In free abrasive grinding, the choice of abrasive material, the compound used as well as process times and parameters should be taken into consideration as process input parameters. The selection of a suitable abrasive material depends above all on the desired result. For a large material removal and short production times, an abrasive chip-type should be used. For workpieces with holes that are true to gauge or tight tolerance limits requiring a fine treatment, or to obtain surfaces for electroplating, plastic-bonded abrasives are to be recommended. The dimensions of the abrasive material should be determined in relation to the workpiece and the facility. Selecting excessively large rhombi or stars in small plants does not guarantee the kinematic sequence of motions necessary for material removal and leads to faulty results. With respect to the shape of the abrasive material, the rule applies that with larger, bulkier chips with sharper cutting edges, grinding efficiency increases and the micrograph is cruder. Moreover, the geometry of the chips is determined by the accessibility of all workpiece contours that are to be processed. The abrasive material may not become wedged neither individually nor in pairs.
In order to guarantee the necessary sequence of motions, the minimal filling quantities of the plants must be taken into consideration. Oscillatory free abrasive grinding facilities are usually filled to 80 %, centrifugal machines and drums from 50 to 70 % [N. N.2].
One essential component of free abrasive grinding is, besides the selection of the right abrasive material, the chemical composition in a water-based solution (compound). This has the task above all of keeping the abrasive material during the process capable of grinding and of guaranteeing removal. In addition, dirt, dust on the workpiece and abrasive material as well as oil and fat residue clinging to the workpiece should be absorbed and a sufficient protection from corrosion provided. Their chemical composition is usually not provided by the manufacturer. The operator must however pay heed to how sanitary it is and clean the compound before disposing.
Compound selection offers many opportunities of navigating the grinding behaviour of the abrasives and of obtaining workpiece surface effects, for example a dull finish and activation or passivation as a prerequisite for further treatment. Compounds are usable in batch quantities in a closed system, in circulating operations or regulated in flow operations.
Besides workpiece and abrasive matter dust, emulsified oil and dissolved metals are often to be found in the free abrasive grinding fluid. Wastewater purification in accordance with legal regulations can only be achieved by means of settling, filtering or centrifuging. The dirty wastewater must therefore be collected and provided with precipitating or flocculating agents in order to separate not only suspended matter but also heavy metals and mineral oil in the form of flakes [N. N.2]. These settle, forming a clear aqueous phase and a layer of sludge. Fi
nally, the sludge is separated from the clear water via filtration, and the water can be used again, e. g. as process water.
The removal efficiency of all method variations of free abrasive grinding is maximal at the start, when the burrs and edges are engaged. Only a small amount of material removal takes place on surfaces and edges that have already been rounded, so that an increase in removal efficiency by means of lengthening the process is almost always uneconomical.
The frequency and amplitude of the oscillatory motion can be exploited as parameters for oscillatory free abrasive grinding units. While the vibration frequency can only be altered in a few machine designs, influencing the amplitude of oscillation by shifting balancing weights is possible. In this way, the grinding effect can be adjusted to the machining task [HINZ76]. The only parameter in the case of abrasive drum tumbling units is the rotational speed. The material removal rate increases with rpm. However, the workpieces are then subjected to larger stresses.