FIELD OF THE INVENTION

This disclosure relates to a method of dressing a grinding wheel using a dressing roller comprising polycrystalline diamond (PCD) segments, particularly but not exclusively for imparting a fir-tree profile into the base of a nickel alloy turbine blade.

BACKGROUND

Dressing is generally understood to mean the mechanical shaping of a rotary grinding wheel, the dressing roller being held against or applied to the working surface of the grinding wheel and producing controlled abrasion on the grinding wheel in such a way that the working surface of the grinding wheel will run perfectly true when rotating. Moreover, a defined profile can be produced in a corresponding manner on the working surface of the grinding wheel. A further objective of dressing is to produce a defined surface roughness. When a workpiece is ground, the grinding wheel is frequently intended to produce a defined roughness on the surface thereof. The degree of this roughness depends on the manner in which the dressing step on the grinding wheel was carried out.

Creep feed grinding technology is characterised by lower workpiece speeds (table speeds) and higher depths of cut. Generally, to keep heat to a minimum, extremely soft and ultra-high porosity aluminium oxide grinding wheels are used. An example is shown in Figure 1. One disadvantage is that these wheels need to be continuously dressed to maintain their form within tolerance. Electroplated diamond dressers are also typically used in creep feed grinding applications. An example is shown in Figure 2.

Dressing is critical to creep-feed grinding because it keeps the grinding wheel in an open and free-cutting condition. The way a diamond roll dresser is used has a significant impact on the quality and efficiency of the creep-feed grinding process. The diamond roll rotary speed and the rotary direction have a critical effect upon creep-feed grinding applications. A positive speed ratio (cp/ = Toresser/ Grinding) means that the dresser roll and grinding wheel are travelling in the same direction at the point of contact. For example, a qa of +0.8 indicates that the peripheral speed of the dresser is 80 per cent of the peripheral speed of the grinding wheel in the same direction. On the other hand, a negative qa implies that the dresser roll and grinding wheel are travelling in a different direction at the point of contact. When qa is between 0 and 1, this is called ‘crushing’ dressing mode or ‘sync dressing’, and when qa is negative or greater than 1, this is called ‘cutting’ dressing mode or ‘async dressing’. The relative linear velocity (RLV) is the difference between both linear speeds - see Figure 3. RLV is the sum of both linear speeds - see Figure 4.

The key dressing operating parameters used in creep feed grinding with diamond roll dressers are qa (Vorinding, Voresser), RL V(V Grinding, Voresser) and ar, where a _r is the infeed rate, as indicated in Figure 3.

Diamond grits wear through both ductile and brittle mechanisms. At low values of positive qa (high RLV), grits undergo ductile failure, and wear flats establish at the tip of the grits - see Figure 5a. A wear flat is generated by mechanical abrasion combined with a high flash of temperatures. At higher values of positive qa (low RLV), grits fail by fracturing in a brittle failure mode. The extent of failure increases with qa, from “micro-fracturing” to “macrofracturing” at larger failure scales - see Figure 5b and 5c respectively.

As qa approaches +1, the force becomes increasingly compressive and leads to large scale diamond crushing “macro-fracturing” - see Figure 5c. Negative qa and, consequently, very high values for RLV cause a severe grit dull (ductile failure), flattening the grinding wheel. This condition causes thermal damage to the workpiece.

An optimum condition is thus typically in the +0.85 and +0.5 ranges when the grinding wheel grits fail by “micro-fracturing”. Figure 6 shows a typical influence of qa on dressing and grinding powers and effective grinding wheel surface roughness for electroplated grits. Previous research in the art introduced interference angle (6) for indicating the severity of dressing - see Equation 1. This parameter describes the relationship between in-feed speed, dresser and grinding wheel peripheral speed. The research showed that a larger interference angle requires less dressing energy and a higher probability of grit fracturing. On the other hand, a smaller interference angle results in grit flattening with higher dressing energy requirements. It was also found that the interference angle affects grinding performance. It was observed that a larger interference angle reduces the grinding forces and increases workpiece surface roughness. The opposite trend was obtained with a smaller interference angle. Thus, a balance between dressing parameters is required to achieve optimised grinding performance (previously mentioned qa is in the range +0.5 to +0.85).

Equation-1 where a _r is the infeed rate, and D is the grinding wheel diameter.

