This invention concerns grinding machines as per the preamble of claim 1. An example of such a machine is disclosed by US 1 740 551 A.
This invention is of particular application where a rough grinding operation is followed by a finish grinding operation which is performed by a smaller grinding wheel the diameter of which is such that one or more concave regions can be formed in the external surface of the cam profile during the finish grinding operation.
Whilst in theory a small diameter grinding wheel could be used for the rough grinding step, the wear which occurs on the grinding wheel when it is used for rough grinding is such that in practice the largest possible diameter grinding wheel is always used for rough grinding since the larger the diameter, the longer the normal life expectancy of the wheel.
In EP 744243A2 which is state of the art under Art. 54(3) and (4) EPC, is disclosed a grinding machine having first and second grinding wheels, the second grinding wheel being carried by an arm pivotable by a ram about the axis of the first grinding wheel.
According to of the present invention, there is 'provided a grinding machine in which a first grinding wheel is mounted on a wheelhead adaped to be moved towards and away from a workpiece for grinding same, a second grinding wheel is mounted at one end of an arm, the other end of which is pivotally mounted to the wheelhead about an axis parallel to the axis of rotation of the first grinding wheel, in which the first grinding wheel is larger than the second grinding wheel, and means is provided for pivoting the arm from an upper parked position into a lower grinding position in which the second grinding wheel engages a peripheral surface of the workpiece, characterised in that for rough grinding and subsequent final profile grinding of a cam including re-entrant positions, the arm is pivoted to the wheelhead to the rear of the first grinding wheel, the length of the arm being such that when the second wheel is in the grinding position, part of the second wheel is closer to the workpiece in the direction of movement of the wheelhead than is the corresponding part of the first grinding wheel so that as the wheelhead is advanced towards the workpiece to perform a grinding operation using the second wheel, the first wheel is kept well clear of the workpiece.
In this primary embodiment, a lateral shift of the wheelhead is necessary to align the second wheel with the same part of the workpiece as was engaged by the first larger grinding wheel before the second grinding wheel can be brought into operation. Alternative arrangements obviate the particular problem.
In the first variant, a long arm is employed so that the second grinding wheel is positioned in between the first grinding wheel and the workpiece with the two grinding wheels in the same plane, so that as the wheelhead is subsequently advanced towards the workpiece, it is the same part of the workpiece which is engaged by the second grinding wheel as was engaged by the first grinding wheel.
In a second variant, the arm is even longer and is still arranged to position the second grinding wheel in the same plane as the first grinding wheel, but because of the longer arm length, the arm is of sufficient length to position the second wheel on the opposite side of the workpiece from that of the first grinding wheel and a sufficient distance is provided between the two grinding wheels to permit the workpiece to rotate and for the wheelhead to move in known manner so as to maintain contact between the workpiece and one of the grinding wheels without the workpiece coming into engagement with the other grinding wheel, whilst one of the grinding wheel is grinding and then vice versa, whilst the other of the grinding wheels is grinding.
In a modification to the second variant mentioned above, the second grinding wheel is located in position beyond the workpiece before the first wheel is advanced and a rough grinding operation is performed, and by appropriate movement of the wheelhead one or the other of the grinding wheels engage the workpiece.
The arm carrying the second grinding wheel may itself be mounted on a slideway on the wheelhead so as to be movable in a direction towards and away from the workpiece on the opposite side thereof from that engaged by the first grinding wheel, in parallel with the sliding movement of the wheelhead. As rough grinding is coming to an end and before the wheelhead is fully retracted so as to disengage the first grinding wheel from the workpiece, the second wheel can be brought into engagement with the workpiece to finish grind the surface thereof (and in the case of a cam profile to introduce into the profile one or more concave regions whose radius is smaller than the radius of the larger grinding wheel).
After the finish grinding operation using the second grinding wheel, the arm mount is moved and the arm is pivoted so as to disengage the second wheel from the workpiece and permit the workpiece to be removed.
If a number of regions along the length of a workpiece are to be rough and finish ground, and if the diameter of workpiece is such as to allow the wheelhead to traverse the workpiece parallel to the axis of rotation of the latter without the grinding wheels or the arm interfering with intermediate sections of the workpiece, then indexing of the wheelhead can be performed so as to reposition the grinding wheels relative to the workpiece ready to rough and finish grind the next region thereof.
Where the two grinding wheels are to be brought into alignment with either the second wheel between the first wheel and the workpiece, or with the first and second wheels on opposite sides of the workpiece, the pivoting arm may be arranged to one side or the other of the first grinding wheel, and a spindle may be provided at the outboard end of the pivoted arm which offsets the second grinding wheel to the side of the arm and into alignment with the first grinding wheel.
Altematively the arm may be configured and shaped so as to circumscribe the upper half of the first grinding wheel and any casing surrounding the latter, when the arm has been pivoted from the parked position into the grinding position.
