Investment casting process

A method of manufacturing one-piece composite drill bits or coreheads suitable for drilling or coring petroleum wells or in mining. A shell (32) of hard wear-resistant and erosion-resistant material is formed by investment casting in a finished form requiring nominal or no finish shaping. A shank (44) of machinable material is then cast inside the shell, in conditions which cause fusion bonding of the two materials to form a one-piece composite drill bit (42) or corehead by the two-stage casting procedure. The shank (44) is finish machined to form a connection for attachment to a drill string. Pockets (20) for cutter inserts and gauge protectors can be pre-formed to final dimensions in the hard shell (32). The investment mold for the shell (24) allows the hard material to be cast in the required shape with little or no final shaping. The method enables drill bits and coreheads to be manufactured at relatively low cast by eliminating most or all skilled manual finishing operations.

This invention relates to a method of manufacturing petroleum/mining drill 
bits/coreheads with synthetic and natural diamond materials by utilising 
investment casting methods. 
Current methods for producing drill bits/coreheads utilise a matrix or a 
steel body. 
In the matrix type, tungsten carbide powder matrix is formed in a thick 
shell around a steel inner core which carries the threaded connection. The 
cutters are then brazed on to the pre-formed matrix shell. 
This method is suitable since the tungsten carbide matrix is very resistant 
to fluid erosion and abrasive wear, natural diamonds can be included in 
the matrix shell for gauge protection, and relatively complex shapes can 
be produced. 
However, the method suffers from the disadvantages that possible breakdown 
of bond between the matrix shell and steel core may occur, manufacture of 
the graphite mould is precision work requiring high labour input, and high 
cost due to quantity of carbide required. 
Also, the differential of contraction between matrix shell or steel core 
may cause cracking especially in the larger products and further, poor 
quality of the matrix body formed necessitates extensive hand fettling. 
In the steel body type, the normal method of manufacture is by machining 
from the solid using multi-axis milling machines and then hard-facing 
using welding or spray metal techniques prior to the installation of the 
cutters. These cutters are either brazed in place or pressed into prepared 
holes and held in place by interference fit. 
The advantages of the steel body type are a single unit construction with 
no possibility of break-up due to bond failure or cracking, low cost 
materials, and CNC multi-axis milling machine techniques give good 
repeatability for batch production. 
However, the steel body type method is labour intensive, in that hard 
facing has to be applied after machining, and any surplus hard facing has 
to be hand-ground away from cutter pockets prior to installation. Also, 
the allowable complexity of shape is restricted by limitations of 
machining capabilities. 
It has previously not been considered a viable solution to manufacture 
drill bits/coreheads utilising investment casting techniques; the matrix 
and CNC machining approach being far more established and understood than 
this hitherto unknown method of manufacturing. 
The accepted standard method of manufacturing an investment casting for 
industrial products such as aircraft turbine blades and engine components 
is as follows: 
A master mould is manufactured to cast accurate wax males of the product 
required. The wax males are then coated with a ceramic material by dipping 
them in a slurry and then raining sand on the wet slurry. This is done a 
number of times, allowing the slurry and sand coating to dry before 
re-dipping. 
In this way, a thick coating of material is built up around the wax male. 
The coated wax male is then furnaced to bake the coating and melt out the 
wax, thus creating an accurate ceramic mould of the product to be cast. 
Under normal circumstances, this method of manufacture would not be used to 
produce a steel-bodied bit or corehead due to the fact that it would 
require subsequent hard facing after casting in order to withstand the 
fluid erosion and abrasive wear experienced downhole. The application of 
this hard facing by spray metal or welding techniques would cover or 
damage the accurately-formed profile of the investment cast product thus 
spoiling the dimensional accuracy and therefore defeating the purpose of 
using this process in the first place. 
It is an object of the invention to obviate or mitigate the above 
disadvantages by utilising the investment casting process in a novel 
method of manufacture to product a highly accurate and, if required, 
complex casting, which needs little refinishing prior to installation of 
the cutters. 
According to a first aspect of the present invention, there is provided a 
method of casting a drill bit or corehead, said method comprising a 
two-stage process wherein the first stage comprises forming a relatively 
hard outer shell by investment casting, and the second stage comprises 
casting a relatively less hard core within the outer shell in conditions 
which cause fusion bonding of shell and core, the outer shell having 
substantially the final form of the outer part of the intended drill bit 
or corehead, and the core having at least principal features of the final 
form of the shank of the drill bit or corehead. 
