WO2008032272A2 - Waterjet nozzle - Google Patents

Abstract

A nozzle for waterjet cutting apparatus comprises a nozzle body in which a hole is formed, the hole comprising a cylindrical bore formed concentrically with an adjacent outwardly tapered bore. The tapered bore can be conically or spherically tapered. The cylindrical bore defines an exit opening of the nozzle, and the tapered bore defines an inlet opening of greater diameter than the exit opening. Preferably the nozzle has a ratio of bore edge width W to hole diameter D (W/D) in the range of 4 to 15%. The nozzle body may comprise various hard materials including ruby, sapphire or diamond. In the case of diamond, the nozzle body can comprise single crystal diamond, polycrystalline diamond, or a sintered diamond material.

Description

WATERJET NOZZLE

BACKGROUND OF THE INVENTION

THIS invention relates to a nozzle for waterjet cutting apparatus.

Water jets are commonly used for cutting applications similar to laser cutting. Typical materials to be cut are sheet metals (steel, titanium), composites, diapers and food. In a typical waterjet pure water is pressurized (up to 4000 bar currently) and exits through a nozzle with a small hole. In the hole a water jet is formed which is used for cutting. In some applications the water jet is mixed with a grit of abrasive particles which is suctioned into the water jet by the vacuum that is produced by the rapid jet. This happens in a mixing chamber (also called a focusing tube) downstream of the nozzle exit. Pure water cutting would be used in the food industry, for diapers, in the semiconductor industry, while for metal cutting abrasive technology is widely used.

In order to obtain efficient cutting the waterjet needs to retain its parallel flow shape over long distances. This is referred to as coherence. Onset of loss of coherence is seen as a spreading of the jet, mist formation around the jet, and bubbles inside the jet (milky appearance). For good cutting aiming is very important so a jet should be very accurately in line with the nozzle hole. Otherwise the jet may hit the walls of the holder and/or the focusing tube (in abrasive cutting applications) and immediately erode the holder/focusing tube. Also, alignment with the cutting head is of importance in order to cut the product material where it needs to be cut.

Nozzles are characterized by their hole diameter (D) and by a value called the discharge coefficient (CD). CD is a function of the edge radius divided by the hole diameter (R/D) and gives the ratio of the actual mass flow divided by a hypothetical mass flow that would occur if the flow would fill the full cross section of the cylindrical hole. This in fact is not what happens: for a sharp edged nozzle the flow lines detach from the nozzle surface and contract to a diameter which is smaller than the actual hole size. Therefore the actual flow is lower than the hypothetical flow and CD is always smaller than 1. The theoretical minimum value of CD for an orifice is 0.5959 (see e.g. R.W. Fox and AT. McDonald, Introduction to Fluid Mechanics, J. Wiley & Sons, New York, 1994, ISBN 0-471-59274-9). For nozzles with R/D larger than approx. 0.15, CD equals 1. For the user of waterjet nozzles the combination of hole diameter and CD determines the capacity of the pump one has to install and the amount of water that is used up. So, for the economy of the process these are crucial parameters.

Another consideration in nozzle design is their lifetime during operation: the lifetime of sharp edged nozzles is extremely short (at least for conventional sapphire and ruby nozzles) due to wear and chipping at the edge. Therefore the edge is somewhat rounded off in order to obtain a stronger nozzle with less wear.

According to current practice, the aim is to produce nozzles with low CD values, typically less than 0.65 for pure water cutting and O.7-0.75 for abrasive cutting.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a nozzle for waterjet cutting apparatus comprising a nozzle body of hard material in which a hole is formed, the hole comprising a cylindrical bore formed concentrically with an adjacent tapered bore, the cylindrical bore defining an exit opening and the tapered bore defining an inlet opening of greater diameter than the exit opening.

The tapered bore is conical or may have a curved profile. For example, the bore may have a spherical or spheroidal taper, which may be due to the use of a conical tool that has been subject to wear, or to the use of a spherical or spheroidal tool.

Preferably, the nozzle has a ratio of bore edge width W to hole diameter D (W/D) in the range of 4 to 15%.

The nozzle body may comprise ruby, sapphire or diamond.

For example, the nozzle body may comprise single crystal diamond, polycrystalline diamond, or a sintered diamond material.

In particular, the nozzle body may comprise single crystal diamond which is a natural diamond, synthetic diamond produced by a High-Pressure-High- Temperature (HPHT) method, or synthetic diamond produced by a chemical vapor deposition (CVD) process.

Preferably, the orientation of the axis of rotational symmetry of the nozzle is chosen to coincide with either the {110} and {100} crystal axes or the {111} crystal axis of the single crystal material.

The nozzle body may instead be formed from polycrystalline diamond, grown by a chemical vapor deposition (CVD) process.

