In a previous article we presented the main aspects of the great variety of abrasive discs that are available today, including super abrasives and diamond blades. These ideal tools for the cutting and roughing They have wide application in grinders and some types of grinders, particularly bench grinders, as well as many other electrical machines.
Let us remember that a disk or grinding wheel is a generally self-sharpening tool made up of abrasive grains (cutting material) joined by an agent binder and reinforced by a composite material structure (mesh) that allows some space free space (pores) between the cutting edges.
The characteristics of a abrasive disc they depend on the combination of grain type and size, binder, structure and grade. Let’s examine each of these parameters in detail.
Abrasive grain type
As we already pointed out in the article cited above, the cutting and grinding wheels They can be composed of different types of grain, which determines two large subgroups:
to) Conventional abrasive discs: this category basically includes discs whose abrasive grain consists of aluminum oxide, Silicium carbide or a combination of aluminum and zirconium oxides. They are used for soft and hard or semi-hard materials, with applications that are detailed in the table below.
b) Super-abrasive discs: This category comprises a more tenacious type of discs, the cutting material of which is the polycrystalline diamond or the boron nitride. They are for exclusive use in high-speed, precision cutting and roughing of high-hardness materials, and we’ll cover them in a future article.
Abrasive grain size
This parameter is stated in terms of mesh: The coarsest grains are represented by low mesh numbers and the finest grains by high numbers, as we see in this table.
The coarse grains They are used in ductile, soft and fibrous materials, for the rapid removal of sanding material, for large contact surfaces and exerting considerable force with the tool. The finer grains They are used to obtain smooth finishes on hard and brittle materials, in small contact areas, and to hold shape in small, narrow corners.
The relative ability of the binder to keep the abrasive grains attached to the disc gives an idea of the hardness of the disk and is measured in terms of binder strength. It is worth saying that if the disc is grade “soft”, The binder has little strength to hold the grains together and the grains will tend to come off easily with use. On the other hand, if the degree is “Lasted”, The grains are strongly bound to the binder and the disc has a longer shelf life.
The “soft” characteristic is not always disadvantageous, as although the peripheral grains are dislodged, they will expose other new grains, so the disc is automatically “sharpened”. This is not the case with the “hard” grade, in which the disc will need to be sharpened by mechanical means.
The grade of a grinding disc is indicated by letters: the lowest of the alphabet represent softer grades, while the highest letters of the alphabet represent harder grades, as shown in the following table.
Represents the spacing between the grains on a disk. Very tight spacing is denoted by low numbers, while more open spacing is denoted by high numbers. Disc structures are dependent on workpiece material, material removal rate, precision, and required surface finish.
The following table shows the numbers covered by the structures dense, half Y open.
It is the material that holds the grains together. The type of binder depends on the working speed of the disc, the type of operation and the required surface finish. The materials used for the binders are symbolized by one or two letters, according to the following table.
Nomenclature of abrasive discs
Both cutting and grinding wheels have an identification based on the parameters we just looked at and therefore includes numbers and letters.
Let’s look at an example.
The manufacturer’s prefix and registration are optional identifications that not all discs include; however, the other parameters are always present. Therefore, a disk that has the nomenclature that we see in the figure above will have the following characteristics:
Type of abrasive = TO: aluminum oxide
Grain size = 36: normal
Grade or hardness = L: medium
Structure = 5: dense or compact
Binder type = V: vitrified
Shapes of abrasive discs
Finally, let’s bear in mind that, as we saw in another article, abrasive cutting and grinding discs for grinders come in at least two different forms, according to use, as shown in the table below.
In a future article we will detail a guide that will help us choose the most suitable abrasive disc for the operation we wish to carry out and the type of material of interest.