GB2057949A - Mechanically removing the oxide layer from the external surface of the ends of polyolefine pipes - Google Patents

Abstract

A device for mechanically removing the oxide layer from the external surface of the end of a polyolefine pipe comprises one or more machining tools (2', 2'') which are arranged on arms (3', 3'') or in a torus, which can be rotated by means of a crank (8), and are resiliently pressed by springs (4', 4'') or spring biassed jaws against a zone (1') of the external surface (1'') of the pipe end (1), in order to form a profiled surface which is suitable for socket welding. For this purpose, the cutting edge of the machining tools (2', 2'') is profiled in the manner of a saw. By means of a bearing (5), a rigid connection (6) and a pipe clip (7), the axis of rotation of the arms (3', 3'') is coaxially aligned to the pipe (1) which is held secure against rotation in the pipe clip (7) and is positioned by means of a stop disc (9). <IMAGE>

Description

SPECIFICATION Device for and method of mechanically removing the oxide layer from the external surface of the ends of polyolefine pipes The invention relates to a device for, and a method of mechanically removing the oxide layer from the external surface of the end of a polyolefine pipe, in order to ensure that this end can be welded to a joint socket.

Polyolefine materials, such as polyethylene and polypropylene, tend to absoption of oxygen and, in some cases to form molecular crosslinks on the surface. This tendency manifests itself at elevated temperature and, in particular, under the action of the untraviolet rays present in sunlight. The addition of stabilisers to the base material can delay this change in the material, but it cannot prevent it completely. Nevertheless, measures can be taken which ensure that, even on long term storage in the open, the molecular changes are restricted to a surface layer of a thickness of hardly more than 1/10 mm.

The reason for this is that, with increasing depth of material, the presence of oxygen and the effect of the UV radiation decrease markedly. The zone of molecular change in the material, commonly called the oxide layer, has an impeding effect on welding and it must therefore be removed beforehand if welding is to be carried out. Since chemical means are unsuitable for this purpose, the oxide layer must be eliminated mechanically, that is to say by machining. In the case of polyolefine pipes, .this problem arises very particularly in connection with sockets which can be welded electrically, as described, for example, in Swiss Patent Specifications No. 349,404 and 396,536.It was hitherto customary to work the circumferential surface, falling within the welding zone, by chip-forming methods, either using a scraping blade uniformly around the circumference in the direction of the pipe axis of with the aid of a coarse emery cloth in the circumferential direction. Both methods are highly labour-intensive and are therefore frequently carried out in practice in only unsatisfactory manner. Moreover, especially when using emery, it is difficult to judge the correct extent of abrasion of material. In view of the serious consequences of an inexpert surface treatment, the use of special machining devices suggests itself.Difficulties arise, however, because relatively large diameter tolerances and considerable deviations from the ideal cylindrical form must be expected in polyolefine pipe parts so that demanding precision work is required in order to maintain a depth of abrasion of a few tenths of a millimetre.

According to a first aspect of the present invention, there is provided a device for mechanically removing the oxide layer from the external surface of the end of a polyolefine pipe, in order to ensure that this end can be welded to a joint socket, the device comprises at least one planer-type machining tool having a limited depth of penetration and which is mounted so as to be resiliently urgeable against the external suface of the pipe end in the envisaged welding zone and so as to be movable around the external surface.

According to a second aspect of the present invention, there is provided a method of mechanically removing the oxide layer from the end of a polyolefine pipe, the method comprising engaging the pipe end with at least one plane-type machining tool having a limited depth of penetration and moving the or each machining tool around the end of the pipe while resiliently urging it against the end of the pipe.

The invention will be further described by way of example with reference to the accompanying drawings, in which: Figures 1 and 2 are side and end views respectively of devices for mechanically removing the oxide layer from they circumferen tiai surface of the ends of polyolefine pipes, embodying the present invention; Figure 3 is a longitudinal section through the wall of a machined pipe end, in an enlarged representation; Figure 4 shows pivotal mounting of the machining tool; and Figures 5a and 5b show the co-operation of a fluted cutting surface with a flat flank and the saw-like cutting edge at the line of intersection of these two surfaces.Fig. 5a showing a cut, at right angles to the pipe axis, through the pipe wall and the tool, and Fig. 5b showing the tool according to Fig. 5a in tangential view in the direction opposite to that. of its movement.

In Fig. 1, 1 denotes a pipe end which is to be machined and which can be either a pipe or the spigot end of a pipe fitting (elbow, Tpiece and the like). 1' indicates a machining zone on the external surface 1" of the pipe end 1, which zone must cover the range of the subsequent (electric) socket weld. Machining is carried out with the aid of a device, by means of which two machining tools 2', 2" are moved without axial feed around the pipe end 1 and thus act as planers. Devices with an axial tool feed are, however, also possible, but they require a more involved construction and turning back into the starting position before each new machining operation. The circumferential movement is caused by the rotation of arms 3', 3", whilst the resilient contact pressure of the tools 2', 2" against the surface 1', which is to be machined, is ensured by springs 4', 4".The axis of rotation, common to both arms 3' and 3", is aligned coaxially to the pipe axis via a bearing 5, a rigid connection 6 and a pipe clip 7. The pipe clip 8 clamps around the pipe end 1 which is to be machined, at a point which, as viewed from the end face, is located beyond the machining zone 1'. The clamping action prevents the device from rotating with the tool and at least partially corrects any non-circularities of the pipe end which may be present.

The drive of the arms 3', 3" is accomplished with the aid of a crank 8. A disc 9 serves as an end face stop for axially positioning the machining tool 2', 2" and the pipe clip 7 relative to the pipe end 1.