The polycrystalline diamond material is doped with a selected impurity, such as boron.

As another alternative, the nozzle body may be formed from a sintered diamond material comprising diamond particles bonded by sintering with a binder such as cobalt, silicon or silicon carbide.

The cylindrical and tapered bores may have values for their root-mean- square roughness corresponding to what can be obtained in a polishing -A-

process (as low as several to tens of nm). However, higher values of the root-mean-square roughness up to several μm are also acceptable as long as the intersection of the tapered bore with the upper plane surface is circular with deviations of less than a few μm.

The cylindrical and/or the tapered bores of the nozzle may be polished, with a root-mean-square roughness value of less than 50nm.

Alternatively, the cylindrical and/or the tapered bores may be unpolished, with a root-mean-square roughness value of less than 2μm.

The intersection of an outermost end of the tapered bore with a planar surface of the nozzle body is preferably circular with deviations of less than 2μm.

Preferably, a line defined by the intersection between the adjacent tapered and cylindrical bores of the nozzle hole is substantially circular, and is sharply defined with edge chipouts of less than 1μm.

In the case where the tapered bore is conical, it may have a cone angle in the range of 30 to 120 degrees.

According to a second aspect of the invention there is provided a method of manufacturing a nozzle for waterjet cutting apparatus, the method comprising providing a nozzle body of hard material, forming a cylindrical bore in the body, and forming a tapered bore adjacent the cylindrical bore and concentric therewith to define a nozzle in which the cylindrical bore defines an exit opening and the tapered bore defines an inlet opening of greater diameter than the exit opening.

The method may comprising forming the cylindrical bore by laser drilling and/or by electric discharge machining/spark erosion. The method may further comprise polishing the cylindrical bore to a root- mean-square roughness value of less than 50nm.

The polishing of the cylindrical bore may be carried out by a wire-polishing technique.

Alternatively, the cylindrical bore may be left unpolished so that it has a root-mean-square roughness value of less than 2μm.

The tapered bore may be formed by chamfering.

The chamfering may be carried out by a mechanical grinding and/or polishing process.

Instead, or additionally, the chamfering may be carried out by laser drilling, and/or by electric discharge machining/spark erosion.

The chamfering may be carried out at one end of a previously formed cylindrical bore.

The method preferably includes forming the tapered bore to define a ratio of bore edge width W to hole diameter D (W/D) in the range of 4 to 15%.

The tapered bore may be formed with a conical shape having a cone angle in the range of 30 to 120 degrees.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a sectional side view of a waterjet cutting head of the type utilising pure water;

Figure 2 is a sectional side view of a waterjet cutting head in which abrasive particles are mixed with water; Figure 3 is a schematic sectional view (not to scale) of a portion of a waterjet nozzle, illustrating the effect of a low discharge coefficient (CD) on the diameter of a waterjet exiting the nozzle;

Figure 4 is a sectional side view of a prior art nozzle for waterjet cutting apparatus, having a small edge radius;

Figure 5 is a sectional side view of a first embodiment of a nozzle for waterjet cutting apparatus according to the invention, with a bore having a conically tapered inlet portion; and

Figure 6 is a sectional side view of a second embodiment of a nozzle for waterjet cutting apparatus according to the invention, with a bore having a spherically tapered inlet portion.

DESCRIPTION OF EMBODIMENTS

Figures 1 and 2 show examples of conventional waterjet cutting heads, the first utilising water only and the second utilising a mixture of abrasive particles and water. In Figure 1, the cutting head comprises a tubular housing 10 with a cap 12 that is screwed onto the end of the tubular housing and holds a waterjet nozzle (or nozzle insert) 14 in place at the lower end of the cutting head. A conduit 16 feeds water under high pressure to the inlet side of the nozzle, and a waterjet 18 exits the nozzle at high velocity.

In Figure 2, the arrangement is generally similar, with the cutting head comprising a tubular housing 20 in which is a conduit 22 that supplies water under pressure to a waterjet nozzle 24. Below the nozzle 24 is a mixing tube 26, which receives the waterjet emitted by the nozzle. An auxiliary conduit 28 feeds abrasive powder into the mixing tube, which is entrained in the water in the mixing tube. At the lower end 30 of the mixing tube, a waterjet 32 containing the abrasive particles is emitted. Waterjet nozzles of the invention can be used in either type of cutting head.

Conventionally, waterjet nozzles (also called nozzle inserts) are formed from materials such as sapphire and ruby. Diamond is also used. The inserts are polished disc-shaped bodies, typically between 0.5 to 1 mm thickness and 2.0 mm in diameter, with a central cylindrical hole of diameter up to typically 200 μm for pure water cutting. Abrasive cutting requires nozzles with a hole diameter typically larger than 200μm. The cylindrical hole usually has a length of less than 100 μm (5-10% of the nozzle body thickness), although larger length values have been used (up to 50% of the nozzle body thickness), and then continues as a conical hole with a typical cone angle of typically 50+/- 15 degrees in the case of diamond nozzles.

