US3453719A

US3453719A - Manufacturing diamond bits

Info

Publication number: US3453719A
Application number: US620839A
Authority: US
Inventors: Robijn Feenstra
Original assignee: Shell Oil Co
Current assignee: Shell USA Inc
Priority date: 1967-03-06
Filing date: 1967-03-06
Publication date: 1969-07-08
Anticipated expiration: 1986-07-08

Description

y 8, 1969 R. FEENSTRA 3,453,719

MANUFACTURING DIAMOND BITS Filed March 6. 1967 Sheet 4 of 2 FIG. I

8 FIG. 2

I II A FIG. 3

IO M I0 11 IO' u I INVENTOR:

- FIG. 4 H15 N y 3, 1969 R. FEENSTRA 3,453,719

MANUFACTURING DIAMOND BITS Filed March 6, 1967 Sheet 2. of 2 INVENTOR:

ROBIJN FEENSTRA 8Y .2 73M HIS AGENT United States Patent York Filed Mar. 6, 1967, Ser. No. 620,839 Int. Cl. B21k 5/02, 23/00 U.S. Cl. 29-473.1 11 Claims ABSTRACT OF THE DISCLOSURE A mixture of graphite particles and liquid is placed in a cup-shaped flexible membrane; a pattern of a bit is inserted into the mixture from the open side; the liquid is pressed out of the mixture by applying fluid pressure on the outer side of the membrane; the mold is removed from the cup and diamonds placed in the mold along circles having their centers on the central bit axis; the mold is filled with powder material to be sintered, binder material is then placed on top of the material; and the mold and its contents are subjected to sintering temperature.

The invention relates to a sintered diamond drilling bit and a method and means for manufacturing such a bit.

Diamond bits are in particular applied when drilling holes through rock material of poor drillability, which is often required when drilling in subsurface formations for searching and/or recovering oil or other valuable products.

These bits are -of the rotary type and the diamonds thereof exert a scraping action On the rock through which a hole is being drilled, while the bit is loaded in an axial direction and rotated around its central axis.

Bits of this type are especially useful for drilling through deep, abrasive, hard formations, since their lifetime when drilling through such formations is greater than with any other type of rotary rock bit such as a roller bit. Consequently, a diamond bit has to be replaced less frequently than a bit of another type. The higher material and manufacturing costs of a diamond bit with respect to other types of bits are easily compensated by the advantages obtained as a result of the reduction in time required to replace the worn bits during drilling operations.

The invention is concerned with the provision of a sintered diamond drilling bit which will be less liable to damage of the diamonds due to overload than known bits.

The invention is also concerned with a method of manufacturing a sintered diamond drilling bit, in which several steps are taken to insure that the arrangement of the diamonds in the sintered bit body is as accurate as possible, in order to divide the load on the bit as evenly as possible over all the diamonds.

A further object of the invention is the provision of a sintered diamond drilling bit, in which an extremely small number of diamonds is required for carrying out the required cutting action.

The method of manufacturing a sintered diamond bit according to the invention comprises the following steps:

(a) Forming a graphite mold by placing a mixture of graphite particles and liquid into a cup-shaped flexible membrane, introducing a pattern of the bit in an opening provided in a plate arrangement at the open side of the cup-shaped membrane, such that the rim of the opening fits closely around the circumference of the pattern, densifying the mixture by pressing part of the liquid out of the mixture by providing a fluid pressure on the outer side of the membrane, reducing the fluid pressure to atmospheric pressure, retracting the pattern out of the opening, and removing the graphite mold out of the membrane;

(b) Vaporizing the remaining part of the liquid from the graphite mold;

c) Placing diamonds in the mold along circles which have their centers on the central bit axis, filling the mold with a mass of powder material to be sintered, and placing a quantity of binder material on top of the mass;

(d) Subjecting the mold with the contents thereof to sintering temperature; and

(e) Removing the sintered diamond bit from the mold after the sintering process is over.

