A robust, untroubled grinding process needs safety parameters for tool production [KLOM86, p. 11]. Tool breakage might be caused by wrong tool design, manufacturing defects, inapproriate choice, faulty handling or storage, improper use, and clamping, etc. [DIN11].
Several organizations define safety measures for grinding wheel use, such as Berufsgenossenschaft or OSHA (Occupational Safety and Health Administration). Different standards exist on tool safety (DIN EN 12413 for conventional grinding wheels, DIN EN 13236 for superabrasive grinding wheels, DIN EN 13743 for coated grinding tools, BVG D12, BGI 543, ANSI B7.1) [DIN07, DIN11].
The bursting speed of grinding tools needs to be larger than the maximum wheel speed. The standard DIN EN 13236 defines safety factors against breakage due to centrifugal forces (Eq. 5.9) [DIN11]. For example, in open grinding machines the grinding wheel speeds need to be 80 m/s or lower and the safety factor is 3.0; in housed grinding machines with safety guards wheel speeds can be up to 320 m/s and the safety factor is 1.75 [DIN11]. For manual grinding tools, safety factors of
3.5 at 80 m/s and 3.0 at 63 m/s and below apply [DIN11].
(5.9)
vbr bursting speed
vs maximum grinding wheel speed
Vitrified bonded grinding tools with a diameter larger than 80 mm have to be tested with a ring test [DIN07]. Here the grinding tool is tapped with a non-metallic item and the tone has to be well-defined, not hollow or rattling like at a broken grinding tool [DIN07]. Spin burst tests and tests with bombardment of a retard plate give the burst energy of grinding wheels [KONI70] Langbein made burst tests with high-speed cameras [EVER06, p. 391, LANG76]. External cylindrical grinding has the highest safety danger [EVER06, p. 391, LANG76].
Malkin and Guo [MALK08, p. 38 f.] show how the bursting speed increases with wheel hardness and decreases with grit size. Mfinnich estimated the bursting speed of grinding wheels [MUNN56, BEHR07]. However, measured bursting speeds and speeds calculated with Munnich’s approach deviate up to 20 % [MUNN56, BEHR07]. Behrens and Kammler derived a new equation based on FEA analysis, which can be used for one-sided recessed grinding wheels [BEHR07].
The guard material has to absorb the energy of broken tool pieces. Safety guards designed with pipes, pipes with intermediate metal sheets or foamed plastic covers with metal sheets provided good results [KONI70].