GB2057944A - Apparatus for smoothing deformed sheet - Google Patents

Abstract

Apparatus for smoothing deformed sheet (2), especially of coachwork, has a transformer (6) a return electrode 33 and an interchangeable deforming electrode (18, 18a, 18b) with contact faces (20, 20a, 20b; 33, 33a), which can be placed on the sheet (2). The deforming electrode (18, 18a, 18b) is under the influence of a spring (14) so that it can bring a force to bear on the sheet (2). The invention consists in the return line electrode (24) surrounding the deforming electrode (18, 18a, 18b), the deforming electrode (18, 18a, 18b) being displaceable from a rest position against the increasing force of the spring (14) to behind the contact face (33, 33a) of the return line electrode (24) and the relative spacing of the contact faces (20, 20a, 20b; 33, 33a) in the rest position of the deforming electrode (18, 18a, 18b) in the direction of the spring action is controllably adjustable. <IMAGE>

Description

SPECIFICATION Apparatus for smoothing deformed sheet, especially of coachwork, by electric heating The invention relates to an apparatus for smoothing deformed sheet, especially of coachwork, by local electric heating of the sheet by passing a current through it, comprising a high current transformer and two electrodes connected to it, which have contact faces for the sheet and of which at least one, a deforming electrode, can be applied to the deformed region of the sheet with mechanical pressure and the other is a return line electrode.

The deformations that can be treated include bulges, that is to say bumps projecting from the surface of the coachwork, such as are principally pushed out by forceful closure of luggage compartment lids on the corners of luggage, as well as instabilities in the sheet, where one can, using thumb-pressure, push in and out a part of the sheet, producing a noise, which can also be termed a "frog".

This latter is the result of high tensions in the sheet, which are produced either by smaller outward impressions or by bulges made by hammer and anvil. The sheet is partially stretched in length and can no longer take up its original mean length, but can at times spring through it.

Electrothermal smoothing with the above described apparatus has in recent times taken on an increased importance, since it was discovered that by a substantially radial current through the sheet emanating from the deforming electrode, not only bulges, especially so-called "pimples", but also the instabilities mentioned can be removed. A marked temperature gradient is set up around the deforming electrode, with a maximum in the middle, and this enables bulges (pimples) to be pushed back in with medium to strong pressure. At instabilities, it is observed that clearly on the basis of specific heat-tensioning appearing in the sheet cooling, a shrinking process takes place which tensions the sheet and tends to draw it back to its original, stable length.

For the above described electrothermal smoothing of deformed sheet an apparatus is known comprising a transformer after the manner of a welding transformer and two electrodes connected thereto by cables, and movable independently of one another, of which the one is attachable at a practicable desired point of the sheet, while the other, held in the hand, serves as a deforming electrode. It is also already known to form the deforming electrode as a percussive tool in order to avoid a simultaneous treatment with an additional tool (West German Auslegeschrift 23 62 103). The known equipment requires considerable experience and skill to leval bumps (bulges). Instabilities such as "frogs" are, with the known apparatus, practically impossible to remove properly on account of the poor control of the amount of force.

A device with co-axial electrodes is known, of which one is lengthwise displaceable against a spring, the electrode arrangement however serving thereby only to guide the current, while the treatment proper of the sheet is carried out by an additional tool (East German Patent Specification 22 617).

The present invention is based on the problem of providing an apparatus of the type described above, with which, without using additional tools, not only raised bumps (bulges) but also instabilities (frogs) can be removed and which makes possible a straightforward and quickly learned operation.

The invention comprises apparatus for smoothing deformed sheet by local electric heating of the sheet by passing a current through it, comprising a high current transformer and into electrodes connected thereto, which electrodes have contact faces for the sheet and of which electrodes at least one (a deforming electrode) can be applied to the deformed region of the sheet with mechanical pressure and the other is a return line electrode, characterised in that the return line electrode surrounds the deforming electrode, the deforming electrode is biassed to project beyond the return line electrode by a spring against which it can be displaced to a position behind the contact face of the return line electrode, and the extent to which the contact face of the deforming electrode projects beyond the return line electrode when no load is applied is adjustable.

By the invention, that is to say a deforming electrode can be delicately controlled by being part of a coaxial electrode arrangement.

