CA1076471A - Torch traversing mechanism - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The invention describes a method of machining a workpiece with a cutting tool arrangement involving relative rectilinear movement between workpiece and tool, a heat source being mounted for transverse movement in a series of arcuate passes across the workpiece to heat and soften the portion thereof immediately ahead of the cutting tool.
In examples given, the heat source is one or two plasma torches and the arcuate passes are part of an oscillatory or of a rotational movement.

Description

--` 10764~
'~he present invention relates to machining and more particularly to a method of hot machining using a plasma torch, for example, as a heat source~

~he term 'hot machining' as used herein refers to a method of machining workpieces in which heat from a heat source, for example a plasma torch, is applied to a work-piece to soften it and thus facilitate machi~ng 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. ~etter Patent No. 19 351,140 to place a heat source i~ the form of a plasma torch ahead of a heat-resistant cutting tool, the plasma torch subjecting only the portion of the workpiece entering the cutting zone to heat so that the remainder of-` the workpiece remains thermally undamaged.
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In general, where hot machining is applied to turning, the heat source is maintained in fixed relation- ?
ship with respect to the turning tool. Difficulty arises when hot machining is applied to milling however because the cutting tool rotates. '~hus some degree of relative movement between the heat source and the workpiece is neceissary. ~
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In accordance with the present invention therefore there i8 provided a method of machining a workpiece with a -~ 2.

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~C~76471 cutting tool or tools which comprises creating relative rectilinear movement between the workpiece and heat supplying means, mounting the heat supplying means for movement in a series of arcuate or substantially arcuate passes transversely of the workpiece with reference to said relative rectilinear movement, the speed of said arcuate movements and the intensity of the heat source being so related to that of said relative rectilinear movement as to produce a substant-ially evenly heated and softened region of the workpiece in advance of the cutting tool or tools the or each of which is arranged to remove heated and softened workpiece material.

Al60 in accordance with the present invention there is provided a method of machining a workpiece with a cutting tool or tools which compris~es creating relative rectilinear movement between the workpiece and a heat source, mounting the heat source for oscillatory movement trans-versely of the workpiece with reference to the direction of said relative rectilinear movement9 the path of the heat source being curv~d and the speed of said oscillatory movement and the intensity of the heat source being so related to that of said rectilinear movement as to produce substantially even heating and softening of a region bf the workpiece in advance of the cutting tool or tools the or each of which i8 arranged to remove heated and softened workpiece material~

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~ he invention further provides a method of machining a workpiece with a cutting tool or tools which comprises creating relative rectilinear movement between the workpiece and a heat supplying means, mounting the heat supplying means for rotational movement in a path~ a portion of which is arranged transversely o~ the workpiece with reference to the direction of said relative rectilinear movement, the path of the heat supplying means being circular or substantially circular and the speed of said rotational movement and the intensity of the heat source being so related to that of said rectilinear movement as to produce substantially even heating and softening of a region of the workpiece in advance of the cutting tool or tools the or each of which is arranged to remove heated and softened workpiece material.
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Conveniently the method includes the step of clamping the workpiece to a movable bed of the machine ~or movement therewith and mounting the,heat source solely for oscillatory or rotational movement across the workpiece.
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Preferably the machine is a milling machine but a broaching, planing or caulking machine may also be used.
Where a milling machine is used the cutting tool is the milling cutter or cutters carried by the milling head~

l'he heat supply means is conveniently at least one plasma torch. In one example,'one torch may be mounted 4.
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1~76471 for arcuate oscillatory movement with respect to the direction of feed of the workpiece by means of a rack and pinion assembly. ~he plasma torch may be carried by the pinion which is itself mounted for oscillatory movement by the rack which is moved reciprocably by a double-acting pneumatic or hydraulic cylinder.

In other examples, one or more torcbes may be carried on a rotary plate for rotational movement about a centre axis of the cutting tool.

Whichever form of heat supply means is employed, the heat source is preferably traversed across the work-piece at a constant speed. In the case of oscillatory movement, this may be achieved by traversing the heat source beyond the edge of the workpiece prior to reversing the direction thereof.

