To correct the volume of a recess, for example to form the portion of the combustion chamber in the cylinder head of an internal combustion (IC) engine, a tool electrode (11) is introduced into the recess, the tool electrode having the shape of the desired recess. The tool electrode is spaced from the recess by a gap (12), which is filled with an electrolyte, such as a solution of NaNO.sub.3. The extent of removal of volume of material from the workpiece is determined by controlling the current-time integral. To obtain an accurate measure of workpiece removal, the resistance across the gap can first be measured, and compared with a standard resistance, from which the current-time integral can then be controlled in accordance with an imperical determination; or, with the voltage across the gap held constant and the spacing between the electrode (11) and the workpiece being predetermined, a final current value will be determinative of the extent of material removal, so that the eventual volume of the recess can be accurately controlled. The method is particularly suitable for machining the cylinder heads of multi-cylinder IC engines, so that all combustion chambers of the cylinder heads will have the same volume, thus making the compression within all the cylinders identical and ensuring the smooth running of the engine.