GB1587195A

GB1587195A - Machine tools

Info

Publication number: GB1587195A
Application number: GB3827876A
Authority: GB
Original assignee: TI MATRIX Ltd
Current assignee: TI MATRIX Ltd
Priority date: 1976-09-15
Filing date: 1976-09-15
Publication date: 1981-04-01
Also published as: DE2741605A1; JPS5336088A

Description

(54) IMPROVEMENTS IN OR RELATING TO MACHINE TOOLS (71) We, Ti MATRIX LIMITED, a British Company, formerly MATRIX MACHINE TOOLS LIMITED, of P.O. Box 39, Fletchamstead Highway, Coventry, CV4 9DA, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:: This invention relates to machine tools and concerns machine tools of the kind having a machining -tool holder, a tool-former holder and positioning means for causing relative movement between the machine-tool holder (and its machining-tool mounted thereon) and a work piece whereby the work piece can be machined by the machining-tool and/or for causing relative movement between the two holders whereby the machining-tool mounted in its holder can be formed to a required shape by a tool-former mounted in its holder.

A typical machine-tool of the kind described above is a grinding machine, such as a cylindrical or form-grinding machine, in which the machining-tool is a grinding wheel and the tool-former is a diamond dressing tool.

The term "tool-forming" is intended to include forming a new machining-tool to a required shape in situ on a machine tool for the operation it is to perform as well as reforming a worn tool to its required shape, and includes dressing in the case of a grinding wheel.

In the operation of grinding machines it is necessary periodically to dress the grinding wheel, that is to say, during operation the surface of the grinding wheel which contacts a work piece wears and eventually it becomes worn to an extent that the work piece is not machined with the required degree of accuracy.

It then becomes necessary to dress the grinding wheel. This is accomplished using a diamond or diamond-tipped dressing tool which re-forms, that is re-cuts or re-machines, the desired profile in the grinding wheel so that the desired profile on the work piece can, in turn, be obtained. It is, of course, necessary to dress a new wheel to the desired shape before any machining operation is performed.

A support structure to carry the dressing tool is necessary and conventionally this takes the form of a structure movably attached to the grinding machine so that the wheel can be dressed to the required profile. A problem with such a conventional arrangement is that in making the dressing apparatus sufficiently rigid to ensure the necessary accuracy the usual result is a relatively large structure which has to be carried by the machine.

A further problem exists in that conventionally the dressing operation is carried out at either the top or bottom of the grinding wheel or on a diametrical plane opposite to the position at which the grinding wheel, in operation, grinds a work piece. This is disadvantageous in that the further away from the operating position the actual dressing operation takes place the less accurate the finished results.

According to the invention, there is provided a method of controlling a machine tool of the kind described, the method comprising storing a plurality of sets of data values from which the X-Y coordinates which define the required shape of the machining surface of the machining-tool can be derived and controlling the positioning means in accordance with the stored sets of data values in a predetermined sequence to cause the tool-former to form the machining surface of the machiningtool in the required shape.

Further, according to the invention there is provided a machine tool of the kind described, the control apparatus comprising control means which, in operation, control operation of the positioning means in the machine tool, the control means comprising storage means which store a plurality of sets of data values from which the X-Y coordinates which define the required shape of the machining surface of the machining-tool can be derived, means which read from the storage means the said sets of data values in a predetermined sequence and which so control, in operation, the positioning means that the tool-former forms the machining surface of the machining-tool in the required shape.

The control apparatus and/or control means of the two immediately preceding paragraphs may be, or may form part of, a computer for controlling the operation of the machine tool.

A computer may be arranged to control operation of the machine tool, the computer being arranged periodically to cause the machine tool to change from a machining mode of operation wherein a work piece is machined by a machining-tool, to a tool-forming mode of operation wherein the machining-tool is formed to its required shape, and then to revert to its machining mode of operation. The computer may comprise control means according to the three immediately preceding paragraphs.

The said sets of data values may be values representative of the X-Y coordinates of the machining-tool.

The said sets of data values may be values representative of the required dimensions of a work piece after it has been machined, and the control means may further comprise means for detennining the actual dimensions of a machined work piece at a plurality of predetermined locations on the work piece, means for comparing the actual dimensions with a plurality of required dimensions at the said locations and for providing a plurality of output values representative of the errors between the said actual dimensions and the said required dimensions, and means coupled to receive said output values and to so control the positioning means thereby that the tool-former forms the machining surface of the machine tool to the required shape to reduce the said errors.

