Speeds and Feeds - Cutting Speed

Cutting Speed

Cutting speed may be defined as the rate (or speed) that the material moves past the cutting edge of the tool, irrespective of the machining operation used – the surface speed. A cutting speed for mild steel, of 100 ft/min (or approx 30 meters/min) is the same whether it is the speed of the (stationary) cutter passing over the (moving) workpiece, such as in a turning operation, or the speed of the (rotating) cutter moving past a (stationary) workpiece, such as in a milling operation. What will affect the value of this surface speed for mild steel, is the cutting conditions:

For a given material there will be an optimum cutting speed for a certain set of machining conditions, and from this speed the spindle speed (RPM) can be calculated. 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 (Carbon steel, high speed steel (HSS), carbide, ceramics)
• 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 are available from the material manufacturer or supplier, the most common materials are available in reference books, or charts but will always be subject to adjustment depending on the cutting conditions. The following table gives the cutting speeds for a selection of common materials under one set of conditions. The conditions are a tool life of 1 hour, dry cutting (no coolant) and at medium feeds so they may appear to be incorrect depending on circumstances. These cutting speeds may change if, for instance, adequate coolant is available or an improved grade of HSS is used (such as one that includes cobalt).