Ageing of resinoid bonded grinding wheels

Status:

completed 02/2000

Aims:

Resinoid bonded grinding wheels which are employed for wet grinding may exhibit a drop in bond strength and hardness under the influence of the cooling agents, which are aqueous and generally alkaline. This ageing phenomenon is now generally attributed to water molecules, and in particular OH ions, penetrating the interface between the abrasive grain and the resin bond, and causing resin molecules to become detached from the abrasive grain surface by a mechanism of displacement. The reduced adhesion may lead to a reduction in the bursting speed. In the course of drafting of European standard EN 12413, "Safety requirements for bonded abrasive products", the question thus arose regarding what reduction in strength might be anticipated as a consequence of moisture absorbed during storage, and whether the use of resinoid bond grinding wheels, in particular wheels for off-hand grinding and off-hand cutting off, should be subject to a time limit.

Activities/Methods:

The ageing behaviour of resinoid bonded cutting, coarse grinding and precision grinding wheels was tested in a normal laboratory atmosphere and a damp alternating atmosphere (FW24) in accordance with DIN 50016, "Testing of materials, structural components and equipment; method of test in damp alternating atmosphere". The duration of ageing in a normal laboratory atmosphere at room temperature and a mean relative humidity of approx. 50 % was up to three years for the cutting and coarse grinding wheels and up to one year for the precision grinding wheels. The cutting and coarse grinding wheels were subjected to an alternating damp climate with cyclical temperature fluctuations between 23 and 40 °C and humidities of between 83 and 92 %, for up to two years in the case of the cutting and coarse grinding wheels and up to one year in that of the precision grinding wheels. The bursting speed and the single side load were measured as the ageing indicators on the cutting and coarse grinding wheels, and the modulus of elasticity on the precision grinding wheels.

Results:

The storage of resinoid bonded grinding wheels under warm, damp conditions may lead to ageing within only a few months. This effect manifested itself in a reduction in the fracture velocity on cutting and coarse grinding wheels and in the single side loading in the case of cutting and coarse grinding wheels. Some test samples failed to meet the safety requirements in terms of the minimum fracture velocity after only a few months in storage. Precision grinding wheels aged faster than cutting and coarse grinding wheels under warm damp conditions. The reason for this is presumably the former's more open grain structure, which favours penetration by moisture. Within a year, the bursting speed had fallen by up to 60 %, depending upon the grinding wheel specification. In the same way, the modulus of elasticity was modified increasingly as the storage time increased. Since the modulus of elasticity can be measured by non-destructive testing, it can therefore be used to obtain an approximate assessment of the ageing condition of resinoid bonded precision grinding wheels. Conversely, resinoid bonded grinding wheels exhibit no safety-critical reductions in strength, even over a period of several years, when stored in areas subject to normal ambient conditions, i.e. temperatures around 20 °C and an atmospheric humidity of approximately 50 %. Under such storage conditions, the ageing of resinoid bonded grinding wheels does not constitute a safety issue.

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