GB2137126A

GB2137126A - Trepanning Cutter

Info

Publication number: GB2137126A
Application number: GB08229887A
Authority: GB
Inventors: Philip Henry Knight
Original assignee: Talbot & Co Ltd F W
Current assignee: Talbot & Co Ltd F W
Priority date: 1982-10-19
Filing date: 1982-10-19
Publication date: 1984-10-03

Abstract

The cutter (19) has a cutting point (35 or 37) located radially inwardly of the outer perimeter of the cutting portion (23). Preferably it is in the form of an aluminium bronze cutter plug (19) having an externally threaded portion (21) and an unthreaded portion (23) wherein may be provided a pair of generally longitudinally extending slots, which serve to retain the material which is removed during the cutting operation. The arcuate edges (29 and 31) so formed are machined at an angle to the horizontal and bevelled at (33) to form the cutting points (35 and 37). Such a cutter plug is especially useful in under-pressure drilling of a plastics (for example, PVC) pipe (5) (Fig. 1 not shown). <IMAGE>

Description

SPECIFICATION Trepanning Cutter This invention relates to trepanning cutters, and more particularly to a trepanning cutter which is especially useful for cutting holes in the walls of plastics pipes.

Plastics pipes are frequently used for conducting gas and liquids under pressure. For example, mains water pipes are now commonly made from PVC. When it is desired to connect a branch pipe to a PVC pipe carrying water under high pressure, it is clearly desirable if the connection can be made in a leak-proof manner without turning off the water supply to the existing pipe. One way in which this may be achieved is described in British Patent Specification No. 837399 which discloses a connection for pipe lines, the connection comprising a pipe saddle which is clamped around the outside of a pipe. The pipe saddle carries a tube extending outwardly with respect to the pipe and a tubular branch opens into the tube a short distance radially outward from the pipe.

The tube is internally screw threaded and carries within it a rotatable plug having external screw threading cooperating with the internal screw threading of the pipe. The rotatable plug is provided with a cutting tool, the arrangement being such that when the connection is mounted on a pipe the plug can be turned so that the cutting tool cuts a hole in the pipe, the plug substantially preventing communication between the tubular branch and the inner end of the tube while the hole is being cut and also on completion of the cutting operation, whereafter the plug can be screwed outwardly so as to permit free passage of the fluid between the inner end of the tube and the tubular branch.

The cutting tool described in Patent Specification No. 837399 is a trepanning cutter having a saw-tooth cutting edge. In order to ensure that the piece of pipe removed by the trepanning cutter does not fall into the bore of the pipe, the internal bore of the trepanning cutter itself is screw-threaded. In use, the internal threads of the trepanning cutter engage with the plastics material of the pipe which is being cut out, and thereby retain it in the bore of the cutter.

More recently, a modified form of trepanning cutter has been used for under-pressure drilling into water mains and the like. This modified form of trepanning cutter is in the form of a metal cylinder (for example, an aluminium bronze cylinder) having an external screw-threaded section, and an unthreaded free end. The free end is provided with a pair of cutting points which are formed as follows: A pair of slots is cut from the free end of the cylinder in the general direction of, but at a slight angle to, its longitudinal axis. This operation leaves a pair of opposed arcuate edges at the free end of the cylinder. Each of the two arcuate edges is then machined at an angle to the axis of the cylinder, so as to form a cutting point at one end of each arcuate edge. The two arcuate edges are in fact angles in opposite senses so as to maintain the rotational symmetry of the cutter.

This design of trepanning cutter ensures smooth and easy cutting as the cutter is screwed down onto a PVC pipe. It will be understood that the rate of penetration of the pipe by the cutter is dependent on the pitch of the external screw thread of the cutter. For cutting plastics pipes, the rate of penetration should not be too high, otherwise cracking of the plastics material may occur. The external screw thread of the cutter should therefore be relatively fine.

The slope and depth of the two slots in the cutter ensure that the strips of material which are removed by the two cutting points at the free end of the cutter are allowed to move quickly away from the cutting area and are held within the cutter.

It has been a natural consequence of the way in which first the slots, and then the cutting points of such a trepanning cutter are formed, that the cutting points lie on the outer edge of the cylinder. This means that a clean hole is formed by the cutting operation. At the moment of final penetration of the cutter through the wall of the pipe, some tearing of the PVC material may occur, but the rags which are caused by this tearing operation are left attached to the disc of the pipe wall which is cut out. This method of cutting has long been thought to be entirely satisfactory for cutting holes in PVC pipes.

According to the present invention, there is provided a trepanning cutter having a cutting point which is radially inward of that part of the trepanning cutter which defines the edge of the hole which is cut by the trepanning cutter. It will be understood that the term "cutting point" as used herein means the point on the trepanning cutter which first comes into contact with the work piece when the cutter is brought up to the work piece along the longitudinal axis of the cutter.

