V - The magazine of the VOLLMER Group - 2019

CONTACT ANGLE AND CONTACT SURFACE The coolant distribution coefficient is just as important as the speed. The larger the contact surface, the more heat can be transferred to the coolant. For example when grinding grooves, the large cutting depth causes a large contact length and the cool- ant can absorb 60% or more heat. With cylindrical outside diameters, however, the contact length is short, so that only 5–15% of the heat is transferred and the coolant functions as lubricant. The contact surface is also proportional to the contact angle. The jet of coolant has to meet the grinding wheel just before the contact point with the workpiece (1–3 mm) so that the coolant can reach the hotspot – according to centrifugal forces, vector speed and Newton's laws of motion V coolant = V grinding wheel (Image 3) . AQUAPLANING FORCES If the permissible tolerances are very small, good cooling can also cause its own problems, in particular, a high aquaplaning/hydrodynamic force, which can lead to deflection, difficulties in adhering to the tolerances and an increased risk of damage. From around 500 N, the spindle is pushed upwards and the workpiece is pushed downwards, which leads to the grinding wheel mandrel and the workpiece becoming deformed. This leads to roundness errors, tolerance errors, tapering and edge deformation. Oils generally perform worse than water-based coolants in this respect. 3 TWO GOLDEN RULES for effective cooling. Firstly, the speed of the grinding wheel and coolant should be almost identical. Secondly, the coolant jet should meet the grinding wheel just before the contact point. Cooling lubricant 3 ROLL THE FILM: Dr J. Badger on cooling 21