GB2504824A

GB2504824A - Aligning abrasive grains to form tools

Info

Publication number: GB2504824A
Application number: GB1310189.4A
Authority: GB
Inventors: John James Barry; Cormac Lee; David Patrick Egan
Original assignee: Element Six Ltd
Current assignee: Element Six Ltd
Priority date: 2012-06-11
Filing date: 2013-06-07
Publication date: 2014-02-12
Also published as: GB201210230D0; WO2013186146A1; GB201310189D0

Abstract

A plurality of abrasive grains 10, each having an aspect ratio substantially greater than 1, are dispersed in a binder material and precursor material from which the a matrix material can be formed, to provide a paste body in which the abrasive grains are substantially randomly oriented, the paste body is then extended along two mutually orthogonal dimensions to provide a precursor structure in which the abrasive grains 10 are substantially non randomly oriented with respect to each other and the binder material is at least partly removed or transformed and the precursor structure is treated to transform the precursor material into matrix material 20 thus providing a tool element. The abrasive grains 10 may be diamond or cubic boron nitride with a means size between 20 and 5000 microns. The abrasive grains 10 may be plate-like or may have a length along a first axis substantially greater than its length along a second axis orthogonal to the first. The paste body may be extended by rolling, pressing or stretching into a sheet or wafer which may then be folded or stacked and then worked or flattened.

Description

METHOD FOR MAKING TOOL ELEMENTS AND TOOLS COMPRISING SAME

This disclosure relates generally to tool elements comprising abrasive grains, particularly but not exclusively super-abrasive grains such as diamond or cubic boron nitride grains, and tool components and tools comprising same.

United States patent application publication number 20100190423 discloses super-abrasive tools having oriented super-abrasive particles held to a substrate by a thin braze layer and related methods. A method for orienting super-abrasive particles in a tool is disclosed. Such a method can include providing a plurality of super-abrasive particles having a preselected average size, preselecting a thickness for an amorphous braze layer to be applied to a substrate, wherein the thickness is based on the average size of the plurality of super-abrasive particles, and applying an amorphous braze layer to the substrate at the preselected thickness.

Japanese patent number 3075288 discloses a method of making a grinding wheel, the method including rolling a billet body comprising whisker fibres and binder material under a constant pressure to form a rolled bar.

There is a need for cutting tool elements comprising abrasive grains and having enhanced performance characteristics, as well as for methods for providing same.

Viewed from one aspect there is provided a method of making a tool element comprising a plurality of abrasive grains dispersed in a matrix material, each abrasive grain having an aspect ratio substantially greater than 1, in which the abrasive grains are non-randomly oriented with respect to one another; the method including: combining the abrasive grains with binder material and precursor material from which the matrix material can be formed, to provide a paste body in which the abrasive grains are substantially randomly oriented; extending the paste body along at least one dimension to provide a precursor structure in which the abrasive grains are substantially non-randomly oriented with respect to each other; at least partly removing or transforming binder material and treating the precursor structure to transform the precursor material into matrix material to provide the tool element.

The characteristics of the paste comprised in the paste body! such as the viscosity or consistency, should be such that the process of extending the paste body will tend to result in the abrasive grains comprised therein becoming more aligned with respect to the direction of extension and consequently with respect to each other. The characteristics of the paste may be selected or controlled such that sedimentation (i.e. settling) of the abrasive grains is retarded sufficiently so that the abrasive grains do not substantially drift downwards under gravity during the manufacturing process between the time the paste is prepared and the time of treating the precursor structure to provide the tool element. The characteristics of the paste may also be selected or controlled such that the abrasive grains tend to remain paced apart from each other within the paste, thus tending to promote uniform distribution of the abrasive grains in the paste. This may be achieved by selecting the type and content of the binder material or combination of binder materials.

Variations of the method are envisaged by this disclosure, of which the following are non-limiting and non-exhaustive examples.

The method may include extending the paste body along two mutually orthogonal dimensions. In other words, the method may include extending the paste body along two orthogonal axes, in two orthogonal directions to flatten it. The paste body may be extended along two orthogonal directions simultaneously or sequentially. The method may include reducing the dimension of the paste body along a first axis (in other words, reducing its thickness) whilst simultaneously extending it along two mutually orthogonal axes, each being orthogonal (perpendicular) to the first axis.

