The present invention relates to machine tools and control thereof. More particularly, it concerns co-ordination of the movement of a tool mount and a workpiece holder during a machining operation.

Background to the invention

Historically, methods employed to finish grind the working surfaces of crankshaft crank pin diameters involved rotating the crank pin surfaces about their own centres. The machine tool was adjusted between each successive crank pin such that the grinding action on each pin was in the form of a cylindrical grind.

Methods were developed to cause grinding wheels to move orbitally following orbital motion of the respect crank pins about a longitudinal axis of a workpiece, as described for example in CA-A-364437.

More recently, machine tools have used Computer Numerical Control ("CNC") systems in which the position of the in-feed machine axis is electronically linked to the rotation of the workpiece under precise servo control. Such an approach is described in US4,375,670 which describes a machine tool including separate controllers for the drive means for rotating the crankshaft and the drive means for displacing a tool carriage. Both controllers are controlled such that irregularities of one movement cannot influence the other movement as one movement is not reliant on the other.

In existing techniques, a tool is brought into engagement with the workpiece whilst the workpiece is stationary at a defined point in its rotation. Once this start position has been reached, the tool and workpiece are driven simultaneously and in synchronism. Summary of the invention

The present invention provides a machine tool for machining a workpiece, comprising:

a machine base;

a workpiece holder supported by the machine base for holding a workpiece, the holder being rotatable about an axis;

a workpiece rotary drive for rotating the holder about the axis;

a tool mount supported by the machine base for carrying a tool to be engaged with a workpiece held in the holder;

a drive arrangement for moving the workpiece holder and the tool mount relative to each other in a plane which is transverse with respect to the axis, by moving at least one of the workpiece holder and the tool mount relative to the machine base; and

a control arrangement configured to control the workpiece rotary drive and the drive arrangement so as to machine a surface profile on the workpiece which is non- equidistant from the axis in a plane which is transverse with respect to the axis, wherein a tool mounted in the mount is engaged with the workpiece to commence the machining operation whilst the workpiece is being rotated by the workpiece drive.

The ability to bring the tool into engagement with the workpiece whilst it is being rotated, even though the surface being machined is non-equidistant from (that is, non- circular with respect to) the axis about which it is rotating, significantly reduces the time taken to machine a workpiece when two or more discrete machining operations are required. It avoids the need to halt the rotation of the workpiece, thereby saving the time otherwise taken to decelerate the rotation of the workpiece to a standstill, bring the tool into engagement with it and then accelerate the workpiece and tool up to their normal machining velocities.

The surface profile being machined onto the workpiece is a profile which is intended to be non-equidistant from the workpiece holder's axis of rotation in the finished workpiece, that is, the desired finished surface is not a cylindrical surface centred on the holder's axis of rotation, but deviates significantly from such a surface. For example, the surface being machined may be the surface of a crank pin orbiting around a crankshaft axis, or a cam lobe on a camshaft.

The machine tool is controlled such that the tool is engaged with a workpiece surface non-equidistant from the workpiece holder's axis of rotation, whilst the workpiece is being rotated (preferably maintaining a substantially constant workpiece rotation velocity) to machine the surface to a profile that is the desired surface finish for the workpiece part being machined, the profile being non-equidistant from the workpiece holder's axis of rotation.

In one preferred embodiment, the axis is a longitudinal axis of the machine tool, the tool mount is movable in a plane which is transverse with respect to the axis, the drive arrangement comprises a tool mount drive for imparting the transverse movement to the tool mount, and the control arrangement is configured to control the workpiece rotary drive and the tool mount drive so as to machine the surface profile on the workpiece.

Alternatively, in another embodiment, the workpiece holder is movable in a plane which is transverse with respect to the axis, the drive arrangement comprises a workpiece transverse drive for imparting the transverse movement to the workpiece holder, and the control arrangement is configured to control the workpiece rotary and transverse drives so as to machine the surface profile on the workpiece.

The invention further provides a method of machining a workpiece with a machine tool, comprising the steps of:

rotating the workpiece about an axis; and

moving one of a tool and the workpiece relative to the other in a plane transverse with respect to the axis so as to machine a surface profile on the workpiece which is non-equidistant from the axis in the transverse plane, such that the tool engages with the workpiece to commence the machining operation whilst the workpiece is rotating. According to embodiments of the invention, whilst the workpiece continues its rotation, the tool mount drive is controlled to bring the tool into engagement with the workpiece to commence a machining operation such that the velocities and accelerations of the respective contact points of the workpiece and tool are matched.