As stated earlier to avoid excessive wear, especially in abrasive applications where, when the flow is interrupted, an ensuing vacuum may suck in abrasive particles upstream of the nozzle entry, the edge of the nozzle hole on the high pressure side is given a radius. This radius typically has a value between 2 and 4% of the nozzle hole diameter and is (partially) determined by the value of CD one wishes to obtain. An embodiment of a known waterjet nozzle as described above is shown in Figure 4.

For sapphire and ruby the radius can, for example, be made by a brushing technique whereby a hair brush with a diamond paste is applied to a tray with many identical nozzles and the polishing action of the hairs with the paste then forms a radius on all edges, including the edge of the nozzle hole. Typically the nozzles themselves have sub 10-micron tolerances on diameter and thickness, while the hole diameter is specified to within 2 microns.

Sapphire and ruby nozzles typically fail within tens of hours due to wear or chipping at the entry side of the hole and due to breakage. Slight variations in the nozzle radius or local chipouts will severely degrade the jet quality with ensuing poor cutting efficiency (wide cuts, jet walkoff which means cutting at the wrong position, insufficient cutting depth). Therefore typical lifetimes in abrasive cutting for sapphire/ruby nozzles are only a few tens of hours.

Diamond nozzles are much more expensive (€100's vs €10's) but they show useable lifetimes of the order of 1000 hours. However, giving diamond nozzles the same shape as sapphire nozzles turns out to be extremely difficult. The small radii required can in general not be made by hair brushing. Other techniques such as polishing with a soft pad also are difficult. Therefore diamond nozzles tended to have large radii with concomitant poor jet quality.

The applicant has made nozzles in diamond plates by first polishing the plates, laser drilling a hole with cylindrical and conical sections, where the cylindrical section has a diameter slightly under the required final size, and then subsequently polishing the inner wall of the hole to its final size using a wire polishing machine to pull a metal (e.g. copper or steel) wire which is impregnated with diamond powder through the nozzle hole. To get uniform polishing the nozzle itself is rotated about its axis during the polishing process. This is a commercial process also known from the production of diamond wire dies.

Since the laser drilling leaves a hole with a large edge radius, which is not removed by wire polishing, and wire polishing itself may lead to chipping at the flat entrance side of the nozzle, one approach has been to polish the plates on the entry side to remove both the large radius and the edge chips. One is then left with nozzles with a very accurate, polished cylindrical hole and with an extremely small edge radius (<0.5 μm, typically 0.2 μm).

These nozzles have excellent beam quality with very low discharge coefficient (close to the theoretical minimum value for CD of around 0.60) and although they show no significant wear in pure water cutting (note that this is in contrast to what is seen in sapphire and ruby), in abrasive cutting they still suffer from edge chipping at the sharp entry of the hole, resulting in lifetimes for abrasive cutting which in some cases were as low as 100 hours instead of the values obtained with the large radii.

The conventional approach of attempting to provide nozzles with the lowest possible discharge coefficient (CD) is indicated by the fact that for higher CD values the waterjet coherence is significantly degraded. However, in the theory there is no direct link between low CD and jet quality. It appears that the need for a low value of CD is primarily caused by the difficulty of providing a uniform radius of curvature R around the circumference of the edge of the nozzle hole. By providing a small radius the locus (line) of flow release remains well defined and circular and thus a good jet quality is obtained.

For a nozzle with a radius R, especially for values of R/D larger than approximately 4%, the line of release is not well defined: local variations in the radius may lead to different radial positions at which the flow detaches from the nozzle surface and finally in a non-parallel jet. This then is characterized as a poor quality jet. Furthermore, the inevitable wear of the surface will lead to a time-varying jet quality (generally degrading as time progresses). Good quality waterjets are said to have good coherence, meaning that they are well focused, so that more water is concentrated in the jet rather than spreading out and losing energy.

According to the present invention, a tapered bore is formed by chamfering or otherwise providing a tapered bore, preferably concentric with the adjacent cylindrical bore that defines the discharge portion of the nozzle, with a bore edge width W in the range of 4 to 15% of the diameter D of the adjacent cylindrical bore. This results in a number of advantages. For a tapered bore however the line of release of the flow is well defined and coincides with the outer diameter of the tapered section. By ensuring that the line of release is a circle a stable well-defined parallel flow is obtained, perpendicular to the entry surface. The upper value of the preferred range for the tapered bore width is determined by the demand that the flow, after releasing from the outer edge of the tapered bore, should stay clear of the nozzle walls, while the lower value of the range is determined by the demands on long lifetimes.

