GB2436723A

GB2436723A - Drill bit with polycrystalline diamond inserts

Info

Publication number: GB2436723A
Application number: GB0706061A
Authority: GB
Inventors: Paul Anthony Graham
Original assignee: Technicut Ltd
Current assignee: Technicut Ltd
Priority date: 2006-04-01
Filing date: 2007-03-29
Publication date: 2007-10-03
Also published as: GB0606620D0; GB0706061D0

Abstract

A drill bit 11 for drilling holes in composite materials such as carbon fibre reinforced composites (CFCs) and glass fibre reinforced composites (GRPs), has a solid carbide body 14 and possesses dagger geometry, with 180{ opposed flats converging towards an apex 8 of the bit to define two cutting edges 19, 20. A lower rotational speed inner zone 15 of each cutting edge comprises exposed carbide, and a higher rotational speed outer zone 16 of each cutting edge is defined by portions of two polycrystalline diamond (PCD) inserts 12 spaced 180{ apart and brazed into a receiving seat 13 of the carbide of the body. Preferably the drill bit has a point angle of 30{.

Description

Title of the Invention

Drill Bit

Field of the Invention

This invention relates to a drill bit particularly, but not exclusively for the machining of composite materials, such as carbon fibre reinforced composites (CFC) or glass fibre reinforced composites (GRP) as used in aircraft components such as panels, details, structures and flying surfaces.

Background to the Invention

Aircraft designers are increasingly making use of CFC and GRP in aircraft to construction. There are numerous advantages to be gained from the introduction of composite materials into aircraft construction: - (I) up to 40% weight reduction over the equivalent metallic component; (ii) maintenance costs are reduced over the equivalent metallic component; (ii) radar cross-section is reduced; and is (iv) increased flexibility of component design.

These performance benefits mean that the use of composite technology is being implemented on all new military and commercial programs. It is also being considered to upgrade existing airframe designs to introduce.composite technology to gain a performance advantage.

Industry trends are such that it is projected that around 35% of the Joint Strike Fighter Aircraft will be produced in CFC's and around 22% of the new Airbus A380 aircraft will be CFC's.

The biggest current disadvantage with the introduction of composites is the high costs associated with processing the components once they are in the cured state. A major activity associated with the processing of cured components is the production of drilled holes. The design demands of a drilled hole in a composite component would be that:- (i) there is no visual evidence of exit face de-laminations around the hole; (ii) there is no visual evidence of entry face de-laminations around the hole; (iii) there is no evidence of the structural integrity of the component being damaged around the hole; (iv) there is no evidence of splintering of the composite around the hole; (v) there is no evidence of de-laminations within the hole; and (vi) surface finish within the hole is clean and un-torn.

Once the above criteria can be satisfied to the demands of the drawing and the design specification, the productivity measure can only be expressed in cost per hole.

The factors that determine the cost per hole are detailed:- (i) drill bit life before quality criteria is not achieved, and (ii) cost of drill bit.

As can be seen from the detailing of factors that effect the cost per hole the primary objective is to create a drill bit geometry that achieves the required component quality, after that, the cost of the drill bit and the number of holes that are produced to the quality requirement will determine and govern the cost per hole.

With conventional drill bit geometry and drill bit materials it is possible to achieve the required quality specifications within composite materials, however because of the abrasive nature of composite materials, there are two main problems that occur:- (i) the achievement of the quality demands requires the quality of the cutting edge to be retained, and (ii) once cutting edges exhibit wear the quality demands will fail. As wear appears on the drill bit, increased cutting forces are required. The inefficiency of the worn drill bit requiring increased force will cause quality failures.

Traditional cutting drill bits for hole production in composites could cost in the region of 50.00 each and only produce 10 holes that met the quality criteria, giving a cost per hole of 5.00 per hole.

Object of the Invention A basic objective of the present invention is the provision of an improved drill bit having geometry that can sustain the quality demands of composite components over an extended number of holes, thus reducing the cost per hole.

Summary of Invention

According to the present invention there is provided a drill bit for the drilling of holes in composite materials such as CFC's and GRP's, wherein the drill bit is of solid carbide and of dagger geometry, with 180 opposed flats converging towards an apex of the bit to define two cutting edges, with a lower rotational speed inner zone of each cutting edge of exposed carbide, and with a higher rotational speed outer zone of each cutting edge defined by portions of two polycrystalline diamond (PCD) inserts spaced 180 apart and brazed into respective receiving seats of the carbide of the body.

Advantages of the Invention The drill bit in accordance with the invention thus ensures holes produced are free from any of the current known quality defects.

Thus, the drill bit in accordance with the invention addresses the shortcomings of the existing conventional drill bits by providing extended cutting life and consequently facilitates reduced cost per hole. Compared to current conventional ho'e production methods the cost per hole can be reduced as low as Ò.40p per hole.

