GB2274796A - Abrasive belt grinding machine - Google Patents

Abstract

An abrasive belt grinding machine wherein a rotary workpiece such as a camshaft is to be mounted in a work holder and is to be treated at two or more axially spaced locations. The workpiece can be simultaneously treated by driven abrasive belts which are mounted on two or more discrete carriages or by one of two belts on a common carriage. The belts 36, 37 on a common carriage 18 can be selected to effect coarse and precision grinding of selected portions of the rotary workpiece 7 in the work holder. While a first shoe 38 urges one of the belts on the carriage 18 into material removing engagement with the workpiece, a second shoe 39 for the other belt is maintained in a retracted position in which the belt is out of contact with the workpiece. The carriage or carriages are movable transversely of the axis of the workpiece in the work holder and the work holder and/or the carriage or carriages are movable in the direction of rotational axis of the workpiece in the work holder to thus place selected portions of the workpiece into alignment with a selected belt on a single carriage or with selected belts on two or more discrete carriages. Each carriage can be moved at right angles to the axis of the workpiece or at an oblique angle to such axis. <IMAGE>

Description

ABRASIVE BELT GRINDING MACHINE The present invention relates to improvements in abrasive belt grinding machines, and more particularly to improvements in machines which can be utilized with advantage as grinders for the cams or lobes of camshafts or other workpieces having a plurality of axially spaced-apart non-circular portions. Machines of the just outlined character are disclosed, for example, in U.S. Pat. No.

4,833,834 granted May 30, 1989 to Patterson et al. for "Camshaft belt grinder" and in U.S. Pat. No. 4,945,683 granted August 7, 1990 to Phillips for "Abrasive belt grinding machine".

Heretofore known belt grinding machines are equipped with a work holder having means for rotatably supporting a workpiece, a motor or other suitable means for rotating a properly mounted workpiece about its rotational axis, a mobile grinding head which carries an endless abrasive belt and means for driving the belt, and means for moving the grinding head transversely of the axis of the workpiece in the work holder so that the belt can impart to a selected portion of the rotating workpiece a predetermined contour, e.g., the shape of a cam or lobe on a rotating blank which is to be converted into a crankshaft.

Additional abrasive belt grinding machines are disclosed in published German patent applications Serial Nos. 40 29 129 Al and 40 03 409 Al. The German application Serial No. 40 29 129 Al discloses a machine with a grinding unit which includes a single abrasive belt. A shoe is provided to urge the belt against a selected portion of the external surface of a rotary workpiece and, when the treatment of such selected portion is completed, the workpiece is shifted axially relative to the single grinding unit to thus locate another portion of the workpiece in proper position for removal of material from its exposed surface. Removal of material is controlled by a motor which moves the belt substantially radially of the rotating workpiece.The German application Serial No. 40 03 409 Al discloses an abrasive belt grinder wherein each of several portions of a rotary workpiece can be moved into engagement with a discrete driven belt. For example, the number of belts can match the number of cams or lobes on a camshaft so that each cam can be ground simultaneously with all other cams. Each of the two German applications discloses the utilization of a single type of abrasive belts, i.e., the quality of the abrasive surfaces of all belts to be used in a machine or which are actually used in a machine is the same. A drawback of such proposals is that the axially spaced-apart portions of a rotary workpiece cannot be treated in an optimal way. Thus, the finish with a coarse abrasive belt is not satisfactory if a precision finish is desired.By the same token, a belt which is designed to precision grind a workpiece cannot meet the requirements for rapid removal of substantial quantities of material from a rotating workpiece.

The invention is embodied in a machine for grinding to a predetermined contour a plurality of axially spaced-apart portions of a rotary workpiece, such as the cams of a camshaft for use in a motor vehicle. The improved apparatus comprises a work holder including means for rotatably mounting a workpiece, means for rotating the workpiece in the work holder, and at least one grinding unit comprising first and second abrasive belts, means for driving the belts, and means for selectively moving at least one of the belts into material removing engagement with a selected portion of a workpiece in the work holder.

The apparatus further comprises means for effecting a displacement of the work holder and the at least one grinding unit relative to each other substantially radially of a workpiece in the work holder. The means for effecting a displacement can include guide means movably mounting the at least one grinding unit and extending substantially transversely of the axis of the workpiece in the work holder.

The at least one grinding unit can further comprise a common support for the belts and rotary pulleys for the belts. The pulleys are rotatably mounted on or in the common support and include at least one idler pulley and at least one second pulley. The means for driving the belts can include means for rotating the at least one second pulley.