The conventional wisdom has been that one needed a low value of CD in order to get good jet quality. This in fact is not true. A high CD value for a nozzle with a large width of the tapered bore is not really a problem since the flow is stable. One only needs to redefine the way a nozzle is specified: instead of giving the nozzle hole diameter and the discharge coefficient, one would only give the flow rate through the nozzle at some pressure and from this the user would know the requirements on his pump capacity in terms of the mass flow rate it has to sustain and on the amounts of water and electricity used.

By making a large width tapered bore we can get around problems of chipping or at least alleviate them: for a nozzle with an edge radius, especially the nozzles with a small edge radius according to the prior art, anything that disturbs the radius (wear or chipouts, for example) will lead to a degradation of the jet quality. However as long as a chipout does not get into or through the line of release, a chipout from the tapered surface itself would not be harmful in terms of nozzle performance. And the relatively large width of the tapered bore itself makes it less susceptible to chipping than an edge with a small edge radius.

Jet aiming is significantly improved with respect to prior art nozzles with an edge radius since it is now possible to make a nozzle with a jet which is always perpendicular to the entry side surface. Any eccentricity of the tapered bore only translates into a shift of the jet from the geometrical center of the waterjet nozzle but does not produce an aiming error. Non- roundness (ellipticity) of the chamfer would still give a stable jet but with a non-round cross section. For cutting applications this is not desirable. Since the jet does not hit the wall of the nozzle hole and thus the nozzle shape and roughness inside the outer edge of the tapered bore is unimportant for the flow, many of the stringent requirements for the finish of the nozzle hole (roundness, absolute diameter, roughness) can be relaxed. Thus, one does not need to polish the tapered edge or the inner walls of the nozzle hole to very high accuracy and low roughness. Wire polishing intended to provide a smooth side wall with a very accurate hole size may not be needed. Instead one could just use a laser-cut or otherwise drilled cylindrical hole (i.e. non-round hole of variable size) and achieve significant cost reduction by not polishing. Also, the tapered bore itself may be rough or even pitted as long as the line of release is well defined.

Figure 3 shows schematically (not to scale) how a waterjet 34 emitted from a nozzle 36 of the invention has a smaller diameter than the internal diameter of the cylindrical portion 38 of the nozzle hole, and leaves the exit opening 40 of the nozzle without contacting the inner surface thereof.

Figure 5 shows an embodiment of a waterjet cutting nozzle of the invention having a conically tapered bore at its inlet end. The nozzle comprises a generally disc-shaped nozzle body 42, preferably formed of diamond material as described herein. The body has a hole comprising a central cylindrical bore 44 which is continuous and concentric with an adjacent tapered bore 46 defining an inlet end of the hole. The bore 46 has a conically outwardly tapered shape. Below the exit end 48 of the cylindrical bore 44, the material of the body is removed to define an outwardly tapering conical hole 50 about the exit end, with a typical cone angle of 50 degrees, +/- 15 degrees (that is, a taper angle of 25 degrees, +/-7.5 degrees). In general, cone angles in the range of 30 to 120 degrees are suitable. The line of intersection between the adjacent tapered and cylindrical bores is substantially circular, and is sharply defined.

An alternative embodiment is shown in Figure 6, which is generally similar to that of Figure 5, but which has a spherically tapered inlet end 52 instead of a conically tapered inlet end. Again, the line of intersection between the adjacent spherical and cylindrical bores is substantially circular, and is sharply defined.

In both cases, the nozzle preferably has a ratio of bore edge width W to hole diameter D (W/D) in the range of 4 to 15%, measured as indicated in Figures 5 and 6.

Since for jet quality chipping is relatively unimportant as long as the line of release is well defined, one possibility to obtain a significant cost reduction is to use polycrystalline diamond material for nozzles. The main question then would be their strength associated with the lifetime of such nozzles but that is easily accommodated by using thicker material. The cost savings with respect to single crystal diamond material are still sufficient to make this attractive.

The use of polycrystalline diamond has not been considered previously for the application in question due to the tendency of this material to chip near the hole edges. The difficulty or even-impossibility of polishing this material using the known wire polishing technique has also contributed to the fact that consideration has not been given to using polycrystalline diamond nozzles.

