Abrasive particles used for grinding wheels are of two types viz.: 1. Natural Abrasive and 2. Artificial Abrasive. Generally for most of the purposes, natural abrasives are not used due to certain advantages of artificial (manufactured) abrasive.
These are produced by uncontrolled forces of nature.
The following are the generally found and used natural abrasives:
(a) Sand stone or solid quartz
(b) Emery (50-60% crystalline A12O3 + Iron oxide)
(c) Corundum (75-90% crystalline A12O3 + Iron oxide
Efficiency of Abrasive Particles:
The efficiency of abrasive particles depends upon:
(ii) Uniformity in composition.
(iii) Hardness – Common rule about it is that hardness of abrasive should be more that of work material.
(iv) Toughness – If wheel is not tough, the abrasive particles will fracture readily and wheel wear will be excessive.
(v) Sharpness of fracture – The better cutting action is obtained by sharp edged abrasives. The natural abrasives give rounded edges and are, therefore, not efficient in cutting.
Advancements in Abrasive Particles:
There has been a significant shift in the work material type from soft towards hard- to-grind materials used for various applications. Any hard material can only be further machined or finished by a grinding process. These newer and harder materials pose a definite challenge to the grinding wheel.
All this requires significant improvement in the grinding wheels, both in terms of wheel specification as well as wheel manufacturing techniques.
Some of these improvements are:
i. Tougher abrasives that remain sharper for a longer period of time
ii. Friable abrasives that continuously re-sharpen to expose newer and sharper cutting edges
iii. Wheels that produce low vibration levels at higher speeds
iv. Tighter geometrical tolerances on wheels.
Abrasive manufacturers have developed various solutions to meet the above requirements, some of them using a combination of newer abrasives (one such product is the 86A range of products), improved bond systems and tighter manufacturing limits, resulting in a significantly superior product for precision grinding applications.
86A abrasive is a revolutionary abrasive for precision grinding applications. There are basically two families of abrasives, viz. Aluminium Oxide (AlO) and Silicon Carbide (SiC).
Aluminium oxide abrasives are used for ferrous grinding applications and Silicon Carbide for non-ferrous grinding applications. Within the family of AlO abrasives there are various types of abrasives based on its chemical composition and crystalline structure.
Monocrystalline AlO (denoted as 32A) and White Aluminium Oxide (38A) are two of the most common AlO types of abrasives. 32A is a fast cutting abrasive used for heavy duty precision grinding applications. 38A is a friable type of abrasive used for precision grinding applications.
The properties of both these abrasives were combined to obtain the revolutionary new conventional abrasive called as 86A. This grain has the ability to retain its edge for a longer period of time, thus resulting in cooler cutting and less metallurgical damage to the work surface. Also, the longer lasting cutting edge means more number of jobs between dressing thereby reducing grinding costs.
The 86A abrasive, is best suited for high productivity applications due to its high material removal rate properties. It has also proved effective on difficult to grind materials. The 86A abrasive is best suited for grinding of high speed steels, cast alloys and cutters, nickel alloys and high chrome steels. The hardness of 86A abrasives is of the order of2150—2250 knoop, whereas hardness of 38A abrasives is around 1900—2100 knoop.
A comparative performance of 86A abrasives versus 38A abrasives is shown in Figs. 20.4 (a) and (b). It is clear that the grinding ratio (defined as the ratio of volume of material removed to the volume of wheel wear) for the 86A wheel is higher over a range of material removal rates as compared to the 38A abrasive.
The specific grain toughness is higher for 86A abrasives than 38A abrasives. Also, the specific grinding energy is lower for the 86A wheel, since for the same power drawn by both the wheels, the material removed by the 86A wheel is substantially higher. In the final count the grind ability (defined as the ratio of grinding ratio and the specific grinding ratio) of the 86A wheels is higher than that of the 38A wheels.
Some of the applications where the 86A abrasives have shown significant superiority are tool room grinding, surface grinding, gear grinding, internal grinding, bearing race grinding and other form grinding applications.
Artificial or Manufactured Abrasive:
The quality and composition of these particles can be easily controlled and their efficiency is far better than that of natural abrasives.
Most commonly used manufactured abrasives are:
(a) Silicon Carbide (SiC):
It is available in variety of colours. A special variety of bluish green is very suitable for grinding tip tools. The trade names of it are ‘Carborandum’, ‘Crystolon’. ‘Electron’ etc.
(b) Aluminium Oxide (Al2O3):
The trade names for fused aluminium oxide are ‘Aloxite’; ‘Alundum’ and ‘Borolon’. Its special form is white Al203 which when pure, looks like brilliant white crystal. It is most suitable for tool steels where heat generation due to grinding is low.
(c) Boron Carbide.
(d) Boron Nitrite (CBN):
CBN grinding wheels are used to grind hardened and difficult to grind steels. These have long life and high grinding ratios. Temperature Encountered in grinding is much less and hence much better finish and quality of surface.
Manufacture of Artificial Abrasives:
(a) Silicon Carbide:
In its manufacture, the following ingredients are thoroughly mixed and heated in electric furnace at about 2320°C for around 36 hours. Then the whole of solid mass is crushed, washed and treated with alkalies.
It is again washed and finally ground in small particles. These are then sieved in different mesh number sieves. For fine grinding, particles (180—200 mesh) are taken.
The various ingredients are:
(i) Silca sand — 25 parts
(ii) Petroleum coke obtained by non-destructive distillation (of very pure form) — 34 parts
(iii) Common salt — 2 parts
(iv) Saw dust (powder from wood) — 12 parts
Out of these silica sand gives silicon, petroleum coke supplies carbon and saw dust burns at high temperature to give porous structure. It is usually of greenish black colour.
(b) Aluminium Oxide (Al2O3):
It is manufactured by fusing mineral bauxite (Hydrated Al2O3 + Si2O3 + titanium oxide) mixed with ground coke and iron scraps. This is fused in electric furnace and after fusion is complete, it is crushed, washed, treated with alkalies, washed again and finally ground and graded. It is of reddish brown colour, and is tough and sharp which has tendency to fracture easily and thus used for grinding tool steels.
Comparison of SiC andAl2O3 (i) Al2O3 is more tough than SiC but its hardness is less. Al2O3 is, therefore very suitable for grinding materials of high tensile strength (2700 kgf/cm2). If Al2O3 be used with material of low tensile strength, then resistance offered by the material on grinding wheel will be less.
In such case, abrasive particles do not fall off from the wheel and keep on getting blunt, so grinding will be poor. Hence all tough materials like high speed steel, tough bronze and copper must be grounded by Al2O3 and not others.
(ii) SiC is more hard and more brittle. If, it be used with materials of high tensile strength, then more resistance will be offered by job on wheel and the abrasive particles fall off rapidly due to quick fractures producing a more rapid self-sharpening.
It is mainly used with materials of low tensile strength as cast iron, carbide tips, tungsten tool and all non-metallic materials like marble stone, hard rubber, plastic, leather etc. With these materials, tougher Al2O3 particles under-go less fractures. Scratching hardness of SiC particle is more. A special form of SiC is used to grind and resharpen cemented carbide tipped tools.