The method may include flattening the paste body whilst increasing its length and breadth. In some examples, the paste body may be extended by working to the paste body, by rolling or compacting for example, to reduce a first dimension of its size (thickness, for example) whilst allowing the paste body to become extended in other dimensions (breadth and width, or diameter for example). For example, the paste body may be substantially unconstrained in dimensions, other than the first dimension, at when the first dimension is being reduced, at least up to a point. This may be described as substantially unconstrained flattening of the paste body, at least until desired dimensions are achieved. In some examples, the extended paste body may be substantially circular or polyhedral.

The method may include repeating the process of extending the precursor structure at least once, which may have the effect of increasing the alignment of the abrasive grains with respect to each other.

The matrix material may be bond material for grinding wheels, such as resin bond, vitrified bond or metal bond material, of a combination of any of these.

The method may include treating the precursor structure by heating and or curing it and hardening it.

The abrasive grains may comprise super-abrasive material such as cubic boron nitride (CBN) or diamond, or ceramic materials such as silicon nitride, silicon carbide (SiC), alumina (A1203) or alumina-based ceramic materials.

The abrasive grains may have a mean size of at least about 20 microns or at least about 50 microns. The abrasive grains may have a mean size of at most about 5,000 microns, at most about 1,000 microns or at most about 500 microns.

In some examples the abrasive grains may be generally plate-like or wafer-like. The abrasive grains may have a mean thickness of at least about 1 micron or at least about 10 microns; and or at most about 500 microns, at most about 50 microns or at most about 20 microns.

In some examples, most of the abrasive grains may be generally plate-like and the method may include rolling or pressing the paste body to provide the precursor structure. For example, at least 50 per cent, at least 70 per cent or at least 90 per cent of the abrasive grains may be plate-like.

In some examples the abrasive grains may be generally elongate or acicular. The abrasive grains may have a maximum length of at least about 50 microns, at least about 100 microns or at least about 1,000 microns.

Each abrasive grain may have a first length dimension along a first axis, a second length dimension along a second axis and a third length dimension along a third axis, the first, second and third axes being orthogonal to each other; in which the first length dimension may be substantially smaller or substantially greater than the second length dimension and or the third length dimension. In some examples, the first length dimension may be the thickness of a substantially plate-like (also referred to as flat) abrasive grain, and the second and third length dimensions may be the length and breadth of the abrasive grain, the first length dimension being substantially smaller than both the second and the third length dimensions. In some examples, the first length dimension may be maximum length of an elongate or acicular abrasive grain and the second and third length dimensions may be substantially smaller than the first length dimension.

In some examples, the first length dimension may be substantially greater than the second length dimension and the method may include extending the paste body until the first axes of at least about 50 per cent of the abrasive grains are oriented within about 45 degrees of each other.

In some examples, most (more than 50 per cent) of the abrasive grains may be generally plate-like and the method may include rolling or pressing the paste body to provide the precursor structure.

In some exampled, most (more than 50 per cent) of the abrasive grains may be generally elongate and the method may include extruding the paste body to provide the precursor structure.

The method may include extending the paste body by means of rolling, stretching.

extruding, press-forming or moulding to provide the precursor structure. The method may include extending the paste body by flattening it to provide a substantially flattened precursor structure, which may be in the form of a sheet (the term "flattened" includes arrangements in which the precursor structure lies substantially on a plane, is coiled, folded! curved and so forth; it is not intended to limit the arrangement to being planar).

Various example methods may include processing the paste body in various ways.

For example, the method may include flattening the paste body to provide a precursor structure in the form of a sheet; the method may include flattening the paste body to provide a sheet, disposing regions of the sheet on top of one another to provide a stacked construction and flattening the stacked construction to provide the precursor structure in sheet form; the method may include folding the sheet to form the stacked construction; and or the method may include providing a plurality of the sheets and stacking them on top of one another to form the stacked construction.

In some examples the thickness of the stacked construction may be at least about 5 millimetres. In some examples, the precursor structure may be cut up and stacked or folded and then rolled again, which process may be repeated several times in order to increase the degree of grain alignment in the direction of the rolling and consequently with respect to each other and provide a precursor sheet.

In some examples, a flattened precursor sheet or a plurality of precursor sheets may be arranged as a multi-layered stack having a desired thickness, depending on the type of tool element to be made. For example, the stacked structure may have an overall thickness of at least about 5 millimetres and at most about 15 millimetres.

Precursor structures (which may also be referred as green bodies) may be cut from the stacked structure to provide precursor segments. In some examples, each precursor segment may have a length of at least about 20 millimetres, width of at least about 5 millimetres and thickness of at least about 1 millimetre. In a particular example, the longest dimension of the precursor structure may lie substantially in the plane defined by the averaged axis along which the longest length dimension of the constituent abrasive grains lie. In some other examples, relatively thin layers of the precursor sheets may be stacked on top of each other to provide a stacked construction having a height such that the longest dimension of the precursor structure lies in a plane substantially normal to the plane defined by the averaged axis along which the longest dimensions of the constituent abrasive particles lie.