The means for driving the belts can comprise at least one rotary pulley which is common to the belts and means for rotating the at least one pulley.

The moving means can include a shoe for the at least one belt and means for shifting the shoe between a first position in which the at least one belt removes material from a rotating workpiece in the work holder and a second position in which the at least one belt is disengaged from the workpiece. The at least one grinding unit can further comprise means for tensioning the belts.

In accordance with a presently preferred embodiment of the invention, the at least one grinding unit comprises a support for the belts and a first shoe for the other of the two belts. The moving means of such machine can include a second shoe for the at least one belt and means for moving the second shoe relative to the support.

The first belt can include or constitute a precision grinding belt, and the second belt can include or constitute a coarse grinding belt.

The moving means can include means for selectively moving each of the belts into material removing engagement with a workpiece in the work holder, and such moving means can include a discrete shoe for each of the two belts and means for shifting each of the shoes between a retracted position and an operative position in which latter position the respective belt removes material from a workpiece in the work holder. The shifting means can include means for maintaining the shoe for one of the two belts in the operative position while the shoe for the other of the two belts is maintained in the retracted position.

The improved machine can further comprise means for displacing one of the two parts including the work holder and the at least one grinding unit relative to the other of such parts in the direction of the axis of the workpiece in the work holder.

The means for effecting displacement can include means for moving one of the parts including the at least one grinding unit and the work holder transversely of the axis of the workpiece in the work holder to thereby effect removal of material from the workpiece by at least one of the belts.

The just mentioned means for effecting displacement can include means for displacing one of the parts including the at least one grinding unit and the work holder at an oblique angle (other than 900) to the axis of the workpiece in the work holder.

The machine can further comprise a base for the at least one grinding unit, and the at least one grinding unit can be turnably mounted on the base for angular movement about a second axis which is substantially normal to the axis of the workpiece in the work holder. The second axis is or can be a vertical axis, and the axis of the workpiece which is installed in the work holder can be or is a substantially horizontal axis.

The improved machine can comprise at least one additional grinding unit which is adjacent the at least one grinding unit as seen in the direction of the axis of the workpiece in the work holder. The at least one additional grinding unit can include means (e.g., one or more abrasive belts) for removing material from a first portion of a workpiece in the work holder while at least one belt of the at least one grinding unit removes material from at least one second portion of a workpiece in the work holder.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved grinding machine itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain presently preferred specific embodiments with reference to the accompanying drawings.

FIG. 1 is a front elevational view of a portion of an abrasive belt grinding machine which embodies one form of the invention; and FIG. 2 is a plan view of the structure which is shown in FIG. 1.

The abrasive belt grinding machine which is shown in FIGS. 1 and 2 comprises a base or bed 1 supporting or embodying elongated guide means or tracks 2 for a work table or work holder 3. The latter is provided with a headstock 4, a tailstock 6 and a set of steady rests 14 to properly support an elongated rotary workpiece 7 having a rotational axis 13 extending in parallelism with the tracks 2. The work holder 3 is movable along the tracks 2 in directions which are indicated by a double-headed arrow 8.

The illustrated workpiece 4 is a blank of a camshaft (e.g., for use in a motor vehicle) having portions in the form of non-circular cams or lobes 9 spaced apart from each other in the direction of the tracks 2. The headstock 4 is provided with or cooperates with means 11 for rotating a properly installed workpiece 7 about the axis 13 in the direction of arrow C, and the tailstock 6 is provided with a center 12 for the adjacent end of the workpiece. The steady rests 14 serve to prop the median portion of the workpiece 7 between the rotating means 11 and the center 12, i.e., to prevent the workpiece from yielding to the pressure of a grinding tool during removal of material from the peripheral surface of a selected cam 9.The means for advancing the work holder 3 and the parts 4, 6, 7, 11, 12, 14 therein in directions indicated by the arrow 8 includes a prime mover for a rotary feed screw 16 which is mounted in the base 1 at a level below the work holder 3.

The base 1 further carries a second set of guide means or tracks 17 which extend transversely of the tracks 2. In the embodiment of FIGS. 1 and 2, the tracks 17 are exactly normal to the tracks 2. However, it is equally possible to install or shape the tracks 17 in such a way that they make an oblique angle with the tracks 2, i.e., with the axis 13 of a workpiece 7 which is properly mounted on the work holder 3. The tracks 2 and 17 will be positioned to make an oblique angle if the grinding tool or tools are to remove material from tapering (e.g., conical) surfaces or to provide selected portions of the workpiece 7 in the work holder 3 with tapering external surfaces.

