Drilling is an important part of material processing, especially during assembly work, whereby holes are created using drills. Sharp-edged burrs often occur as a result of drilling, particularly for metals, and are removed using countersinks. Taps are special type of drilling tool. With these tools, internal threads can be cut into existing drill holes and screws threaded in. Hand drilling tools allow almost silent drilling and are battery operated for independent operation. Milling is another type of material processing. The advantage of milling is that the material can be smoothed in several directions.