GB2350315A

GB2350315A - Cutter for forming polygonal holes

Info

Publication number: GB2350315A
Application number: GB0024234A
Authority: GB
Inventors: John Francis Collins; Roger Fields
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-10-25
Filing date: 2000-10-03
Publication date: 2000-11-29
Anticipated expiration: 2020-10-03
Also published as: WO2001030548A1; AU1041001A; GB0024234D0; GB9925048D0; GB2350315B

Abstract

A hole cutter 10, for forming a square hole in masonry or the like to receive an electrical switch or socket box, has a rotary cutter 15 operating within four chisel blades 20 each of which has a recess (21, figure 1) enabling the rotary cutter to sweep a circle X of diameter equal to or greater than the width of the hole. The recesses (21, figure 1) are preferably rounded and formed with cutting edges so that each straight side of the hole is cut progressively. Cut material is able to escape from the box formed by the chisel blades 20 through the recesses (21, figure 1) before the chisel blades 20 sink fully into the masonry.

Description

2350315 CUTTER FOR FORMING POLYGONAL HOLES This invention relates to a

cutter for forming polygonal holes and more particularly to an improved cutter for forming blind polygonal holes, such as rectangular or square holes, in masonry, brickwork and the like.

Mortising tools are known for example from US-A-2,996,090 and US-A5,297, 903 which comprise a hollow, square chisel within which a drill bit rotates. Simultaneously with rotating the bit the chisel is subjected to hammer action to produce a square hole. As used in carpentry, for example, such mortising tools are of relatively small cross sectional area.

In order to sink rectangular electrical socket or switch boxes into plasterwork or masonry relatively large rectangular holes "t be made. Existing mortising tools are not suitable for adaptation for this task simply by enlarging the chisel because the bore drilled by the rotary cutter should be kept as close as possible to the cutting edges of the chisel and the resistance to penetration of a cutting edge increases with its length as does its tendency to crumble or damage masonry and fail to make a clean, straight-line cut.

Another problem associated with existing mortising tools is the difficulty of providing egress from within the square chisel of the material being cut. This is generally done by providing slots in the sides of the chisel remote from its cutting edges and the drill bit serves to convey scrap material up the chisel to the slots. A tool suitable for making a large polygonal hole will employ not a drill bit but a rotary cutter with radially extending blades, and this will be less efficient in displacing cut material upwardly within the chisel.

A principal object of the present invention is to provide solutions to the above problems and to provide an improved hole cutter suitable for forming relatively large polygonal holes in masonry and the like.

In accordance. with the present invention there is provided a cutter for forming polygonal holes in masonry and the like, comprising a base, chisel means defining at least the corners of the polygonal hole to be formed extending from one side of the base, a rotary cutter having a spindle which extends through and is rotatable relative to the base and 2 - means for transmitting percussive blows to the chisel means through the base, wherein the circle swept by the rotary cutter is not less in diameter than the distance between parallel sides of the polygonal hole to be formed such that the rotary cutter acts on material to be cut in the interval between adjacent corners of the hole.

By this arrangement the plasterwork or other material to be cut is initially contacted only by relatively short chisel cutting edges only iii corners of the hole being formed and is therefore less likely to fracture or crumble. Between the comer-defining chisel cutting edges the material is cut by the rotary cutter, which is less likely to cause such damage.

In a preferred embodiment of the present invention the said diameter is up to 5% greater than said distance between parallel sides of the hole to be formed.

The chisel means may def ine at least the corners of a square hole to be formed.

The chisel means may comprise a polygon of blades each of which has in its cutting edge remote from the base a recess permitting passage of the rotary cutter. The inner end of each said recess is preferably rounded.

Each said recess is preferably of depth sufficient to allow egress of cut material f rom the polygon def ined by the chisel means between the rotary cutter and the base and each said recess preferably has a cutting edge continuing the cutting edge of the blade in which it is formed. Thus a straight line is cut between adjacent corners of the polygon in a progressive sequence calculated to provide a clean cut. First the material is cut at the corners by the chisel means and between the corners by the rotary cutter, then as the cutter penetrates the material being cut its surface rises to the inner ends of the recesses and lines are progressively and smoothly cut tangential to the circle cut by the rotary cutter.

The blades may be releasably engaged in respective slots in the base and each preferably has chamfered side edges to abut with similar edges of contiguous blades.

3.

The base may be of greater cross-sectional area than the chisel means to limit the depth of the hole being cut.

A preferred embodiment of the present invention will now be described by way of non-limitative example with reference to the accompanying drawings, in which: Figures 1 and 2 illustrate a cutter in accordance with the invention respectively in a partial exploded view and in cap assembled condition, and Figure 3 is an underplan view of the cutter of Figures 1 and 2.