The tracks 17 support and guide a carriage 18 which is reciprocable in directions indicated by a doubleheaded arrow 19 and forms part of or carries a grinding unit 22. The means for effecting a movement of the grinding unit 22 and the work holder 3 relative to each other transversely of the axis 13 includes a variable-speed reversible electric motor 21 or another suitable prime mover. In the illustrated grinding machine, the means for effecting such relative movement of the work holder 3 and carriage 18 is designed to move the carriage 18 (and hence the grinding unit 22) transversely of the axis 13, i.e., along the tracks 17. The aforementioned prime mover 21 can transmit motion to the carriage 18 by a suitable feed screw (not specifically shown but corresponding to the feed screw 16).

The grinding unit 22 comprises a composite frame including two parallel upright plate-like frame members 23, 24 and a substantially horizontal connecting member 26 between the members 23, 24. The frame member 23 is affixed to or forms part of the carriage 18, and one end of the connecting member 26 is affixed to the frame member 23 so that the connecting member is maintained in an overhung position. That end of the connecting member 26 which is remote from the frame member 23 is rigid with the frame member 24. FIG. 2 shows that the frame members 23, 24 are normal to the tracks 2 and hence parallel to the tracks 17.

In FIG. 1, the frame member 24 has been partly broken away to reveal at least some of those components which form part of the grinding unit 22 and are installed between the frame members 23 and 24.

The grinding unit 22 further comprises idler pulleys or rollers 27, 28 and driver pulleys or rollers 29, 31 having shafts with end portions mounted in the frame members 23 and 24. The means for transmitting torque to the driver pulleys 29, 31 includes a prime mover 34 (e.g., a variable-speed electric motor) and belt transmissions 32, 33. The transmission 32 transmits torque to the driver pulley 29, and the transmission 33 transmits torque to the driver pulley 31. Other means for rotating the pulleys can be utilized with equal or similar advantage.

The grinding unit 22 further comprises two grinding tools 36 and 37 each of which constitutes an endless abrasive belt. The two belts are disposed in two parallel vertical planes and each of these belts is trained over the idler pulleys or rollers 27, 28 as well as over the driver rollers of pulleys 29, 31. In the illustrated grinding machine, the planes of the belts 36, 37 are normal to the axis 13 of a workpiece 7 which is properly mounted on the work holder 3.

The grinding unit 22 also comprises two shoes 38 and 39 which can respectively engage the internal surfaces of the belts 36, 37 to move the material removing external surfaces of the belts into engagement with the peripheral surfaces of the aligned cams 9 on the workpiece 7 in the work holder 3. The shoes 38, 39 are designed to bias the respective belts 36, 37 against the adjacent cams 9 with a requisite force to ensure removal of material from the peripheral surfaces of the engaged cams 9 while the workpiece 7 rotates about its axis 13 and is propped by the steady rests 14 on the work holder 3. The shoe 38 is mounted on or forms part of the free end of one arm of a bell crank lever 41.The free end of the other arm of the lever 41 is pivotably mounted on a horizontal shaft 42 in the carriage 18, and this lever can be pivoted between the illustrated extended or operative position and a retracted or inoperative position by a fluid-operated motor 43 here shown as a hydraulic or pneumatic cylinder and piston unit.

A stop 44 is provided on the carriage 18 to arrest the lever 43 and the respective shoe 38 in the illustrated operative position. A second bell crank lever 46 mounts the shoe 39 and is pivotable about the axis of the shaft 42 by a second fluid-operated motor 47. FIG. 1 shows the second shoe 39 (for the belt 37) in the retracted or inoperative position. The arrangement is preferably such that the motor 47 maintain the shoe 39 in the retracted position of FIG. 1 when the motor 43 maintains the shoe 38 in the illustrated extended or operative position, and vice versa. The stop 44 is designed to serve as a means for limiting the forward movement of the shoe 38 or 39 beyond the operative position. The shaft 42 has end portions journalled in the frame members 23, 24. The illustrated stop 44 is also connected to and is disposed between the frame members 23, 24.