According to a second aspect of the present invention, there is provided a 
method of casting a drill bit or corehead, said method comprising the 
steps of forming or providing at least the basis of a shank of the drill 
bit or corehead, the shank or proto-shank being of a relatively less hard 
material, buttering the shank (or proto-shank) with weld material or spray 
metal deposit of lower melting temperature than that of a material 
subsequently to form an outer shell of the drill bit or corehead, forming 
or providing a ceramic mould of the bit head, suspending the pre-buttered 
shank or proto-shank in the ceramic mould, pre-heating the ceramic mould 
and shank (or proto-shank) to a predetermined casting temperature, and 
casting the relatively hard material around the shank (or proto-shank) to 
be fusion bonded thereto and to form a relatively hard outer shell around 
the shank. 
According to a third aspect of the present invention there is provided a 
drill bit or corehead, manufactured by the method according either to the 
first aspect of the present invention, or to the second aspect of the 
present invention. 
The method of the invention combines the advantages of both matrix and 
steel bodied type production, substantially reducing the labour content 
per manufactured unit, thus greatly enhancing the possibilities of mass 
production. 
In order to achieve a product which would fulfil the requirement of the 
industry, it was necessary to devise a method of investment casting a hard 
bit body whilst retaining a tough machinable central core. This was 
achieved by casting the bit body utilising investment casting methods. 
In accordance with the invention, the drill bit/corehead is made by two 
separate casting stages in a two-part manufacturing process, the body 
being cast in separate casts as follows: 
Cast 1: to create a very hard and fluid erosion resistant outer shell which 
has the accuracy of outer form that the investment casting could produce. 
Cast 2: to cast within this shell a central core which was tough yet 
machinable, in such a way that fusion bonding of the two materials is 
achieved and the final casting is a single piece of material incorporating 
a tough central core and having an outer casing of hard material which is 
highly resistant to abrasive wear and erosion wear. 
The purpose of producing the bit in a two-step casting is that the bit 
shank requires different properties to the bit head i.e. the bit head 
requires to be resistant to abrasive wear and to be resistant to fluid 
erosion whereas the shank requires to be easily machinable and to have the 
capability of withstanding high stress/fatigue levels. 
These properties are not realistically achievable from one material. 
The complex form of a drill bit head is difficult and expensive to machine 
and therefore lends itself to the investment casting process. The bit 
shank on the other hand is less critical and can be sand cast or 
investment cast and machined to size at a later stage. 
In addition to creating an investment cast drill bit/core head with the 
hard facing in situ, the internal hydraulic manifolding required to direct 
fluid to the nozzles in the bits used for cooling and cleaning, could be 
cast in situ in the second cast by installing this prefabricated ceramic 
into the shell of the first cast and casting around; thereby creating the 
bit complete with its manifolding in a two-step casting process. 
However, inclusion of the manifold may be omitted if so necessitated by the 
design. 
In carrying out this novel manufacturing process, a preferred preliminary 
stage is to produce an accurate male wax model of the bit head to be cast. 
This can be achieved in a number of ways: 
Method 1--it can be machined from the solid piece of wax attached to a 
mandrel. (This is a particularly useful approach for prototyping or batch 
production.) 
Method 2--wax injection mould dies can be manufactured for the particular 
component and injection mould wax males can be produced. (Suitable for 
mass production). 
Method 3--a combination of methods 1 and 2 can be used i.e. injection mould 
the basic shape and carry out minor machining on the wax. (This allows for 
greater flexibility for cutter and gauge protection slug positioning while 
maintaining the advantages of relatively low cost mass produced waxes). 
Method 4--wax injection mould can be produced for the bit in component form 
and these mass produced component parts assembled at the wax stage to 
produce a variety of bits. (This allows for mass production of a variety 
of products at relatively low cost). 
The preferred second stage of manufacture is to produce a ceramic mould 
from the wax male which has been produced by one of the above methods. 
This may be done by the conventional investment casting method as 
previously described. 
The preferred third stage is to make an investment casting by pouring 
molten alloy into the prepared ceramic mould thus producing an exact copy 
of the original wax male. This casting material should be highly resistant 
to abrasive wear and fluid erosion in its cast stage e.g. the high-cobalt 
alloys such as stellite. The resultant casting preferably incorporates all 
the cutter and gauge slug pockets to a high degree of accuracy. It may 
also include fluid porting and nozzle positions together with an internal 
attachment profile such as thread slots or keyways. 
The preferred fourth stage, after casting and cleaning, is to fit the 
internal ceramic components into position within the hard shell and 
prepare the hard shell for the second casting operation. The shell from 
the first cast is therefore now set in a sand mould bed with the shank 
form created using sand or ceramic moulding techniques. 
