Working on copper. These copper are called as electrodes and used for doing edm in cavity and core. EDM is a machine that passes electric current to copper or other metals.
These electrodes has two function first used for cutting sharp edges and small radius where machining is not possible.
secondly it is used for mat finishing on final product. Electrodes are used on EDM machine for cutting grains on cavity so that the final outcome of it will have mat surface finish.