Wear resistant tools

Wear resistant rotary machining tools including twist drill bits, slot drill bits, thread taps, end mills and reamers are disclosed. The tools comprise a body of cemented tantalum, titanium tungsten carbide (1) having a layer of abrasive polycrystalline diamond or cubic boron nitride compact (2) bonded to a convex curved surface of the tool body. The twist drill bits of the invention comprise a layer of abrasive compact of substantially uniform axial thickness extending over substantially the whole top surface of the tip of the drill bit. Methods of manufacturing the wear resistant tools are also disclosed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to wear-resistant tools for cutting and drilling and 
in particular to wear-resistant rotary cutting tools for machining metals 
and other materials. 
2. Description of the Related Art 
Almost every product made by the engineering industries requires at some 
stage in its manufacture the machining of metals or other materials. 
Typical machining operations include drilling, milling, reaming, thread 
cutting, slot cutting, and turning. Typically, these operations are 
carried out by automated machine tools which are fitted replaceably and 
often interchangeably with tool bits such as drill bits, end mills, thread 
taps, slot drills and reamers. The preferred shapes of the various cutting 
tools are well known in the art and will not be described further here. 
The material from which the cutting tools are made must combine as far as 
possible the properties of resistance to deformation (hardness), 
resistance to fracture (toughness), and resistance to wear (durability). 
The most widely used materials for this purpose are steel and tungsten 
carbide. However, both steel and tungsten carbide are prone to wear in the 
operating environment and as a result cutting tools made of these 
materials need to be removed for sharpening or replacement quite 
frequently. The expense of sharpening or replacing the steel or tungsten 
carbide tool bits, and the machine tool downtime that is needed for their 
frequent replacement, add considerably to the total cost of the 
manufacturing process and interfere with the smooth operation of 
production lines. 
Abrasive compacts are also well known in the art and are used extensively 
in industry for the abrading of various workpieces. They consist 
essentially of a mass of abrasive particles present in an amount of at 
least 70 percent, preferably 80 to 90 percent, by volume of the compact 
bonded into a hard conglomerate. Compacts are polycrystalline masses and 
can replace single large crystals in many applications. The abrasive 
particles of compacts are invariably ultra-hard abrasives such as diamond 
and cubic boron nitride. Compacts containing diamond abrasive particles 
are known in the art by the initials PCD. Compacts containing cubic boron 
nitride abrasive particles are known as PCBN. 
Abrasive compacts generally contain a second phase or bonding matrix which 
contains a catalyst (also known as a solvent) useful in synthesising the 
particles. In the case of cubic boron nitride, examples of suitable 
catalysts are aluminium or an alloy of aluminium with nickel, cobalt, 
iron, manganese or chromium. In the case of diamond, examples or suitable 
catalysts are metals of Groups VIII of the Periodic Table such as cobalt, 
nickel or iron or an alloy containing such a metal. 
As is known in the art, diamond and cubic boron nitride compacts are 
manufactured under conditions of temperature and pressure at which the 
abrasive particle is crystallographically stable. 
Abrasive compacts may be bonded directly to a tool or shank for use. 
Alternatively, they may be bonded to a backing such as a cemented carbide 
backing prior to being mounted on a tool or shank. Such backed compacts 
are also known in the art as composite abrasive compacts. 
The backing will typically be made of cemented carbide such as cemented 
tungsten carbide, cemented tantalum carbide, cemented titanium carbide or 
a mixture thereof. 
Hitherto, machining tools comprising PCD or PCBN cutting edges have 
generally been made from flat pieces of PCD or PCBN or their composites. 
Tools with more complex shapes have generally been made by brazing flat 
pieces of PCD or PCBN/tungsten carbide composite onto tungsten carbide 
tool bodies followed by machining the body and composite together to form 
the desired tool. The limitations that result from using a planar geometry 
for the PCD layer can readily be appreciated by considering the case of 
the most widely used machining tool: the twist drill bit. 
At present, standard twist drill bits having PCD or PCBN cutting surfaces 
are mainly of two types, as described in U.S. Pat. Nos. 4,679,971 and 
4,527,643. In the first type, a solid disc of PCD or PCBN/tungsten carbide 
composite is brazed to the tip of a coaxial tungsten carbide shank of 
similar diameter, resulting in a cylindrical blank tipped at one end with 
a thin layer of PCD or PCBN. This blank is then machined to the desired 
drill bit shape. Since the tip of a standard drill bit is preferably 
pointed with a tip angle of approximately 118.degree., it follows that 
drill bits made in the above way are characterised by a PCD or PCBN layer 
at the cutting edge whose thickness decreases linearly with increasing 
distance from the rotary axis of the drill bit. Typically, the PCD or PCBN 
layer near to the distal edge of the tip is extremely thin and therefore 
relatively weak. Furthermore, resharpening the tool will remove this thin 
layer entirely. 