Alternatively a bifurcated arm may be employed, the two parts lying on opposite sides of the first wheel.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
Referring first to Figures 1A and 1B there are shown side views of a cam 10 being ground respectively by a large grinding wheel 12 and a small grinding wheel 14. The wheels are fed in a horizontal direction towards and away from the cam to enable the wheels to follow the profile of the cam including its lobe portion 16.
The large wheel 12 of Figure 1 A is arranged to rough grind the cam to a modified profile in which only just sufficient stock is left for finish grinding. As the final profile of the cam has two concave or re-entrant portions 18, which have a radius smaller than that of the wheel 12, the latter cannot reach into the concave portions, so that a larger amount of stock is left in those portions at the completion of rough grinding.
The diameter of the large wheel 12 is chosen to be the largest which is compatible with an economic production of the cams. In contrast, the diameter of the small wheel 14 is determined by the minimum radius of the concave portion 18 of the cam profile.
Referring now to Figure 2 there is shown a grinding machine in accordance with one embodiment of the invention, having a wheelhead 20 on which is rotatably mounted the large wheel 12. Pivotally mounted on the wheelhead about an axis parallel to the rotational axis of the wheel 12 is a swinging arm 22 at whose outboard end is rotatably mounted the small grinding wheel 14. The arm 22 is offset from the wheel 12 and from any guard around the wheel. The small wheel 14 is likewise offset from the wheel 12, so that there is a lateral clearance between the wheels.
In the lowered position of the arm 22, shown in Figure 2, the wheel 14 arranged to finish grind a workpiece, for example a cam 10. Subsequently the arm 22 with the wheel 14 is adapted to be swung into an upper parked position, as shown in dotted lines, by retraction of a hydraulic ram 24 whose rod end is pivotally connected to an upper cranked portion 22A of the arm, and whose opposite end is pivotally connected to a remote part of the wheelhead 20, not shown.
In operation, rough grinding is performed with the arm 22 in its upper position, the wheelhead 20 being advanced forwards from the position shown until the large wheel 12 comes into engagement with the workpiece or cam 10. On completion of rough grinding, the wheelhead is retracted sufficiently to enable the ram 24 to bring the arm 22 into the lowered position in which the small wheel 14 is able to engage the cam for the finish grinding operation. Due to the offset between the two wheels it is necessary for the cam or the wheelhead to be moved laterally so that the small wheel 14 is brought into alignment with the cam.
Figure 3 shows an embodiment of the invention which is a modification of that described with reference to Figure 2.
Here the outboard end 22B of the arm is extended sufficiently to enable the small wheel 14 to be positioned in front of the large wheel 12 and in alignment therewith. With the use of this long arm, operation of the grinding machine is similar to that described above, except that when the small wheel 14 is lowered for finish grinding there is no need to realign the cam 10 with wheel 12, since the two wheels 12 and 14 are already in the same plane.
Referring now to Figures 4 and 5 there are shown side and plan views of a further embodiment. The swinging arm is here formed effectively in two halves as a bifurcated arm 30, the two halves being spaced apart to straddle each of the grinding wheels 12 and 14 with a sufficient clearance from the sides of the wheels.
The outboard end 30A of the arm 30 is extended even further than the end 22B of the swinging arm 22 of Figure 3, such that the small wheel 14 is disposed on the side of the cam 10 opposite the large wheel 12. The distance between the two wheels requires to be such that, during rough grinding by the wheel 12, the small wheel 14 stands sufficiently clear of the cam 10 for it not to come into contact with the cam.
It should be noted that none of the rotary drives for the wheels 12 and 14 are shown, since such drives per se do not form part of the present invention.
The horizontal drive for the wheelhead 20 is shown in Figure 4 at reference 34. Although such drive may be used (as previously) for wheelhead movement for both grinding wheels, Figure 4 also shows a modification for driving the small wheel 14 into and out of engagement with the cam 10.
In this arrangementthe arm 30 is pivoted about a housing 32 which is slidably mounted on the wheelhead 20, movement of the housing being imparted by the drive mechanism 34 secured to the wheelhead.
Since the mechanism 34 moves the arm to the right, (as viewed in Figure 4), during an infeed movement of the small wheel 14 towards the cam, the rod of the ram 24 will be retracted so that any friction in the ram will not tend to cause a pivoting movement of the arm 30.
However, in a modification the pivoted mounting for the ram 24 may instead be formed as an extension of the housing 32 so that the ram moves with the housing during actuation of the drive mechanism 34, obviating any tendency for the arm 30 to be pivoted during such actuation.
In operation, as rough grinding is being completed before the wheelhead 20 retracts to disengage the large wheel 12 from the cam 10, the drive mechanism 34 begins to draw the small wheel 14 in to engagement with the cam for finish grinding. When finish grinding is complete the ram 24 retracts the arm 30 and wheel 14 to allow the cam to be removed and replaced by another workpiece or cam to be ground.