In this article we will discuss about:- 1. Manufacture of Grinding Wheels 2. Processes of Grinding Wheels 3. Grade 4. Structure 5. Area of Contact.
Manufacture of Grinding Wheels:
(i) The abrasive particles are first crushed to powder form and passed over magnetic separators for removing iron impurities.
(ii) These are then washed with water to remove foreign elements like dust or impurity and then with chemical compound to remove grease.
(iii) The particles are then graded according to their grain size by passing through proper sieves.
(iv) Abrasive particles are then mixed with proper bonding materials (the bonding material is based on abrasive and the process used) and moulded in proper shape and then dried.
(v) After drying, it is baked (heating depends upon process used). After baking, the binding material becomes set and holds the particles together.
(vi) It is then cut and given final shape.
(vii) Lastly it is tested for cracks, leakage and balance.
Since wheel has to revolve at very high speeds cracks can’t be tolerated. For leakage etc. it is tested hydraulically or as may be possible. Finally it is balanced statically as well as dynamically. Generally static balancing is done before using it by mounting on a mandrel.
Tight control during manufacturing in terms of density control, mix flow and geometrical accuracy results in more uniform and consistent wheel. These goals are achieved by automation of wheel presses, free-flowing mixes, better size control during moulding, microprocessor control of kilns, and CNC dressing facilities.
Processes of Grinding Wheels:
1. Bonding Process:
A bond is a material that holds the abrasive grains together enabling the mixture to be kept in a desired shape in the form of wheel.
The bonds most commonly used during manufacture of grinding wheels are:
(i) Vitrified Bond (denoted by V)
(ii) Silicate Bond (denoted by S)
(iii) Shellac Bond (denoted by E)
(iv) Rubber Bond (denoted by R)
(v) Bakelite or resinoid Bond (denoted by B).
In these, vitrified, silicate, rubber and Bakelite bonds are denoted by the process and shellac bond is denoted by its property, i.e., elasticity (E) in order to avoid confusion with silicate bond.
To obtain the maximum out of the abrasive, it is important that the bond system is strong, versatile and has superior corner holding properties. The idle bond system should facilitate a uniform grain release, resulting in the wheel remaining free cutting for a longer period of time.
Extremely load resistant and free cutting bond systems having increased form holding capability result in reduction of dressing frequency. This results ill significant increase in wheel life. This also improves the parts produced per hour due to savings in dressing time and increased wheel life.
2. Moulding Process:
(i) Vitrified Process (for Vitrified Bond):
This process is used for making most of the wheels. In this process, after manufacture of abrasives all types of grains are mixed with ceramic porcelain, moulded in moulds, dried, set properly and fired at 715°C for 12 to 14 days.
(a) The grinding wheels produced by this process are very strong and porous. Due to porosity, rate of removal of metal is very high.
(b) The wheels are not affected by acids, alkalies and are chemically inert.
(a) Due to high temperature of fusion, there are greater changes of thermal distortion including cracking. Thus, the wheels produced by this process are available in different grades depending upon the degree of distortion.
(b) Due to difficulty in manufacturing, the diameter of the wheel can’t be too great and is limited to 90 cm.
(c) The wheel can’t be worked under severe working conditions, because due to forced fluctuations and self- vibrations in the machine, the wheel being brittle is likely to break.
(d) Manufacture of wheel takes a long time of the order of 30 days.
(e) It can’t be used at more than 2000 m/min for cutting purposes. (Parting off operation by abrasive grinding wheel is very fast operation and gives very good finish).
(ii) Silicate Process (for Silicate Bond):
In this process sodium silicate is mixed with abrasive grains; mixture is moulded in a mould, dried for several hours and finally baked at temperature of 270°C for about 20-80 hours.
(a) Since it is processed at a low temperature, therefore free cutting action is possible.
(b) Process is rapid, taking only a few days.
(c) Bigger wheels i.e. greater than 90 cm can also be made.