With this process it is possible to insure that the distances between the central bit axis of the diamond bit and the cutting tops of those diamonds located on a common circle vary at most 0.1 millimeter, when measured in directions rectangular to the bottom profile to be cut by the bit.

A means for carrying out part of the method according to the invention, comprises a housing open to one side, a plate member provided with an opening therein arranged on the open side of the housing, a flexible membrane sealingly connected at the rim thereof to the open end of the housing, and means for connecting the space between the housing and the membrane to a pressure source.

The invention may be carried into practice in various ways but two specific embodiments will now be described by Way of example with reference to the accompanying drawings in which:

FIGURE 1 shows a view of the cutting end of a sintered diamond bit according to the invention taken in the direction of the axis in FIGURE 2;

FIGURE 2 shows a cross-section of this bit on the line 22 of FIGURE 1;

FIGURE 3 shows cross-sections of ridges of the bit on a larger scale;

FIGURE 4 shows cross-sections similar to those shown in FIGURE 3, but with ridges of different form; and

FIGURE 5 shows a molding apparatus for forming a graphite mold of the drilling bit, which mold is to be used in the sintering process.

As shown in FIGURE 2 the drilling bit 1 comprises a shank 2 provided with coupling means, such as screwthread 3, for coupling the bit via a sub (not shown) to a drill string. The shank 2 is connected to a sintered mass 4, which is provided with ridges 5, arranged as shown in FIGURE 1, radially with respect to the central axis of the bit. A conduit 6 is arranged in the sintered body 4, which conduit communicates with the cavity 7 of the shank 2 for passing drilling fluid out of the drill string to the lower side of the bit 1 during drilling operations carried out thereby.

Diamonds 8 are placed in the ridges 5 in such a way that the diamonds are located on circles which have their centers on the central bit axis. Preferably each diamond 8 forms together with diamonds 8 arranged in other ridges 5, a set of diamonds which is arranged on a common circle. By way of example, FIGURE 3 shows cross-sections of three ridges 5 at the locations of diamonds belonging to a common circle.

As will be seen from the following description of the method of manufacturing the present drill bit, the diamonds are arranged during the manufacturing of the bit on the bottom of channels provided in a graphite mold, which mold is subsequently filled with a mass of powder material suitable to be sintered at a suitable temperature to a single body by the addition of a binder material. Thus, the diamonds 8 will, as shown in FIG- URE 3, contact the lower boundaries of the ridges 5.

During the drilling operation of a bit having diamonds 8 arranged in ridges 5 as shown in FIGURES 1-3, each set of diamonds located on a common circle will cut the formation aong a common track. An equal distribution of the load over the diamonds equally spaced along a common circle is only possible if the distances D between the central bit axis AA and the cutting tops of the diamonds belonging to a common circle are, when measured in directions perpendicular to the bottom profile to be cut by the bit, are equal to each other. It will be clear that this is impossible due to the nature of the processes, which have to be followed in manufacturing the bit. By applying the method according to the invention, however, these distances D will vary within extremely small tolerances. In fact the maximum variation in these distances will be at most 0.1 millimeter. Thus, in the bits according to the invention the diamonds equally spaced along a common circle and cutting in a single track will be substantially equally loaded. Consequently, no overload of diamonds will occur, which overload would normally result in a consecutive breaking of the diamonds, unless the total load on the bit was reduced, which would, however, cause a reduction in penetration rate of the bit.

In using the bit according to the present invention, however, the load is evenly distributed over all the diamonds arranged in the bit, which allows the maximum load to be applied to each diamond. Thus, a considerable penetration rate can be obtained with a minimum number of diamonds.

The number of diamonds belonging to a common circle need not be equal to the number of ridges. In a bit having nine ridges, the diamonds may e.g. be placed on common circles containing 1, 3, or 9 diamonds, which are evenly distributed with respect to the bit axis.