The preferred fifth stage is to pre-heat the combined mould before the 
second cast to such a level that it takes into account the temperatures, 
masses and specific heat values of the two materials being combined such 
that a percentage of the inner skin of the outer shell is melted down to 
form a fusion bond between the two materials. This will cause alloying of 
the two materials causing fusion bonding to take place between the hard 
outer shell and the softer but tougher inner core material. Latent heat of 
fusion plays a major role in this process, ensuring that fusion bonding 
can take place without total melt-down of the shell. A suitable pre-heat 
temperature of the shell can be determined by taking into account the 
relative masses and respective temperatures of the shell and the material 
poured to product the inner core. 
After this second casting process, a product will have been manufactured 
which has an accurately-formed, hard wear-resistant outer shell fusion 
bonded onto a tough machinable inner core. 
This will have particular application to the manufacture of drill 
bits/coreheads for the petroleum and mining industries. 
It should also be noted that the manufacturing process described above is 
flexible and is capable of being reversed, by casting the hard material 
around the shank as follows: 
Step 1: form or provide a shank (or at least the basis of a shank, to be 
finished subsequently), by casting or by any other suitable process; 
Step 2: butter the shank with weld material or spray metal deposit, of 
lower melting temperature than the material of the cast shell; 
Step 3: form or provide a ceramic mould of the bit head; 
Step 4: suspend this pre-buttered shank in the ceramic mould of the bit 
head; 
Step 5: pre-heat the ceramic mould and shank assembly to correct casting 
temperature; and 
Step 6: cast the hard material around the shank to form a hard shell. 
Embodiments of the manufacturing process will now be described, by way of 
example, with reference to the accompanying drawings, in which:

Referring first to FIG. 1, an alloy mandrel 10 has an attachment thread 12 
formed on one end. A wax block 14 (shown in ghost outline) is cast around 
the thread 12 to form an assembly ready for machining to shape. 
FIG. 2 shows a wax shell 16 as typically machined from the block 14, and 
unscrewed from the thread 12 to leave an internal attachment thread 18. 
The cutter shell 16 has a four-bladed form, with pockets 20 on the blade 
edges for subsequent mounting of cutter inserts, and side-face pockets 22 
for subsequent insertion of hard inserts to maintain cutter gauge against 
diameter reduction by wear. As an alternative to being machined, the wax 
shell 16 could be formed by injection moulding. 
A ceramic mould 24 (FIG. 3) is formed from the wax shell 16, the mould 
including runners 26 and a riser 28 for the pouring in of liquid metal. 
The ceramic mould 16 is mechanically supported in a bed of sand 30 during 
the first stage of the casting process. 
FIG. 4 shows the second stage of casting, in which the first-stage casting 
32 (with risers removed) is placed against a ceramic shank mould 34. A 
ceramic manifold insert 36 is placed within the casting 32 to form a 
manifold in the second-stage casting. The assembly of first-stage casting 
32 and shank mould 34 is mounted within and supported by sand 38 held in a 
drum 40. 
FIGS. 5 and 6 show the composite casting 42 resulting from the second stage 
of the moulding process. The composite casting 42 includes a bit shank 44 
fusion bonded to the hard first-stage casting 32 along a fusion bond line 
or zone 46. A central conduit 48 runs from a connector 50 on the bit shank 
44 through to a flow manifold chamber 52 and thence to nozzles 54, these 
passages being formed in the second stage of casting (FIG. 4) by the 
inclusion of the ceramic manifold insert 36. PDC cutters 56 are mounted in 
the pre-formed cutter pockets 20 (FIG. 4) in the blade edges, and hard 
slugs or inserts 58 are fitted in the pre-formed pockets 22 outer edges of 
the blades, to act as gauge protectors. 
The process of the invention has the advantage that highly accurate 
investment casting requires a minimum of hand grinding, machining etc, 
prior to cutter installation, thus substantially reducing labour content 
involved in the standard method of producing drill bits/coreheads. 
Fusion bonding ensures integrity of bond between the shank and bit head. 
The casting method allows for greater flexibility in the design of fluid 
porting, and in cutter and gauge insert installation. The inherent 
accuracy of the casting process gives better quality control of cutter 
pockets and braze bond integrity due to the fine clearances achievable, 
giving good capillary action of the braze material and better 
self-distribution. 
Injection moulded wax ensures consistency of cutter positioning and 
therefore of bit performance. 
Thus, there has been described a method of manufacture which utilises the 
investment casting process to give the degree of accuracy required for 
producing drill bits/corehead bodies, and enables the hard facing to be 
applied in a two-part manufacturing process. 
Modifications and variations of the above-described processes and products 
can be adopted without departing from the invention as defined in the 
appended claims.