To exploit the advantage of not having to polish the inside wall of the cylindrical hole of the nozzle the inventors have produced nozzles using polycrystalline diamond grown by the chemical vapor deposition. (CVD) process. These nozzles were made from DiaFilm PC™ as manufactured by Element Six Ltd. of Ascot, United Kingdom. DiaFilm PC™ is a polycrystalline CVD grown type of diamond which has a very high, reproducible yield strength. It is however electrically insulating, since it is a pure diamond variety. By deliberately introducing impurities into the diamond during the CVD growth-process the polycrystalline diamond can be made electrically conductive. For instance in DiaFilm PE™, as manufactured by Element Six Ltd. of Ascot, United Kingdom, the diamond has been made electrically conductive by the uptake of boron. This material has the advantage that in addition to laser drilling and cutting, products from it can also be shaped using Electric Discharge Machining techniques (also known as spark erosion). In nozzles according to the invention made from DiaFilm PE™, EDM technology was used to drill the cylindrical and conical holes, trim the outer edge of the nozzles and/or apply a shaped chamfer or spherical tapered bore to the entry side of the cylindrical hole. Furthermore it was found possible to round off or chamfer the outer edges of these nozzles using EDM technology. It is noted that an improved lifetime may be obtained using boron-doped CVD-grown polycrystalline diamond with respect to the undoped variety. This is caused by the fact that boron-doped CVD-grown polycrystalline diamond has higher resistance to wear than undoped CVD-grown polycrystalline diamond.

Other grades of polycrystalline CVD, which are either electrically insulating or conductive, and/or with different grain sizes and preferred crystal orientations with respect to the growth direction in the CVD reactor, may also be used for waterjet nozzle applications.

Further nozzles were produced from sintered diamond material where the diamond particles are bonded by sintering with a binder such as cobalt (e.g. Syndite™) and silicon (e.g. Syndax™). Another very attractive material is Skeleton™ where the diamond particles are bonded using silicon carbide (SiC). All these materials are manufactured by Element Six Ltd. of Shannon, Ireland.

These materials combine the advantages of high strength and high toughness leading to very desirable behavior in terms of reduced tendency to form chipouts with respect to single crystal and polycrystalline CVD diamond, and in the case of cobalt and silicon bonded sintered materials they can also be processed using EDM techniques. For all these sintered materials it has also been found possible to obtain polished holes using wire-polishing technology. However as stated above, according to the present invention it is preferred not to do this and to just coarsely drill a hole with either a laser or using EDM techniques, or a combination of these techniques, to obtain a significant cost reduction in the manufacture of these products.

In addition to these hole drilling techniques it is also possible to shape these sintered diamond nozzles by conventional techniques known to those skilled in the art, such as but not limited to grinding, lapping and polishing.

In some applications it was also found to be attractive to use different types of single crystal diamond. Most prior art diamond nozzles were made from natural diamond of type Ia, which contains high concentrations of nitrogen impurities (typically hundreds of ppm N). These impurities are not evenly distributed in the diamond crystal but tend to cluster in so-called platelets which are of sub-micron or micron size.

In some cases the inventors have produced nozzles from natural type Ha diamond, which contains very low concentrations (typically less than 20 ppm) of nitrogen which is not clustered but homogeneously distributed in the crystal. Also synthetic single crystal type Ha diamond as grown by the CVD process was used. Use of the type Ha diamond and especially the synthetic CVD grown form of it had the advantage that this material is known to be harder and therefore shows less deformation at the high pressures it is subjected to in waterjet cutting applications. Synthetic CVD grown diamond has the additional advantage of being an engineered material with reproducible material properties and so it shows far more consistent, reproducible behavior than natural diamond.

Further tests were made with single crystal, synthetic type Ib diamond as grown by the High-Pressure-High-Temperature (HPHT) method. This material contains high concentrations of nitrogen impurities (typically hundreds of ppm N), which are uniformly distributed in the diamond crystal lattice. This material is known to be tougher than other forms of single crystal diamond and thus the tendency to chip and/or crack is reduced. As discussed above chipping is an important factor limiting the useful lifetime of waterjet nozzles, and the combined application of a chamfer according to this invention with the use of type Ib diamond leads to an exceptionally long lifetime of these nozzles.

In the case of single crystal diamond, chipping and wear near the entrance of the cylindrical hole could be further reduced by carefully choosing the orientation of the crystal axes to the geometric axes of the nozzle. In particular it was found advantageous to choose the orientation of the axis of rotational symmetry of the nozzle to coincide with either the {110} or the {100} axes. Even better performance was found for an orientation of the symmetry axis which coincided with or was close to a {111} axis.

The tapered bore may be formed by laser cutting. Another method for forming a conical bore would be to use a conical metal tip and polishing the bore with diamond grit, either by mechanically rotating the tip at ultra high speed or by vibrating the tip ultrasonically, or a combination of these techniques as is used in wire die polishing. Instead of a metal tip and diamond grit, conical tips could be formed from resin bonded or sintered diamond material where the diamond particles are bonded in the material by a resin or by sintering with a binder such as cobalt (e.g. Syndite™) or silicon (e.g. Syndax™). For the electrically conductive materials EDM techniques may also be applied to form the tapered bore as discussed earlier. Note that one could also apply the techniques of conical tip polishing to non-diamond nozzles. Although in principle this could work, the fact that this is a non-batch process, processing single nozzles at a time, does not make this a preferred route in terms of production costs.