In the case of a grinding machine, a diamond dressing tool can be attached in any convenient manner to the work table of the machine. Thus, if the grinding machine is under computer control, the computer program can be arranged periodically to set the machine to a "tool-dressing mode" of operation in which the work piece holding table and the grinding wheel are moved relative to the other through a prearranged sequence under the control of the computer program, so that the diamond dressing tool is made to follow a pre-arranged sequence of movements relative to the grinding wheel to thereby dress the wheel.

The invention will now be described by way of example with reference to the accompanying drawing, in which: Figure 1 is a simplified, schematic plan view of part of a grinding machin tool embodying the invention, and Figure 2 shows an end view of the machine of Figure 1.

Figure 3 is a block diagram of part of the computer of Figure 1.

Referring to the drawing, there is shown a part of a grinding machine tool 10 including a work piece holding table 12. Work piece holding centres 14, 16 are provided on the table 12 for holding a work piece 18 securely but freely rotatable;the centre 14 being retractable for mounting and dismounting the work piece on the machine. A grinding wheel 20, which can be for stright cylindrical grinding bevel, screw or other complicated shapes as shown, is rotatably supported on a holder 22, and means (not shown) is provided for rotating the grinding wheel at a desired speed.

Positioning apparatus, shown schematically at 24, is provided for controlling the relative positions of, and movements between, the grinding wheel 20 and its holder 22 on the one hand and the work-holding table 12 on the other.

The positioning apparatus 24 is arranged to control the usual machining operations, such as advancing the grinding wheel 20 towards the work piece 18 and retracting it away from the work piece, and traversing the wheel 20 across the work piece 18, or vice versa, or any combination thereof. Such movements may be effected hydraulically for example. The positioning apparatus, is in turn controlled over a line 25 by a control apparatus 26 in the form of a computer. Thus the computer 26 can be programmed to cause the machine tool 10 to perform a sequence of operations whereby the work piece 18 is ground to its required shape by the grinding wheel 20.

As aforementioned, the grinding wheel 20 is subject to wear during use and has to be dressed periodically.

Mounted on the work table 12 is a dressing tool support 28 for rigidly supporting a diamond dressing tool 30. The support 28 is movable relative to the work table 12 so that any degree of angular movement, for example, to facilitate bevelling, is possible. Movement of the support 28 relative to the table 12 could be controlled manually but is preferably controlled by the computer 26. From Figure 2 it will be clear that the apparatus according to the invention allows the dressing tool 30 to dress the grinding wheel at substantially the same position as that at which the wheel 20 grinds the work piece 18. Also the relatively large support structure required for the dressing equipment in a conventional machine tool is no longer required.

Thus, the relative positions and movements of the grinding wheel 20 and the work piece 18 on the one hand and the grinding wheel 20 and the dressing tool 30 on the other hand can be controlled by the computer 26. The computer 26 can be programmed to cause the grinding machine 10 to operate in a "machining mode" in which a work piece is, or work pieces are, machined by the wheel 20, and periodically to operate in a "toolforming or -dressing mode" in which the wheel 20 is dressed by the dressing-tool 30.

The method of programming the computer 26 to change from one operative mode to the other and to cause the required relative movements between the grinding wheel 20 and the dressing tool 30 in the tool-dressing mode can of course be effected in avariety of ways.

In the following description, it will be assumed that the grinding wheel 20 can be advanced towards and retracted from the table 12 with its axis parallel to the table, and the table can be traversed on an axis parallel to the axis of the wheel 20 under the control of the positioning apparatus 24, so that most grinding operations will require a combination of movements of the grinding wheel 20 and the table 12.

In a simple arrangement, the computer 26 is programmed to cause the machine 10 to operate in its machining mode, when the table 12 is traversed to the position in Figure 1. After a predetermined number of such machining operations, the computer is programmed to cause the machine to operate in its tool-dressing mode when the table 12 is traversed to a position at which the wheel 20 can be dressed by the dressing-tool 30. The dressing tool 30 can be made to execute any movement in relation to the grinding surface 20a of the wheel 20 as demanded by the computer program.