In the bit manufactured according to the present method, the distances between corresponding points of the ridges of the bit axis AA vary at most 0.05 millimeter when measured in directions perpendicular to the bottom profile cut by the bit.

The penetration rate can even further be improved by arranging the diamonds 10 in the manner indicated in FIGURE 4. Ridges 11 are used, which instead of having flat bottoms have bottoms consisting of a portion 12, and a portion 13 arranged at an angle to the portion 12. It will be clear, that if the channels applied in the graphite mold in which the bit is to be sintered, have

bottom portions

12 and 13 identical to those shown in FIGURE 4, the diamonds 10 which are placed in these channels will contact, provided that they are neither too great nor to small, both

portions

12 and 13 in each ridge 11. Consequently, the contact points 14 lying on bottom portions 12, will be less blunt than the contact points 9 in the construction shown in FIGURE 3. The width of the portions 12 has to be chosen in relation to the dimensions of the diamonds 8 which are to be used. To this end, all the diamonds have to be within a certain size range if they are to be applied to a bit having bottom portions 12 of a certain width. If the dimensions of a diamond are too great, there will be no contact point with the bottom portion 12 as indicated by diamond 10' in ridge 11'. If desired, the side walls of the ridges may be sloped in the way as indicated in FIGURE 3 to give the ridge a wider base and more strength.

It will be clear that the invention is not limited to the bit as shown in FIGURES 1-4 of the drawing and that several modifications may be made. Thus, the ridges 5 (or 11) need not be arranged radially with respect to the central bit axis as shown in FIGURE 1, but they may also be arranged such that in the view taken along the axis toward the bottom, they touch a common circle which has its center on the central bit axis. Further these radially arranged ridges may be curved. In another embodiment the ridges may be arranged more or less concentrically with respect to the bit axis.

Although the flushing conduit 6 has been shown in FIGURE 1 to be arranged concentrically with respect to the central axis of the bit 1, this conduit may as well be placed eccentrically, which may be desirable for example when no core should be left in the rock which is being drilled.

The shape of the bit body parts lying between the ridges 5, may be in any convenient way provided that it is suitable for guiding the flow of flushing liquid out of the opening 6 towards the sides of the bit body and along the diamonds 8, as well as for enabling an easy removal of the chips cut by the action of the diamonds 8. Further, the invention is not limited to the shape of the shank 2, which shank is used for connecting the bit to a drill string via a sub. If desired, the shank 2 may have a cylindrical form. In another manner, several small cylinders are arranged in the body 4, which cylinders are provided with screw threaded openings for connecting a flange thereto by means of bolts. The flange may be provided with means for coupling the same to a drill string, drill collar or sub, or may form part of these latter means. A very suitable manner of mounting the shank 2 in the body 1 has been shown in copending U.S. application Ser. No. 594,839, filed Nov. 16, 1966.

The method of manufacturing a sintered diamond bit will now be described. To obtain the required accuracy in the setting of the diamonds, a special molding technique has been developed to form the graphite mold in which the bit is to be sintered. An apparatus in which such a technique may be carried out is shown in FIGURE 5 of the drawing. The molding apparatus comprises a housing 21 having a cup-shaped interior which is open at its upper side. A plate 22 provided with an opening 23 is arranged on the upper side of the housing and centered and supported by the housing 21. A ring member 24 is connected to the hosuing 21 by suitable means, such as bolts 25 for clamping a flexible membrane 26 (e.g. manufacturer of rubber or other similar material) to the housing 21.

On the housing 21 and the plate 22 there is mounted a bridge member 27 by means of bolts 28 (of which only one is shown) which bolts are arranged in a flange 29 of the bridge member 27 and the vertical wall of the housing 21.

