In this article we will discuss about the factors to be considered while selecting grinding wheels.
Selection of Grinding Wheels:
The proper selection of grinding wheels is very important for getting good results (i.e. obtaining better finish and at the same time having more life of the wheel). In order to meet all these requirements, the various elements that influence the process must be considered.
Selection mainly depends upon the following factors:
(a) Constant factors.
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(b) Variable factors.
Constant factors include:
(i) Work material. It should be remembered that for grinding a soft material, hard wheel should be used and vice versa,
(ii) Amount and rate of stock removal,
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(iii) Area of contact between work and wheel.
(iv) Condition of grinding machine. A softer grade of wheel is used on robust and heavy machine,
(v) Finish and accuracy required on the job.
Variable factors include:
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(i) Wheel speed,
(ii) Work speed,
(iii) Condition of grinding machine (state of the wheel spindle bearing),
(iv) Skill of operator (personal factors).
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From above it is obvious that several factors are to be considered for the proper selection of the right wheel. The different wheels are constituted by different combinations of abrasive materials, grain size, type of bond, hardness of bond, structure etc. Thus the difficulty in choosing right wheel for any particular job can be gauged from the fact that more than 10,000 different combinations are obtainable in one wheel.
Work Material:
It will influence the following elements:
(a) Abrasive material,
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(b) Grain size of grit number (mesh number),
(c) Grade (strength of bond),
(d) Structure.
(a) Abrasive:
This choice of right abrasive is to some extent determined by the type of material only to be ground, which will decide whether the abrasive is Silicon Carbide (SiC) or Aluminium Oxide (Al2O3) as these are most commonly used abrasives in different varieties. SiC is the best suited abrasive for brittle and hard materials like grey cast iron castings, chilled iron, tungsten carbide, hard steels, stone, porcelain and other ceramic substances.
SiC is also recommended for low tensile strength material such as non-ferrous metals, bronze, brass, copper, aluminium and plastic materials, A1203 is better for tough materials having high tensile strength like mild steel, alloy steel, high speed annealed malleable iron, tough bronze, wrought iron, etc.
(b) Grain Size:
For softer materials, it is a general practice to use coarse grain size and for harder materials, fine grains. Coarser grain is used for high rate of stock removal. Fine grain is used if the work size or the work surface finish is important. Grain size is determined by the mesh number by which it is retained when passed through a series of meshes in a vibrating sieve.
(c) Grade:
The hard materials and materials having high strength offer more resistance to wheel while grinding operation is performed. Thus if hard grade of wheel is used then wheel will get blunt soon and the grinding will not be good. Therefore, for better results on such materials, the abrasive particles should break and fall quickly so that new sharp faces of the particles do the work and they never get blunt.
For softer materials, high or harder grade, i.e. good bond is used. The grading is done by capital alphabets, the first alphabets being used for softer grade and last ones for harder grade.
(d) Structure:
This represents the void between the abrasives and is influenced by the work material. In the case of harder materials the chips are of small size and also the rate of metal removal is low. Thus a small reservoir is needed to remove the chips from the hard material, and the dense structure is desirable for it.
For softer materials, the open structure is prescribed as the rate of metal removal is high and size of chips is also big. The structure is denoted by numbers from 1 to 15.
Amount and Rate of Stock Removal:
It does not influence the abrasive material but the
(a) Grain size,
(b) Grade,
(c) Structure.
For fast removal of metal, coarse gain size is required and vice versa. As regards grade, soft grade is used for fast removal of metal, of course at the cost of wheel life. With softer grade, the abrasive particles fall off quickly and wheel keeps on sharpening, thus removing more quantity of material. Also in order that metal may be removed at faster rate, more space is required for chip removal and hence open structure is desirable for fast removal of metal and vice versa.
Area of Contact:
It mainly influences grade and to some extent grain size also. When the area of contact in grinding operation is large, total grinding pressure is distributed over a larger area and the pressure per unit area is less and hence a softer wheel is needed for it. Thus for internal grinding where arc of contact is more, softer wheel is used and for external grinding, harder wheel.
Condition of Grinding Machine:
Heavy rigid machines demand the softer grade of wheel than the light machines. If condition of grinding machine is such as to cause vibration, harder grade is used compared to one where complete freedom from vibrations is there.