It is possible to modify the illustrated grinding machine by fixedly securing one of the shoes 38, 39 to the carriage 18 for the grinding unit 22 or to a fixed part of the grinding unit (e.g., to the frame including the members 23, 24 and 26). The corresponding belt 36 or 37 is then movable into material-removing engagement with a selected cam 9 on the workpiece 7 in the work holder 3 by moving the carriage 18 along the tracks 17 in a direction toward the axis 13, i.e., radially of the workpiece 7. Such modification constitutes a substantial simplification of the improved grinding machine because one of the levers 41, 46 and one of the motors 43, 47 can be omitted or deactivated.

The grinding unit 22 also comprises means for tensioning the abrasive belts 36, 37, at least while the belts are in the process of removing material from a workpiece. Such tensioning means serves an important function in actual use of the band 36 or 37 as well as during retraction of one of the shoes 38, 39 to the inoperative position while the other shoe is being moved to its operative position. The illustrated tensioning means comprises idler rollers 48, 49 for the belts 36, 37, respectively, levers 51 carrying the rollers 48, 49 and pivotably mounted between the frame members 23, 24 and resilient means 52 (e.g., one or more coil springs) for biasing the levers 51 in a direction to maintain the respective rollers 48, 49 in tensioning engagement with the belts 36, 37.The spring or springs 52 are designed to invariably maintain the belts 36, 37 under a certain initial tensional stress.

In accordance with a feature of the present invention, the characteristics of the material removing external surfaces of the belts 36, 37 are not or need not be identical. For example, the belt 36 or 37 can be used for coarse grinding (scrubbing) of external surfaces of the cams 7, and the belt 37 or 36 can be used for precision or final grinding of such surfaces. An advantage of such selection of the belts 36 and 37 is that it is possible to install in one and the same grinding unit 22 a first belt whose abrasive external surface is best suited for coarse or preliminary grinding of selected portions of a rotary workpiece on the work holder 3 together with a second belt whose abrasive external surface is best suited for precision or finish grinding of surfaces which were previously ground by the first belt.The just described selection of the belts exhibits the advantage that the cost of grinding is reduced because a workpiece need not undergo preliminary grinding in a first machine and final grinding in a second machine, i.e.., it is not necessary to transfer a workpiece from a first machine into a second machine.

Moreover, such construction and assembly of the grinding unit 22 reduce the likelihood of inaccurate treatment of workpieces because a workpiece 7 which is properly held between the headstock 4 and the tailstock 6 of the work holder 3 during preliminary grinding remains in the same position relative to the work holder 3 during subsequent precision or final grinding.

As already mentioned above, the controls (not specifically shown) for the motors 43 and 47 are or can be constructed in such a way that the shoe 39 for the belt 36 is maintained in the extended position of FIG. 1 (so that the external surface of the belt 36 can remove material from the adjacent cam 9 of the workpiece 7) while the other shoe 41 is maintained in the inoperative position of FIG.

1, and vice versa.

The exact configuration of the external surface of a cam 9 which is being treated by the belt 36 or 37 is determined by the motor 21, i.e., by the extent of movement of the carriage 18 in the directions indicated by the double-headed arrow 19 while the belt 36 or 37 is in the process of removing material from the adjacent cam 9.

When the coarse or preliminary grinding (e.g., with the belt 36) is completed, the motor 43 is caused to pivot the bell crank lever 41 so that the shoe 38 is moved to the retracted or inoperative position while the corresponding lever 51 pivots under the bias of the spring or springs 52 to move the tensioning roller 48 outwardly and to thus continue to maintain the belt 36 under requisite tension. The workpiece 7 is thereupon moved by the feed screw 16 through the medium of the work holder 3 so that the cam 9 which was treated by the belt 36 moves into register with the belt 37. The motor 47 is then caused to pivot the lever 46 so as to move the shoe 39 from the retracted position of FIG. 1 to the extended or operative position.The spring 52 yields and enables the roller 49 to move downwardly (as viewed in FIG. 1) while the shoe 39 moves the belt 37 into material removing engagement with the external surface of the cam 9 which was already treated by the belt 36. The forward movement of the shoe 39 to the operative position is completed when the shoe 39 and/or the respective lever 46 reaches and is arrested by the stop 44. The belt 37 then proceeds to remove material from the adjacent lobe 9, i.e., to carry out the precision grinding operation while the workpiece 7 rotates about its axis 13.

As already mentioned above, the tracks 17 need not be exactly normal to the tracks 2. If the tracks 2 and 17 make an oblique angle, the belts 36 and 37 can be used to provide the cams 9 of a rotating workpiece 7 or selected portions of another rotary workpiece with tapering (e.g., conical) external surfaces or to subject tapering external surfaces to a rough and/or precision grinding treatment.