Further advantages of the PCD tipped solid carbide dagger drill bit in accordance with the invention compared to a conventional drill bit are that:-s (I) cutting forces are reduced such that no exit face de-laminations are seen; (ii) cutting forces are reduced such that no entry face de-laminations are seen; (iii) cutting forces are re-directed, such that axial forces are converted to radial forces from the design of the drill bit geometry (see below). The result is the removal of damage to the structural integrity of the composite being drilled, due to reduced axial forces; (iv) PCD has the hardness and wear resistance characteristics to maintain a sharp cutting edge for extended periods of machining, thus ensuring the quality of the holes produced is maintained over the prolonged life of the drill bit; and (v) PCD is only applied over part of the cutting edge to provide maximum impact with limited restriction (see below). Drill bit design is such that peripheral surface speeds increase as the diameter of a drill bit increases. The higher the peripheral speed, higher heat temperatures are generated and at higher temperatures greater wear is seen on any cutting tool. The addition of the PCD insert at the location specified in a dagger geometry solid carbide drill bit in accordance with the invention, provides increased wear resistance where it is required.

Similarly, the same properties that make the PCD resistant to wear also make it susceptible to failure in the mode of chipping caused by impact damage or shock loads.

The centre of a drill bit, where the peripheral speed is small, minimal, or zero at the very point of the drill, is subjected to impact or shock loads during the drilling process. * 5

However, the drill bit construction in accordance with the invention, is such that the centre of the drill bit subjected to irn pacts or shock loads is solid tungsten carbide.

Preferred or Optional Features of the Invention The drill bit has a 30 point angle.

Whilst the ratio of PCD to solid carbide basically needs to suit the drill bit diameter as well as the nature of the material to be drilled, some 20% to 70% of the drill bit diameter is in the PCD zone, and some 80% to 30% of the drill diameter is of exposed carbide.

The diameter of the drill bit is in the range of 2mm to 50mm.

The overall length of the drill bit is in the range of 20mm to 330mm.

Each PCD insert has an arcuate seating face, to seat on a correspondingly arcuate receiving face of a recessed, receiving seat in the carbide.

Each PCD insert has a major cutting edge extending towards the apex of the drill bit, and a minor cutting edge at the outer diameter of the drill bit.

is Brief Description of the Drawings

Figure 1 illustrates the geometry of a conventional fluted drill bit currently employed for the drilling of CFC's and GRP's; Figure 2 illustrates the geometry of a dagger drill bit; Figure 3 illustrates a drill bit in accordance with the invention; Figure 4 is a perspective view of a drill bit in accordance with the invention; Figures 5 and 6 are side elevations of the drill bit of Figure 4; Figure 7 is an enlargement of portion B of Figure 5; Figure 8 is an enlargement of portion A of Figure 6; Figure 9 is an enlarged end view of the drill bit of Figures 4 to 6; and Figure 10 shows a PCD insert.

Detailed Description of the Drawings

A conventional fluted drill bit 1 comprises a body 2 with an axis of rotation 3. The cutting end of the drill has a point angle a' typically of 120 , with a cutting force shown by arrow Fl at an angle B1, being 300 to the axis 3.

A conventional dagger drill bit 4 is shown in Figure 2, and comprises a body 5 having an axis of rotation 6, the cutting end having dagger geometry with two 180 opposed flats 7 converging towards a point or apex 8 to define two cutting edges 9 and 10. The cutting end has a point angle 2 of 30 to the apex 8, so that the cutting force io shown by arrow F2 is at and angle B2, being 75 to the axis 6.

A dagger drill bit 11 as illustrated in Figures 3 to 10, has the same reference numerals as Figure 2, and in accordance with the invention, has two 180 opposed PCD inserts 12 each brazed into a receiving seat 13 of a solid carbide body 14 of diameter 2mm to 50mm, and of length 20mm to 330mm. The location and geometry of is each insert 12 provides a lower rotational speed inner zone 15 and a higher rotational speed outer zone 16.

The inserts 12 are of such dimensions and geometry that some 20% to 70% of the drill bit diameter D is in the PCD zone, and some 80% to 30% of the drill diameter d is of exposed carbide.

In detail, each insert 12 has an arcuate seating face 17 to seat on a correspondingly arcuate receiving face 18 of the seat 13, and each insert 12 also has a major cutting edge 19 extending towards the apex 8 of the drill bit 11 and a minor cutting edge 20 at the outside diameter of the body 5.

Claims

CLAIMS

1. A drill bit for the drilling of holes in composite materials such as CFC's and GRP's, wherein the drill bit is of solid carbide and of dagger geometry, with 180 opposed flats converging towards an apex of the bit to define two cutting edges, with a S lower rotational speed inner zone of each cutting edge of exposed carbide, and with a higher rotational speed outer zone of each cutting edge defined by portions of two polycrystalline diamond (PCD) inserts spaced 1800 apart and brazed into respective receiving seats of the carbide of the body.

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2. A drill bit, wherein the drill bit has a 30 point angle.

3. A drill bit as claimed in Claim I or Claim 2, wherein some 20% to 70% of the drill bit diameter is in the PCD zone, and some 80% to 30% of the drill diameter is of exposed carbide.

4. A drill bit as claimed in any preceding claim, wherein the diameter of the drill bit is in the range of 2mm to 50mm.

5. A drill bit as claimed in any preceding claim, wherein the overall length of the drill bit is in the range of 20mm to 330mm.

6. A drill bit as claimed in any preceding claim, wherein each PCD insert has an arcuate seating face, to seat on a correspondingly arcuate receiving face of a recessed, receiving seat in the carbide.

7. A drill bit as claimed in any preceding claim, wherein each PCD insert has a major cutting edge extending towards the apex of the drill bit, and a minor cutting edge at the outer diameter of the drill bit.

8. A drill bit substantially as herein before described with reference to Figures 2 to of the accompany drawings.