Finish and Accuracy Required:
For high degree of accuracy and fine finish requirement, small sized grain wheels should be used.
Variable Factors:
i. Wheel Speed and Work Speed:
These are the most predominant factors and about 70% of the complaints can be improved by proper selection of work and wheel speed e.g. if one gets burnt surface then speed of the wheel may be reduced. If there is excess wheel wear, it indicates that either wheel is running too slow or the work too fast.
Wheel speed affects the grade to a considerable extent and for higher wheel speed, soft wheel (soft grade) should be used. Wheel speed depends upon type of grinding operation e.g. external or internal grinding or parting off operation. Work speed depends upon type of work, type of grinding and finish required. It also affects the grade, and for higher work speed it is desirable to use harder wheel and vice versa.
ii. Condition of Grinding:
(By condition of grinding we mean whether the grinding is done in wet conditions or dry conditions.) In dry conditions with hard wheel the heat generation is more and thus soft wheel is required and vice versa.
iii. Skill of Operator:
An unskilled worker can’t handle soft wheels and he is likely to break them. Thus unskilled worker should be allowed to work only in those conditions which require a hard wheel.
Selection of Grinding Wheels for Thread Grinding and Tool Sharpening:
The factors influencing the type of abrasive for thread grinding wheels are the material of workpiece, its hardness, pitch and profile of the threads. Al2O3 wheel is preferred for most of the applications. For grinding titanium, SiC wheel is used and for grinding carbide and ceramic materials, diamond wheel is used. Finer grit size is used for finer pitch.
If fine grit it used then harder wheel is employed. For high precision thread grinding, and where lead errors in pre-cut threads are to the corrected, vitrified bond wheels are used which are more rigid also. Resinoid bond wheels are very flexible and can remove stock rapidly. However, these can’t correct the lead errors in pre-cut threads because of their flexibility.
For tool sharpening, Al2O3 wheels are used for H.S.S; silicon carbide wheels are used for carbide-tipped tools. The operation of lapping and fine finish is done by diamond wheel. CBN wheel is well suited for grinding a variety of difficult to machine tool steels. Other considerations are same as for general grinding applications.
Selection of Grinding Wheel According to I.S. Specifications:
Various elements are put in systematic manner as follows:
Compulsory Elements:
Following have to be mentioned in all the wheels:
(1) Abrasive,
(2) Grain size,
(3) Grade,
(4) Type of Bond.
Optional Elements are:
(1) Prefix,
(2) Structure,
(3) Suffix.
Abrasive:
These are denoted by:
A—for Al2O3, C—for SiC
WA—for white Al2O3, GC—for green grit SiC.
The last two are sometimes put under prefix also.
Grain Size:
It is denoted by grit number.
The various numbers for different types of grain size are given below:
Coarse grain: 8, 10, 12, 14, 16, 24
Medium grain: 30, 36, 46, 54, 60
Fine grain: 80, 100, 120, 150,180
Very fine grain: 220, 240, 280, 320, 400,500,600.
For all types of grinding higher limit is upto 180. The grit number above 200 is recommended for lapping operation etc.
Grade:
The following classification is employed for grade:
A—E: Very soft,
G—K: Soft,
L—O: Medium,
P—S: Hard,
T—Z: Very hard.
Type of Bond:
The following notations are followed:
V—Vitrified,
B—Resinoid,
BF—Resinoid reinforced
R—Rubber,
RF—Rubber reinforced
E—Shellac,
S—Silicate,
Mg—Magnesia
Prefix:
It denotes manufacturer symbol for exact nature of abrasive e.g. GC. Here G is prefix and C stands for silicon carbide.
This varies from manufacturer to manufacturer and they have their own code numbers. Sometimes mixture of two varieties may also be used in abrasives.
Structure:
It is denoted by number from 1 to 15.
1—8: Dense structure
9—15: Open structure.
Suffix:
It is manufacturer’s own identification mark (trade secret) and depends upon the process and type of manufacturer.
Thus according to above sequence a grinding wheel will be completely specified as follows: (Refer Fig. 20.7)
Recommendations for selection of grinding wheels as per I.S. 1249.
