-by-step toward the fixed reversal point.
To the extent that the axial contour profile in a bore to be honed cannot be described as a portion across the complete length of said axial contour profile in which the diameter continuously changes always in the same direction (decrease or increase) between an extreme value and another extreme value, the bore for the purpose of controlling can be divided into a plurality of portions to which said preconditions then apply again. Said plurality of portions can then be sequentially worked according to the method described. A special case is present, for example, when there are two such portions and one of the portions in terms of the diameter grows in a downward manner, and the lower portion in terms of the diameter grows in an upward manner. In this case, both reversal points at the top and at the bottom can be simultaneously shortened in a manner corresponding to the current measured results and the predefined profile curve (axial nominal contour profile).
A honing operation by way of which a bore having a purely conical contour profile has been generated in the context of an experiment will now be explained by means of
In the case of the experiments, various process parameters that are relevant here were detected with the aid of a system for diagnosing machine parameters and the results evaluated.
In the honing operation shown, the rotating speed of the honing spindle in an initial phase was increased to a nominal value and then remained substantially constant during the entire honing operation. The reciprocating controller was set such that the stroke length (axial spacing between the upper and the lower reversal point) in an initial first honing phase PH1 was so large that the honing tool with the aid of the annular cutting group machines the entire bore length between the bore entry and the bore exit by means of a few complete double strokes. The axial positions of the upper reversal point UO and of the lower reversal point UU herein remained constant across a plurality of double strokes.
The subsequent second honing phase PH1 here is referred to as the stroke modification phase since the stroke orientation of the honing tool and/or the stroke length are modified, or can be modified, respectively, from one stroke to another stroke in said second honing phase. The term “stroke orientation” herein refers to the region between the upper reversal point UO of a reciprocating movement (close to the bore entry) and the lower reversal point UU of the reciprocating movement closer to the end of the bore that is remote from the entry, in each case in relation to a fixed machine coordinate system. Each axial relocation of the position of a reversal point thus also changes the stroke orientation. In the exemplary case of
The general trend in terms of the temporal profile of the upper reversal point herein is such that the axial position thereof across the honing time in a step-by-step manner approximates the lower reversal points such that the stroke length was reduced in a step-by-step manner from one stroke to another stroke. In this way, the portion of the bore that is more remote from the entry is machined by way of more strokes than the portion closer to the entry such that a plurality of overlapping honing actions take place in the portion that is more remote from the entry and thus more material is subtracted there than in the region closer to the entry.
A particularity of the method now lies in that the extent of the stroke shortening between successive strokes, or in more general terms: the absolute size of the stroke increment, respectively, is not fixedly predefined but can vary as a function of the results of the in-process diameter measurement.
Those diameter measurement values in the course of a stroke which were detected when the honing tool was situated at the upper reversal point UO or close thereto were in each case selected for the diameter measurement. Under these conditions, the diameter measurement value utilized for controlling is detected in a phase of a very low axial velocity of the honing tool such that the measuring sensors (air measuring sensors) are situated substantially in the same axial portion, or in an axially narrow measurement zone of the bore, respectively, over a comparatively long period, and the diameter present therein can thus be determined with high accuracy (possibly by way of forming a mean value). The axial extent of the measurement zone can be, for example, 10 mm or less, and in particular be in the range from 3 mm to 8 mm.
From the profile of the flaring position AP it can be seen that constant flaring, that is to say a constant actuation rate, was used in this exemplary embodiment such that of all of the process variables illustrated only the axial position of the upper reversal point varied as a function of the diameter measurement.
The honing operation is terminated when the diameter measurement indicates that the targeted contour profile has been reached within the tolerances. The switching-off thus takes place as a function of the in-process measurement.
In order for the control procedures to be better visualized, fragments of the diagram of
It can be particularly readily seen in the diagram of
In addition to the two curves mentioned above in another region of the stroke modification phase, the nominal value of the diameter measurement (DM-S) as well as the diameter actual value (DM-I), thus the result of the current diameter measurement, are illustrated in the region of the respective upper reversal point of the reciprocating movement in
This finely tuned controlling of the stroke lengths during the stroke modification phase leads to an axial contour profile which corresponded to the nominal contour profile with high accuracy (within a range of a few micrometers) across the entire bore length.
There are a plurality of possibilities in terms of the implementation of the concept of controlling. The variant described in more detail here provides a fixed predefining of nominal stroke shortenings (corresponding to the nominal contour profile) and a correction for determining the stroke shortenings actually implemented or set, respectively, in the process by way of the offset or the correction value, respectively, according to the current measured results. A calculation of the stroke shortening according to the removal, and predefining the contour without a nominal stroke shortening, is also possible, for example.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2017 210 187.3 | Jun 2017 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/065619 | 6/13/2018 | WO | 00 |
