GB1586559A

GB1586559A - Hot machining

Info

Publication number: GB1586559A
Application number: GB2129176A
Authority: GB
Original assignee: PRODUCTION ENG RES
Current assignee: PRODUCTION ENG RES
Priority date: 1977-08-22
Filing date: 1977-08-22
Publication date: 1981-03-18

Description

(54) IMPROVEMENTS IN OR RELATING TO HOT MACHINING (71) We, THE PRODUCTION ENGINEERING RESEARCH ASSOCIATION of GREAT BRITAIN, a British Company, of Melton Mowbray, Leicestershire, 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:- The present invention relates to a method of hot machining using a plasma torch, for example, as a heat source.

The term "hot machining" as usedherein refers to a method of machining a workpiece in which heat from a heat source, for example a plasma torch, is applied to a workpiece to soften it and thus facilitate machining by a separate cutting tool such as a turning tool in the case of a lathe.

It is known from the Applicants prior U.K. Letters Patent No. 1,351,140 to place a heat source, in the form of a plasma torch ahead of a heat-resistant cutting tool, the plasma torch subjecting only the portion of the workpiece to be removed by the tool to heat so that the remainder of the workpiece remains thermally undamaged.

Hitherto difficulties have arisen in setting the heat input to a workpiece from a plasma torch for a machining operation. The torch has been set to an optimum setting, judged in the light of the experience of an operator, and once machining has been commenced this setting has been maintained. Thus certain areas of the workpiece have tended to be underheated, and therefore insufficiently softened, whereas other areas have tended to be overheated giving rise to distortion and thermal damage in a completed workpiece.

This problem has been particularly acute when hot machining workpieces of irregular cross section, or when workpieces were to be machined to non-circular crosssections such as elliptical sections.

According to the broadest aspect of the present invention therefore there is provided a method of hot machining (as defined herein), comprising detecting by detector means located between the heat source and the cutting tool a condition of said workpiece which varies in dependence upon the heat applied thereto as a measure of the machinability of the workpiece, issuing a signal representative of the detected condition to a control system, the control system including means for deriving a difference signal representative of the difference between the detected condition and an empirically derived reset, adjustable condition, and controlling the heat applied to the workpiece by supplying said difference signal as a control signal to the heat source.

Preferably the control signal is used to control the magnitude of the heat derived from the heat source. The control signal may alternatively be used to alter the position of the heat source with respect to a workpiece. Alternatively again the control signal may be used to adjust both the magnitude of the heat derived from the heat source and the position of the heat source with respect to the workpiece.

Where the heat source is a plasma torch the electrical power input to the torch may be conveniently controlled. In addition, or alternatively, the gas flow supply to the torch may be controlled. In this connection it is preferred that the plasma utilises argon gas rather than a mixture of gases although a plasma utilising a gas mixture of hydrogen, nitrogen and/or argon may be used. Where a gas mixture is employed, if desired, the proportions of the constituent gases may be individually controlled. It will be appreciated however that the heat source may be other than a plasma torch, for example an oxyacetylene or propane torch, in which case the gas supply to the torch is the only variable available for control.

Preferably, the detected condition of the workpiece is its temperature. Preferably also, the empirically derived preset, adjustable condition is a temperature selected within a range at which the material of said workpiece approaches superplasticity, i.e. exhibits abnormally high tensile elongation.

The method may include the steps of sensing the temperature of the machined surface of the workpiece by means of sensor means and comparing a signal representative of the sensed temperature with a predetermined temperature signal indicative of the permissible temperature to which the machined surface of the workpiece may be subjected, the control system being operative to issue a signal to prevent any increase to the input of the heat source in the event of the sensed temperature attaining the predetermined temperature.