The cutter 10 illustrated comprises a square base 11 formed with a central aperture 12 through which a spindle 13 extends, the end 14 of the spindle being adapted to be engaged in the chuck of a hammer drill (not shown) and having fixed thereto below the base 11 a rotary cutter 15. Optionally the spindle 13 may also engage a drill bit 16 which in use will lead the rotary cutter 15. This will form a central pilot hole which may receive a plug, enabling a socket box to be screwed in position within the larger, square hole.

Radial arms of the rotary cutter 15 carry respective, oppositely facing carbide blades 16. The spindle 13 has a thrust ring 17 which in use will deliver percussive blows to a hardened anvil ring 18 secured to the upper side of the base 11.

Slightly inward of and parallel with its sides the underside of the base 11 is formed with grooves 19 each of which receives a respective chisel blade 20. The blades are held in place in the grooves by cap screws penetrating holes 23 in the sides of the base 11. Each chisel blade 20 has a rounded recess 21 in its cutting edge 22, the cutting edge and its recess being continuously chamfered at approximately 45 degrees to the plane of the blade. In the assembled condition of the cutter 10 the blades 16 of the rotary cutter lie generally in the same plane perpendicular to the axis of spindle 13 as the cutting edges 22 of the chisel blades. The recesses 21 are of sufficient depth to provide gaps in the square formed by the blades 20 between the rotary cutter 15 and the underside of the base 11.

4.

Each chisel blade 20 is also chamfered along its side edges 24 so that, as best seen in Figure 3, the blades 20 are in contact with one another throughout their thicknesses in the corners of the "box" formed by the blades.

Where it extends beyond its grooves 19 the peripheral region 25 of the underside of the base 11 serves as a stop limiting the depth of the square hole cut by the cutter 10.

In use whether or not a pilot hole is first drilled by the drill bit,36 the blades 16 of the rotary cutter and the chisel blades 20 will engage the wall or the like in which a square hole is to be formed more or less simultaneously. The base 11 will reciprocate as the rotary cutter 15 rotates so that as a circular hole is formed approximately of the same diameter as the width of the square hole to be formed simultaneously the tips of the chisel blades 20, spaced apart by the recesses 21, will form the corners of the hole. As the cutter sinks into the hole it is forming the cutting edges of the recesses 21 progressively cut the remainder of each side of the square from the corners inward toward the central cut made by the rotary cutter, until finally the whole of each chisel blade 20 is received into the wall.

In the meantime material cut by the rotary cutter 15 can escape from the "box" formed by the chisel blades 20 through the gaps between the rotary cutter and the base 11 defined by the recesses 21.

Thus although making a large square hole the material being cut is not subjected to the action of chisel blade cutting edges as long as the sides of the hole at least until the hole is practically finished and in this way the damage e.g. to plaster surrounding the hole is minimised and clean, straight cuts are made.

As will be seen from Figure 3, the extremities of the blades 16 of the rotary cutter sweep a circle X to which the sides Y of the square hole being formed are substantially tangential, although it is found desirable that the diameter of this circle X should be slightly greater than the width of the hole so that a greater proportion of each side of the hole is cut by the rotary cutter 15 before the cut is completed by the respective chisel blade 20.

5.

Claims

CLAIMS:

1. A cutter for forming polygonal holes in masonry and the like, comprising a base, chisel means defining at least the corners of the polygonal hole to be formed extending from one side of the base, a rotary cutter having a spindle which extends through and is rotatable relative to the base and means for transmitting percussive blows to the chisel means through the base, wherein the circle swept by the rotary cutter is not less in diameter than the distance between parallel sides of the polygonal hole to be formed such that the rotary cutter acts on material to be cut in the interval between adjacent corners of the hole.

2. A cutter as claimed in claim 1, wherein said diameter is approximately 5% greater than said distance between parallel sides of the hole to be formed.

3. A cutter as claimed in claim 1 or claim 2, wherein the chisel means defines at least the corners of a square hole to be formed.

4. A cutter as claimed in any one of the preceding claims, wherein the chisel means comprises a polygon of blades each of which has in its cutting edge remote from the base a recess permitting passage of the rotary cutter.

5. A cutter as claimed in claim 4, wherein the inner end of each said recess is rounded.

6. A cutter as claimed in claim 4 or claim 5, wherein each said recess is of depth sufficient to allow egress of cut material from the polygon defined by the chisel means between the rotary cutter and the base.

7. A cutter as claimed in any one of claims 4-6, wherein each said recess has a cutting edge continuing the cutting edge of the blade in which it is formed.

8. A cutter as claimed in any one of claims 4-7, wherein the blades are releasably engaged in respective slots in the base and each has chamfered side edges to abut with similar edges of contiguous blades.

6.

9. A cutter as claimed in any one of the preceding claims, wherein the base is of greater cross-sectional area than the chisel means to limit the depth of the hole being cut.

10. A cutter for forming polygonal holes in masonry and the like substantially as hereinbefore described with reference to and as shown in the accompanying Drawings.