~ ince the workpiece is substantially evenly heated to avoid thermal damage thereto, it i9 preferred that two successive passes of the heat source over the workpiece overlap. However, the degree of overlap between two successive passes should be such that substantially even heating results and the workpiece is not thermally damaged.
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It will also be found convenient under certain 5.
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circumstances to provide a leading edge of a workpiece with an additional sacrificial portion having a lesser degree of hardness than the main workpiece. ~his lessens the shock- -loading on the cutting tool at the commencement of machining and also causes the pilot arc of the plasma torch to transfer to the sacrificial portion ahead of the main workpiece, thereby improving the heating and softening of the leading edge portions thereof.

Where the size of the cutting tool permits, reduction both in the risk of damage to the plasma torch and in the distance between plasma arc impingement and the actual cutting region, is achieved if the plasma torch is received in a recess in the body of the cutting tool.

~ he invention will ~ow be further described by way of example with reference to the accompanying drawings in which~

~ igure 1 is a general side elevational view of part of a vertical milling machine;

~ igure 2 is an underside plan view of the apparatus shown in ~'iKure 1;

~ igure 3 is a diagrammatic illustration of the torch traverse mechanism as used in ~igures 1 and 2, ., .
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~ 'igure 4 is a diagr~mmatic side view, part in section, showing a second milling machine according to the invention.

~ igure 5 is an end view of the machine of Figure 4 in the direction of arrow V.

~ igure 6a and 6b show alternative arrangements of mounting the torch in the machine of ~'igure 4.

Figure 7 is a diagrammatic illustration of a power cut-off arrangement; and Figure 8 shows a sacrificial workpiece portion.

In Figure 1 the machine head of a vertlcal milling machine is shown as 1. The machine head rotatably mounts a cutter body at a conventional, shallow angle to the horizontal for movement in the direction of the arrow 'C', the cutter body carrying one or more cutting inserts 4 in known manner. Although four cutting inserts 4 are shown in Figures 1 and 2 it will be appreciated that any number of such cutting tips may be used.
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A workpiece 3 is clamped on the milling machine carriage (not shown) in known manner for movement i~ the direction of the arrow '~' in ~'igure 2. ~he workpiece is 7.
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connected electrically to the main circuit of the plasma arc. A plas~a torch 5 is mounted for oscillatory move~ent by means of a rack and pinion mechanism located between the machine head and the cutter body 2 as may be seen in ~igure 2. ~be rack 6 of the mechanism is arranged to reciprocate between a pair of limit switches 8, 9 (Figure 3) by means of a double-acting pneumatic or hydraulic cylinder 10. Movement of the rack 6 in turn causes the pinion 7, and hence the plasma torch, to traverse the work-pice 3 in a series of arcuate angular passes.

It is preferred that the torch is arranged to travel at a constant speed as it passes over the workpiece ~-3. This is achieved by spacing the limit switches 8, 9 at a greater distance apart than the width of the workpiece 3 so that reversal of the direction of movement of the torch 5 occurs beyond the edge of the workpiece. In any e~ent the speed of the oscillatory movement of the torch 5 is so related to the rate of feed of the workpiece as to sub-stantially evenly heat the workpiece in advance of the cutting tips 4, if necessary by arranging for subsequent passes of the torch to overlap. ~
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A workpiece 3 is clamped on the milling machine carriage (not shown) in known manner for movement in the direction of the arrow 'B' in ~'igure 2. In use, the plasma torch 5 is energised with the torch lying displaced from :.:
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the workpiece so that the resulting plasma arc is non-transferred (pilot arc). ~he machine is switched on so that the cutter body 2 rotates. The plasma torch is then caused to commence its oscillatory movement by activating the cylinder 10. hs the torch 5 passes over the edge of the wor~piece a transferred arc to the workpiece is auto-matically established. ~his is retained until the torch Passes over the opposite edge of the workpiece when the torch again becomes non-transferred. It will be appreciated that movement of the table and the oscillatory movement are adjusted so that optimum heating takes place as the carriage moves towards the cutter.

~ he heat from the plasma torch is arranged to heat only those regions of the workpiece entering the cutting zone. ~hereafter the cutting inserts in the cutting tool remove the heated and therefore softened portions of the workpiece in the manner previously proposed by the Applicants.

~ he torch traverse rate is adjusted according to the table traverse rate of the milling machine so that the arcuate bands produced by the plasma torch overlap. It has been found satisfactory if successive traverses of the heat source are spaced apart not more than 100 per cent of the effective width of the heat source. ~o allow heat to penetrate into the depth of cut of the cutting inserts 4, the torch applies heat ahead of the milling cutter, the .: " : ' , , , : .