The relationship between 6, qa and RLS, assuming a constant ar, is shown in Figure 7.

Polycrystalline diamond (PCD) blades represent an improvement over existing electroplated diamond roll dressers technology. PCD blades are also referred to as ‘abrasive segments’. An example of a PCD based diamond roll dresser is provided in GB2574492. PCD is an example of a superhard material (also called a superabrasive or ultra-hard material) comprising a mass of substantially inter-grown diamond grains, forming a skeletal mass defining interstices between the diamond grains. PCD material typically comprises at least about 80 vol.% of diamond and is conventionally made by subjecting an aggregated mass of diamond grains to an ultra-high pressure of greater than about 5 GPa, and temperature of at least about l,200°C, for example.

For grits sized from 20 to 100 mesh (typical values for electroplated wheels), the number of grits on the wheel surface per unit of length, Ni, is of the order of O (10°— 10 ¹ ). In the case of PCD blades, and assuming the maximum and minimum quantity of blades is within 24 and 117 which is based on previous work by the Applicant, Ni corresponds to the number of blades per unit of length and is of the order of O ( 10" ² l 0 ¹ ). The optimum value for qj is affected by many parameters, and one of them is Ni.

The wear mechanisms and, consequently, the tool life of the grinding wheels and dressers are thus associated with 8 and Ni.

Use of PCD blades requires a different strategy to electroplated grits, the incumbent technology. Although values of 8 can be adjusted to the same values used for electroplated diamond roll, the depth of cut per blade, which is an indication of cutting aggressiveness, (dr = ar/Ni), can be an order of magnitude larger. In this case, PCD dressers need to be operated differently in order to improve tool life.

It is an object of this invention to provide optimised dressing conditions for a rotary abrasive machining tool comprising a plurality of PCD abrasive segments.

STATEMENT OF INVENTION

In accordance with the invention, there is provided a method of dressing a grinding wheel, the method comprising providing a dressing roller and a grinding wheel, wherein the dressing roller comprises a hub and a plurality of polycrystalline diamond (PCD) segments mounted peripherally about the hub, each PCD segment having a pair of side surfaces extending generally radially and an end surface extending generally circumferentially between the side surfaces, the method comprising the steps of: a. rotating the dressing roller and/or the grinding wheel, and b. engaging a periphery of the dressing roller and a periphery of the grinding wheel, wherein a speed ratio qa between the dressing roller and the grinding wheel is below 0 or above +1, and wherein a rake face is located on one of said side surfaces of each PCD segment and a corresponding flank face is located on the end surface.

Preferable and/or optional features of the invention are provided in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS

A version of the invention will now be described by way of example and with reference to the accompanying drawings in which:

Figure 1 shows an example of a prior art aluminium oxide grinding wheel;

Figure 2 shows an example of a prior art electroplated diamond roll dresser used in creep feed grinding applications;

Figure 3 is a schematic indicating the conditions between a grinding wheel and a dressing roller when operating with a positive speed ratio, qa;

Figure 4 is a schematic indicating the conditions between a grinding wheel and a dressing roller when operating with a negative speed ratio, qa;

Figures 5a, 5b and 5c are high resolution images showing ductile and brittle mechanisms of grit wear, in particular with Figure 5a showing a wear flat, Figure 5b showing micro-fracturing, and Figure 5 c showing macro-fracturing;

Figure 6 schematically shows the influence of qa on dressing and grinding powers and effective grinding wheel surface roughness;

Figure 7 show two graphs indicating the relationship between 5, RLS and qa;

Figure 8 is a perspective view of an example of a rotary abrasive machining tool for use with the invention;

Figure 9 is an end view of the tool of Figure 8;

Figure 10 is a front view of the tool of Figure 8; Figure 11 is a cross-sectional view taken through the line A-A of Figure 10,

Figure 12 is an enlarged view of encircled zone D from Figure 11, in which the enlarged zone is drawn to a scale of 1.5:1;

Figure 13 is an enlarged view of encircled zone B from Figure 11, in which the enlarged zone is drawn to a scale of 1.5:1;

Figure 14 is a partial perspective view of the tool of Figure 8;

Figure 15 is a cross-sectional view through the tool of Figure 8;

Figure 16 is a close-up partial perspective view of abrasive segments mounted on the hub in the tool of Figure 8;