The precursor structure may be formed according to the intended final shape of the tool element. For example, if the tool element is for a grinding wheel then the precursor structure may be curved somewhat, being provided with a curved surface for matching the curve of the grinding wheel hub prior to the step of curing the green body. The precursor structure may then be cured to provide a segment by subjecting it to a controlled heating and cooling cycle, avoiding substantial deleterious effects that may potentially arise from rapid thermal expansion or cooling, such cracking and distortion, which may occur in some examples at or above about 900 degrees centigrade. The precise curing conditions and cycle will likely be different for different examples, depending at least on the type and content of the precursor materials comprised in the precursor structure.

In some examples, the method may include combining the tool element with at least one other element to provide a tool or a component for a tool. The method may include configuring the precursor structure to be suitable for attachment, abutment or coupling to a component of a tool such as a hub, carrier body or substrate. For example, the method may include configuring the precursor structure to be suitable for a grinding wheel and joining the tool element to a hub of a grinding wheel. In one example, the abrasive grains may comprise CBN material, the precursor material may be suitable for transforming into binder material for a vitrified bond CBN grinding wheel.

The method may include providing a plurality of abrasive grains having various shapes and separating abrasive grains into a plurality of populations, each population having a different average shape in terms of a shape characteristic, such as aspect ratio, acicularity, flatness and so forth.

In some examples, the method may include shape sorting abrasive grains by means of a shape sorting table, in which a slightly canted table is vibrated according to a suitable wave form. The abrasive grains may be fed onto the table at a proximate end and allowed to migrate to the opposite distal end under the effect of the vibrational movement. Owing to the canted disposition of the table surface, the abrasive grains also may also tend to migrate laterally on the table surface, according to the shape of the grains. For example, flatter abrasive grains and blocky grains tending are likely to tend to travel in opposite directions, with blockier abrasive grains tending to roll downward and flatter abrasive grains tending to travel upwards as a result of the characteristics of the vibrational cycle. Thus, as the abrasive grains travel further away from the point at which they are fed onto the table they are likely to tend to diverge laterally according to their flatness. By the time the grains reach the distal end of the table, a substantial degree of divergence is likely to have occurred (depending on various factors such as the configuration of the table and the operating conditions used) according to the distribution of shapes of the abrasive grains, and the abrasive grains may be collected by containers located along the end of the table, each container collecting grains having a shape falling within a range.

Thus, several batches of abrasive grains having different degrees of flatness, blockiness, acicularity and or other shape characteristics can be obtained. In particular, abrasive grains having the shape characteristic that the length along one axis is at least 1.5 times the lengths along the other two orthogonal axes.

In some examples, the abrasive grains of one or more of the populations may be combined with the binder material and precursor material. For example, a population comprising substantially acicular abrasive grains may be selected and grains from that population used to make the paste body. In another example, a population comprising grains having a plate-like shape may be used. In some examples, respective populations comprising acicular and plate-like abrasive grains may be selected abrasive grains from each may be combined with the binder material and precursor material to provide the paste body.

In some examples, the method may include combining a plurality of precursor structures, at least a first and a second precursor structure differing substantially in at least one characteristic to provide a precursor construction, and treating the precursor construction to provide the tool element. For example, the precursor structures may be in the form of sheets or wafers and the method includes stacking the sheets or wafers on top of each other to provide the precursor construction. The first and second precursor structures may differ substantially in a characteristic selected from the mean shape or crystal habit of the respective abrasive grains, the content of the respective abrasive grains within the paste (for example the volume or weight per cent of the grains in the paste), the mean strength of the respective abrasive grains, the internal crystal structure of the respective abrasive grains or an aspect of the defects present in the respective abrasive grains.

In some examples, the method may include providing a plurality of extended paste bodies and stacking them on top of one another to form a stacked construction, and working the stacked construction to provide the precursor structure. For example, working may include compacting the stacked construction to reduce its thickness.