The computer 26 includes a store 27 for storing a plurality of sets of data values representative of the X-Y coordinates of the surface 20a of the grinding wheel as measured at discrete positions axially along the surface from a datum position. In the tool-dressing mode, the program causes a command circuit in the computer 26 to read-out these values one set at a time in a predetermined sequence starting at the datum position and causes the positioning apparatus 24 to move the tip of the dressing tool 30 over the surface 20a of the wheel 20 in the same sequence and in a path determined by the X-Y coordinate values to thereby dress the wheel to its required profile.

After the wheel 20 has been dressed the computer program causes the wheel support 22 to be re-positioned to compensate for the reduction in diameter of the wheel 20 and the program then directs the machine to revert to its machining mode.

In another embodiment in which the computer is programmed to cause the grinding machine 10 periodically to operate in a "gauging mode" in which the actual dimensions of the machined workpiece are measured, the store 27 (Figure 3) is arranged, as before, to store a set of data values representative of the required dimensions of a machined workpiece measured at discrete positions along its length from a datum position. Means 31, such as a gauge sizing device which can be applied to the workpiece after a machining operation, is provided on the machine 10 for measuring, in the gauging mode of operation, the actual dimensions of a machined workpiece and for providing electrical signals representative of these dimensions to a store 33 in the computer 26.In the computer the values representative of the actual dimensions read out from store 33 are compared in a comparator circuit 32 with the corresponding required dimensions read out from store 27 in a predetermined sequence starting at a datum position and under the control of a command circuit 34 to produce corresponding error signals (which might be the differences between corresponding actual and required dimensions) and the error signals are used to so control the positioning apparatus 24 that the dressing tool 30 follows a path across the surface 20a of the wheel 20 to thereby dress the wheel to its required shape.

The means 31 for measuring the actual dimensions of a machined workpiece can also be used to determine when the machine 10 is to be switched to its tool-dressing mode of operation. In this case, the error values could be compared in a comparator 36 with a set of corresponding tolerance values for a machined workpiece read out from a store 38 in the required sequence under the control of the command circuit 34 and it the magnitude of the error values exceeds that of the tolerance values at one or more predetermined positions on the workpiece the output of the computer 36 is fed to an operational control circuit 40 which is arranged to command a tool-dressing operation.

The datum position for the dressing-tool 30 can be established in a number of ways and it is essential that the tool 30 be accurately positioned at the commencement of the toolforming mode of operation even when the tool 30 wears or is replaced. One way of determining the datum position is to grind a diameter after dressing the grinding wheel with the dressing diamond tool. This diameter or surface can then be measured and used as the datum within the machine. Another way is to mount an optical tool setting unit 35, such as one marketed by us and known as the Automatic Tool Setting Unit Mk3 (ASTU), or a similar device on the grinding machine 10. The tip of the dressing tool 30 is then positioned so that the tool tip profile coincides with the datum lines and the X-Y coordinates at that position are stored in the computer 26. Thereafter, prior to a tool-forming operation the tip of the tool 30 is set to these X-Y coordinates under the control of the computer.

Various other modifications and additions can of course be made to a machine tool according to the invention. For example, although it is apparant from the figures that the dressing-tool support 28 can be carried on the top surface of the work table 12 with the grinding wheel 20 adjacent one side thereof, other configurations are possible, and for example, to allow surface grinding the grinding wheel will be above the work table and the dressing-tool can be supported accordingly.

Also, although a machine 10 comprising movement in the X and Y axes has been described, the invention is believed to be equally applicable to machines having a third axis of movement as well as multi-axis machines.

Claims

WHAT WE CLAIM IS:

1. A method of controlling a machine tool of the kind described, the method comprising storing a plurality of sets of data values from which the X-Y coordinates which define the required shape of the machining surface of the machining-tool can be derived and controlling the positioning means in accordance with the stored sets of data values in a predetermined sequence to cause the tool-former to form the machining surface of the machining-tool in the required shape.

2. A method according to Claim 1 , in which the said sets of data values are values representative of the X-Y coordinates of the said machining surface of the machining-tool.

3. A method according to Claim 1, in which the said sets of data values are values representative of the required dimensions of a workpiece after it has been machined, and the method further comprises the steps of determining the actual dimensions of a machined workpiece at a plurality of predetermined locations on the workpiece, comparing the actual dimensions with a plurality of the said required dimensions at the said locations and providing a plurality of output values representative of the errors between the said actual dimensions and the said required dimensions, and using said output values to so control the positioning means that the tool-former forms the machining surface of the machine tool to the required shape to reduce the said errors.