A rod 30 is provided with a flange 31 on which a pattern 32 of the bit to be manufactured is connected by bolts 33. The rod 30 is slidably arranged in the bridge member 27, and rotation thereof is prevented by a slide wedge 34 connected to the flange 31 by a bolt 35 and cooperating with a guide member 36 connected to the bridge member 27 by bolts 37.

The rod 30 can be moved vertically with respect to the bridge member 27 as well as to the housing 21 on which the bridge member 27 has been mounted, by rotation of a nut member 38 cooperating by screw threads with the bridge member 27. The nut member 38 is locked in an axial sense with respect to the rod 30 and can be freely rotated therearound.

On the inner wall of the housing 21 a network of shallow channels 39 is arranged with at least one point thereof communicating via a bore 40 with a pressure source (not shown). Bore 40 passes through the wall of the housing 21, and suitable conduits 41 communicate with the channels 39, and with the bore 40 leading to the pressure source.

If desired, the membrane 26 may be flat in the nonstressed condition. By lowering the pressure in the bore 40 to below atmospheric pressure, the membrane will be pressed against the inner wall of the housing 21 in the position as shown in FIGURE 5. The pressure source (not shown) communicating with the conduit 40 is then of a type which can produce pressures higher as well as lower than atmospheric pressure.

To obtain in the graphite mold an exact negative form of the bit to be manufactured, a pattern of the bit is made, which pattern has exactly the same exterior shape (ridges 5, channel 6) as the bit shown in FIGURE 1. For sake of simplicity, however, only the channel 6' has been shown in the pattern 32 in FIGURE 5. The pattern 32 is preferably made of metal, and has been machined very carefully. The variation in the distances between the central axis of the pattern and the places on the ridges where (in the bit) the diamonds which are to cut a common track, are to be located is maximum 0.02 millimeter when measured in a direction perpendicular to the bottom profile of the pattern.

In the position as indicated in FIGURE 5, the pattern 32 fits accurately with its upper end in the opening 23 in the plate 22 Without, however, sealing this opening. Suitable channels and conduits (such as 42, 43, and 44) are provided in the plate 22, pattern 32 and between pattern 32 and flange 31 respectively for draining the liquid which is pressed out of the liquid/ graphite mixture which will finally form the graphite mold 45.

In the particular pattern 32 indicated in the drawing, the lower inner part has been conically shaped. It will, however, be clear that the method according to the invention is not limited to the use of a type of pattern as shown in the drawing, but may be applied for any other pattern as well. If desired, other channels than channel 6' may be provided in the pattern 32. These channels will, in the sintered bit body, act as flushing channels for guiding the flow of drilling fluid to the required parts of the bit and/or the hole. Since the shape and arrangement of such flow channels is known per se and do not form part of the invention, these channels are not described in detail. It is noted, however, that the channels arranged in the pattern 32 have to be shaped such that the pattern can be easily removed out of the graphite mold after the molding operation is over, which removal should not damage the channels formed in the mold by the ridges on the pattern 32.

The method of forming a graphite mold by means of the apparatus as shown in the drawing, will now be described in detail.

With the bridge member 27 and the pattern 32 removed from the housing 21, the space inside the cupshaped flexible membrane 26 is partially filled with a mixture substantially consisting of graphite particles and liquid, such as water. Thereafter the bridge member 27 having the pattern 32 either in the extended position, or in some other position, is placed on the housing 21 and connected thereto by the bolts 28. If the pattern 32 is in the non-extended position with respect to the bridge member 27 when the latter is mounted on the housing 21, the pattern is moved into the position as indicated by FIGURE 5 by rotation of the nut 38.

Subsequently, a fluid, preferably oil, is supplied under pressure from a suitable (not shown) pressure source via the

conduits

40 and 41 to the channels 39 arranged in the inner wall of the housing 21. By the network of channels 39, the fluid is initially distributed over the surface of the membrane 26, thereby exerting compression forces on the graphite/liquid mixture arranged within the membrane 26. By this action, part of the liquid is displaced from the mixture and drained via

conduits

42, 43, 44 and the narrow gap present between the wall of the opening23 and the pattern 32, which gap allows the passage of water and air, but prevents the passage of graphite particles. The mixture is thereby densified.

