in a turning operation, or the speed of the cutter moving past a workpiece, such as in a milling operation. The cutting conditions will affect the value of this surface speed for mild steel. Schematically, speed at the workpiece surface can be thought of as the tangential speed at the tool-cutter interface, that is, how fast the material moves past the cutting edge of the tool, although "which surface to focus on" is a topic with several valid answers. In drilling and milling, the outside diameter of the tool is the widely agreed surface. In turning and boring, the surface can be defined on either side of the depth of cut, that is, either the starting surface or the ending surface, with neither definition being "wrong" as long as the people involved understand the difference. An experienced machinist summed this up succinctly as "the diameter I am turning from" versus "the diameter I am turning to." He uses the "from", not the "to", and explains why, while acknowledging that some others do not. The logic of focusing on the largest diameter involved (OD of drill or end mill, starting diameter of turned workpiece) is that this is where the highest tangential speed is, with the most heat generation, which is the main driver of tool wear. There will be an optimum cutting speed for each material and set of machining conditions, and the spindle speed (RPM) can be calculated from this speed. Factors affecting the calculation of cutting speed are: The material being machined (steel, brass, tool steel, plastic, wood) (see table below) The material the cutter is made from (High-Carbon Steel, high-speed steel (HSS), Carbide, Ceramics, and Diamond tools) The economical life of the cutter (the cost to regrind or purchase new, compared to the quantity of parts produced) Cutting speeds are calculated on the assumption that optimum cutting conditions exist. These include: Metal removal rate (finishing cuts that remove a small amount of material may be run at increased speeds) Full and constant flow of cutting fluid (adequate cooling and chip flushing) Rigidity of the machine and tooling setup (reduction in vibration or chatter) Continuity of cut (as compared to an interrupted cut, such as machining square section material in a lathe) Condition of material (mill scale, hard spots due to white cast iron forming in castings) The cutting speed is given as a set of constants that