For example, the tracks 17 can be adjustably affixed to the base 1 so that the carriage 18 can be caused to move at right angles or at an oblique angle (other than 900) relative to the tracks 2. The same result can be obtained if the tracks 17 are mounted on a turntable (not specifically shown) which is turnable relative to the base 1 about an axis 53 normal to the axis 13, i.e., about a vertical axis if the axis 13 is horizontal. Such mounting of the tracks 17 exhibits the advantage that they can be selectively moved to positions at 900 or at an oblique angle relative to the tracks 2.Another advantage of mounting of the tracks 17 for angular movement about the axis 53 is that the person in charge can select any desired (even very small) departure of the inclination of tracks 17 from a position at 900 to the tracks 2, depending on the nature of work which is to be performed by the belt 36 and/or 37. Thus, by the simple expedient of making the tracks 17 turnable about the axis 53, one can greatly enhance the versatility of the improved grinding machine in that the machine is capable of providing selected portions of rotary workpieces with cylindrical, oval or tapering external surfaces, depending on the selected inclination of the tracks 17 relative to the tracks 2. Such greatly increased versatility can be achieved at a relatively low cost, namely by providing means for changing the inclination of the tracks 17 relative to the tracks 2 and/or vice versa.

It is further within the purview of the invention to provide two or more grinding units 22 which are mounted on the base 1, or on one or more additional bases, next to each other and next to the illustrated grinding unit 22 as seen in the direction of the axis 13. This renders it possible to first treat one or more selected portions 9 of the workpiece 7 by the belt 36 and/or 37 of the illustrated grinding unit 22 and to thereupon treat selected portions of the workpiece 7 by the belt or belts of one or more additional grinding units behind or in front of the grinding unit 22 in FIG. 1. The workpiece 7 remains attached to its work holder 3 if the tracks 2 are sufficiently long to permit movements of such workpiece along the illustrated grinding unit 22 as well as along one or more additional grinding units. Furthermore, one or more belts of a first grinding unit can remove material from one or more first portions 9 of a rotary workpiece 7 (or an analogous workpiece) while one or more belts of one or more additional grinding units simultaneously remove material from one or more additional portions 9 of the same workpiece. This renders it possible to complete the treatment of two or more axially spaced apart portions of a rotary workpiece within surprisingly short intervals of time. In other words, the output of a machine employing two or more grinding units for simultaneous treatment of axially spaced apart portions of a rotary workpiece can complete the treatment of a workpiece within a fraction of time which is required to complete such treatment in one or more conventional grinding machines wherein the grinding tools include or constitute abrasive belts.

An important advantage of the improved grinding machine is that it renders it possible to carry out a variety of different grinding operations without even temporarily disconnecting the workpiece from the work holder 3. Moreover, each of a plurality of operations can be carried out by resorting to an abrasive belt which is best suited for a particular grinding operation. Still further, and since a plurality of different grinding operations can be performed upon a workpiece which remains affixed to one and the same work holder, the precision of finish of each selected portion of a rotary workpiece is more satisfactory than in heretofore known machines wherein a workpiece must be transferred from a first work holder to at least one additional work holder.Furthermore, it takes less time to replace a damaged belt in a single machine with a plurality of belts than in two or more discrete machines. This, too, contributes to a higher output of the improved machine.

The provision of a single prime mover 34 to drive the belts 36, 37 contributes to simplicity, lower cost and compactness of the described grinding machine. Each of the belts 36, 37 can be moved into material removing engagement with the adjacent portion of a workpiece 7 in the work holder 3 irrespective of whether one of the shoes 38, 39 is non-movably affixed to the carriage or whether each of the two shoes is movable relative to the carriage 18 in a manner as actually shown in the drawings.

If one of the belts 36, 37 constitutes a coarse grinding tool and the other belt constitutes a precision grinding tool, the dimensions of a blank which has been treated by the coarse grinding tool can approximate (i.e., only slightly exceed) the dimensions of a finished workpiece. Such mode of operation is particularly desirable when the dimensions and surface finish of a finished workpiece must match or very closely approximate optimal dimensions and/or an optimal surface finish.

