Surface Coating. While selecting the correct type of tap for a job, the material to be tapped should also be considered. This may determine the surface coating that should be applied to the tap in order to extend its life. Most taps are supplied with no surface treatment. They are referred to as Bright Finish. These taps are mainly for use on non-ferrous materials, or steels that do not cold weld. Bright finish taps are therefore suitable for all hand operations, where speeds are too low for cold welding to occur, and for most machine operations. Trapezoidal screw threads were devised to allow rotary and transversing motion on machines; and are also used in jacks, valves, presses and other mechanisms where heavy loads are encountered. The trapezoidal thread is characterized by a 30 included angle. Trapezoidal taps typically require specialized engineering and design due to the nature and severity of cut required in producing trapezoidal threads. (these are the metric equivalent of Acme Taps but with a 30 included angle) All size and variety available with us.