The control system may include an indicator for providing an audible or visual signal until the preset temperature is achieved. In the event of the predetermined temperature being achieved before the preset temperature an alarm signal will indicate that optimisation of machining cannot be achieved under the existing operating conditions. This signal may be used to shut down the hot machining process until the operating conditions have been altered. The conditions which may be altered for a turning operation for example are: a) The position of the heat source, either circumferentially, axially or in attitude, or, where approptiate, radially, b) Cutting conditions: (i) Cutting speed, (ii) Tool feed rate, and (iii) Depth of cut.

c) The tool variables: (i) Tool material, and (ii) Tool shape and geometry.

Both the detector means and the sensor means are preferably optically-operable devices, such as an infra-red sensor, which may be spaced from the workpiece surface and sighted on to a portion of the surface to detect the temperature of that portion. A suitable infra-red sensor or detector is that sold under the trade name Thermodot (Registered Trade Mark) by Messrs.

Infrared Industries Inc. of Box 989 Santa Barbara, California, U.S.A..

The invention will now be described further by way of example with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic plan view of a lathe being used to hot machine a workpiece, and Fig. 2 illustrates a control system in accordance with a preferred embodiment of the invention.

The lathe illustrated in Figs. 1 and 2 is a known centre lathe having a lathe bed defining a lathe axis and supporting a headstock and a tailstock at either end thereof. A carriage (not shown) is mounted on the bed for movement therealong between the headstock and tailstock. A chuck, rotatably mounted at the headstock, carries a workpiece whilst a cutting or turning tool supported in a tool-post is mounted on a cross-slide on the carriage.

Thus, the carriage is mounted for longitudinal movement of the lathe axis, the cross-slide is mounted for transverse movement of the lathe axis so that a large number of tool positions may be obtained.

Fig. 1 shows the lathe diagrammatically as 10, the headstock as 12, the chuck as 14 and the tailstock as 16. A workpiece W is shown supported in the chuck 14, the free end of the workpiece being carried in a centre mounted in the tailstock 16. The workpiece is mounted for rotation between the chuck and the tailstock about the machine's axis 18 in known manner.

A heat source 20, which in the illustrated embodiment, is a plasma torch, is mounted to the side of the workpiece W and a cutting tool 22, mounted in a tool post 24, is disposed so as to cut the material of the workpiece heated by the heat source 20.

For convenience in Fig. 2 the plasma torch is shown above the workpiece.

Referring now to Fig. 2 the heat input to the workpiece material to be removed by the cutting tool 22 is detected and monitored by an infra-red temperature detector 26. The detector is sighted on to a portion of the workpiece between the plasma torch 20 and the cutting tool 22. An electrical signal Tl representative of the temperature of the workpiece portion is fed to an integrator circuit 28 which serves as a smoothing filter to ensure that the control system does not respond to small irregularities in the signal which might otherwise cause "hunting".

The output from the integrator is fed to an input terminal of a summing amplifier 32. A second input terminal of the summing amplifier is fed with a signal Oi representative of an empirically derived temperature. This temperature preferably corresponds to a temperature selected within the range at which the workpiece material approaches superplasticity, but also depends upon the machining conditions including the size of the workpiece, the material from which it is made, its rotational speed, etc. The temperature signal Oi is set by means of a potentiometer 30.A signal representative of the difference between the detected temperature and the preset temperature is derived in the summing amplifier 32 and this signal, in dependence upon its sign and magnitude, is used to control the power input to the plasma torch 20 through a servo mechanism to be described hereafter.

In some machining applications, it may be advantageous to provide means for preventing a critical temperature of the machined surface from being exceeded. To this end, an infra-red temperature sensor 34 is arranged to monitor the temperature of the machined surface of the workpiece immediately after metal has been removed.

A signal T2 representative of this temperature is passed through a further integrator 36 which is used as a smoothing circuit in a similar way to the integrator 28.

The smoothed output from the integrator 36 is fed to a comparator 38 where the signal, representative of a predetermined temperature, is compared with a signal O2 and set by means of a potentiometer 39.

This temperature corresponds to a maximum permissible residual temperature in the particular workpiece. It will be appreciated that the signal representative of the maximum permissible residual temperature will usually be lower than the signal representative of the empirically derived temperature 01, so that the signal 82 will normally be less than the signal 01.

