The screwdriver is divided into a manual screwdriver and an electric screwdriver depending on the power source. According to the head, it's divided into the FLATHEAD and cross (PHILLIPS). There are a variety of specifications and models. In the field, there are two standards. One is the metric system, that is, we often say to MM as a unit. The other is British system, inche is used as a unit, such as 1/4, 3/16, the number of 25 is the metric size.
1) the flat head screwdriver model for the Arbor width × Arbor. For example, 3 × 75MM, then the blade width of 3MM, rod length of 75MM (non-full-length).
2) Phillips screwdriver model that the size of the knife bit × Arbor. For example, 2 # × 75MM, then the head for the No. 2, the metal rod length of 75MM (non-full-length). Some manufacturers to PH2 to represent 2 #, the actual is the same. You can use the thickness of the arbor to roughly estimate the size of the head, but the industry is to distinguish the size of the head. Model number 0 #, 1 #, 2 #, 3 # corresponding to the thickness of the metal rod roughly 3.0MM, 5.0MM, 6.0MM, 8.0MM.