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distanced measured along a radius from the cutting tips to the arc axis being referred to as the 'lead'. Preferably the torch is mounted in advance of the cutting tips by a distance of up to twice the diameter of the plasma torch nozzle orifice, although up to six times may be permissible under certain circumstances. Whilst the machining apparatus has been described as a millin~ machine it will be under-stood that this term is to be construed generically as including milling machines capable of, for example, face milling, end milling, side millin~ and slab milling. -~
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Moreover whilst in the described embodiment the workpiece is clamped on a milling machine carriage it will be appreciated that where, for example, the workpiece is of sufficient size, the workpiece may remain stationary and the cutting tool and heat source moved relatively thereto. In addition to the relative movement, the heat source will also be oscillated about the direction of the relative movement.

Whereas in the above described embodiment of the invention the heat source is mounted for arcuate oscillatory movement transversely of the workpiece, it is to be under stood that the heat source could be mounted for movement in a series of arcuate passes in the same direction across ~: .
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Figures 4 and 5 show a further milling machine 11 set up for a face milling operation in which heat supply means are passed i~ an arcuate path across a workpiece.
In this example the arcuate path is part of a circular pat~
described by two plasma arc torches 12 mounted for rotation with a cutting tool head 13 having four cutters 14.

A workpiece 15 is clamped on the milling machine carriage 11a in known manner and the torches are operated with the same requirements in mind as already explained above.

In a rotational torch mode, the torch or torches will preferably rotate at the same speed as the cutter, the table traverse rate of the milling machine being adjusted so that the arcuate bands produced by the plasma torch overlap. It has been found with milling cutters of small diameter, say, up to 4 inches, that a single torch produces the right amount of oYerlap. With longer cutters the use of two or more torches is preferable, the number increasing as the diameter of the cuttex increases, so as to space apart successive heated traverses not more than 100 per cent of the width of the heat source. In the present example, the use of two torches is shown.
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'~he cuttin~ head 13 is mounted for rotation on a slipring 16 and power for the heat source is supplied ., .
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through a brush gear device 17. '~he conventional drawbolt which passes through the spindle 1~ of a milling machine is, in the present example, replaced by a hollow distributor tube 18a comprisin~ three tubes (not illustrated in detail) for supplying the services (gas~ coolin~ water and heated return water) to the plasma torch or torches through a rotary union 19. ~he tube 18a also functions as a drawbolt in the usual manner.

The rotary union 19 performs not only the function of providing the services but only ensures that the power to the heating source is only supplied as the appropriate torch traverses the arcuate pass across the workpiece and not during the remainder of the rotational movement. Where the number of torches is low, say two or three, each torch will be supplied in turn, but where this number increases, say to six, it is possible that there will be perhaps two - -torches in the course of traversing the ~orkpiece at a ~iven moment. It would then be impracticable to divide a single power source between the torches, and it is .
probable that a separate power source would have to be provided for each torch.

~ igure 6a shows an arrangement of torch mounting in which there is a considerable distance between the plasma impingemerlt point 20 and the cutters 14. As can be seen from ~'igure 6b this distance may be considerably re-: . .
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In ~`igure 7, it will be observed that a stand-off distance x is provided between the torch 12 and the work-piece 15, which, in practice, is small enough to e~sure consistent striking of the main arc onto the workpiece.
However, when the trailing end of the workpiece is reached and the torch traverse path extends beyond the workpiece, there is arranged to be a marked increase in stand-off distance to a distance ~. ~his sudden increase is sensed by the power unit and the power to the heat source is automatically cut off, although the pilot arc remains active.

~ igure 8 shows the workpiece 15 provided with a mild steel bar 22 secured to its leading edge so as to lie . .
flush with the surface to be milled. The plasma torch 12 ~ ;
therefore traverses the bar 22 before it reaches the main workpiece. This causes the main transferred arc to become established and to commence heating the bar. Thus the heating operation has already begun ahead of the main work-piece and a more even heating effect is obtained. Morever, because the bar is chosen to be of softer metal than the workpiece, the shock loading on the cutting tool is reduced.

~he following test results were obtained in .. , -., .
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~7~471 experiments designed to contrast the method of hot machining in accordance ~ith the invention with conventional machining.