Upon the temperature monitored by the sensor 34 reaching the predetermined value a signal is issued to either a power shut down circuit or a power control circuit (not shown) to prevent a further increase in the electrical power supplied to the plasma torch 20.

The summing amplifier works out the difference between the two input signals supplied thereto and provides an output signal dependent thereon to a servo amplifier 42 of a servo mechanism. It is this signal fed through an appropriate servo drive motor 44 to effect mechanical displacement of a saturable reactor transformer 46 which supplies the input power by known means to the torch 20 through a power line 48.

Until the desired preset temperature represented by signal Oi is reached, an audible or visual signal may be generated by means not shown. In the event that the predetermined temperature represented by the signal 82 is reached before this preset temperature, an alarm signal (audible or visible) will indicate that optimisation of machining cannot be achieved under the operating conditions pertaining. Thus, it will be necessary to implement a change in the machining set-up, e.g. change of position of plasma torch either circumferentially, axially or in attitude, a change of workpiece speed, tool feed rate etc.Likewise, if the preset temperature represented by signal 0, cannot be achieved under the operating conditions pertaining because the heat source cannot provide sufficient heat, it may be necessary to change the machining set-up as discussed previously.

The relative axial positions of the plasma tdrch and temperature sensors, with respect to the cutting tool, are important, whereas there may be some latitude in their circumferential spacing, always provided that the detector 26 lies between the heat source 20 and the cutting tool 22. The detector should be sighted towards a small area on the transitional surface 25 (Fig. 1) generated by the main cutting edge of the cutting tool. Preferably, its axial position should be as close as is practicable to the cylindrical surface 27 (Fig. 1) generated by the cutting tool.

As regards the axial position of the heat source, this will usually be such as to provide a "lead" if the workpiece is of small or moderate size, that is, the centre of a hotspot from the plasma torch will be several revolutions ahead of the portion monitored by detector 26. For large depths of cut on large workpieces the heat source will preferably be inclined so that it is normal to and heats transitional surface 25 generated by the main cutting edge of the cutting tool, but preferably is displaced axially with respect to the portion of the detector 26.

The sensor 34 is directed on to the cylindrical surface 27 generated by the cutting tool but displaced axially the minimum distance necessary to avoid spurious readings caused by heated metal yet to be removed by the cutting tool.

In operation a cold workpiece W is - inserted into the lathe in the manner described and the lathe energised to rotate the workpiece. The main transferred arc of the plasma torch is then energised in known manner at a low energy level to heat a selected portion of the workpiece W. The detector 26 detects the temperature of the heated portion of the workpiece and once this reaches a preselected value the cutting tool 22 is caused to engage and machine the workpiece. After the cutting tool has engaged with the workpiece the sensor 34 is activated to sense the temperature of the machined surface of the workpiece. As the control loop operates, the energy level fed to the plasma torch, and hence the workpiece, is increased.The energy level is allowed to increase until the optimum machining conditions corresponding to the preset temperature represented by signal 8i have been obtained. It will be understood that as the tool 22 feeds to machine the workpiece the detector 26, sensor 34 and plasma torch 20 move in synchronism with the tool feed. The control circuitry then operates in the manner described.

It will be appreciated that whereas in the described embodiment the condition of the workpiece sensed by the detector 26 is a temperature condition, it may be possible to measure other conditions which vary in dependence on the heat applied.

WHAT WE CLAIM IS: 1. A method of hot machining (as herein defined) a workpiece, comprising detecting by detector means located between the heat source and the cutting tool a condition of said workpiece which varies in dependence upon the heat applied thereto as a measure of the machinability of the workpiece, issuing a signal representative of the detected condition to a control system, the control system including means for deriving a difference signal representative of the difference between the detected condition and an empirically derived preset, adjustable condition, and controlling the heat applied to the workpiece by supplying said difference signal as a control signal to the heat source.