Drill bits of diameter greater than 5 mm are generally furnished with PCD 
or PCBN cutting edges by forming a suitable blank of cemented tungsten 
carbide or similar material, cutting one or more slots into the tip of the 
blank, and brazing one or more flat pieces of PCD or PCBN/tungsten carbide 
composite into the slot or slots, followed by machining the blank and 
inserts jointly to form a drill bit with cutting edges composed of PCD or 
PCBN. This method of manufacture is expensive. Furthermore, drill bits 
made in this way are characterised by a non-ideal tip configuration 
imposed by the flat PCD or PCBN insert. The cutting edges are at the wrong 
angle and not properly aligned relative to each other, which results in 
uneven machining and excessive heat generation. The imperfect tip 
configuration interferes with the removal of swarf. 
Drill bits with diameters greater than 5 mm made by the processes described 
above are currently in use. Their superior wear resistance outweighs their 
cost and disadvantageous properties. Typically, holes are drilled 
initially with a PCD or PCBN tipped drill, which drills unevenly for the 
reasons set out above, and the holes are then given a smooth finish with a 
conventional steel or tungsten carbide drill. 
U.S. Pat. No. 4,713,286 discloses twist drills in the diameter range 
0.15-3.2 mm for use in printed circuit board manufacture. These drills are 
machined from drill blanks, which blanks comprise one or more veins of PCD 
or PCBN bonded in situ to a cemented tungsten carbide body, the veins of 
PCD or PCBN positioned to extend longitudinally from the conically shaped 
point region of the drill blank. Once again the machining required to form 
the twist drill is complex, and the twist drills provided in this way have 
non-ideal tip configurations because the cutting edges on opposite sides 
of the drill tip are not aligned. 
The absence of satisfactory PCD or PCBN tipped drills and other rotary 
cutting tools represents a serious and long-felt deficiency in machine 
tool technology in view of the outstanding importance of rotary machining 
operations in most manufacturing processes. 
Accordingly, it is an object of the present invention to provide twist 
drill bits having cutting edges of PCD or PCBN with improved cutting 
properties, improved toughness and improved durability relative to 
existing cutting tools of this type. 
It is a further object of the present invention to provide slot drills, end 
mills, thread taps and reamers having the advantageous properties listed 
above. 
It is a further object of the present invention to provide methods of 
manufacturing the twist drill bits, slot drills, end mills, thread taps 
and reamers according to the present invention. 
SUMMARY OF THE INVENTION 
The present invention provides twist drill bits comprising a layer of PCD 
or PCBN of substantially uniform axial thickness extending over 
substantially the whole top surface of the tip of the drill. The PCD or 
PCBN layer is bonded to a drill body comprising cemented tungsten carbide, 
cemented tantalum carbide, cemented titanium carbide or similar materials. 
The body may comprise two or more pieces joined together by brazing or 
other means. The thickness of the PCD or PCBN layer is typically between 
0.1 and 2.0 mm and preferably between 0.5 and 1.5 mm. 
The present invention further provides a slot drill, end mill, thread tap 
or reamer comprising a layer of PCD or PCBN bonded to a convex curved 
surface of a tool body, the layer being shaped to provide a cutting edge 
composed of PCD or PCBN. 
The tool body may comprise cemented tungsten carbide, cemented tantalum 
carbide, cemented titanium carbide or similar materials. The tool body may 
comprise two or more pieces joined together by brazing or other means. The 
thickness of the PCD or PCBN layer normally does not exceed 2.0 mm. 
The present invention further provides a method of manufacturing a twist 
drill bit comprising the following steps: providing a drill blank body 
having a conical tip; fabricating a drill blank by bonding a layer of PCD 
or PCBN to substantially the whole surface of the conical tip, said layer 
having substantially uniform axial thickness; and machining the drill 
blank to form the twist drill bit. Once again, the drill blank body may 
comprise steel or tungsten carbide or tantalum carbide or similar 
materials. The thickness of the PCD or PCBN layer is preferably 0.5 to 1.5 
mm. 
The present invention further provides a method of manufacturing a slot 
drill, end mill, thread tap or reamer comprising the following steps: 
providing a suitably shaped tool blank body; fabricating a tool blank by 
bonding a layer of PCD or PCBN to a convex curved surface of the body; and 
machining the tool blank to form the slot drill, end mill, thread tap or 
reamer.

DETAILED DESCRIPTION 
Referring to the drawings, the standard twist drill bits of the invention 
comprise a body 1 of cemented tungsten carbide or similar material bonded 
to a layer 2 of PCD or PCBN. An example of a suitable PCD is SYNDITE, a 
product of the De Beers Industrial Diamond Division (SYNDITE is a 
Registered Trade Mark of De Beers Industrial Diamond Division). The layer 
of PCD or PCBN is of substantially uniform axial thickness and extends 
across the top surface of the tip of the drill bit from the centre of the 
tip of the drill bit 3 to approximately the distal edge 4 of the tip of 
the drill bit. This configuration results in a cutting edge at the tip of 
the drill bit 5 (sometimes referred to in the art as a cutting lip) that 
is composed of PCD or PCBN, the thickness of the PCD or PCBN underlying 
the cutting edge being substantially uniform along the cutting edge. The 
thickness of the PCD or PCBN layer may be chosen so as to optimise the 
toughness and durability of the drill bit, and is typically 0.1 to 2 mm 
and preferably 0.5 to 1.5 mm. 
