This invention concerns a grinding machine and grinding method as per the preamble of claims 3 and 1. An example of such a method and machine is disclosed by DE-A-4 133 754.
The use of CBN wheels in grinding processes has resulted in the requirement for large volumes of coolant to be delivered at ever higher pressures and velocity. CBN wheels tend to have a higher density and lower porosity than conventional Aluminium Oxide wheels. Grinding forces are higher and it is possible that the coolant contributes to the generation of high dynamic forces between the grinding wheel and the workpiece.
In order to grind at ever higher production rates and optimise maximum wheel life, it has been necessary to continue to increase coolant flows and pressures. In parallel with this it has been observed that profile quality and consistency in ground parts has deteriorated.
Document DE-A-4133754 discloses a control system for a grinding machine in which the changes in feed rate between rough grinding, finish grinding and spark-out are automatically determined as a result of signals from a grinding position sensor and a grinding load sensor.
According to one aspect of the invention there is provided a method of grinding an external surface of a workpiece, comprising the steps of directing liquid coolant onto the external surface being ground, and monitoring the grinding process by means of a probe for detecting the dimension of the surface, characterised in that during a predetermined final stage of a grinding cycle, before final size is reached, the rate and/or direction of flow of liquid coolant onto said surface is altered for at least part of the duration of said final stage, so that a reduced flow of the coolant reaches said external surface, whereby the accuracy of final grinding of the workpiece is improved.
The method is particularly applicable when grinding with a CBN grinding wheel.
Using the method of the invention, it has been found that a cam lobe profile can be improved and grinding errors in the final size reduced from 10's of microns to 2 or 3 microns during finish grinding, if the coolant flow is reduced or at least in part directed away from the region of engagement during the grinding of the final 20 microns down to final size.
In a similar manner, using the grinding method of the invention, the out-of-roundness of a crankpin has been reduced from a 10 micron envelope to an envelope of less than 1 micron, by reducing or deflecting or even removing the high pressure coolant flow when the component was 25 microns above final size, and maintaining the reduced or deflected flow (or turning off the flow) for the duration of the final 25 micron grind.
Even more surprisingly the width between side cheeks of a crankpin bearing even during a rough grind operation has been maintained more constant, therefore maintaining them parallel, by reducing or deflecting or removing the coolant flow during the last stage of grinding the two parallel faces.
According to another aspect of the invention there is provided a grinding machine comprising a grinding wheel, drive means for rotating the wheel to effect grinding of an external surface of a workpiece, means for rotating the workpiece during grinding, means for effecting relative movement between the grinding wheel and the workpiece to bring the wheel and the external surface of workpiece into engagement for grinding and to maintain such engagement for the duration of a grinding cycle, means for directing a jet of liquid coolant onto the wheel or the workpiece or both, at least in the region of the engagement of the wheel and the workpiece, sensing means for sensing when the grind cycle is nearing completion and the workpiece has a predetermined oversize condition remaining to be ground, said sensing means comprising a workpiece engaging probe for detecting the dimension of the external surface being ground, and means for generating a control signal responsive to signals from the sensing means, characterised by coolant control means for altering the rate of flow of coolant from the jet and/or the direction of the jet, sensing means for means for supplying the control signal to the coolant control means to alter the jet of liquid coolant so as to deflect, redirect, or reduce the jet when said predetermined oversize condition is sensed and for at least part of the duration of the remainder of the grinding cycle.
Alternatively or in addition, the sensing means may comprise means responsive to the advance of the grinding wheel during the grinding cycle, which is programmed to deliver a signal when the grinding wheel has advanced through a given distance thereby indicating that said predetermined oversize condition has been reached during which coolant flow is to be altered.
Where the grinding process is computer controlled and the position of the wheel relative to the workpiece and the distance to final size are both available to the computer, the coolant flow control means may be controlled by signals from the computer.
The invention will now be described, by way of example only, with reference to the accompanying drawings in which:-
Referring first to Figure 1, there is shown part of a grinding machine including a CBN grinding wheel 10 rotating clockwise, as indicated by the arrow. The wheel is shown in grinding contact with a contra-rotating cylindrical workpiece 12, in this case a crankpin rotating about the main axis of a crankshaft (not shown).
In order for the wheel to follow the orbiting movement of the crankpin 12, the wheel is moved horizontally backwards and forwards under computer control by means known per se.
A coolant nozzle 14 feeds a jet of coolant liquid down into the zone of contact between the wheel and the crankpin. The nozzle is mounted on a wheelhead (not shown) on which the wheel 12 itself is rotatably mounted.
In view of the high metal removal rates achievable with CBN wheels, it is necessary to have a large coolant flow from the nozzle 14 during the main grinding feed. It has been found that the jet of coolant flow increases the force exerted on the crankpin being ground, and hence tends to reduce the grinding accuracy which this invention seeks to mitigate.
Figure 2 shows the control circuit for the coolant, in which a pump (not shown) feeds coolant under pressure along a supply line 16 to a solenoid operated selector valve 18 which, in this example, is under the control of the computer. The selector valve has two positions, a first normal position (as shown) in which a full flow of coolant passes through it, and is fed via line 19 to the nozzle 14, and a second position in which the flow is throttled so that a reduced flow is fed to the nozzle. The reduced flow can be adjusted by a flow control valve 20 which connects the pump to the selector valve 18 along a line 21 parallel to the line 16.
A typical cycle of grinding feed against time is shown in Figure 3. The feed rate progressively reduces from an initial fast feed 22 and a medium feed 24 down to a slow feed 26, after which there is a dwell period 28 in which the feed ceases and sizing of the workpiece occurs, in this case the crankpin 12.
In accordance with the invention, and as illustrated in Figure 4, the coolant flow is maintained at a full rate 30 during the fast feed and up to the end of the slow feed 26, whereupon the computer issues a command signal to the selector valve 18 to cut the flow down to a reduced rate 32 for the duration of the dwell period 28. At the end of the dwell period, the coolant flow is completely stopped while the grinding wheel executes a rapid retract, as shown at 34 in Figure 3. Adjustment of the control valve 20 enables the ratio of the full flow to reduced flow to be varied to cater for differing conditions.
Since the crankshaft is not uniformly stiff when subjected to side forces, there is a tendency for different amounts of material to be ground away in different planes, causing the crankpin to be ground to a non-round shape. Reducing the flow rate 32 during the final stage of grinding reduces the coolant pressure at the nozzle 14, and hence the force on the crankpin (when grinding forces are low already), and it has been found that the out-of-roundness of the crankpin can be reduced from approximately 10 microns to less than 1 micron, in terms of its true diameter. Thus, the invention enables a higher accuracy and truer grinding profile to be achieved than previously.
As an alternative to reducing the flow rate, as aforesaid, the coolant could be directed away from the wheel during the final grinding stage. However, it is believed that this alternative would be more complicated and less practical than reducing the flow rate.