Guide to select of grinding wheels

The selection of bond used determines the type of technology in which the grinding wheels are to be made. In contrast, its quantity in the abrasive mass influences the hardness of the grinding wheel, i.e. its ability to hold the grain. Hard wheels have a high grit holding capacity while soft wheels have a weaker grit holding capacity.

The table below shows the types of bonds used :

The above symbols from the table must appear when ordering a particular grinding wheel range. For examples of correct marking, see the sample orders for the individual ranges

The type of abrasive grit used is determined by the type of material that the wheel is intended to machine.

As a general rule of thumb, CBN is the superhard equivalent of electrocorundum and diamond of silicon carbide.

Grinding wheels have a structured designation consisting of three parts:

The creation of the abrasive type follows strictly defined principles. The rules for creating the correct symbol of the grinding wheel are presented in the table below:
BODY SHAPE • ABRASIVE LAYER SHAPE • POSITION OF THE ABRASIVE LAYER

Body shape	Abrasive layer shape		Abrasive layer position
1	A	F	1
3	B	FF	2
4	BT	H	3
6	C	L	9
9	D	M
11	E	Q
12	EE	U
14	ET	V

Example: 14A1

This abrasive wheel has a body with the shape /14/, an abrasive layer with a rectangular cross-section /A/, positioned on the periphery of the body /1/

Example: 11V9

This abrasive wheel has a truncated cone-shaped body /11/ and an abrasive layer with a parallelogram cross-section /V/, positioned in its corner /9/.

The grit size has a decisive influence on the grinding process, so its correct selection has a decisive impact on the results achieved.

The correct choice of grit size ensures that the wheel operates correctly and achieves the desired smoothness of the machined surfaces. In general, the smaller the grain size, the smoother the machined surface. However, the aim should not always be to achieve the smoothest possible surface, but always to achieve the desired results in the shortest possible time. This means that as coarse a grain as possible should be used which allows acceptable smoothness to be achieved.

Excessive allowances should not be used when grinding with fine grit grinding wheels, as this increases the wear of the abrasive layer and degrades the quality of the machined surfaces. For roughing, the coarsest possible grit should always be selected in order to maximise grinding efficiency.

It is recommended to use a grinding depth of no more than 1/3 of the nominal grain size stated in the grinding wheel characteristics. For example, for grit D126 according to FEPA, the grinding allowance should not exceed 0.042 mm.

The following criteria should be taken into account when selecting the grain size:
– type of machining (roughing, finishing)
– the desired smoothness of the workpiece surface
– expected grinding performance

Grains below a granulation of 46 are called micropowders. The size table of these grains is as follows:

Concentration determines the amount of diamond or boron grain per unit volume of the grinding wheel layer. The standard grain concentration values for resin bonded grinding wheels are shown in the table below.

The concentration of abrasive grit in the working layer is one of the most important parameters of a grinding wheel. It affects the grinding wheel’s ability to grind, its service life, the temperature of the workpiece, and also the accuracy of the machining. Like any parameter, the concentration should be properly selected to suit the grinding process conditions. It should be remembered that the optimum concentration value depends on the other parameters of the grinding wheel, i.e. grit size, bond hardness, etc.

High concentration (K100, K125; V240, V300) is recommended for:
– high demands on the behaviour of the grinding wheel profile during operation
– low abrasive layer height
– hard bond
– coarse grain
– deep grinding

A standard concentration (K50, K75; V120, V180) is recommended for:
– grinding of planes and cylindrical surfaces
– medium abrasive layer height
– soft bond
– fine grain

A low concentration (K25; V60) is recommended for:
– very wide abrasive layers
– very fine grain

A high grain concentration increases tool life, which is particularly important for contour grinding and for grinding workpieces with very small diameters. The benefits of a high tool life generally offset the higher cost of the tool.

A disadvantage with high grain concentration is the occurrence of higher cutting forces and an increase in the temperature of the machining process. A high concentration of grain is not always the most favourable and technologically best solution, but it certainly requires good cooling.

Body material
The grinding wheel body can be made from a variety of materials. The material of the body, through its vibration damping or heat dissipation properties, fundamentally influences the grinding process. Therefore, its selection should depend on the expected processing parameters.

The following materials are available:
– aluminium, resin tools, ceramics
– moulding compound, resin tools
– steel
– ceramics, ceramic tools

A comparison of the characteristics of the available materials is shown in the table below:

The primary criterion for diameter selection is the type of grinder being used. If there is a choice, large diameter grinding wheels should be used. The advantage of this is the improved quality of the machined surface and the higher cost-effectiveness of the work due to the higher machining performance.

When grinding holes, make sure that the diameter of the grinding wheel is between 60 and 80% of the diameter of the hole being ground. Smaller-diameter grinding wheels prevent the achievement of a high quality machined surface, while larger wheels make it difficult to remove the spoil.

We define the hardness of a grinding wheel as the ability of the wheel to hold the grain. Hard wheels have a high grit holding capacity while soft wheels have a weaker grit holding capacity.

The selection of wheel hardness depends on a number of grinding wheel operating parameters. Commonly used selection criteria are presented in the table below:

Cubic boron nitride (CBN) is produced similarly to synthetic diamond. CBN is the second hardest artificially produced abrasive. Unlike diamond, it is not adversely affected by iron, making it ideal for machining all types of alloy steels.

CBN tools have a higher wear resistance compared to conventional tools, and it is easier to achieve the desired dimensions and surface quality of workpieces using them. The aforementioned features make it possible to achieve significantly higher productivity and lower costs in the grinding process when using CBN tools.

