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Briefing February 2020

Hand-operated machines - angle grinders

What is an angle grinder:

Angle grinder (angle sander, angle saw, disc grinder, or just called a flex) was introduced in 1954 from the company, FLEX Power Tools GmbH from Steinheim an der Murr. The angle grinder is a mostly electric-driven hand machine with a fast-rotating, round grinding or cutting disc. The disc is driven by an angular gear, which gives the angle grinder its name.

The principle of an angle grinder:

The disc rotates at up to 13,300 rpm at 115 mm Disc diameter (at 125 mm up to 12,200 rpm) and is therefore exposed to very high centrifugal forces. The theoretical cutting speed of the discs on the circumference is usually about 80 m/s, the idle speeds for the different disc diameters (115, 125, 150, 180, 230, and 300 mm are common, the latter only as a cut-off) differ accordingly. The larger the wheel, the lower the idle speed. For metals, metal discs are used and for stones, concrete etc., stone discs are used.

What should be considered when working with angle grinders:

When working with angle grinders, tight-fitting safety goggles, fire-retardant work clothes and possibly a dust mask, among other things, should be worn. In advance, a permit for hot work is usually required; only in special workshops can this be waived. Angle grinders should always be held with both hands; this also applies to the smaller so-called "one-hand angle grinders". Due to the high noise level, ear protection is necessary. Never work without the right protective cover; this is not only an important safeguard if the disc bursts, it also prevents the disc from touching the handles and controls of the machine. Also, hands should only be taken off the Flex when the disc has come to a standstill. It is imperative to avoid "rough grinding" with cutting discs - here, there is an increased risk of bursting, since in these discs, the stabilizing glass fiber fabric is applied only on the outside and would be the first thing to be sanded off during rough grinding. The disc then has nothing to resist the strong centrifugal forces. All discs have a sell-by date, stated by the manufacturer on the disc, which is usually three years. When working with angle grinders, as with all grinding processes, abrasion occurs. Due to the high speed, a lot of heat is released at the machining location. In metalworking, especially when grinding steel and cast iron, the chips are therefore thrown off at a high speed (flying sparks). If these glowing particles concentrate in a great quantity on combustible material (including inappropriate clothing of the operator), there is an acute fire hazard. Similarly, grinding sparks can penetrate into crevices or cavities and lead to hidden smoldering fires, which may only show flames hours later. That is one reason for the "hot work" permit. Inspection periods are in accordance with § 3 BetrSichV by a determined risk assessment.

Never mount a large disc on a small flex. Not only because the protection no longer fits, but also the peripheral speed described to the left, which results from the higher speed, is much too high for the disc. Thus, the disc could break even at idle speed.

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