Electrically Heated Cutting Tool

The invention relates to an electrically heated cutting tool for cutting rubber and plastics, the tool comprising a blade of flat material through which current flows, the blade being substantially U-shaped and having two limbs, the free ends of which are connected with the two poles of a power supply. In a known tool of this art (German Pat. No. 1 943 189) the knife takes the form of a flat blade the limbs of which end in the connecting region, i.e. in the region where the limbs are connected together, in a V-shaped or acute angled cutting edge. A cutting tool of this kind makes it possible to cut rubber and plastics without waste so long as the location of the cut and/or the line along which the cut is to be made are freely accessible. For cutting locations which are difficult to reach, for example when cutting adhesively secured panes for vehicles, the use of the known blade is difficult if not indeed impossible.

The principal object underlying the invention is to provide an electrically heated cutting tool of the initially named kind which also makes it possible to cut material in locations where access is difficult.

This object is solved in accordance with the invention, by bending the blade at right angles to the plane of the material in a region between the free ends of the limbs and the connecting region.

Advantageous modifications and developments of the invention are set forth in the appended subordinate claims.

The cutting tool of the invention makes it possible, as a result of its angled form, to cut even in locations which are difficult to reach and makes it possible, by way of example, to cut adhesively secured panes away from vehicles.

In accordance with an advantageous modification of the invention, the foremost limb, as seen in the cutting direction, is of reduced width in the region of the bend and adjacent thereto so that the strongest heating occurs in this region. This is particularly advantageous for the named application, namely the cutting tool of adhesively secured window panes, because in this application material accumulates in the region of the bend, i.e. in the edge of the recess which accommodates the pane and leads to rapid heat dissipation and thus to cooling of the blade. The reduction of the width of the limb at this location, or at another location at which, depending on the application, increased heat dissipation occurs, allows the distribution of the heat to be very accurately controlled.

In accordance with a further advantageous embodiment of the invention, a cooling device is provided which cools the part of the blade which lies outside of the cutting region so that a further concentration of heat occurs in the cutting region itself because the hotter regions of the knife have a larger resistance.

As a considerable power is required to sufficiently heat the knife, the cutting tool is preferably fed via leads connected with a power supply, as otherwise the arrangement of a transformer in the cutting tool itself would make the weight of the cutting tool too large. This arrangement presupposes that the power supply to the blade must be capable of being switched within the current circuit of the blade itself i.e. on the secondary side of a transformer which forms the power supply. In order to avoid problems with establishing contact with such high currents, the switch is preferably constructed in the form of an actuating lever biased in the direction of switching off with the actuating lever carrying a contact plate which is provided with a contact at one end, with this contact cooperating with a fixed contact. This contact is, for example, connected with the feed lead and the contact plate itself is in this case connected with one of the contact pieces via a flexible wire. In this manner the two current junctions across contact surfaces necessary with other switches are avoided.

The cooling device can preferably be switched on and off in dependence on the actuation of the switch and for this purpose detectors, known per se, are provided for the voltage drop along the secondary winding of the transformer and/or for the curent flowing on the secondary side.

In order to reduce the heat transfer as far as possible, cut-outs which reduce the heat conduction can also be provided in the contact pieces in the region between the housing and the clamping plates which clamp the blade.

The invention will now be described in the following in more detail with reference to the embodiment illustrated in the drawing.

In the drawings:

FIG. 1 shows a side view of an embodiment of the cutting tool,

FIG. 2 shows a partly sectioned plan view of the cutting tool of FIG. 1,

FIGS. 3a-3d show a first embodiment of a cutting blade for the cutting tool of FIGS. 1 and 2,

FIGS. 4a-4d show a second embodiment of blade for the cutting tool of FIGS. 1 and 2, and

FIG. 5 is a schematic view of the electrical circuit for the blade and cooling apparatus of FIGS. 1 and 2.

The embodiment of the electrically heated cutting tool shown in FIGS. 1 and 2 has a housing 1 to which contact pieces 2 are secured. The contact pieces 2 cooperate with clamping plates 3 in order to clamp a blade 6. The clamping blades 3 are pressed with the aid of screws (set bolts) 4, 5 against the blade which is in contact with the contact pieces 2, thus providing a reliable electrical contact.

Electrical feed lines or leads 9, 10 and also a hose 11 which supplies a cooling device 7, which will later be explained in more detail, with cooling air extend through the housing. The lead 9 is directly connected with one of the contact pieces 2 whereas the other lead 10 can be connected with the other contact piece 2 via a switch actuatable by an actuating lever 8. Having regard to the relatively large currents flowing through the blade, the switch consists of a contact plate 16 embedded in the actuating lever 8 with the contact plate 16 being connected with the associated contact piece 2 via a flexible wire 12 and having a contact which can be brought into contact 17 with a fixed contact 13 on actuation of the actuating lever. The actuating lever is biased by a spring 14 in such a way that the electrical circuit from the lead 10 to the associated contact piece 2 is interrupted. The free end of a cooling tube 7 extends above the blade 6 and is provided with air outlet openings 18 which are directed towards the clamping plates and the blade. In operation the cooling air is introduced into the cooling tube 7 via the hose 11 so that parts of the blade 6 which are not in cutting engagement are cooled. In this manner the regions of the blade which are located outside of the actual location of the cut are held at a low temperature so that the heat generation of the remaining regions of the blade due to the increase in resistance is concentrated in these other regions.

As can be recognized from FIGS. 1 and 2, and also in particular from FIGS. 3a-d and 4a-d, the blade 6 has an angled portion 6a which can extend at any desired angle to the remaining region of the blade. The case of a right angled bend illustrated in the drawings represents only a special case which is suitable, by way of example, for the already named application, namely the cutting out of adhesively secured vehicle window panes. As can be recognized from FIGS. 3a and 3b and also FIGS. 4a and 4b, the width of the foremost limb 6b of the blade as seen in the cutting direction, is reduced at area 6f in the region of the bend and also adjacent thereto, so that heat concentration occurs in this region. This is in particular advantageous because during cutting, an accumulation of material occurs in this region and this accumulation results in a comparatively large heat dissipation.

Naturally this reduction of the width can also be extended to the bend region of the other regions of the foremost limb seen in the cutting direction so that a corresponding adaptation of the blade to the cutting conditions and to the heat dissipation which occurs is possible.

As can further be seen from FIGS. 3b and 4b, the reduction of the width results in a cutting point in the bend region 6a which facilitates the penetration of the blade into the material to be cut.

The angled region 6a which connects the two limbs 6b and 6c of the blade can either be acutely angled with equal sides in the manner shown in FIG. 3b or can be asymmetrically constructed depending on the application, in accordance with FIG. 4b.

In order to avoid heat transfer from the blade and/or from the contact pieces 2 to the housing, the contact pieces 2 are provided with cut-outs which are shown in FIG. 2 as circular openings. These openings 15 considerably reduce the heat transfer through the contact pieces, do not however lead to a significant increase in the resistance of the contact pieces, the dimensions of which are principally determined by the mechanical loads which occur during cutting. These bores 15 result, in particular in combination with the cooling air from the cooling tube 7 in significant reduction of the heating up of the housing 1. The feeding of the cooling air into the hose 11 can be controlled in dependence of the actuation of the switch via the electrical feed circuit for the blade, as shown in FIG. 5. The supply 19 of cooling air can, if necessary, be interrupted in the switched off condition or the quantity of cooling air can be reduced. The supply of cooling air can be switched on and off in dependence on the actuation of switch operator 8 which closes the electrical circuit through the blade.