What you should know about grinding disks

Grinding disks are tools made of resin-bonded abrasive grains, reinforcing glass fabrics that give them mechanical strength and pores. The difference between them and other cutting tools is that they are self-sharpening.

Abrasive substances are materials that, due to their hardness, eliminate either by rubbing or friction, a part of an object's surface to obtain accurate dimensions or a better finish.

Disks can be used to cut, in which case they are called cutting disks or to snag, when they are called snagging disks.

Grinding disks get their properties by combining abrasive grains, binders and porosity. The most important features of abrasive grains are the grain size, usually called grit, hardness, toughness and friability.

The number that identifies grit corresponds to the Tyler mesh size of the grain. Hardness on the Knoop scale, standardized by the National Institute of Standards and Technology, is used for grinding disks.

Friability is the grain capability of fracturing when it loses cutting capacity during use, creating new edges that restore it. Friable grains are suited for operations that require physical integrity of the piece being handled.

Grinding disks are basically composed of three materials: grains, resin bond and reinforcement fabric with pores.

Abrasive grains are responsible for cutting and snagging. They behave as saw teeth on the material being worked on. During use they wear out and lose their abrasive capability. This demands more power, which in turn causes the grains to break, exposing new cutting edges.

The most used materials for cutting disks in industrial practice are aluminum oxide (Al₂O₃) and silicon carbide (SiC).

The purpose of a bond is to keep the grains together. This capacity is called hardness. The higher the amount of binder, the higher the grain retention capacity and the higher the disk hardness. Phenol resin binders are used for cutting operations.

There are two types of disks: hard and soft, depending on the bonding material to grains ratio. Hard disks are suited for soft materials or small contact areas while soft disks are suited for hard materials or large contact areas.

Reinforcing glass fabrics increase the disks mechanical strength, supporting bending efforts and preventing rupture (Fig. 1).

All cutting disks use reinforcing glass fabrics in their construction. The fabrics are distributed throughout the disks together with abrasive grains and a binder (Fig. 2).

Porosity is the volume of voids in the disk. It is related to the hardness and distance between abrasive grains within the disk. Pores help in cooling the disk material during operation and for removal of the chips generated.

There are two types of porosity: high and low. High porosity is required for a good finish and for hard and friable materials, while low porosity is suitable for soft and resistant materials as well as for large contact areas.

Cutting disks are used to cut metallic and non-metallic materials, such as steel, cast iron, bronze, brass, titanium, ceramics and glass. The work pieces may be bars, pipes, plate and structural shapes. They are also used to open grooves.

Disks are able to cut hard and tough materials that steel saws are unable to cut. They can be used in portable or stationary machines and should be always placed perpendicular to the work piece.

Incorrect practices and improper operational conditions lower the efficiency of the cutting disk, reducing its performance significantly.

Choosing a good disk

A good cutting disk choice depends on the material to be cut and on the cutting section. As a general rule, hard or coarse grain disks are used to cut soft materials while soft or fine grain disks are used to cut hard materials.

Aluminum oxide disks are used for high tensile strength materials such as steel and its alloys. Silicon carbide disks are used for low tensile strength materials such as cast iron and non-ferrous and non-metallic materials.

An important factor for choosing the right cutting disk is the length of the contact arc between the work piece and the disk, since this is what determines the length of the chips produced. Large chips obstruct the path of abrasive grains on the disk periphery, reducing cutting performance.

Disks behave differently when cutting thin walled or solid work pieces. This is evident when using a straight cut machine, which is the most used type for small work pieces. A straight cut is perpendicular to the work piece surface, while a deflected cut is angled.

For the same diameter, the contact arc of pipes is much smaller than that of solid materials (Fig. 3).

Since the unit pressure between the disk and the work piece is very different for thin-walled and solid pieces, the disks requirement is different.

The increase in unit pressure causes early loosening of abrasive grains from the disk surface before they have been fully used, accelerating their wear. High hardness disks should be used to solve this problem.

In thick-walled parts, where the contact arc is increased, the unit pressure between the disk and the work piece is lower. In this case, a free cutting disk, with lower hardness is required.

Free-cut is when the piece offers lower resistance against the disk advance, with less production of heat and burrs. Free-cut disks tend to perform cuts with no deflection.

It is impossible to estimate the mean working life of grinding disks by means of a formula or table. The variety of materials and applications and the manner in which the disks are used, influence their wearing out. Disk manufacturers recommend that users carry out their own tests to determine the mean working life in their particular cases, or estimate a statistical value through direct observation.

Storage location should be dry, free from vibration and sudden temperature changes and as close as possible to the point of use.

Use grinding disks in the order they come in. They should be stored in a horizontal position, on flat shelves, preferably in their original packaging, following the manufacturer recommendations (Fig. 4).

• Grinding disks are self-sharpening.
• Choosing the right disk depends on the material to be cut and the length of the contact arc.
• Disks should be stored in dry, vibration-free cabinets.
• Giovanni S. Crisi, is with Mackenzie Presbyterian University, Sao Paulo, Brazil, and Thais Maldonado is a materials engineer in Sao Paulo. The authors can be contacted at (11) 6128-0461 or gscrisi@mackenzie.com.br. The authors wish to thank Saint Gobain Abrasives Ltd. for their authorization to reproduce the figures shown in this article.