A grinding wheel is a highly specialized tool which is mostly used in grinding machines for the precise hard-fine machining of metallic materials. To be able to observe the high production tolerances during grinding and to achieve cost-efficient machining times, grinding wheels have developed into high-performance tools.

Grinding wheel design and materials

Grinding wheels are rotationally symmetric tools which are usually joined via a centric mount with the drive spindle of a machine tool by means of a frictional connection. A grinding wheel consists of abrasive grits held in a bond. The abrasive grits provide the grinding wheel with its cutting action, as they splinter off upon contact with the workpiece, thereby continually forming new sharp cutting edges. The abrasive grits used are generally classified according to their cutting properties or their hardness. The categories are as follows:

Classification of abrasives according to workable materials:

For use with long-chipping, tough materials and particularly suitable for steel.

A corundum
B cubic boron nitride (cBN)

For use with short-chipping, brittle materials, e.g. for glass, ceramic, cemented carbide, grey cast iron.

C silicon carbide (SiC)
D diamond

Classification of abrasives according to their hardness:

Conventional abrasives

A corundum
C silicon carbide (SiC)

Superabrasives

B cubic boron nitride (cBN)
D diamond

A distinction is made between natural abrasives (e.g. quartz or corundum) and synthetic abrasives such as silicon carbide (SiC), cubic boron nitride (cBN) or synthetically produced diamond. The performance characteristics of grinding wheels can be set in a concrete way during production by the addition of certain auxiliary materials, thereby adapting them to the relevant application purpose. Certain fillers and additives are used which can influence heat resistance, toughness or strength, for instance. Precision is particularly important – right from production of the grinding wheel. The run-out of a grinding wheel must be perfect, so that no unwanted vibrations influence the subsequent production process.

Grinding wheel requirements

It is with good reason that stringent requirements are placed on grinding wheels and on the entire grinding production process. By the time a part is ground, it has already gone through the majority of its required machining steps. This is why the damage caused by grinding burn or non-observance of tolerances is particularly great. Accordingly, it is important to avoid grinding burn. At the same time, a process with zero grinding burn must run at an appropriate machining speed in order to be cost-efficient. The grinding wheel plays a considerable role in fulfilling the requirements of the grinding process. The main requirements demanded of grinding wheels may be summarized as follows:

Service life

A grinding wheel is an expensive wear part within the machine tool. For this reason, increasing the service life of the grinding wheel is particularly important, so that the wheel does not have to be replaced frequently. Dressing has a significant influence on service life. This is used whenever the grinding wheel no longer demonstrates the specified shape or cutting action. During the dressing process, part of the abrasive layer is removed in order to eliminate contaminants. In addition, abrasive grits which have become dull are dislodged during dressing to expose fresh grains for renewed cutting action. Frequent dressing thus leads to increased grinding wheel wear and undesired non-productive process times, as grinding cannot take place during the dressing process.

Cutting action

The abrasive grits protruding from the grinding wheel bond lend the grinding wheel its cutting action. Through contact with the material during grinding, these abrasive grits become worn, however. This causes the cutting action of the grinding wheel to diminish. The longer a grinding wheel can maintain the required cutting action in the grinding process, the greater the benefit in terms of cost-efficiency. The cutting action of a grinding wheel can be influenced by the selection of suitable grit material as well as by the type of bond used, enabling a self-sharpening action of the grinding wheel to be achieved. Restoring the cutting action can also be achieved by means of dressing – but with the above-mentioned drawbacks.

Application

The grinding wheel used must be suitable for the material to be machined. For this, some basic aspects need to be considered in advance with respect to the bond and grinding wheel abrasive used, as well as with respect to run-out accuracy and the desired grinding quality (abrasive grit concentration, average abrasive grit size, etc.). The grinding wheel manufacturers usually state which grinding wheel is most suited to which machining task.

Mountability

Furthermore, the selected grinding wheel must fit into the machine tool with respect to its dimensions and to the planned connection dimensions for machine fastening. It is also important to enable additional elements such as lubricoolant nozzles to be mounted in such a way that they do not collide with the grinding wheel. Here, it is important to consider the freedom of movement required by loading devices or other fixtures in the machine space.

Tasks of the grinding wheel bond

The task of a grinding wheel bond sounds trivial at first. Its purpose is to maintain the shape of the grinding wheel and hold the abrasive grits in place. However, the tasks of the bond go far beyond this.

• Fixing the abrasive grits in the bond matrix of the grinding wheel – however only for as long as the cutting edges of the grit demonstrate a cutting action.
• Removal of the abrasive grit from the bond matrix of the grinding wheel as soon as the cutting edges of the grit are dull. This is referred to as the self-sharpening effect.
• Ensuring that it achieves a sufficient level of self-wear. This serves to maintain the required grain protrusion.
• Protection from abrasive stress to guarantee that the grinding wheel demonstrates a high level of profile accuracy.
• Heat removal to avoid grit damage.
• Good profile property for avoiding unnecessary non-productive times (particularly dressing).