4. A method according to Claim 1,2 or 3 for controlling a machine tool of the kind described in which the positioning means is arranged to cause relative movement between the machine-tool holder and a work piece in a machining mode of operation and to cause relative movement between the said two holders in a tool-forming mode of operation, the method being further used to cause the machine tool to change from the machining mode of operation to a tool-forming mode of operation wherein the machining-tool is formed to its required shape, and then to revert to its machining mode of operation.

5. A method according to Claim 4 in which the method is used periodically to cause the machine tool to change from the machining mode to the tool-forming mode of operation.

6. A method according to Claim 4, as dependent upon Claim 3, further comprising the steps of comparing the said output values representative of the errors between the actual dimensions with a set of corresponding tolerance values and, if the magnitude of the error output values exceeds that of the tolerance values at one or more predetermined positions on the work piece, causing the machine tool to change to its tool-forming mode of operation.

7. A method according to any one of the preceding Claims, in which the machine tool is a grinding machine.

8. A method of controlling a machine tool substantially as hereinbefore described with reference to the accompanying drawings.

9. Control apparatus for controlling the operation of a machine tool of the kind described, the control apparatus comprising control means control which, in operation, of the positioning means in the machine-tool, the control means comprising storage means which store a plurality of sets of data values from which the X-Y coordinates which define the required shape of the machining surface of the machining-tool can be derived, means which read from the storage means the said sets of data values in a predetermined sequence and which so control in operation, the positioning means that the tool-former forms the machining surface of the machining-tool in the required shape.

10. Control apparatus according to Claim 9 in combination with a machine tool of the kind described.

11. Control apparatus according to Claim 9 or 10, in which the said sets of data values are values representative of the X-Y coordinates of the machining-tool.

12. Control apparatus according to Claim 9 or 10, in which the said sets of data values are values representative of the required dimensions of a work piece after it has been machined, and the control means further comprises means for determining the actual dimensions of a machined work piece at a plurality of predetermined locations on the work piece, means for comparing the actual dimensions with a plurality of required dimensions at the said locations and for providing a plurality of output values representative of the errors between the said actual dimensions and the said required dimensions, and means for coupling said output values to so control the positioning means thereby that the tool-former forms the machining surface of the machine tool to the required shape to reduce the said errors.

13. Control apparatus according to Claim 9, 10, 11 or 12, in which the positioning means of the machine tool is arranged to cause relative movement between the machine-tool holder and a work piece in a machining mode of operation and to cause relative movement between the said two holders in a tool-forming mode of operation, the control apparatus further comprising operational control means for causing the machine tool to change from a machining mode of operation wherein a work piece is machined by a machining-tool, to a tool-forming mode of operation wherein the machining-tool is formed to its required shape, and then to revert to its machining mode of operation.

14. Control apparatus according to Claim 13, in which the operational control means is arranged periodically to cause the machine tool to change from the machining mode to the tool-forming mode of operation.

15. Control apparatus according to Claim 13, as dependent upon Claim 12, in which the operational control means comprises means for comparing the said output values representative of the errors between the actual dimensions and the said required dimensions with a set of corresponding tolerance values and for causing the machine tool to change to its toolforming mode of operation when the said error output value exceeds a corresponding tolerance value at one or more predetermined positions.

16. Control apparatus according to any one of Claims 9 to 15, in the form of a computer for controlling the operation of a machine tool.

17. Control apparatus according to any one of Claims 9 to 16, in combination with a machine tool, in which the machine tool is a grinding machine and the tool former is a diamond dressing tool.

18. A combination according to Claim 17, in which the diamond dressing tool is arranged to be mounted on a support which is movable relative to the work table.

19. A combination according to Claim 18, as dependent upon Claim 16, in which the position bf said support is controlled by the computer.

20. A combination according to Claims 17, 18 or 19, in which the machine tool is so arranged that, in a tool-forming mode of operation, the tool former forms the machining tool at substantially the same position as that at which the machining tool machines a workpiece in a machining mode of operation.

21. Control apparatus for controlling the operation of a machine tool substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

22. A machine tool substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.