Through the controlled adjustment of the spacing of the contact faces in the direction of the line of action of the spring, different regions of the spring characteristic (load-extension line) can be brought into operation according to the size of the spacing. With the exception of a necessary small spring pretension, the spring, in its rest state, that is to say when the deforming electrode is fully extended, is in a substantially untensioned position, which, by a preferably easily adjustable stop, is constant in all conditions of use.

The maximum possible spring extension and therewith the maximum attainable restoring force at the limit of this spring extension are now primarily determined by such extension as the contact faces can hold in reserve relative to one another when the apparatus is pressed on to the sheet. Here it is to be noted that both electrodes faces must lie on the sheet if a current is to flow. It is possible therefore, by deliberate or defined adjustment of the spacing of the electrode faces, to specify the maximum spring force within narrow limits, in which naturally also the type and size of the deformation of the sheet plays a positive, but subsidiary, role.

A large spacing of the electrode faces corresponds to greater spring extension with a high maximum force. Such an adjustment of the electrode spacing serves especially for the removal of pointed, projecting bulges, socalled "pimples". For the tensioning of the sheet in instabilities, a correspondingly smaller axial spacing of the electrodes will be selected, so that the spring extension and with it the maximum force to put the return line electrode in contact with the sheet will be suitably reduced. Here it should be observed that the spring force of the deforming electrode in shrinking or tensioning necessarily opposes this process. Once the relative spacing of the electrode faces has been preset the apparatus can be used thereafter for completely specific working purposes without the operator being able to apply a force much different from the intended force.In removing instabilities by shrinking, the apparatus can be moved smoothly in particular surface designs over the sheet, so that the instabilities can be removed stepwise with reduced pressure of the deforming electrode.

The adjustment of the axial spacing of the electrode faces may be effected in different ways. Thus, for example, the return line electrode can be extended in the direction of the spring extension, as, for example, by an attachable ring, if the return line electrode is formed as a hollow cylinder. This attachable ring has the additional advantage that it can be thrown away without great expense if after a specified period of use it shows appearance of oxidation on its front face, which in this case forms the electrode face of the return line electrode. It is, however, also possible to secure the deforming electrode exchangeably in a sprung holder and exchange it for a shorter or longer deforming electrode. The longer electrode necessitates a greater spring extension and serves, therefore, for the levelling of projecting bumps (pimples).Such an electrode has, appropriately, a substantially plane front or contact face. The shorter electrode necessitates a shorter spring extension and serves, therefore, principally for the shrinking of instabilities. Since this process, as previously described, is usually carried out stepwise over a larger surface, the contact surface is preferably convex, whereby the gliding motion is facilitated and an excessive oxidation by point or edgc contact on tilting the apparatus can be avoided. Obviously, also, the exchangeable deforming electrodes can be replaced by new electrodes on corresponding oxidation.

The apparatus according to the invention can be advantageously made so that the electrode arrangement is disposed in a housing of a pistol shaped hand tool, in which the housing can also hold the high-current transformer.

In this manner there is provided an easily manipulated and portable, compact apparatus, which is also immediately deployable on awkwardly accessible coachwork. The apparatus referred to can be used like a hand drill.

One embodiment of the invention and the different possible variations for adjusting the axial electrode spacing will be more particularly described with reference to the accompanying drawings, in which: Figure 1 is a longitudinal section through a complete apparatus in the form of a pistol, Figure 2 is an attachable ring for the return line electrode, Figure 3 is an exchangeable deforming electrode, Figures 4 to 6 are those part sections of Fig. 1, which relate to the electrode arrangement, with different axial spacings of the electrode faces, and Figure 7 is an extension-force diagram (characteristic) for the devices of Figs. 1-6.

In Fig. 1 there is shown an apparatus 1 which is specially intended for the treatment of coachwork panels 2. The apparatus comprises a using 3 consisting of two half shells, which is shown opened along its joint and has the form of a pistol with a hand grip 4 (shown only in part). Through the hand grip 4 passes an electric cable 5 to the primary winding of a high current transformer 6.

Switching on and off is effected by means of a push button switch 7 arranged in the hand grip, the connections to which are not shown.

An overload switch 8 included in the circuit protects the transformer 6.