Figure 17 is a perspective view from the front of the hub in Figure 8;

Figure 18 is a perspective view from the rear of the hub in Figure 8;

Figure 19 is a perspective view of an individual abrasive segment from Figure 8;

Figure 20 is a side view of the abrasive segment of Figure 19;

Figure 21 is a cross-sectional view through the tool of Figure 8 when incorporating spring pins;

Figure 22 is a graph showing the relationship between a thickness (in millimetres, mm) of an individual abrasive segment and the quantity of abrasive segments (referred to in the graph as ‘blades’) required; Figure 23a is a schematic drawing indicating the flank and rake faces of the abrasive segment when operating in crushing mode, and Figure 23b is a graph depicting the corresponding value of qa;

Figure 24a is an image showing spalling at the top of the PCD blade when operating in crushing mode and Figure 24b is a schematic indicating the direction of the associated velocity vector;

Figure 25a is a schematic drawing indicating the flank and rake faces of the abrasive segment when operating in cutting mode, and Figure 25b is a graph depicting the corresponding value of qa;

Figure 26a is an image showing wear at the top of the PCD blade when operating in cutting mode and Figure 26b is a schematic indicating the direction of the associated velocity vector;

Figures 27a and Figure 27b are schematic drawings indicating how the interference angle (5) can be modified by changing qa and a _r ;

Figure 28 is a schematic comparing average grits protrusion (h), a characteristic of electroplated diamond dressers, with PCD blades;

Figure 29 is a heat map showing the relationship between 8 = f(ar, qd) and RLV for an early single PCD blade test; and

Figure 30 is a heat map showing the varying levels of surface roughness when the PCD blade is operating in cutting mode, and within the preferred ranges for ar and RLV, i.e. where a _r is in the range of 0.001 to 0.006 pm/rev and RLV is in the range of 4 to 20 m/s.

DETAILED DESCRIPTION

Example rotary abrasive machining tool

Referring to Figures 8 to 21, a rotary abrasive machining tool is indicated generally at 100. For the purpose of the invention, the rotary abrasive machining tool 100 is configured as a dressing roller. The rotary abrasive machining tool 100 comprises a hub 102 with a plurality of axially extending radial slots 104 in an outer circumference thereof, and a plurality of abrasive segments 106 located in the radial slots. Each abrasive segment has a body 108 for mounting the abrasive segment in the hub and further comprises an abrading edge 110. Each abrasive segment is individually secured to the hub using a pin element 112 that extends at least partially through the abrasive segment and/or at least partially through the hub adjacent the abrasive segment.

The pin element extends axially, partially through the abrasive segment and partially through the hub adjacent the abrasive segment.

The hub is annular with a central aperture 114 for mounting onto the rotatable shaft of a rotary dressing machine (not shown). The general shape of the hub is akin to a pipe flange, in that it has a ring portion 116 and a raised surface 118 to one side, best seen in Figure 15. The hub comprises opposing first and second major axial surfaces 120, 122 - see Figures 17 and 18. An outer circumferential surface 124, which connects the first and second major axial surfaces, generally tapers radially inwardly from one side to the other.

The slots extend axially between the first and second major axial surfaces. The slots also extend radially into the hub, thereby defining a series of supports 126 between the slots. For each slot, there is an adjacent support. Each support is generally L-shaped with a first support leg portion 128 that extends radially and a second support leg portion 130 that extends axially. The first support leg portion is shorter than the second support leg portion. The first support leg portion is located adjacent to the first major axial surface and the second support leg portion terminates at the second major axial surface.

A first pin recess 132 (see Figure 14) for partially receiving the pin element extends along the longitudinal extent of each support. The first pin recess has a semi-circular lateral cross-section and is intended to become complete, i.e. fully circular, when aligned with another pin recess having a semi-circular lateral cross-section. This is explained in further detail below. Each abrasive segment is also generally L-shaped, best seen in Figure 19. As such, the abrasive segment comprises a first segment leg portion 134 extending from a second segment leg portion 136. The first segment leg portion is shorter than the second segment leg portion. The first segment leg portion extends at an angle X to the second segment leg portion, and angle X is in the range of 75 to 100 degrees. Angle X is measured between outer surfaces of the first and second segment leg portions, as indicated in Figure 20. Preferably, angle X is around 80 degrees.