The pressure exerted on the outer side of the membrane 26 should be as constant as possible. If a pump is used for raising the pressure of the fluid, care has to be taken that the variations in pressure are relatively small, say only 5% of the average pressure. It is preferred, however, to use a pressurized gas stored in a gas bottle, as a pressure source. If desired, this gas may be supplied directly to the space between the membrane 26 and the housing 21, or it may be applied to a vessel containing a liquid, which vessel communicates at its lower end via a conduit to the said space.

If the variations in pressure are too great, the graphite particles will not be evenly compressed, which results in deviations in the mold with respect to the pattern 32,

which deviations unfavorably affect the life time of the bit sintered in said mold.

It has been found that the most homogeneous structure of the graphite mold will be obtained, if the pressure rise preceding the pressurizing period of the operation takes place very quickly, preferably within a few seconds.

The length of the period over which the mold is subjected to compressive forces is determined experimentally for each mixture. The compression period is over after maximum densification of the mixture has been reached, and no more liquid escapes from the above-mentioned

conduits

42, 43, 44 and the narrow gap between the opening 23 and the pattern 32.

Thereafter the pressure is removed from the outside of the cup-shaped membrane 26, and the pattern 32 is carefully lifted and retracted from the opening 23 by rotating the nut 38. This will only be possible if the parts of the pattern 32 have been designed such that they are slightly conical in downward direction, and will, during upward movement of the pattern easily free themselves from their corresponding parts of the mold without damaging them.

After the pattern 32 has been lifted through the opening 23, the bridge member 27 and the plate 22 are removed and the graphite mold 45 is removed from the housing 21, such as for example, by loosening the ring member 24 and lifting the membrane 26 together with the mold 45 from the housing 21, or by inverting the housing 21 together with the mold or by injecting fluid via the conduit 40. If the membrane 26 is of the type which is flat in the non-stressed condition, the mold will be lifted from the housing 21 by the elasticity of the membrane 26.

The remaining part of the water, which has not been removed by the pressurizing operation of the mold is now removed therefrom, for example by drying the mold at ambient temperature or by exposing the mold to a moderate heating.

Thereafter the mold is ready for the sintering process. To this end, diamonds are carefully placed along circles in the channels which have been produced in the mold by ridges of the pattern 32. Due to the molding process according to the invention, the mold is a very accurate negative of the pattern 32.

By the application of small quantities of adhesive material, the diamonds are kept in place with the powder material suitable to be sintered is introduced into the mold. If desired, a shank suitable for connecting the sintered bit to a drill string, etc., is placed in the mold (compare for example FIGURE 2, which shows the position of the shank 2 with respect to the bit body 4, consisting of the sintered mass).

The shank 2 may be made of a material which has the same or about the same expansion coeflicient as the powder to be sintered. Hereby, undesirable stresses within the mass to be sintered, as well as within the mold, which would occur as a result of the temperature changes during the sintering process, are prevented. Such stresses would distort the bit contour, thus displacing the diamonds in such a way that the load exerted on the bit during drilling will be unevenly distributed over the diamonds.

The powder mass to be sintered may consist of tungsten and tungsten carbide, and the shank 2 may be formed by tungsten, either in solid form or in powder form. The shank 2 may be sintered to the tungsten. The screw thread 3 is to be cut after the sintering process.

If the shank 2 is formed by a powder mass, a graphite mold (not shown) may be used for forming the outer surface of the shank. The screw thread 3 may be formed on this latter graphite mold or may be cut on the shank later on.

The shank 2 is preferably mounted in an annular groove which is formed in the sintered body 4, whereafter the shank 2 is brazed to the body 4, preferably by the process described in co-pending US. patent application Ser. No. 594,839, filed Nov. 16, 1966.