In carrying out the method using an oscillatory movement, the workpiece was heated with a constricted-arc plasma torch used in conjunction with a 300 amp transformer-rectifier power unit having an open circuit voltage of 80 volts and a dropping power characteristic. Asingle stream of argon gas was passed through a nozzle attached to the torch body at rates of up to 9.5 litres per minute. The plasma nozzle orifice had a diameter in the region of 3/16 inch. It will be understood that the gas may be a commercial grade inert ~ ;
gas, argon or helium, or may include proportions of other suitable gases, e.g. hydrogen or mitrogen.
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~he workpiece was mounted and clamped to the table of a horizontal milling machine, and the torch was set to ~-oscillate across the workpiece close to the periphery of a ~;
3-tooth face milling cutter of approximately 6~ inch ~`
diameter and mounted on the horizontal spindle of the milling machine. ~he heated material was removed by the cutting inserts mounted in the milling cutter body.

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The torch was operated in the transferred-arc mode, provision being made to complete the current circuit through the workpiece and machine table. ~'he torch was mounted so as to present the arc at an angle of approximately 14.

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60 to the workpiece.

l`he working current was set on the rectifier power unit and various workpieces were face milled using ceramic cutting tips.

~ES~ No. 1 ~ork material: Alloy steel ~n.40B (Har~ness 280 VPN, as cast) 4 in. wide x 6 in. long Cutting inserts: 'Nippon Tungsten' NPC-A2 - ceramic Plasma settings:
Current: 55A
Voltage: 45V
Power: 2.5 kw Cutter speed: 914 rpm (1620s.f.p.m.) ~orch oscillation rate: 160 passes per minO ~
~able traverse rate: 11 in. per min. (=0.069 in. ,' per torch pass) ~eed per tooth: 0.004 in. "
Depth of cut: 0.075 in.
Total length of workpiece face milled satisfactorily: 24 in.
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To contrast the result obtained above with results ~ , ' using conventional cold machining, ~est 1 was repeated, but without plasma heating. ~1he table traverse rate was set ' at 6-~ in. per min. instead of 11 in. per min. The total length face milled satisfactorily was only 6 in. Also, ~,l whereas the wear,on the cutting inserts was severe after ", ~,~
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~ ~107~71 cutting 6 in. without plasma heating, only slight wear had occurred by the end of the test using plasma heating.

To contrast the results obtained previously with those for the system using rotational torch movement, ~est 1 was repeated but with the following differences, due in part initially to practical speed limitation of the system.
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Cutting inserts: Sandvik GC315 Titanium-coated carbide Plasma settings:
Current: 120A
Voltage 58V
Power: 7 kw Cutter speed: 280 r.p.m. 436 s.f.p.m.
Plasma torch speed: 560 passes per min (2 torches x 280 r.p.m.) Table traverse rate: 22 in./min (=0.04 in~ per `~
~orch pass) ~eed per tooth: 0.03 in.
Depth of cut: ,075 in.
; Total length of workpiece face milled satisfactorily: 102 in.
~ool life: 4.6 minutes '.
Thus, superior results to conventional cold machining were ,~ achieved with both methods of plasma keating, the rotational ~, torch being the better of the two heating systems. In both cases, the cutter was capable of further cutting, the tool ' wear being negligible.
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, i471 TEST No. 2 ~ his test used as workpiece an iron based alloy having a high chromium content and good abrasion resistance.
The workpiece was in the fo~ of a cast block 12" long x 6"
wide with a hardness of 835 HV.

~ es~ carried out using an oscillating torch system resulted in the following conditions being achieved:-Cutting inserts: 'Nippon Tungsten' NPC-A2 Ceramic Plasma settings:
Current: 150A
Voltage: 60V
Power: 9 kw Cutter speed: 416 r.p.m. 663 a.f.p.m.
~orch oscillation rate: 120 passes per minute ~able travers rate: 10 in./minute (=0.08 in. per torch pass) ~eed per tooth: 0.002 in.
Depth of cut: 0.1 in.
Total length of workpiece face milled satisfactorily: 36 in.
Corresponding tool life: 3~ minutes Milling tests were then conducted using the equipment with a rotating plasma torch. ~esults achieved were as follows:-, Cutting inserts: 'Nippon ~ungsten' NPC-A2 Ceramic .
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~lasma settings:
Current: 120A
Voltage: 64V
Power: 7.7 kw Cutter speed: 344 r.p.m. 547 s.f.p.m.
Plasma torch speed: 688 passes per minute (2 torches x 344 r.p.m.) Table traverse rate: ll in./minute (=0.016 per torch pass) Feed per tooth: 0.003 in.
Depth of cut: 0.1 in.
~otal length of workpiece face milled satisfactorily: 48 in.
Corresponding tool life: 4~ minutes Although in both cases the tool life was comparatively short, it was advantageous to mill the material much more quickly than milling conventionally at a table tr~ve~se rste of 2.5 in. per mi~ute.