The L-shaped configuration makes the resulting rotary abrasive machining tool particularly suitable for machining fir-tree profiles. The L-shape helps to minimise the volume of material required in the abrasive segment for the machining operation. This is especially important when more expensive superhard materials such as PCD are required for maximum wear resistance and prolonged service life.

Each abrasive segment is inserted into a slot, in between two supports. Once in its final position, the first segment leg portion aligns with the first support leg portion of the hub, and the second segment leg portion aligns with the second support leg portion. The L-shaped configuration of the supports helps to minimise the mass of the hub, providing support only where it is needed.

As shown in Figures 19 and 20, the abrasive segment further comprises a nesting surface 138 intermediate the outer surfaces of the first and second segment leg portions. The nesting surface is important for maximising the quantity of abrasive segments that can be extracted from a blank 140 during manufacturing. Typically, the blank is a circular disc of abrasive material, such as PCD backed with a carbide layer. By incorporating a nesting surface, when determining the appropriate nesting configuration, the quantity of abrasive segments that can be overlaid on to the blank is increased compared to overlaying abrasive segments that do not have a nesting surface. The nesting surface extends at an angle Y in the range of 30 to 50 degrees from the outer surface of the second segment leg portion, as indicated in Figure 20. Preferably, angle Y is around 45 degrees.

In the hub of Figures 8 to 16, the quantity of slots and the corresponding quantity of abrasive segments is 80. This quantity had been determined by taking into account factors such as the target quantity of wheels to be machined by the tool, the rotational speed and the feed rate. There are also geometrical constraints to be consider such as the minimum spacing between abrasive segments (e.g. 15 mm) and/or the radial thickness of the supports (e.g. 0.75 mm).

The quantity of abrasive segments required is related to the total thickness, 1, of each abrasive segment and the diameter, D, of the hub. From experiments, the relationship between the quantity of abrasive segments, the thickness of the abrasive segments and the diameter of the hub has been captured empirically and can be defined by the two equations below: nD

^{Max = ,nt} w nD M ⁱⁿ = ^lnt lsf

In practice, where the hub is tapered (as in the first example), the diameter used is actually the diameter measured to the minimum height of the profiled abrading edge. For hubs that do not taper, the diameter dimension is much simpler to identify.

For example, in the graph of Figure 22, where 1 = 1 mm and D = 150 mm, the quantity of abrasive segments required on the hub falls between the maximum, indicated at line L _max , and the minimum, indicated at line L _m in. It is possible to use quantity of abrasive segments outside of these two lines L _m in and L _max , but at some point it will comprise life and the number of wheels that can be machined by the tool.

For completeness, the total thickness of the abrasive segment in the example rotary abrasive machining tool is around 3 mm and the diameter of the hub is around 140 mm. This gives a working range for the quantity of abrasive segments that may be used as 24 to 117, in which 80 was selected by way of example. Preferably, the thickness of the abrasive segment is in the range of 1 to 4 mm.

A second pin recess 142 having a semi-circular lateral cross-section extends along the longitudinal extent of the abrasive segment. In the aforementioned final position, the second pin recess of the abrasive segment aligns with the first pin recess of the adjacent support, and together form a hole 144 with a circular lateral cross-section. When the pin element is inserted into this hole, it secures the abrasive element within the slot - see Figure 21. The abrasive element can be removed from the hub simply by withdrawing the pin element.

The pin element is a spring pin 146 (also known as a slotted spring tension pin) and is made from, e.g. galvanised spring steel. The spring pin is elongate and comprises a single coil 150 with an open gap 152 in an uncompressed state. When compressed, as occurs when the spring pin is driven into the hole created by the aligned first and second pin recesses, the spring pin reduces in diameter and due to its inherent spring bias urges to try and regain its uncompressed state. By this behaviour, the spring pin acts as a fastener between the abrasive segment and the hub. In the compressed state, the gap in the spring pin is aligned with surfaces of the abrasive segment and the support.