Alternatively actual mechanical polishing of spherical chamfers using steel (or other material) balls and adding a diamond paste is possible. This technique could then also be applied to sapphire or ruby nozzles. By applying pressure for the polishing using a rubber pad pressing down simultaneously on many balls and preventing them from rolling out of the holes one could polish a spherical chamfer on many nozzles at a time. The exact profile or shape of the chamfered bore is not critical, and the chamfered bore can be tapered with either a convex or concave curvature which can be spherical or spheroidal in shape. It is only important that the intersection with the upper surface of the nozzle body is circular.

EXAMPLES OF EMBODIMENTS

Example 1

A single crystal diamond nozzle was produced from synthetic CVD-grown type Ma diamond in the following manner: a rectangular plate was sawn from a CVD grown stone and subsequently polished flat on both sides to an approximate thickness of 0.6 mm. It was then subsequently cut to the required diameter of 2 mm and a central cone with a full cone angle of 50° was cut out with a laser.

Subsequently over the last 100 μm a cylindrical hole was drilled with the laser with a diameter of 220 μm. The diamond plate was subsequently mounted in a wire polishing machine and the inside walls of the cylindrical section of the hole were polished to a final diameter of 250 μm, with a mirror shine surface finish. Chipouts near the intersection of the cylindrical section with the top plane of the nozzle were then removed by diamond polishing on a cast-iron rotating wheel, impregnated with diamond powder, and the nozzle was then polished down to its final thickness of 0.510 mm by polishing the opposite flat side. At this stage the intersection between the cylindrical hole and the top flat plane had a radius of curvature of less than 1 μm.

A tapered section of width 30 μm was then added to the cylindrical hole by polishing with a 00.6 mm steel sphere impregnated with fine diamond grit powder, which was mounted on a high speed rotational axis. The wear on the steel ball was such that the resulting polished surface was not an exact sphere but the intersection line with the top surface was a circle with a diameter of 310 μm, with deviations from an exact circle of less than 3 5 μm. The average full cone angle of the polished surface was 120°. Surface roughness on the tapered surface was less than 50 nm.

Example 2

A batch of six polycrystalline diamond nozzles was produced from synthetic CVD-grown polycrystalline diamond in the following manner: from a rectangular plate, polished on both sides, with a thickness of 0.9 mm, six round plates were cut out to the required diameter of 2 mm and a central cone with a full cone angle of 40° was cut out with a laser in each round.

Subsequently over the last 80 μm a cylindrical hole was drilled with the laser with a diameter of 370 μm. At this stage the wall of the cylindrical section had a typical roughness of approximately 2 μm and the intersection between the cylindrical hole and the top flat plane had a radius of curvature of approximately 12 μm.

A tapered section of width 40 μm was then formed in the cylindrical hole of each nozzle by polishing with an initially conical polishing tip made from a material consisting of resin impregnated diamond particles, which was mounted on a high speed rotational axis. The wear on these tips was such that they had to be replaced several times before the final result was obtained and the resulting polished surface was not an exact cone, but the intersection line with the top surface was a circle with a diameter of 450 μm, with deviations from an exact circle of less than 6 μm. The variation in the diameters of these circles was less than 10 μm. The average full cone angle of the polished surface was 120°. Surface roughness on the tapered surfaces was less than 100 nm.

Example 3

A batch of eight polycrystalline diamond nozzles was produced from sintered diamond with a cobalt binder and with average diamond particle size prior to sintering of 2 μm in the following manner: from a round plate, polished on both sides, with a thickness of 0.5 mm and a roughness on the polished planes of less than 150 nm, eight round plates were cut out to the required diameter of 1.85 mm and a central cone with a full cone angle of 50° was cut out with a laser in each round.

Subsequently over the last 60 μm a cylindrical hole was drilled with the laser with a diameter of 200 μm. At this stage the wall of the cylindrical section had a typical roughness of approximately 5 μm and the intersection between the cylindrical hole and the top flat plane had a radius of curvature of approximately 10 μm.

A tapered section of width 25 μm was then formed in the cylindrical hole of each nozzle by polishing with an initially conical polishing tip made from the same material as the nozzle, i.e. cobalt bonded sintered diamond particles, which was mounted on a high speed rotational axis. The wear on these tips was such that the resulting polished surface was not an exact cone, but the intersection line with the top surface was a circle with a diameter of 250 μm, with deviations from an exact circle of less than 5 μm. The variation in the diameters of these circles was less than 3 μm. The average full cone angle of the polished surface was 80°. Surface roughness on the tapered surfaces was less than 80 nm.