In a forward facing wall 9 of the housing 3 is carried, by means of an annular groove 10, a rotationally symmetrical insulating body 11 of plastic, which has a coaxial longitudinal bore 12. At the inner end of the longitudinal bore is a collar 13, which serves as an abutment for a pressure spring 14. The other end of the spring 14 pushes against a shoulder 15 of a bolt-shaped holder 16, which is slidable in the longitudinal bore 12 against the force of the spring 14. At its outer end, the holder 16 has a hollow cone shaped bore 17, in which a deforming electrode 18 is located by means of a complementary cone 19. A contact face 20 serves to carry the current to the coachwork panel 2. At this end of the holder 16 is a screw thread, with two nuts, which serves for axial adjustment of the holder 16 and for fixing the pre-loading of the spring 14. The holder 16 is connected to one output terminal of the secondary of the transformer 6 by two pivotally connected conducting straps 22 and 23.

In the insulating body 11 is located, coaxially therewith and with the deforming electrode 18, a return line electrode 24 in the form of a hollow cylinder. This is connected to the other output terminal of the secondary winding of the transformer 6 by a conducting strap 25, the fastening of the strap 25 to the return line electrode 24 being effected on the blind side of the drawing. The conducting components, and especially the electrodes 18 and 24 are of copper. A pilot light 26 shows the state of connection of the transformer 6.

The apparatus can be hung up in the workshop by an eye 27.

In Fig. 2 is shown a ring 28, which has a wider bore 29 and a narrower bore 30, which are connected with one another at a radial shoulder 31. Slits 32 made at the periphery, of which only the one at the back can be seen, impart to the ring 28 in the region of the bore 29 a certain elasticity, so that the ring can be pushed in as far as the shoulder 31 on the return line electrode 24 (Fig. 1) and this extended up to the axial length of the bore 30. At the position of the contact face 33 of the return line electrode 24 appears in this way the contact face 33a of the ring 28 which also comprises copper.

A deforming electrode 18a with a convex contact face 20a is shown, of which the cone 19a corresponds to the cone 19 of Fig. 1. In this way, the deforming electrode 18 can be located in the holder 16. The deforming electrode 18 a however, has as shown by the part beyond the holder 1 6a substantially smaller length than the deforming electrode 18, so that by abutting the return line electrode 24 on the coachwork panel 2, only a markedly smaller spring extension is possible.

The location of the objects shown in Figs. 2 and 3 in connection with the subject matter of Fig. 1 will be explained in more detail with reference to Figs. 4, 5 and 6. The pictorial representation is similar save for a 2:1 increase in scale, so that a recapitulation of the description can be substantially avoided.

According to Fig. 4, a coachwork panel 2 has a pimple-form, projecting bulge 34 such as is typically put into a luggage compartment lid. In this case, the longer deforming electrode 18 according to Fig. 1 is inserted into the holder 16, the contact face 20 being plane. In the illustrated case, at least the spring-extension S2 is available for the deformation of the spring 14. If now a pressure is exerted on the apparatus in the direction of arrow 35 the return line electrode 24 moves to the left until the electrode face 33 comes into contact with the coachwork 2. This is after covering a distance marginally greater than the extent S2. From this moment on, a current can flow from the deforming electrode 18 through the coachwork 2 to the return line electrode 24, which has the effect of correspondingly heating the coachwork in the region of the bulge 34.As now the deforming electrode 18 is under the influence of the large force of the spring 14, the bulge 34 is levelled.

In the embodiment illustrated in Fig. 5, the coachwork 2 has an instability not apparent in the drawing. In this case the return line electrode 24 is extended, by the attachment of the ring 28 as shown in Fig. 2, by an amount X, which corresponds to the length of the bore 30. A deforming electrode 18b has in this case the same free length as the deforming electrode 18 in Fig. 4, but in this regard, its contact face 20 is made convex.If such equipment according to Fig. 5 is moved in the direction of arrow 35, the contact face 33a of the ring 28 already comes into contact with the coachwork after a very much shorter stroke Si. From this moment on, can likewise flow a current from the deforming electrode 18b through the coachwork 2 to the ring 28 and from there to the return line electrode 24, which has the result that it correspondingly heats up the coachwork 2 in the region of the instability. Now, the spring 14 exerts, on account of its smaller compression only a substantially smaller force on the coachwork, the pressure being only just enough to ensure passage of the current. Under the influence of the current heating and the subsequent cooling, the instability is shrunk out. If necessary this process can be assisted by moving the apparatus across the coachwork 2.The contact face 2b is made convex to facilitate this movement.