Before placing the mold together with the diamonds and the powder to be sintered into a sintering oven, an amount of binder material (such as German silver when sintering tungsten and/or tungsten carbide powder) is placed in the powder material. To prevent distortion during the cooling process following the sintering process, it has been found that the amount of binder material must not be greater than the volume of the pore space of the powder mass. If greater amounts of binder material are applied, the binder material remains in liquid form on top of the powder material and solidifies in situ thereby causing distortion of the bit body, which results in an uneven load distribution over the diamonds when the bit is rotated under load when drilling a hole through rock material.

If desired, the graphite mold may be strengthened by supporting it at the outer side thereof, e.g. by placing it in a cup-shaped supporting member, which member may be made of graphite. Preferably there is used a graphite cup, which is made of a solid block of graphite, for example by turning it on a lathe. If required, the outer side of the graphite mold is brought into a form to match the interior wall of the cup-shaped supporting member by scraping any undesired graphite therefrom.

Strengthening of the mold by inserting reinforcements in the graphite/liquid mixture before pressurizing it, is considered unattractive since this may give rise' to distortions of the mold during the manufacturing process and/or during the sintering process.

The invention is not limited to a particular shape of the interior of the housing 21, and/ or of the member 26. This latter need not be flat in the non-stressed condition. The housing, or the membrane is pre-formed, is preferably symmetrical with respect to the central axis thereof and has a cross-section as indicated in FIGURE of the drawing. Preferably, the central axes of the pattern 32 and the housing 21 coincide.

The graphite mold, optionally supported by a cupshaped member, and containing diamonds, powder material, binder material, and optionally a shank for coupling purposes, after being placed in the sintering oven is heated to a temperature suitable to sinter the powder material to a common unit, without burning the diamonds. As such processes are known per se, no details need to be given here, regarding the sintering temperatures and sintering periods required for the various types of sinter material and binder material, which may be applied.

By way of example, some details will be given of a molding and sintering operation of a 6" diamond bit. The bit had a shape corresponding to the shape of the pattern 32 as shown in FIGURE 5. The membrane 26 had a diameter of about 210 millimeters, was about 1 millimeter thick and was made of rubber. The deviation of the tops of the ridges 5 (FIGURE 1) on concentric circles (along which the diamonds are to be placed) were measured, and the greatest variation occurring in the distances between these tops and the central axis of the pattern 32 (FIGURE 5), measured in a direction perpendicular to the bottom profile to be cut by a bit according to pattern 32, was 0.02 millimeter.

A mixture consisting of 60% graphite particles and 40% water was manually introduced in the membrane 26. Thereafter, the pattern 32 was clamped in the position as indicated in the drawing, and oil under pressure was supplied to the channel 40. Within 5 seconds the pressure within the space bounded by the membrane 26 and the inner wall of the housing 21 was raised from 1 to 30 kilograms per square centimeter, which latter pressure was applied over a period of 120 minutes during period water was pressed out of the mixture and the' mixture was densified. Thereafter, the pressure was brought down to 1 kilogram per square centimeter. Subsequently, the pattern 32 was retracted from the graphite mold and the plate 22 and the ring member 24 were removed so as to enable the lifting of the mold 45 from the apparatus.

After removal of the graphite mold the remaining part of the water contained within the pores of the graphite mold was removed by heating the mold gradually over a period of 40 hours from ambient temperature to C. The mold was thereafter cooled down slowly. Eventually this drying may take place in the sintering oven by slowly heating up the mold containing the diamonds, sintering mass and binder.

The diamonds used in this particular bit were between and carat and applied in channels of the mold which had the shape of the ridges 11 shown in FIGURE 4. The width of the portions 12 as 0.8 millimeter.

