This separates the physical conditions in the note area from the surrounding surface so that the note will be able to vibrate freely. When the crafting is done the note will be a strained buckle of softer metal, while the surrounding metal will be tempered and stiff. A practical function of the groove is also to define the working area for the later tuning. There is a special model of pans where the groove is complemented with a line of bore-holes. The head of the punch should have a diameter of 4-7 mm.
Some tuners use punches of different size for different notes in the same pan.
The edge of the head have to be fairly sharp to make distinct marks in the metal. A longer punch would be hard to control. The grooving hammer should be a hammer that you are well acquainted with - you must know exactly how hard you are hitting. It is also important that the head is tightly fastened to the handle to get high precision in the hammering. The weight of the hammer should be approximately 0.5 kg.