Example 4

A single crystal diamond nozzle was produced from synthetic HPHT-grown type Ib diamond in the following manner: a rectangular plate with orientation of the surface normal parallel to the [110] crystal axis was sawn from a HPHT grown stone and subsequently polished flat to an approximate thickness of 0.45 mm. It was then subsequently cut to the required diameter of 1.9 mm and a central cone with a full cone angle of 50° was cut out with a laser.

Subsequently over the last 80 μm a cylindrical hole was drilled with the laser with a diameter of 180 μm. The diamond plate was subsequently mounted in a wire polishing machine and the inside walls of the cylindrical section of the hole were polished to a final diameter of 200 μm, with a mirror shine surface finish. Chipouts near the intersection of the cylindrical section with the top plane of the nozzle were then removed by polishing down to its final thickness of 0.40 mm on a cast-iron rotating wheel, impregnated with diamond powder. At this stage the intersection between the cylindrical hole and the top flat plane had a radius of curvature of less than 1 μm.

A tapered section of width 20 μm was then formed in the cylindrical hole of this nozzle by polishing with an initially conical polishing tip made from a material consisting of resin impregnated diamond particles, which was mounted on a high speed rotational axis. The wear on this tip was such that the resulting polished surface was not an exact cone, but the intersection line with the top surface was a circle with a diameter of 310 μm, with deviations from an exact circle of less than 3.5 μm. The average full cone angle of the polished surface was 90°. Surface roughness on the tapered surface was less than 50 nm.

Example 5

A single crystal diamond nozzle was produced from synthetic HPHT-grown type Ib diamond in the following manner: an octagonal shaped plate was cleaved from a HPHT grown stone with an orientation of the surface normal in the direction of the crystal [111] axis. It was subsequently polished flat on both sides to an approximate thickness of 0.45 mm, at an off-angle of approximately 5° from the exact [111] direction, since polishing at exactly 0° off-angle is not possible. It was then subsequently cut to the required diameter of 1.9 mm and a central cone with a full cone angle of 50° was cut out with a laser.

Subsequently over the last 80 μm a cylindrical hole was drilled with the laser with a diameter of 180 μm. The diamond plate was subsequently mounted in a wire polishing machine and the inside walls of the cylindrical section of the hole were polished to a final diameter of 200 μm, with a mirror shine surface finish. Chipouts near the intersection of the cylindrical section with the top plane of the nozzle were then removed by polishing down to its final thickness of 0.40 mm on a cast-iron rotating wheel, impregnated with diamond powder. At this stage the intersection between the cylindrical hole and the top flat plane had a radius of curvature of less than 1 μm.

A tapered section of width 20 μm was then formed in the cylindrical hole of this nozzle by polishing with an initially conical polishing tip made from a material consisting of resin impregnated diamond particles, which was mounted on a high speed rotational axis. The wear on this tip was such that the resulting polished surface was not an exact cone, but the intersection line with the top surface was a circle with a diameter of 310 μm, with deviations from an exact circle of less than 5 μm. The average full cone angle of the polished surface was 90°. Surface roughness on the tapered surface was less than 50 nm.

Example 6

A batch of eight polycrystalline diamond nozzles was produced from sintered diamond with a cobalt binder and with average diamond particle size prior to sintering of 2 μm in the following manner: from a plate, polished on both sides, with a thickness of 0.5 mm and a roughness on the polished planes of less than 150 nm, eight round plates were cut out using EDM wire sawing to a diameter of 1.85 mm and from the centre of one side a small hole was drilled with a laser in each round at an angle of approximately 25°. Subsequently an 80 micron diameter copper wire was inserted through the hole and a cone with a full cone angle of 50° was cut out in each round using EDM wire sawing.

Subsequently over the last 100 μm a cylindrical hole with a diameter of 350 μm was drilled with the laser. At this stage the wall of the cylindrical section had a typical roughness of approximately 1.5 μm and the intersection between the cylindrical hole and the top flat plane had a radius of curvature of approximately 5 μm.

A tapered section of width 35 μm was then formed in the cylindrical hole of each nozzle by EDM removal with a conical electrode tip made from copper, which was mounted on a rotational axis and centered with respect to the hole to within 3 μm. The wear on these electrode tips was such that the resulting surface was not an exact cone, but the intersection line with the top surface was a circle with a diameter of 420 μm, with deviations from an exact circle of less than 10 μm. The variation in the diameters of these circles was also less than 10 μm. The average full cone angle of the tapered surfaces was 90°. Surface roughness on the tapered surfaces was less than 500 nm.