Fig. 6 shows a reversal of the relationships of Fig. 5. The type of deformation is the same as in Fig. 5. The length of the return line electrode 24 is unchanged, that is to say, it is the same as that in Fig. 4. However, in the holder 16 a deforming electrode 18a according to Fig. 3 is inserted, which has a substantially smaller free length than the deforming electrodes 18 and 1 8b in Figs. 4 and 5. The contact face 20a is likewise made convex. The load-extension relationships are substantially the same as in Fig. 5, that is to say, the contact face 33 of the return line electrode 24 comes after a similarly small travel into contact with the coachwork 2 as in Fig. 5, so that the shrinking process also operates in the same way.

Fig. 7 shows the characteristic of the spring 14. The pretension is effected by a compression of the spring 14 by about 6 millimetres by means of a nut 21 (Fig. 1) up to SO. In this way, there-is created a preloading PO of about 1.3 kilogrammes. The available spring extension is in general limited by the maximum spacing of the contact faces and lies between SO and S2. The maximum spring force thus attainable is indicated by P2 and amounts to some 8 kilogrammes. This is a value which is then reached when, in the arrangement according to Fig. 4, the contact faces 20 and 23 lie in a plane. Since on account of the type of bulging a marginally greater insertion of the deforming electrode 18 into the return line electrode 24 results, the characteristic extends marginally into the shaded region.It can be seen from this that the maximum force can fluctuate. The available force region is shown hatched in Fig. 7 (above).

In one arrangement of the apparatus according to Fig. 5 or 6, there is a small average spring extension S, that gives rise to an average spring force P,. Also in this case the spring force attainable at the beginning of the shrinking process depends on the geometric relationships of the deformation of the sheet. As a result, Fig. 7 also shows a shaded region for the spring force P1. It is apparent, however, in spite of the scatter within the region, that through the positive adjustment of the spacing of the contact faces to take account of the characteristic, it is possible to make a positive adjustment of the deforming forces or contact forces, to achieve the desired levelling effect, according to the direction the levelling should go.

The ring 28 (Fig. 2) that serves as an extension can be made in different lengths of the bore 30, that is to say, of the spacing of the contact face 33a from the shoulder 31. In this event, the apparatus can be supplied with two or more rings 28 and the ring with the shortest spacing 33a/31 will normally be used in order to provide a renewable face that, when oxidised or after a certain period of use, is thrown away and replaced by a new one. It may also be useful, however, in the smoothing of indentations. In this event, the length of the return line electrode 24 about the spacing 33a/31 is reduced. The spacing 33a/31 of the ring to be used for the shrinking process must then be greater to the extent the return line electrode 24 is shorter, in order to restore the old relationship. The positive adjustment of the spacing of the contact faces (20, 20a and 20b) from the contact face 33a follows easily by exchanging the ring.

Claims (8)

1. Apparatus for smoothing deformed sheet by local electric heating of the sheet by passing a current through it, comprising a high current transformer and two electrodes connected thereto, which electrodes have contact faces for the sheet and of which electrodes at least one (a deforming electrode) can be applied to the deformed region of the sheet with mechanical pressure and the other is a return line electrode, characterised in that the return line electrode surrounds the deforming electrode, the deforming electrode is biassed to project beyond the return line electrode by a spring against which it can be displaced to a position behind the contact face of the return line electrode, and the extent to which the contact face of the deforming electrode projects beyond the return line electrode when no load is applied is adjustable.

2. Apparatus according to Claim 1, characterised in that the return line electrode is extendable in the direction of the line of action of the spring.

3. Apparatus according to Claim 2, characterised in that the return line electrode is formed as a hollow cylinder and is extendable by an attachable ring.

4. Apparatus according to Claim 1, characterised in that the deforming electrode is exchangeably secured in a sprung holder and is exchangeable for a shorter or longer deforming electrode.

5. Apparatus according to Claim 1, characterised in that the spring has a progressive characteristic.

6. Apparatus according to Claim 1, characterised in that the deforming electrode and the return line electrode are carried coaxially with one another in an insulating body which at the same time surrounds the spring for the deforming electrode.

7. Apparatus according to one or more of the foregoing Claims, characterised in that the electrode arrangement is disposed on a housing of a pistol-form hand-tool.

8. Apparatus according to Claim 7, characterised in that a high current transformer is also arranged in the housing.