Brief description of the drawings

Embodiments of the invention will now be described by way of example and with reference to the accompanying schematic drawings, wherein:

Figure 1 is a block diagram representing a control arrangement of a machine tool embodying the invention in combination with a workpiece and grinding wheel;

Figure 2 is a graph plotting work head rotation angle and in-feed axis position during a machining operation according to processes embodying the present invention; and

Figures 3 and 4 are plots of work head rotation angle and in- feed axis position during a machining operation taken from the data log record of the CNC system controlling the machining operation, with Figure 4 representing part of the plot of Figure 3 in greater detail.

Detailed description of the drawings

In the following embodiments, the workpiece holder is arranged to rotate the workpiece about a longitudinal axis of the machine tool, that is a reference axis which is held in a fixed position during machining relative to the machine base. The tool mount moves towards and away from this axis during the machining operation. In other implementations embodying the invention, the workpiece holder is arranged to move the rotating workpiece towards and away from a tool which is held in a fixed position during machining relative to the machine base. The latter approach is more suited to relatively small and/or lightweight workpieces having a relatively low inertia. For instance, this may be appropriate for tool grinders. Figure 1 depicts components of a control arrangement embodying the invention which is arranged to machine a workpiece 2 with a grinding wheel 4. The workpiece is mounted in a workpiece holder 14 and the grinding wheel is carried by a tool mount in the form of a spindle head 16. The workpiece holder and spindle head are supported by a machine base 18 of a machine tool. The workpiece holder is rotatable about an axis 15.

A computer numerical control (CNC) system 6 is coupled to a workpiece drive and a tool mount drive via a digital drive control bus 8. The workpiece drive comprises a work head drive amplifier 10, a servo drive motor (Mw) and a workpiece position encoder (E _w ).

The tool mount drive comprises an in-feed drive amplifier 12, a grinding wheel servo drive motor (M _G ) and a grinding wheel position encoder (E _G ).

The CNC system 6 performs calculations to generate control command parameters which are transmitted to the workpiece and tool mount drives via the drive control bus. The control bus 8, which could be any one of a SERCOS, Ethernet, Profidrive or similar configuration, delivers demand data packets and receives status data packets from the drives.

The work head drive amplifier 10 and in-feed drive amplifier 12 update their respective internal axis demand registers using the data received via the drive control bus 8. The servo drive motors M _w , M _G receive control signals from the respective drive amplifiers 10, 12 and impart corresponding motions to the workpiece 2 and grinding wheel 4, respectively. The position encoders E _w , E _G deliver position data back to the associated amplifiers. Servo control loops within the drive amplifiers perform any calculations necessary to stabilise the drive motors and minimise any errors between the received demands and the actual positions of the drive motors.

Figure 2 is a synchronisation chart showing plots of work head rotation angle and in- feed axis position during three machining processes embodying the present invention. The saw tooth plot 20 represents the work head rotation angle, indicating continuous rotation of the work piece at a constant rotational velocity. The plots marked "Case 1", "Case 2" and "Case 3" illustrate three different scenarios embodying the invention in which a tool is brought into contact with the workpiece at different points in the rotation of the workpiece.

In Case 1, the respective contact points of the tool and workpiece are brought into engagement at a point where their velocities along a direction of reciprocation of the tool is zero, at a point of reversal of the velocity of the workpiece along this direction. Immediately following this engagement, the tool mount drive is synchronously constrained to match the motion of the workpiece.

When the point of contact of the workpiece is accelerating away from the tool, as in Case 2, the tool mount drive is controlled to accelerate to a velocity greater than the linear velocity at the point of contact, until it is able to decelerate to a velocity where the tool contacts the moving workpiece at a calculated position, velocity and acceleration.

In Case 3, the workpiece is accelerating towards the tool at the point of contact. The tool mount drive is caused to decelerate, reverse direction and accelerate to a velocity and acceleration which matches the velocity of the point of contact of the workpiece. Synchronism is then maintained once the workpiece effectively catches up with the accelerating tool mount drive.

A recording of a machine cycle embodying the invention is shown in Figures 3 and 4. It was recorded using the data logging capability of a CNC control system. It corresponds to dynamic machine synchronisation in a section of a full crankshaft grinding cycle which lies between the end of the first crank pin grind cycle and the start of the subsequent, second crank pin grind cycle. It can be seen that the position of the in-feed axis undergoes a small reversal as it is brought into synchronisation with the workpiece, consistent with "Case 3" above. This portion of the machining operation is shown in greater detail in Figure 4.