While wishing not to be bound by a particular theory, sufficiently extending a paste containing a plurality of randomly oriented grains having sufficiently high aspect ratio is likely to impose a rotational moment on any grains not already generally aligned with the direction of extension of the paste. In general and all else being equal, the greater the degree by which a grain is misaligned with the axis along which the paste is extended, the greater the angle through which the grain is likely to be rotated by the paste as it (the paste) moves responsive to the extension action. The degree of rotation and alignment of the grain responsive to the extension of the paste is likely to depend on various factors such as the aspect ratio of the grain, particular aspects of the shape of the grain, the viscosity of the paste and the adherence of the paste to the surface of the grain. Other factors associated with the process are also likely to affect the degree of alignment of the grains in the precursor structure. For example, the greater the extension of the paste or the more times an extension step is repeated, the greater the degree of alignment that may be expected. The method or methods used to perform the extension of the paste may also affect the degree of alignment of the grains. For example, the aspect ratio of the grains may be selected depending on the viscosity or other characteristics of the paste and in some examples, or the process used to extend the paste, and in some example an aspect ratio of at least about 1.2 may be sufficient while in other examples the aspect ratio may need to be higher.

Viewed from a second aspect there is provided a component for an abrasive tool complising a tool element made using a method according to this disclosure. For example, the component may be a segment for a saw, grinding or cutting tool.

Non-limiting example methods and arrangements will be described below with reference to the accompanying drawings, of which Fig. 1 shows schematic drawings of three example abrasive grains having three different archetypal shapes, with orthogonal axes x, y and z superimposed on the drawings; and Fig. 2 shows a cross section through an example tool element.

The shape of the grains may be characterised in terms of their acicularity or flatness, for example. In principle, the shapes may be described with reference to ratios of pairs of three orthogonal axes, x, y and z as illustrated in Fig. 1. For example the ratios x/y, xlz and ylz may be used to characterise the shapes of the grains semi-quantitatively as follows: As illustrated by the schematic diagram A in fig. 1, grains for which the dimensions along all of the three axes are not substantially different from each other may be characterised as "blocky". There would be no axis through the grain for which the aspect ratio would be relatively much greater than (or less than) 1.

As illustrated by the schematic diagram B in fig. 1, grains for which the dimensions along two of the three axes are not substantially different from each other, but for which the dimension along one of the axes is substantially greater than those of the other two axes may be characterised as "elongate' or acicular. There would one axis through the grain for which the aspect ratio with respect to two other orthogonal axes would be relatively very much greater than 1.

As illustrated by the schematic diagram C in fig. 1, grains for which the dimension along one of the three axes is substantially less than those along the other two axes may be characterised as "flat".

In one example, a plurality of substantially flat CBN grains having size falling within the U.S. Mesh size band of 120/140 were provided as follows. A plurality of CBN grains having a range of sizes and shapes may be provided and sieved using a stack of U.S. Mesh sieves, and the plurality of grains remaining within the 120/1 40 band may be isolated from the rest. These grains may then be shape sorted by means of a shape sorting table. Thus, several batches of grains having different degrees of flatness and blockiness shapes can be obtained. In particular, grains having the shape characteristic that the length along one axis is at least 1.5 times the lengths along the other two orthogonal axes.

An example tool element was made as follows. CBN grains within the U.S. Mesh size range of 120/140 were sorted according to shape as described above to provide a plurality of the grains having substantially plate-like shapes of the kind illustrated in drawing C of Fig. 1. The selected CBN grains were blended with powders suitable as precursor material for a vitrified bond to form a powder blend. In this particular example, the CBN grains were combined with the following powders (the weight per cent of which in the blend -excluding the grains -are indicated in parentheses): Sic2 (61.4 weight per cent), A1203 (17.0 weight per cent), B203 (10.1 weight per cent), Fe203 (0.4 weight per cent), CaO (3.2 weight per cent), MgO (0.1 weight per cent) and Na20 (3.1 weight per cent); see for example the disclosure of matrix material by Jackson M.J. and Mills B., (2000), Materials Selection Applied to Vitrified Alumina and cBN Grinding Wheels, Journal of Materials Processing Technology, Volume 108, pagesll4tol2l.

Binder and plasticiser material for forming a paste capable of being extruded or pressed to form a green body was introduced into the powder blend. In this particular example, the following materials were introduced into the powder blend (the weight per cent of which are indicated in parentheses): cellulose powder (about 4.5 weight per cent of the combined mass of the powder blend and additives) as bindei material, polyethylene glycol (PEG, about one third of the weight of the cellulose) and de-ionised water (about 75 millilitres per kilogram of the powder and CBN grains combined). The resultant paste had the general consistency of dough.