After the diamonds had been glued in place by a suitable adhesive, a mass of tungsten carbide powder was poured into the mold. Subsequently an amount of .3 kilo grams of German silver was added on top of the mass and the mass was heated in an inert atmosphere up to a temperature of 1120 C. over a period of 2.5 hours. Subsequent cooling down of the sintered mass to room temperature took 12 hours.

Thereafter, the sintered bit was placed on a measuring stand, for measuring the greatest variation in the distances between the cutting tops of diamonds belonging to a common circle and the central axis, when measured in a direction perpendicular to the bottom profile to be cut by the bit. The greatest variation was 0.1 millimeter. The variation was 0.05 millimeter or less for about 75% of the diamonds.

The method according to the present invention allows the manufacture of graphite molds suitable for sintering purposes, which molds have dimensions which are so close to the design dimensions, that the diamonds which are applied in the various channels formed by the ridges of the pattern 32 before the sintering material is applied to the mold, have in the sintered bit such a mutual position that the load is evenly distributed over all the diamonds which are arranged over the ridges 5 and equally spaced along the circles which have their centers on the central bit axis. The drawback of an uneven load distribution over the diamonds which results in a breaking out of the diamonds one after the other is thereby prevented. It will be clear that the present invention is not limited to the example given but that various alternatives may be applied in the molding and sintering process, as well as in the constructive details of the bit.

I claim as my invention: 1. In a method of manufacturing a sintered diamond drilling bit, comprising the steps of:

forming a graphite mold by placing a mixture of graphite particles and liquid into a cup-shaped flexible membrane, introducing a pattern of the bit in an opening provided in a plate arranged at the open side of the cup-shaped membrane, such that the rim of the opening fits closely around the circumference of the pattern, densifying the mixture by pressing part of the liquid out of the mixture by providing a fluid pressure on the outer side of the membrane, reducing the fluid pressure to atmospheric pressure, retracting the pattern out of the opening; and removing the graphite mold out of the membrane;

vaporizing the remaining part of the liquid from the graphite mold;

placing diamonds in the mold along circles which have their centers on the central bit axis, filling the mold with a mass of powder material to be sintered, and placing a quantity of binder material on top of the mass;

subjecting the mold with the contents thereof to sintering temperature, and

removing the sintered diamond bit from the mold after the sintering process is over.

2. A method as claimed in claim 1, wherein the liquid is water.

3. A method as claimed in claim 1 comprising the further step of providing the pattern with ridges.

4. A method as claimed in claim 1 including the step of maintaining the fluid pressure on the outer side of the membrane substantially constant over the pressurizing period.

5. A method as claimed in claim 4, wherein at the beginning of the step of densifying the mixture in which liquid is displaced from the mixture, raising the fluid pressure at the outside of the membrane substantially instantaneously.

6. A method as claimed in claim 1 in which the step of vaporizing the liquid from the mold is carried out by:

subjecting the mold and its contents to the sintering temperature.

7. A method as claimed in claim 1 comprising prior to placing diamonds in the mold, supporting said mold in a support member.

8. A method as claimed in claim 7, wherein the supporting member is in the form of a cup-shaped body of graphite.

9. A method as claimed in claim 8, wherein the body of graphite is formed on a lathe.

10. A method as claimed in claim 1 including the further step of placing a shank in the upper part of the mass of powder material arranged in the graphite mold, which shank is suitable to form a connection between the sintered bit and a drill pipe, drill collar or sub, the shank References Cited UNITED STATES PATENTS 2,125,332 8/1938 Bursell 76108 2,147,843 2/1939 Jamar et a1 76-108 2,187,384 1/1940 Maier 76-108 2,200,281 5/1940 Koebel 76-108 2,511,991 6/1950 Nussbaum 76108 3,271,822 9/1966 Rhino 29473.1 X 3,296,693 1/1967 Carter 29473.1

JOHN F. CAMPBELL, Primary Examiner.

R. B. LAZARUS, Assistant Examiner.

U.S. Cl. X.R.