Common bonds for grinding wheels consist of ceramics, plastics or metal.
The suitability of the grinding wheel for the material to be machined also depends on the type of bond used. Based on the bond, the following classification can be made in terms of material suitability:

Galvanic bonds

Grinding wheels with a galvanic bond are usually suitable for grinding steel, cemented carbides, ferrite, CFRP, glass-fibre reinforced plastic, synthetic resin, ceramics, semiconductor materials, quartz glass, graphite, artificial and natural stone, gypsum, rubber and foodstuffs.

Sintered metal

Grinding wheels with a sintered metal bond are usually suitable for grinding glass, ceramics, semiconductor materials, silicon, precious stones, quartzes, concrete, artificial and natural stone, grey cast iron, carbide formers, cemented carbides, hard ferrites.

Plastic

Grinding wheels with a plastic bond are usually suitable for grinding cemented carbides, ceramics, glass, quartzes, carbide formers, steel and stellite.

Ceramic

Alloyed and unalloyed steels, high-speed steels, cast materials, Ni-based alloys, non-ferrous metals, sintered metals, polycrystalline diamond.

(Schleifscheibe: Freundliche Leihgabe der Fa. Theleico)

Application and processes

Grinding wheels are used in almost all grinding processes. As a large diversity of grinding processes are used on machine tools, diverse grinding wheel geometries are available from various manufacturers. Almost all grinding wheel forms are available, ranging from the common ones used for cylindrical grinding through to cut-off wheels for use on hand-guided machines and to cup wheels.

Interactions between the grinding wheel and the lubricoolant

Within the machine tool, the function, service life and reliability of the grinding wheel and the entire process depend on many different factors and the interaction thereof. The interactions between the part, grinding wheel and the lubricoolant will be outlined in more detail here, as the targeted and precise use of lubricoolant can enable maximum productivity to be achieved for the grinding wheel and thus for the grinding machine:

Thermal influences

During grinding, a considerable amount of thermal energy is generated through friction. Around 92% of the process energy occurs as friction heat and must be transported away, otherwise undesired part damage (e.g. grinding burn) may occur, which leads to expensive scrap volumes. Besides this, an excessively high thermal load is likely to damage the grinding wheel (abrasive grits or bond), which leads to premature tool failure and thus higher wear costs. Both problems need to be avoided in order to maximise the productivity of the grinding process.

Temperatures and lubricoolant supply

To remove the heat from the machining zone, lubricoolant is used. Particularly important here is the targeted and requirements-based supply of lubricoolant into the machining zone in order to achieve efficient and functional cooling. The use of too much lubricoolant causes high fluid pressure between the grinding tool and the part (colloquially referred to as the “aquaplaning effect”), and this can have a negative impact on the roundness of the part. By contrast, if too little lubricoolant enters into the machining zone, the process heat can only be removed to an insufficient extent.

The thermal energy which is generated by means of friction during grinding may lead to thermal damage of the part if it is not removed with the chip or the lubricoolant but via the rim zone of the part. The temperatures generated are often so high that grinding burn occurs on the ground parts. Grinding burn refers to thermal rim zone damage to a part which negatively influences subsequent part functionality and may thus lead to the part being scrapped. This costs time and money and should thus be avoided. An optimum interplay between the grinding wheel and its circumferential speed as well as the exit speed of the lubricoolant from the nozzle are important factors for achieving sufficient cooling of the machining task. The correct setting of these parameters can be precisely defined and monitored using suitable measurement and control systems. Here, the Coolant Pointer as well as the Coolant Display from Grindaix are suitable.

Summary

To ensure the seamless, cost-efficient and reliable functionality of a grinding wheel, various factors need to be considered. Initially, basic requirements and the correct choice of grinding wheel play a role in correctly setting up the grinding task. To optimally define the process and achieve maximum cost-efficiency, various parameters are relevant. To correctly determine these parameters, simply using a trial and error approach for such a demanding process is not efficient and often not expedient. Instead, it takes appropriate knowledge and experience, taking into account many different dependencies and correlations.

As an experienced lubricoolant system service provider, Grindaix GmbH can support you in this optimization process or in tackling the problems of your grinding task by drawing on extensive know-how and professional production engineering expertise. We have already optimized several thousand processes prone to grinding burn, thereby significantly reducing the cycle time of the manufacturing process for our customers, while at the same time avoiding grinding burn. In this way, cost-efficiency was increased.

The products of Grindaix GmbH render the handling of your lubricoolant system more efficient, robust and predictable. We have a comprehensive portfolio of lubricoolant nozzles as well as measurement and control systems on offer.

Feel free to contact us – we look forward to your inquiry!