BACKGROUND ART Thus, for example, drill bits commonly called twist drill bits have a helical spiral form with cutting edges at the cutting end of the bit, with flutes between helical edges providing for the passage of drill scarf up the shaft of the drill towards the chuck end during cutting, so as to clear the hole.

The cutting edges are angled at an angle to the longitudinal axis of the bit which is less than ninety degrees, so that the rotating edges cut a conical base to the hole being drille. This has the effect of centring the bit to restrict a tendency to wander.

The bit may be re-sharpened many times, until the bit becomes too short for convenient use. Such drills are used for drilling in metals and other materials and are widely used.

Wood drill bits, suitable also for other materials of similar hardness and strength, also have a helical shape with flutes for clearing the scarf, though often the cutting edges are orthogonal to the longitudinal axis of the bit and a lead-in screw thread is provided at the end of the bit on the bit axis to give the centring action and also draw the bit into the work.

Other drill bits for wood and similar materials comprise merely a flat butterfly wings at the end of a shaft and having no helical form, presenting cutting edges at the forward edges of the flat wings and having a tip at the cutting end of the bit on the longitudinal axis to give a centring action.

However, the shaft is usually smaller in diameter than the diameter of the hole cut by the edges on the wings, so as to allow space for the scarf to be cleared from the hole.

Masonry drill bits commonly used are like twist drills, but are characterised by inserts of hardened material at the cutting edges, such as tungsten carbide, for example.

The scope of the invention is broadly applicable to all drill bit types, including these examples.

Problems related to all drill bits, include that a set of different size drill bits must be kept in order to cover a range of sizes of holes to be drille.

Another problem is that it is often convenient for a lead hole to be initially drilled and then enlarged, but this convenience is detracted from since the drilling must be interrupted for drill bits to be changed in the drill chuck.

DISCLOSURE OF THE INVENTION A drill bit in accordance with this invention comprises a chuck end of the bit, a shaft, cutting edges at a cutting end of the bit, and a means at the cutting end of centring, characterised in that additional cutting edges are provided at least at one further position on the bit between the cutting end and the chuck end, and that the additional cutting edges are dimensioned to cut a larger diameter than the cutting edges at the cutting end.

In accordance with preferred embodiments, there is provided additional cutting edges at two positions between the cutting end and the chuck end of the bit, the edges giving successively larger holes, moving from the cutting end towards the chuck end.

In a preferred embodiment of the invention, the drill bit is made integrally, from a single piece of metal. However, in an alliterative embodiment the drill bit is made as a composite product, where a portion of smaller diameter is set into a bore provided in a portion of larger diameter. The portions can, for example, be braised together or bonded in another analogous way. Alternatively, a portion of smaller diameter can be fitted into a bore with the outer surfaces of the smaller portion and the inner surfaces of the larger portion having matching splines, or analogous formations, even a keyed connection, for example. Manufacturing considerations may be a prime basis for a choice between these options.

In accordance with an alternative embodiment of the invention, the portions which fit one into another may be made so that the depth to which a smaller diameter portion penetrates into a larger diameter portion is adjustable. For example, a smaller portion may be slideable on splines into the larger diameter portion, with a grub screw or other means of locking the portion at a selected depth.

As an alternative to splines, the smaller portion may be slid into a bore of the portion of larger diameter, which bore has helical formations of the same dimensions and pitch as those outside the portion of smaller diameter and fixed by brazing or by grub screws or other means of setting an ajustable position.

Where the drill bit has helical flutes a choice must be made between the flutes on the portions of successive size coinciding at the transition from a smaller size to a larger size, or the flutes being out of synchronisation, e. g. phase shifted. This choice, while impacting clearly on the performance of the bit in clearing scarf, may also be found to influence cutting efficiency or freedom from problems such as tool chatter and other stability related issues.

Drills according to the invention have at least two advantages. They allow a collection of fewer drill bits to be kept while still catering for a good incremental range of holes that can be drille. They allow holes to be drille, e. g. in difficult material with a lead hole of smaller diameter, followed by one or more enlargements of the hole diameter, without changing the bit.

The present invention further provides a set of drill bits of the kind described, each drill bit providing at least two sizes, preferably three sizes.

Reference to sizes is to the sizes of holes that can be cut with the bit, according to the cutting edges at the cutting end and the additional cutting edges provided.

A method of drilling in accordance with the invention comprises using a drill bit according to the invention and feeding the bit into the work to a selected depth, according to the size of hole required to be drille, within the range provided by the single drill bit. An NC machine can be programmed to drill to the required distance of the bit, to give the required size of hole.

Preferably the length of drill bit between the cutting edges at the cutting end and the additional bits is selected according to a depth of hole required to be cut by the bit at a selected size.

The invention can be carried out fitting the drill bits to hand drills or electric or other powered drilling machines, according to the requirements of different applications.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fully described by way of example, with reference to the accompanying drawing, in which a preferred embodiment is shown.

As shown in the drawing, the drill bit comprises a helical fluted bit of three diameters, as indicated at 1,2 and 3. The chuck end 4 of the bit is in this example of reduced size to permit fitting in a smaller drill chuck. The shaft 5 has the helical flutes 6 in it, in this type of bit. The cutting end 7 of the bit has cutting edges there and in accordance with the invention additional cutting edges 8 and 9 are provided at two further positions, dimensioned to cut larger diameter holes.

As an example, the drill bit can be made by the portion of diameter 1 set into a bore in the portion of diameter 2 which in turn is set into the portion of diameter 3, the portions brazed together. Heat treatment follows after the brazing step.

The flutes 6 in this example run continuously with the flutes in each portion synchronised so as to run into the next at the transitions from one size to the next.

The lengths 10,11 and 12 of each portion of the drill bit can be selected according to the depth to which the hole is required to be drilled in each size. An NC machine which uses such bits can have these length dimensions and diameters programmed into it, to allow correct indexing of the bits to required applications.