The paste was flattened to form a sheet having thickness in the range from about 10 millimetres to about 20 millimetres and then rolled to form thinner sheets having thickness in the range from about 1 millimetre to about 3 millimetres. A section was cut from the sheet and subjected to heat treatment to sinter the binder material and produce an example element. Fig. 2 shows a cross section through the example element after sintering, in which the plate-like abrasive grains 10 are dispersed within the matrix 20 and are generally aligned with each other.

Various concepts and terms as used herein will be briefly explained.

The aspect ratio of a body is the ratio of the maximum length dimension of the body defining a first axis to the minimum length dimension of the body along a second axis perpendicular to the first axis.

The grain size of abrasive grains may be expressed in terms of U.S. Mesh size, in which two mesh sizes are provided, the first being a mesh size through which the grains would pass and the second being a mesh size through which the grains would not pass. Mesh size may be expressed in terms of the number of openings per (linear) inch of mesh.

Claims

CLAIMS1. A method of making a tool element comprising a plurality of abrasive grains dispersed in a matrix material, each abrasive grain having an aspect ratio substantially greater than 1, in which the abrasive grains are non-randomly oriented with respect to one another; the method including: combining the abrasive grains with binder material and precursor material from which the matrix material can be formed, to provide a paste body in which the abrasive grains are substantially randomly oriented; extending the paste body along two mutually orthogonal dimensions to provide a precursor structure in which the abrasive grains are substantially non-randomly oriented with respect to each other; at least partly removing or transforming binder material and treating the precursor structure to transform the precursor material into matrix material and providing the tool element.
2. A method as claimed in claim 1, in which the abrasive grains comprise cubic boron nitride or diamond material.
3. A method as claimed in claim 1 or claim 2, in which the abrasive grains have a mean size of at least about 20 microns.
4. A method as claimed in any of the preceding claims, in which the abrasive grains have a mean size of at most about 5,000 microns.
5. A method as claimed in any of the preceding claims, including extending the paste body by means of rolling or stretching to provide the precursor structure.
6. A method as claimed in any of the preceding claims, in which most of the abrasive grains are generally plate-like and the method includes rolling or pressing the paste body to provide the precursor structure.
7. A method as claimed in any of the preceding claims, including flattening the paste body to provide a precursor structure in the form of a sheet.
8. A method as claimed in any of the preceding claims! including flattening the paste body to provide a sheet, disposing regions of the sheet on top of one another to provide a stacked construction and flattening the stacked construction to provide the precursor structure in sheet form.
9. A method as claimed in claim 8, including folding the sheet to form the stacked construction.
10. A method as claimed in any of the preceding claims, including providing a plurality of extended paste bodies and stacking them on top of one another to form a stacked construction, and working the stacked construction to provide the precursor structure.
11. A method as claimed in any of the preceding claims, including combining the tool element with at least one other element to provide a tool or a component for a tool.
12. A method as claimed in any of the preceding claims, including configuring the precursor structure to be suitable for attachment, abutment or coupling to a component of a tool.
13. A method as claimed in any of the preceding claims, including configuring the precursor structure to be suitable for a grinding wheel and joining the tool element to a hub of a grinding wheel.
14. A method as claimed in any of the preceding claims, including providing a plurality of abrasive grains having various shapes, separating the abrasive grains into a plurality of populations, each population having a different average shape in terms of a shape characteristic.
15.A method as claimed in claim 14, in which the abrasive grains of one of the populations are combined with the binder material and precursor material.
16. A method as claimed in any of the preceding claims, in which each abrasive grain has a first length along a first axis and a second length along a second axis, the first and second axes being orthogonal to each other, the first length being substantially greater than the second length; the method including extending the paste body until the first axes of at least about 50 per cent of the abrasive grains are oriented within about 45 degrees of each other.
17. A method as claimed in any of the preceding claims, including combining a plurality of precursor structures, at least a first and a second precursor structure differing substantially in at least one characteristic to provide a precursor construction, and treating the precursor construction to provide the tool element.
18. A method as claimed in claim 17, in which the precursor structures are in the form of sheets or wafers and the method includes stacking the sheets or wafers on top of each other to provide the precursor construction.
19. A method as claimed in claim 17 or 18, in which the first and second precursor structures differ substantially in a characteristic selected from the mean shape or crystal habit of the respective abrasive grains, the content of the respective abrasive grains within the paste, the mean strength of the respective abrasive grains, an aspect of the internal crystal structure of the respective abrasive grains or an aspect of the defects present in the respective abrasive grains.
20. A component for an abrasive tool comprising a tool element made using a method as claimed in any of the preceding claims.
21. A component as claimed in claim 20, in which the component is a segment for a saw, grinding or cutting tool.