In some embodiments, a smaller taper or cone angle than specified above may be preferred in order to improve the quality of the waterjets produced by the nozzles of the invention, even at the expense of a possible reduction in nozzle service lifetime. Tapers down to 15 degrees (that is, cone angles down to 30 degrees) can be used to produce nozzles that generate waterjets with better coherence.

Claims

1. A nozzle for waterjet cutting apparatus comprising a nozzle body of hard material in which a hole is formed, the hole comprising a cylindrical bore formed concentrically with an adjacent tapered bore, the cylindrical bore defining an exit opening and the tapered bore defining an inlet opening of greater diameter than the exit opening.

2. A nozzle according to claim 1 wherein the tapered bore is conical.

3. A nozzle according to claim 1 wherein the tapered bore has a curved profile.

4. A nozzle according to claim 3 wherein the tapered bore has a spherical or spheroidal taper.

5. A nozzle according to any one of claims 1 to 4 which has a ratio of bore edge width W to hole diameter D (W/D) in the range of 4 to 15%.

6. A nozzle according to any one of claims 1 to 5 wherein the body comprises ruby, sapphire or diamond.

7. A nozzle according to claim 7 wherein the nozzle body comprises single crystal diamond, polycrystalline diamond, or a sintered diamond material.

8. A nozzle according to claim 7 wherein the nozzle body comprises single crystal diamond which is a natural diamond, synthetic diamond produced by a High-Pressure-High-Temperature (HPHT) method, or synthetic diamond produced by a chemical vapor deposition (CVD) process.

9. A nozzle according to claim 8 wherein the orientation of the axis of rotational symmetry of the nozzle is chosen to coincide with either the {110} and {100} crystal axes or the {111} crystal axis of the single crystal material.

10. A nozzle according to claim 7 wherein the nozzle body is formed from polycrystalline diamond, grown by a chemical vapor deposition (CVD) process.

11. A nozzle according to claim 10 wherein the polycrystalline diamond material is doped with a selected impurity.

12. A nozzle according to claim 11 wherein the selected impurity is boron.

13. A nozzle according to claim 7 wherein the nozzle body is formed from a sintered diamond material comprising diamond particles bonded by sintering with a binder.

14. A nozzle according to claim 13 wherein the binder is cobalt, silicon or silicon carbide.

15. A nozzle according to any of claims 1 to 14 wherein the cylindrical and/or the tapered bores are polished, with a root-mean-square roughness value of less than 50nm.

16. A nozzle according to any of claims 1 to 14 wherein the cylindrical and/or the tapered bores are unpolished, with a root-mean-square roughness value of less than 2μm.

17. A nozzle according to any one of claims 1 to 16 wherein the intersection of an outermost end of the tapered bore with a planar surface of the nozzle body is circular with deviations of less than 2μnn.

18. A nozzle according to any one of claims 1 to 17 wherein a line defined by the intersection between the adjacent tapered and cylindrical bores of the nozzle hole is substantially circular, and is sharply defined with edge chipouts of less than 1 μm.

19. A nozzle according to any one of claims 1 to 18 wherein the tapered bore is conical and has a cone angle in the range of 30 to 120 degrees.

20. A method of manufacturing a nozzle for waterjet cutting apparatus, the method comprising providing a nozzle body of hard material, forming a cylindrical bore in the body, and forming a tapered bore adjacent the cylindrical bore and concentric therewith to define a nozzle in which the cylindrical bore defines an exit opening and the tapered bore defines an inlet opening of greater diameter than the exit opening.

21. A method according to claim 20 comprising forming the cylindrical bore by laser drilling and/or by electric discharge machining/spark erosion.

22. A method according to claim 20 or claim 21 comprising polishing the cylindrical bore to a root-mean-square roughness value of less than 50nm.

23. A method according to claim 22 comprising polishing the cylindrical bore by a wire-polishing technique.

24. A method according to claim 20 or claim 21 wherein the cylindrical bore is left unpolished so that it has a root-mean-square roughness value of less than 2μm.

25. A method according to any one of claims 20 to 24 comprising forming the tapered bore by chamfering.

26. A method according to claim 25 wherein the chamfering is carried out by a mechanical grinding and/or polishing process.

27. A method according to claim 25 or claim 26 wherein the chamfering is carried out by laser drilling, and/or by electric discharge machining/spark erosion.

28. A method according to anyone of claims 25 to 27 wherein the chamfering is carried out at one end of a previously formed cylindrical bore.

29. A method according to any one of claims 20 to 28 including forming the tapered bore to define a ratio of bore edge width W to hole diameter D (W/D) in the range of 4 to 15%.

30. A method according to any one of claims 20 to 29 including forming the tapered bore with a conical shape having a cone angle in the range of 30 to 120 degrees.