Grinding Under Pressure

Grinding is a thermally dominated process, meaning a high percentage of process heat initially enters the part before coolant quenches it. Unless the coolant is applied at the correct flow rate and pressure, and the proper input conditions are selected, this process can lead to undesirable rehardening burn, thermal softening and tensile residual stresses. By comparison, in a well-designed machining process, the majority of heat typically goes into the chips, not the part. Therefore, less cooling is needed, but the process requires more lubrication and effective chip evacuation, often involving a coolant pressure of 1,000 psi (69 bar) or higher and a low flow rate—8 gpm (30.3 L/min.) or less. In addition,

grinding consumes three to five times more specific cutting energy than other types of machining operations to remove the same volume of material.

Considerably more heat must be removed from the process to maintain part integrity. This big difference in energy consumption is because of the undefined cutting edges of the grinding wheel grit, the smaller chips produced and the friction among the bond, chips and workpiece material. A well-tested flow-rate model for grinding applies 1.5 to 2 gpm (5.7 to 7.6 L/min.) per spindle horsepower. An aggressive grinding process that consumes 20 hp would, therefore, require 30 to 40 gpm (113.6 to 151.4 L/ min.) to keep the process cool. Clearing Air Barriers Because grinding wheels reach far higher peripheral speeds than cutting tools, wheels have a more pronounced boundary layer of air surrounding them during operation. This air barrier can deflect coolant from the grinding zone unless the pump pressure is sufficient for the coolant jet from the nozzle to match the wheel speed and penetrate the barrier... read more