2. A method as claimed in claim 1, wherein the detected condition of the workpiece is its temperature.

3. A method as claimed in claim 1 or 2, wherein the empirically derived preset, adjustable condition is a temperature selected within a range at which the material of said workpiece approaches superplasticity, i.e. exhibits abnormally high tensile elongation 4. A method as claimed in claim 2, including the steps of sensing the temperature of the machined surface of the workpiece by means of sensor means and comparing a signal representative of the sensed temperature with a predetermined temperature signal indicative of the permissible temperature to which the machined surface of the workpiece may be subjected, the control system being operative to issue a signal to prevent any increase to the input of the heat source in the event of the sensed temperature attaining the predetermined temperature.

5. A method as claimed in claim 2, wherein the control system includes an indicator for providing suitable audible or visual signal until the preset temperature is achieved.

6. A method as claimed in claim 5, wherein in the event of the predetermined temperature being achieved before the present temperature an alarm signal indicates that optimisation of machining cannot be achieved under the existing operating conditions.

7. A method as claimed in claim 6 wherein the alarm signal is used to shut down the hot machining process until the operating conditions have been altered.

8. A method as claimed in any one of the preceding claims, wherein the detector means and sensor means are optically operable.

9. A method as claimed in any one of claims 1 to 7, wherein the detector means and sensor means are operable by means of infra-red radiation.

10. A method as claimed in any one of the preceding claims wherein the control signal is used to control the magnitude of the heat derived from the heat source.

11. A method as claimed in any one of the preceding claims, wherein the control signal is used to alter the position of the heat source with respect to the workpiece.

12. A method as claimed in any one of the preceding claims, wherein the heat source is a plasma torch.

13. A method as claimed in claim 12, wherein the control signal is used to control the electrical power input to the plasma torch.

-14. A method as claimed in claim 12 or claim 13, wherein the control signal is used to control the flow of gas to the plasma torch.

15. A method as claimed in any one of claims 12, 13, or 14, wherein the plasma torch utilises argon gas.

16. A method as claimed in any one of claims 12, 13 or 14, wherein the plasma torch utilises a mixture of hydrogen, nitrogen and/or argon gas.

17. A method as claimed in claim 16, wherein the control signal is used to control the proportions of the individual gases.

18. A method as claimed in any one of claims 1 to 11, wherein the heat source is an oxyacetylene torch.

19. A method as claimed in any one of claims 1 to 11, wherein the heat source is a propane torch.

20. A method of hot machining (as herein defined), substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the tool feed. The control circuitry then operates in the manner described. It will be appreciated that whereas in the described embodiment the condition of the workpiece sensed by the detector 26 is a temperature condition, it may be possible to measure other conditions which vary in dependence on the heat applied. WHAT WE CLAIM IS:

1. A method of hot machining (as herein defined) a workpiece, comprising detecting by detector means located between the heat source and the cutting tool a condition of said workpiece which varies in dependence upon the heat applied thereto as a measure of the machinability of the workpiece, issuing a signal representative of the detected condition to a control system, the control system including means for deriving a difference signal representative of the difference between the detected condition and an empirically derived preset, adjustable condition, and controlling the heat applied to the workpiece by supplying said difference signal as a control signal to the heat source.

3. A method as claimed in claim 1 or 2, wherein the empirically derived preset, adjustable condition is a temperature selected within a range at which the material of said workpiece approaches superplasticity, i.e. exhibits abnormally high tensile elongation

4. A method as claimed in claim 2, including the steps of sensing the temperature of the machined surface of the workpiece by means of sensor means and comparing a signal representative of the sensed temperature with a predetermined temperature signal indicative of the permissible temperature to which the machined surface of the workpiece may be subjected, the control system being operative to issue a signal to prevent any increase to the input of the heat source in the event of the sensed temperature attaining the predetermined temperature.

-

14. A method as claimed in claim 12 or claim 13, wherein the control signal is used to control the flow of gas to the plasma torch.