Referring again briefly to Figure 20, the abrading edge forms part of the second segment leg portion. In the final position, the abrading edge protrudes radially past the second support leg portion in order to function as intended. The abrading edge has a profile that is shaped into the second segment leg portion, for example, using laser machining. Since this profiling operation preferably takes place once the abrasive segments are located in-situ within their respective slots, as described in GB 2574492, a typical outline of the abrasive segment before and after profiling is shown at P and Q respectively. Outline P is essentially artificial and phantom, depicting the outline at a specific point in time. Ultimately, outline Q is (one of) the desired profile bestowed onto e.g. a wheel. In practice, the desired profile may be shaped into the abrading edge at any depth between lines P and Q, since the initial profile may be subsequently repeated at a lower depth during a reprofiling situation. Thus, the depth of abrasive material between lines P and Q may also be considered as a resharpening allowance.

A flange 154, also known as a backing plate, is mounted co-axially onto the hub, against the first major axial surface - see Figure 15. The flange is secured in place using a plurality of screws 156 and threaded holes 158 provided in the hub, spaced apart from the abrasive segments. The flange helps prevents axial movement of the abrasive segments in extreme operating conditions. Optionally, the flange is an annular plate with a patterned surface (not shown). The patterned surface on or in the flange engages with a corresponding pattern on the hub in a mating arrangement. The cooperating paterns minimise relative rotation between the hub and the flange. Typically, the patern is a series of recesses and/or protrusions. An example is shown in Figure 18, in which the patern includes a pair of inscribed arcuate recesses 160. The rotary abrasive machining tool may be configured as a grinding wheel, a rotary dressing tool or any other similar form of machining tool. As mentioned previously, the rotary abrasive machining tool is particularly useful for the dressing of grinding wheels having profiles of complex geometry, such as fir-tree profiles.

PCD Blade Operating Conditions

The flank and the rake face of the tool are different, depending on the value of qa - see Figures 23 and 25. The cuting mode (i.e. qa < 0 and qa > 1) is ideal for PCD dressers (see Figures 25 and 26). Due to the velocity vector direction and the rake face at the top of the blade, severe spalling occurs on the flank face in crushing mode (i.e. 0 < qa < 1). Consequently, this condition should be avoided for PCD dressers.

Based on Eq-1, the interference angle (5) can be modified by changing c and ci _r . Thus, if the same value for 8 is required for PCD dresser (when compared with electroplated wheels) qa and ci _r can be adjusted (see Figure 27). In the case of electroplated diamond dresser, the values for ci _r are constrained (typically 0.5 - 1 pm/rev) by the average grits protrusion (A) - see Figure 28. In the case of PCD dresser, however, any possible value for ci _r can be considered (e.g. 1 - 10 pm/rev) due to the defined edge characteristics of PCD cuters.

The wear rate of PCD dressers is a function of several parameters 5, RLV and Ni.

For a single blade test, the relationship between 8 = f(ar, qd) and RLV is shown in Fig 27.

For any given quantity of PCD segments intended for use on the rotary abrasive machining tool, heatmaps such as those shown in Figures 29 and 30 can be produced. The heatmap in Figure 29 indicates that to minimise wear on the PCD segments and therefore prolong service life, if for example the RLV is around 12 m/s, the infeed rate a _r should be kept above 0.002 mm/rev. The heatmap in Figure 30 indicates that to minimise surface roughness at the same RLV, the infeed rate ar should be kept below 0.005 rev/min. Thus, when the two heatmaps are used in conjunction with each other, the RLV and infeed rate ar are optimisable for wear rate and surface roughness, for any given quantity of PCD segments.

In a preferred embodiment, when the dressing roller has an infeed rate (a _r ) measured in pm/rev, the infeed rate (in absolute terms) per PCD segment is between one fortieth (i.e. 0.025) and one fifth (0.2), or in other words, in the range of 0.2 to 0.025. As a first example, if the quantity of PCD segments is 80, and the infeed rate is 3 pm/rev, the infeed rate per PCD segment would be 3/80 = 0.0375. As a second example, if the quantity of PCD segments is 30, and the infeed rate is 6 pm/rev, the infeed rate per PCD segment would be 6/30 = 0.2.

Preferably, the infeed rate (ar) is in the range of 0.001 to 0.006 mm/rev, and preferably the RLV is in the range of 10 to 30 m/s for qa < 0 and 2 to 10 m/s for qa > 1. Within these ranges, the quantity of PCD segments can be iterated and further optimised to potentially reduce the quantity of PCD segments, thereby reducing the overall cost of the rotary abrasive machining tool.

While this invention has been particularly shown and described with reference to embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.