Diamond has the highest hardness of any abrasive known to man. Its hardness and wear resistance, as well as its high thermal resistance, make it particularly suitable for use in grinding difficult-to-machine materials.

Today, 90% of industrial diamond is produced synthetically from graphite. Under high pressure and temperature in the presence of catalysts, the crystallographic lattice of graphite is transformed, resulting in synthetic diamond. As a result of this controlled process, diamonds with different properties can be obtained, enabling the precise selection of the type of abrasive grain to meet the customer’s requirements.

Electrocorundum is a synthetic abrasive consisting of a crystalline aluminium oxide (α-Al2O3) called alumina and a small amount of additives. Depending on the content of foreign oxides of TiO2, Si02, Fe2O3, CaO, MgO or NaO2, the following types of electrocorundum are distinguished:

In addition to the aforementioned types of alumina, several special grades are produced by smelting, such as chrome alumina (pink in colour), zirconia, titanium and others.

In addition to electrocorundum, silicon carbide is a commonly used abrasive. Silicon carbide comes in two varieties:

Types of silicon carbide

In terms of chemical composition and physical properties, the two silicon carbide varieties differ slightly; however, green silicon carbide contains fewer additives, making it more brittle and giving it better abrasive properties.

General recommendations indicate the need to use the smallest W layer widths as possible. The width of the grinding wheel working layer must always be smaller than the workpiece width. If this is not the case, a fault is created on the working surface of the grinding wheel contributing to increased wear:

The height of the tool’s abrasive layer does not fundamentally affect the grinding process, but only the price of the tool itself. Taking into account the economic aspect, it is advantageous to use a higher X layer if the processing conditions allow it.

Cooling during machining
The wet grinding process (with cooling) is superior to the dry grinding process (without cooling) in terms of both wheel life and cutting performance. Cooling contributes to improved grinding conditions by improving spoil removal and lowering the temperature in the grinding zone. Therefore, wet sanding should be used wherever possible. Several per cent oil-water emulsions or mineral oils with some additives to increase cooling efficiency are used as coolants.

During grinding operations, the grinding speed, which is the linear velocity of the grains on the surface of the abrasive layer, plays a very important role. The correct selection of this speed, depending on the workpiece material and type of machining, is a fundamental issue when grinding.

Recommended grinding speeds for bond types ‘B’, ‘P’, ‘M’, ‘G’ depending on the grinding conditions are given in the table below:

Note, for ‘V’ type vitrified bonds made on ‘KC’ ceramic bodies, grinding speeds of 35 m/s should not be exceeded. For grinding speeds above 35 m/s, use ‘KA’ type aluminium wheel bodies.

The speed, denoted as ‘n’, indicates the number of revolutions/minute of the abrasive tool, i.e. how many revolutions the abrasive tool makes in one minute. Grinding speed, designated ‘Vc’, is the speed at which a single abrasive grain cuts the workpiece.The two speeds correspond to each other according to the following formula:

π – ~3,14
d [mm]- disc diameter
n [rpm ⁄ min] – machine spindle speed with abrasive tool

For diamond or CBN, the peripheral velocity ‘Vc’ should be in the range: 20/30 [m/s]

Example – speed calculation [rpm]
for an abrasive tool diameter d = 22 mm, assuming that the disc grinds with the peripheral

The grinding wheel performance can be defined as the ratio of volume of material removed in a particular grinding operation to the used volume of the abrasive layer of the grinding wheel:

G = VuVz

where:
G, grinding performance factor
Vu , volume of material removed [mm ]
Vz , used volume of the abrasive layer of the grinding wheel [mm ]

A higher ratio of these values means higher performance of a particular grinding wheel, resulting in a reduction in unit costs of the product in question.

In a properly used grinding wheel, the grain ‘protrudes’ above the surface of the bond, allowing it to work properly. If the working surface is ‘stuck’, the efficiency of grinding decreases dramatically. In this case, the structure of the grinding wheel should be ‘opened up’ with a ceramic whetstone. The essence of the operation is presented in the figure:

At the end of the production process, grinding wheels are dynamically balanced to ensure:
– optimum wheel life
– minimal wear on the grinder bearings
– the desired machining accuracy

When working with an unbalanced grinding wheel, there is partial contact between the abrasive layer and the workpiece. This causes wear of the grinding wheel in a short period of time at a particular location, which exacerbates the unbalance and increases the roughness of the machined surface.

A balanced grinding wheel is considered to be one whose centre of gravity coincides with the geometric centre of the disc’s axis of rotation.

If the grinding results do not produce the desired results, it is important to ensure that the process parameters have been chosen correctly. If so, and the problems persist, the cause should be determined.

A list of the most common problems encountered during hole machining and possible ways of eliminating them is presented in the table:

The probable causes listed next are not the only ones that can cause certain abnormalities, the reasons listed are simply the most common ones.

The grinding wheel selection scheme is as follows:
geometrical parameters such as:
– type of wheel, dimensions of the abrasive layer and diameter of the bore or shank
– depending on the material to be machined and the processing, the type, concentration and size of the grit and hardness of the bond should be selected
– specify operating conditions with cooling (wet) or without cooling (dry)

If possible, please specify in your order the type of material and machining, as well as its conditions and the type of machine for which the grinding wheels are to be used. This will allow us to adapt the wheel as closely as possible to your needs and reduce selection time.

If the grinding wheel you are interested in is not available on our website, we can manufacture it to your special order. This page contains examples of the most popular products in our range.