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Claims (15)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:-

1. A method of machining a workpiece with a cutting tool or tools which comprises creating relative rectilinear movement between the workpiece and heat supplying means, mounting the heat supplying means for movement in a series of arcuate or substantially arcuate passes transversely of the workpiece with reference to said relative rectilinear movement, the speed of said arcuate movements and the intensity of the heat source being so related to that of said relative rectilinear movement as to produce a substantially evenly heated and softened region of the workpiece in advance of the cutting tool or tools the or each of which is arranged to remove heated and softened workpiece material.

2. A method of machining a workpiece with a cutting tool or tools which comprises creating relative rectilinear movement between the workpiece and a heat source, mounting the heat source for oscillatory movement transversely of the workpiece with reference to the direction of said relative rectilinear movement, the path of the heat source being curved and the speed of said oscillatory movement and the intensity of the heat source being so related to that of said rectilinear movement as to produce substantially even heating and softening of a region of the workpiece in advance of the cutting tool or 19.

tools the or each of which is arranged to remove heated and softened workpiece material.

3. A method as claimed in claim 2 which includes the step of clamping the workpiece to a movable bed of a machine for movement therewith and mounting the heat source solely for oscillatory movement across the workpiece.

4. A method as claimed in claim 1 in which both the speed at which the heat source traverses the workpiece and the speed of the relative rectilinear movement are substantially constant.

5. A method as claimed in claim 2 in which a substantially constant heat source speed is achieved by traversing the heat source beyond the edge of the workpiece prior to reversing the direction of movement thereof.

6. A method as claimed in claim 1 in which the heat source traverses the workpiece at a speed related to the rectilinear speed of the workpiece such that successive traverses of the heat source are spaced apart at not more than 100 per cent of the width of the path heated by the heat source.

7. A method as claimed in claim 1 in which the heat supply means is a plasma torch, the torch being mounted 20.

in advance of the cutting tool or tools by a distance of up to six times the diameter of the plasma torch orifice.

8. A method of machining a workpiece with a cutting tool or tools which comprises creating relative rectilinear movement between the workpiece and heat supplying means, mounting the heat supplying means for rotational movement in a path, a portion of which is arranged transversely of the workpiece with reference to the direction of said relative rectilinear movement, the path of the heat supply means being circular or substantially circular and the speed of said rotational movement and the intensity of the heat source being so related to that of said rectilinear movement as to produce substantially even heating and softening of a region of the workpiece in advance of the cutting tool or tools the or each of which is arranged to remove heated and softened workpiece material.

9. A method as claimed in claim 8 in which the heat supply means comprises at least one plasma torch, the or each torch being mounted for rotation with the cutting tool or tools.

10. A method as claimed in claim 9 in which the plasma torch or torches is/are mounted in recesses in the cutting head to minimise the distance between the plasma 21.

arc impingement point on the workpiece and a cutter of the cutting tool(s).

11. A method as claimed in claim 1 in which a leading edge of the workpiece is provided with a sacrificial portion Or metal of lesser hardness than that of the work-piece itself said portion being secured to lie flush with the workpiece surface to be cut.

12. Apparatus for carrying out the method accord-ing to claim 2, comprising a machine tool having a machine table, a cutting tool or tools, means for bringing about relative movement between the table and the tool(s), a heat supplying means and means mounting the heat supplying means for oscillatory movement transversely of a workpiece on the machine table.

13. Apparatus for carrying out the method accord-ing to claim 8, comprising a machine tool having a machine table, a cutting tool or tools, means for bringing about relative movement between the table and the tool(s), a heat supplying means and means mounting the heat supplying means for rotational movement in a path, a portion of which traverses a workpiece on the machine table.

14. Apparatus as claimed in claim 13, in which the heat supplying means is mounted in a recess in a cutting 22.

head of the machine.

15. Apparatus as claimed in claim 12, in which the heat supplying means comprises at least one plasma arc torch.

23.