It will readily be appreciated that, in the twist drill bit of the 
invention, the substantially uniform thickness of the PCD or PCBN layer at 
the cutting edges enables repeated resharpening of the tool. Furthermore, 
the thickness of the PCD or PCBN layer can be chosen so as to optimise the 
toughness of the cutting edges. In addition, the limited thickness of PCD 
or PCBN layers that can be made by existing technology does not impose a 
limitation on the diameters of the drill bits of the invention. It will 
also be appreciated that it is straightforward to machine the drill bits 
of the invention to the optimised configuration for a twist drill bit, or 
to other desired configurations. 
Optionally, the body of the drill bit may be in two or more parts joined 
together by brazing or other joining means. For example, the tip of the 
drill bearing the PCD or PCBN layer 6 may be brazed to another piece of 
the body 7 that forms the shank of the drill bit. 
The twist drill bit of FIG. 5 is machined from the cylindrical blanks 
having a conical pointed tip of FIG. 6. The cross-sectional angle of the 
conical pointed tip (the tip angle) may be approximately 118.degree., the 
optimum cross-sectional angle for the tip of the finished drill. The 
cylindrical drill blank comprises a body 8 of cemented tungsten carbide or 
similar material having a layer of PCD or PCBN 9 in approximately a shape 
of a hollow cone of substantially uniform thickness extending from the 
centre of the tip of the drill to approximately the distal edge of the tip 
of the drill. The thickness of the PCD and PCBN layer is typically 0.1 to 
2.0 mm, and preferably 0.5 to 1.5 mm. The twist drill bit of FIG. 5 is 
made from the drill blank by machining the body and PCD or PCBN layer of 
the drill blank as a whole into the desired shape for the twist drill bit. 
This machining may be carried out by grinding with diamond grinding wheels 
or by electric discharge machining or by other methods known in the art 
for the machining of very hard and wear-resistant materials. 
The blanks for the manufacture of the twist drills of the invention are 
made by subjecting a layer of diamond powder or cubic boron nitride powder 
combined with a suitable catalyst-solvent onto the appropriate area of the 
surface of a tungsten carbide body of the tool blank in a suitably shaped 
mould to high temperature and pressure. 
The PCD or PCBN layer is thus formed in situ, bonded to the appropriate 
area of the surface of the body of the tool blank. A method of 
manufacturing PCD layers bonded in situ onto shaped cemented tungsten 
carbide bodies in disclosed, for example, in European Patent Application 
No. 89308730.4 (De Beers Industrial Diamond Division (Proprietary) 
Limited) filed on 30th Aug. 1989. 
Typically, the requirements of the high temperature, high pressure 
apparatus mean that a PCD or PCBN layer is bonded in the bonding step to 
only a part of the body of the blank 10, corresponding to the tip of the 
finished drill. The part of the blank with the PCD or PCBN layer bonded to 
it is then attached to the remainder of the body of the blank 11 by 
brazing or other appropriate means. 
The slot drill of FIG. 7 comprises a body 12 of cemented tungsten carbide 
or similar material bonded to a layer of PCD or PCBN 13. The layer of PCD 
or PCBN extends across the top surface of the tip of slot drill and some 
distance down and around the sides of the slot drill below the tip, 
whereby the cutting edges of the slot drill are substantially composed of 
PCD or PCBN. 
Optionally, the body of the slot drill bit may be in two parts joined 
together by brazing or other joining means. For example, the tip of the 
slot drill bit bearing the PCD or PCBN layer (14) may be brazed to another 
piece of the body (15) that forms the shank of the slot drill bit. 
The slot drill bit of FIG. 7 is manufactured by machining it from the 
cylindrical blank of FIG. 8. The blank comprises a body of cemented 
tungsten carbide or similar material 16 and a layer of PCD or PCBN 17 of 
substantially uniform thickness and approximately in the form of a cup 
surrounding and bonded to the top part of the body. The thickness of the 
PCD or PCBN layer is typically between 0.1 and 2.0 mm, and preferably 
between 0.5 and 1.5 mm. The slot drill bits of the invention are made from 
the blanks by machining the body and the PCD or PCBN layer of the drill 
blank as a whole into the desired shape for the slot drill bit. This 
machining is carried out as described above. 
The blanks for the manufacture of the slot drill bits of the invention 
according to the process of the invention are made as described above by 
use of a suitably shaped tungsten carbide body and mould in a high 
temperature, high pressure synthesis step. The blanks may comprise two or 
more pieces brazed together as described above. 
It will be readily apparent to persons skilled in the art that the other 
tool bits of the invention can be made by variations of the methods of 
manufacture described above.