We have found that the trepanning cutter of the present invention is of particular advantage in cutting PVC pipes. Rather than the "rags" being left on the disc which is cut out by the cutter, such "rags" remain around the perimeter of the hole which is formed. Although this might have been regarded hitherto as a disadvantage, we have found that it actually reduces the likelihood of nicks or notches being formed on the perimeter of the hole at the moment of final penetration of the cutter through the wall of the pipe. Instead, such nicks or notches are formed, if at all, around the perimeter of the disc which is cut out by the cutter.

The presence of the nicks or notches around the perimeter of a hole in a PVC pipe is of very great importance, in that it is believed that such nicks or notches form a nucleus for progressive weakening of the material of the pipe, resulting eventually in pipe failure.

Preferably, the trepanning cutter is in the form of a cutter plug for use in under-pressure drilling of PVC pipes. The cutter may be formed from a conventional cutter plug by a further step of bevelling the cutting edges of such a plug. It is important that such bevelling is not so acute as to reduce the strength of the cutting point.

The trepanning cutter of the present invention is now described, by way of example, with reference to the drawings, in which:~ Fig. 1 is a perspective view, partially cut away for clarity, of a trepanning cutter being used for under pressure drilling of a PVC water main; Fig. 2 is a side elevation of the trepanning cutter of Fig. 1, to a larger scale, and Fig. 3 is a section of line AA of Fig. 2.

Referring to Fig. 1, a pair of straps 1, 3 are clamped around a PVC pipe 5 by means of a pair of bolts 7 and 9. One of the straps 1 carries a connecting piece 11 which includes tube or ferrule 13 extending radially outwardly from the pipe 5. A tubular branch 1 5 opens into the tube 1 3 a short distance radially outwardly from the pipe 5. The tube 13 is internally screw-threaded and provided with a cutter plug 17 which is externally screw-threaded. The arrangement is such that the cutter plug can be turned so as to cut a hole in the pipe 5. The cutter plug can then be screwed outwardly so as to permit free passage of fluid between the inner end of the tube 13 and of the tubular branch 15.

The cutter plug 17 is shown in greater detail in Figs. 2 and 3. As can be seen from Figs. 2 and 3 the cutter plug 17 is in the form of a cylinder 19 having a threaded portion 21 and an unthreaded free end 23. A pair of slots 25,27 is cut from the free end 23 in the general direction of the longitudinal axis, but at a slight angle thereto. This angle is preferably from 50 to 100, (for example, 7 2 ) to the longitudinal axis of the cutter plug.

The cutting of the slots 25, 27 leaves a pair of arcuate edges 29, 31, which are then machined at an angle relative to the longitudinal axis of the cylinder. A suitable angle, is for example, 50 from the plane perpendicular to the longitudinal axis of the cylinder. The edge which is formed by the angling operation is then bevelled as at 33, to form a pair of cutting points 35, 37. It will be seen that the cutting points 35, 37 are disposed inwardly of the outer perimeter of the cylinder, and hence inwardly of the perimeter of the hole cut by the cutter plug 17.

it will be appreciated that many other designs of trepanning cutter in accordance with the present invention are possible. For example, the trepanning cutter could have a saw-tooth cutting edge, in which each saw-tooth provides a cutting point disposed inwardly of the outer perimeter of the cutting tool.

Claims

1. A trepanning cutter having a cutting point which is radially inward of that part of the trepanning cutter which defines the edge of the hole to be cut by the trepanning cutter.

2. A trepanning cutter according to claim 1 in the form of a cutter plug having an externally threaded portion and an unthreaded cutting portion.

3. A trepanning cutter according to claim 2 having formed therein a pair of slots which extend generally longitudinally away from the cutting portion towards the threaded portion, the cutting portion thereby comprising a pair of free arcuate edges.

4. A trepanning cutter according to claim 3 wherein said slots extend at an angle of from 50 to 100 to the longitudinal axis of the trepanning cutter.

5. A trepanning cutter according to claim 4 wherein said angle is substantially 7 2 730.

6. A trepanning cutter according to any of claims 3-5 wherein the free arcuate cutting edges of the cutting portion are angled relative to said longitudinal axis and bevelled to form a pair of cutting points on the inner periphery of the edges.

6. A trepanning cutter according to any of claims 3 to 5 wherein the free arcuate edges of the cutting portion are angled relative to said longitudinal axis and bevelled to form a pair of cutting points.

7. A trepanning cutter according to any preceding claim which is formed from aluminium bronze.

8. A trepanning cutter substantially as hereinbefore described with reference to the accompanying drawings.

New Claims or Amendments to Claims Filed on 10th February 1984 Superseded Claims 1,3 and 6 New or Amended Claims:~

1. A trepanning cutter having at least one cutting point which is radially inward of that part of the trepanning cutter which defines the edge of the hole to be cut by the trapanning cutter, said at least one cutting point being the point on the cutter which first comes into contact with a workpiece when the cutter is axially advanced to form a hole in the workpiece.

3. A trepanning cutter according to claim 2 having formed therein a pair of slots which extend generally axially of the cutting portion towards the threaded portion, the cutting portion having a pair of free arcuate cutting edges.