A coarse grinding belt can be designed to remove desired (e.g., substantial) quantities from a blank which is to be converted into a camshaft or the like. This reduces the dimensions of the once treated mark close to those which are required for a finished workpiece. The precision grinding belt thereupon treats the exposed surface of the adjacent portion of a rotating (once ground) blank to impart the required surface finish. The useful life of each of the two belts can be prolonged if such belts are selected by full consideration of the specific type of work to be performed thereby (i.e., coarse grinding and precision grinding). Moreover, such selection of the belts prolongs their useful life because each of the selected belts is merely required to carry out that operation for which its material removing external surface is ideally suited.For example, a precision grinding belt need not remove much material from the adjacent portion of a rotating workpiece if the task of removing larger quantities of material has already been performed by the other (coarse grinding) belt. The time which is required for removal of the major percentage of material from a blank is shortened if it can be performed by a properly selected coarse grinding belt so that the additional treatment of the coarsely ground surfaces can be finished by the other belt (i.e., a belt which is ideally suited for precision grinding) within a short interval of time. Thus, the overall time for subjecting a cam 9 or another portion of a rotating workpiece blank to coarse and precision grinding is much shorter than in heretofore known machines.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic and specific aspects of our contribution to the art and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the appended claims.

Claims (17)

CLAIMS:

1. Machine for grinding to a predetermined contour

portions of a rotary workpiece, comprising a work holder including means for rotatably mounting a workpiece; means for rotating the workpiece in said holder; at least one grinding unit comprising first and second abrasive belts, means for driving said belts, and means for selectively moving at least one of said belts into material removing contact with a selected portion of the workpiece in said work holder; and means for effecting a displacement of said work holder and said at least one grinding unit relative to each other substantially radially of the workpiece in said work holder.

2. The machine of claim 1, wherein said means for effecting said displacement includes guide means movably mounting said at least one grinding unit and extending substantially transversely of the axis of the workpiece in said work holder.

3. The machine of claim 1, wherein said at least one grinding unit further comprises a common support for said belts and rotary pulleys for said belts, said pulleys being rotatably mounted on said common support and including at least one idler pulley and at least one second pulley, said means for driving said belts including means for rotating said at least one second pulley.

4. The machine of claim 1, wherein said means for driving said belts comprises at least one rotary pulley common to said belts and means for rotating said at least one pulley.

5. The machine of claim 1, wherein said moving means includes a shoe for said at least one belt and means for shifting said shoe between a first position in which said at least one belt removes material from a workpiece on said work holder and a second position in which said at least one belt is disengaged from the workpiece.

6. The machine of claim 5, wherein said at least one grinding unit further comprises means for tensioning said belts.

7. The machine of claim 1, wherein said at least one grinding unit comprises a support for said belts and a first shoe for the other of said belts, said moving means including a second shoe for said at least one belt and means for moving said second shoe relative to said support.

8. The machine of claim 1, wherein said first belt is a precision grinding belt and said second belt is a coarse grinding belt.

9. The machine of claim 1, wherein said moving means includes means for selectively moving each of said belts into material removing engagement with a workpiece in said work holder, including a discrete shoe for each of said belts and means for shifting each of said shoes between a retracted position and an operative position in which the respective belt removes material from a workpiece in said work holder, said shifting means including means for maintaining the shoe for one of said belts in operative position while the shoe for the other of said belts is maintained in the retracted position.

10. The machine of claim 1, further comprising means for displacing one of said work holder and said at least one unit relative to the other of said work holder and said at least one unit in the direction of the axis of the workpiece in said work holder.

11. The machine of claim 1, wherein said means for effecting displacement includes means for moving one of said at least one unit and said work holder transversely of the axis of the workpiece in said work holder to thereby effect removal of material from the workpiece by at least one of said belts.

12. The machine of claim 1, wherein said means for effecting said displacement includes means for displacing one of said at least one unit and said work holder at an oblique angle to the axis of the workpiece in said work holder.

13. The machine of claim 1, further comprising a base for said at least one unit, said at least one unit being turnable relative to said base about a second axis which is substantially normal to the axis of the workpiece in said work holder.

14. The machine of claim 13, wherein said second axis is a vertical axis.

15. The machine of claim 1, further comprising at least one additional grinding unit adjacent said at least one unit as seen in the direction of the axis of the workpiece in said work holder.

16. The machine of claim 15, wherein said at least one additional unit includes means for removing material from a first portion of a workpiece in said work holder while at least one belt of said at least one unit removes material from at least one second portion of a workpiece in said work holder.

17. A grinding machine substantially as herein described with reference to the accompanying drawings.