(d) The wheels prepared by this process are very efficient. This is because of the fact that binding force is not as strong as in vitrified process, and thus the abrasive particles fall off rapidly, thereby leaving no chances of any particles getting blunt. It is due to this reason that friction in grinding action will be less, and there will be less heat generation.
Due to less heat generation, these wheels are more suitable for grinding cutters, blades, knives, etc. as they will not lose their property acquired after heat treatment process. Other wheel generate lot of heat and are, therefore, not much suited for this purpose.
This can’t be used for common grinding process because of the rapid wear of wheel, for which the vitrified bond is most suitable.
(iii) Shellac Process (for Shellac Bond):
In this process the abrasive particles are coated with shellac and mixture heated to give uniform mixing and then mixture is rolled. The resulting mixture is very sticky and, therefore, can’t be moulded. The mixture is then pressed to give the desired set and finally baked at about 300°C temperature for less than few hours.
This process gives considerable elasticity to wheel and thus can be used for grinding under severe working conditions.
Wheels of bigger diameter can’t be produced.
(a) Wheels produced by this process are used as cutting or slitting wheels (> 0.80 mm thick).
(b) Wheels produced by this process are used for fine finish grinding such as cam shaft grinding, ball race grinding etc.
(c) Very good polished finish can be obtained by these wheels.
(iv) Rubber Process (for Rubber Bond):
In this case bonding material is pure rubber with some amount of sulphur which acts as vulcanisation agent. The abrasive grains are spread between rubber sheets and they are then rolled to the desired thickness and finally vulcanised. By vulcanisation, the whole mass becomes joined and acts as a solid wheel, rubber acting as bond.
(a) The wheels produced by this process are very hard and tough.
(b) Wheels as thick as 0.1 mm can be made by this process and are, therefore, most suitable for fine parting off operation. The wheels can be operated at speeds ranging from 3000 to 5000 metre/min.
(c) These are also used as snagging wheels, i.e. for the removal of outside scale, runners, risers, slags etc.
(d) These are used for centreless grinding control wheels.
(v) Bakelite and Resinoid Process (for B Bond):
In this process, the abrasive particles are powdered and mixed with synthetic resin and a liquid solvent which dissolves resin. The mixture is rolled or pressed to the desired shape and backed for a few hours at temperature of 205 to 260°C.
This bond is very hard and strong. Operating speed of 3000 to 5000 m/min can be achieved. The wheels produced by this process are used as snagging wheels (i.e. for rough grinding for removal of sand inclusions, scales, etc.). It gives a very rapid stock removal rate.
Grade of a Grinding Wheel:
The grade of a wheel indicates the strength of the grains, and the holding power of the bond. It is usually referred to as hardness of the wheel. The thickness of the bonding layer holding abrasives controls the grade of the grinding wheel.
A hard wheel wears down slowly and a soft wheel wears down readily. Hard wheel is used for precision grinding and for softer materials and also when area of contact of wheel with work is small.
The hardness of grinding wheel is classified as very soft (A to G), soft (H to K), medium (L to O), hard (P to S) and very hard (T to Z).
Structure of Grinding Wheel:
Structure of a wheel refers to the voids between abrasive particles. For a given bonding material thickness of void size is controlled by the spacing of the grains and this structure may be dense or open.
Open structure wheel (having finer grains per unit volume) are used for high stock removal and dense structured wheel for holding precision forms and profiles. The structure is represented by numbers ranging from 0 to 15, the lower numbers indicating a dense structure and higher numbers represent open structure.
Area of Contact:
If area of contact between wheel and work is more as in internal grinding operation, then stress will be less and wheel tends to act harder, and thus softer wheel should be used. This aspect becomes important in selection of a wheel for a specific operation.
If the area of contact is large, chip clearance should be more and structure open type. If area of contact is small, the many fine cutting points are required and grains should be of fine size.