Fig. 2 shows an embodiment which is characterised in that a torus 10 is rotated around the pipe end 1, which is to be machined, the latter serving as the core of rotation. The drawing represents a section at right angles to the pipe axis and shows the machining device as viewed in the direction of the end of the pipe part 1. The.direction of movement of the torus 10 is indicated by an arrow. The torus 10 is of cup-shaped design and is closed by a disc 10' in the form of a circular ring, the central hole of which is slightly larger than the pipe end 1 which is to be machined. The bottom of the cup-shaped torus 10 serves as an end face stop for the purpose of axially positioning a machining tool 2.Between the cup bottom and the disc 10' in the form of a circular ring, the tool 2, a pair of jaws 1 1', 11" and a spring 12, joining the two jaws 11', 11", are accommodated. The jaws 11', 11" are rotatable about axes 13', 13" which are fixed in the cup bottom of the torus 10 and in the disc 10' in the form of a circular ring. The spring 12 exerts a torque, directed against the pipe 1, on the jaws 11', 11". The jaws thus come to bear against the pipe 1 and force the torus 10 in the zone of the tool 2 against the surface which is to be machined, and this is tantamount to a resilient contact pressure of the tool 2.

Fig. 3 shows an enlarged longitudinal section through the wall of a pipe end 1 which has been machined in the zone 1' with the aid of a machining tool having saw-type profiling. In contrast to machining tools having a smooth cutting edge, which can likewise lead to useful results, special constructional measures to limit the depth of penetration are unnecessary in the present case of a profiled cutting edge 2"', and additionally the profiling favours the formation of uniform chips.

The depth of penetration can be about 2/10 to 3/10 mm. The fact that theoretically a small amount of oxidised material remains on each ridge, is not serious since the remaining part of the exposed surface is at least as large as that surface area which a planer having the same width and a smooth cutting edge would be able to produce. Furthermore, the profiling limits the volume of the gap to a size which is tolerable in welding technology. The risk of a notch effect is insignificant since the lining-up of grooves has the effect of relieving notch stresses.

Fig. 4 shows pivotal mounting of the machining tool 2 in order to enable the latter to adapt itself to generating lines of the pipe end 1 which run obliquely to the pipe axis. An axle 14 here serves as the fulcrum.

Figs. 5a and 5b represent a planer with a fluted cutting face 2x and a flat flank 2xx (Fig.

5a). The interpenetration of the two tool surfaces result in a cutting edge 2'" having a row of cutting points which project in the manner of a plough. The depth of the grooves in the machined zone 1' is determined by the depth of fluting, the rake angle a and the clearance angle ss. Due to the special design of the cutting edge 2t ", significant deepening of the grooves is not to be expected, even if the machining tool 2 is moved several times around the pipe end 1. The fact that the position of its cutting face 2x is inclined forward in the direction of cutting means that the rake angle a is negative. This arrangement has proved to be an effective measure against vibration of the machining tool 2, a rake angle a of a few degrees already giving satisfactory results.

Claims (13)

1. A device for mechanically removing the oxide layer from the external surface of the end of a polyolefine pipe, in order to ensure that this end can be welded to a joint socket, the device comprising at least one planer-type machining tool having limited depth of penetration and which is mounted so as to be resilient urgeable against the external surface of the pipe end in the envisaged welding zone and so as to be movable around the external surface.

2. A device according to claim 1, wherein the or each machining tool is set so as to be at a negative rake angle against the external surface of the pipe end which is to be machined.

3. A device according to claim 1 or 2, wherein the or each planer-type machining tool has a cutting edge which is profiled in the manner of a saw such as to produce a group of preferably mutually touching grooves in the external surface which is to be machined.

4. A device according to claim 3, wherein the machining tool has a cutting face with fluts which, in co-operation with the flank of the machining tool, result in both the sawtype profiling of the cutting edge and in the limitation of the depth of penetration.

5. A device according to any one of claims 1 to 4, wherein the or each machining tool is pivotally mounted so that it can adapt itself to generating lines of the pipe end which run obliquely to the pipe axis.

6. A device according to any one of claims 1 to 5, wherein the or each machining tool is fixed on a rotatable torus such that, in use, the pipe end which is to be machined, serves as a core of rotation and, in use, or each machining tool is resiliently pressed against the external surface preferably by the forces of springs which are supported on the torus and, in the zone of the tool, force the torus against the core of rotation.

7. A device according to claim 6, wherein jaws for providing the resilient contact pressure of the or each machining tool are arranged in the torus, approximately symmetrically with respect to the tool, the jaws being pivotally mounted and being resiliently supportable on the external surface of the pipe end.

8. Device according to claim 1, wherein the or each machining tool is fixed on an arm, rotatable about an axis which is rigidly positioned opposite a pipe clip which is clampable around the pipe end which is to be machined, at a point which, as viewed from the end face, is located beyond the machining zone, preferably at a distance which is determined by an end face stop.

9. A method of mechanically removing the oxide layer from the end of a polyolefine pipe, the method comprising engaging the pipe end with at least one plane-type machining tool having a limited depth of penetration and moving the or each machining tool around the end of the pipe which resiliently urging it against the end of the pipe.

10. A device for mechanically removing the oxide layer from the external surface of the end of a polyolefine pipe, such device being constructed and arranged substantially as hereinbefore described with reference to and as illustrated in Figs. 1 and 2 of the accompanying drawings.

11. A device according to claim 10 incorporating the modifications substantially as described with reference to Fig. 4 or Figs. 5a and 5b of the accompanying drawings.

12. A method of mechanically removing the oxide layer from the external surface of the end of a polyolefine pipe, such method being substantially as hereinbefore described with reference to the accompanying drawings.

13. A pipe machined using the apparatus of any one of claims 1 to 8, 10 and 1 1 or the method of claim 9 or 12.