Speeds and feeds

Cutting speed is the speed difference between the cutting tool and the surface of the workpiece it is operating on. Cutting speeds are a critical part of productive and economical machining in the field of manufacturing. They are equally important for the safe operation of any applicable machinery, especially in metalworking and woodworking usage.

The term speeds and feeds may sometimes be used when discussing this value.

Cutting speed

For a given material there will be an optimum cutting speed 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 available from the material manufacturer or supplier, the most common materials are available in reference books, or charts. The following table gives the cutting speeds for a selection of common materials.

Cutting speeds for various materials (Based on High Speed Steel milling cutter)

Material type	meters per min	feet per min
Steel (tough)	15 - 18	50 - 60
Mild steel	30-38	100-125
Cast iron (medium)	18-24	60-80
Bronzes	24-45	80-150
Brass (soft)	45-60	150-200
Aluminium	75-105	250-350

Spindle speed

The spindle speed is the revolutions per minute (RPM) of the spindle of a machine and is calculated using the recommended cutting speed of the material being worked on.

The spindle may hold the:

• Drill bit in a drill
• Milling cutter in a milling machine
• Router bit in a wood router
• Shaper cutter/knife in a wood shaper/spindle moulder
• Grinding wheel on a grinding machine.
• Or it may hold the chuck which then holds the workpiece in a lathe. In these cases the tool bit remains stationery although exceptions may be found such as in thread milling.

Excessive spindle speed will cause premature tool wear, breakages, and can cause tool chatter, all of which can lead to potentionally dangerous conditions. Using the correct spindle speed for the material and tools will greatly affect tool life and the quality of the surface finish.

For a given machining operation, the cutting speed will remain constant for most situations; therefore the spindle speed will also remain constant. Facing operations on a lathe however involve the machining of a constantly changing diameter. Ideally this means changing the spindle speed as the cut advances across the face of the workpiece, this was harder to do in practice and was often ignored unless the work demanded it. The introduction of CNC controlled lathes has solved this awkward problem as variable speed electric motors are a practical solution to the problem.

Grinding wheels are designed to be run at a maximum safe speed, the spindle speed of the grinding machine may be variable but this should only be changed with due attention to the safe working speed of the wheel. As a wheel wears it will decrease in diameter, and its effective cutting speed will be reduced. Some grinders have the provision to increase the spindle speed which corrects for this loss of cutting ability, however increasing the speed beyond the wheels rating will destroy the wheel and create a serious hazard to life and limb.

Generally speaking, spindle speeds and feed rates are less critical woodworking than metalworking. Most woodworking machines including power saws such as circular saws and band saws, jointers, thicknesser/planer rotate at a fixed RPM. In those machines, cutting speed is regulated through the feed rate. The required feed rate can be extremely variable depending on the power of the motor, the hardness of the wood or other material being machined, and the sharpness of the cutting tool.

In woodworking, the ideal feed rate is one that is slow enough not to bog down the motor, yet fast enough to avoid burning the material. Certain woods, such as cherry and maple are more prone to burning than others. The right feed rate is usually obtained by "feel" if the material is hand fed, or by trial and error if a power feeder is used. In thicknessers or planers, the wood is usually fed automatically through rubber or corrugated steel rollers. Some of these machines allow varying the feed rate, usually by changing pulleys. A slower feed rate usually results in a finer surface as more cuts are made for any length of wood.

Spindle speed becomes important in the operation of routers, spindle moulders or shapers, and drills. Older and smaller routers often rotate at a fixed spindle speed, usually between 20,000 and 25,000 RPM. While these speeds are fine for small router bits, using larger bits, say more than 1 inch or 25 millimeters in diameter, can be dangerous and can led to chatter. Larger routers now have variable speeds and larger bits require slower speed. Drilling wood generally uses higher spindle speeds than metal, and the speed is not as critical. However, larger diameter drill bits do require slower speeds to avoid burning.

Cutting feeds and speeds, and the spindle speeds that are derived from them, are the ideal cutting conditions for a tool. If the conditions are less than ideal then adjustments are made to the spindle's speed, this adjustment is usually a reduction in RPM to the closest available speed, or one that is deemed (through knowledge and experience) to be correct.

Some materials, such as machinable wax, can be cut at a wide variety of spindle speeds, while others, such as stainless steel require much more careful control as the cutting speed is critical, to avoid overheating both the cutter and workpiece. Stainless steel is one material that work hardens very easily, therefore insufficient feed rate or incorrect spindle speed can lead to less than ideal cutting conditions as the work piece will quickly harden and resist the tools cutting action. The liberal application of cutting fluid can improve these cutting conditions however the correct selection of speeds is the critical factor.

Spindle speed calculations

Most metalworking books have nomograms or tables of spindle speeds and feed rates for different cutters and workpiece materials; similar tables are also likely available from the manufacturer of the cutter used.

Alternatively, the following formulae may be used to estimate this value:

$RPM={k\times Speed \over Diameter}$

or for accurate calculations:

$RPM={Speed \over \pi Diameter}$

where:

• RPM is the rotational speed of the cutter or workpiece.
• k is a constant, (0.320 for metric units, 4 for imperial unit units)
• Speed is the cutting speed (depending on k) in meters/minute or feet/min
• pi is the constant pi (3.14)
• and Diameter (again, depending on k) in meters or inches

Feed rate

Feed rate is the distance a cutting tool advances per revolution. The metric units are millimeters per revolution.

Feedrate is dependent on the:

• Surface finish desired.
• Power available at the spindle (to prevent stalling of the cutter or workpiece).
• Rigidity of the machine and tooling setup (ability to withstand vibration or chatter).
• Strength of the workpiece (high feed rates will collapse thin wall tubing)
• Characteristics of the material being cut, chip flow depends on material type and feed rate. The ideal chip shape is small and breaks free early, carrying heat away from the tool and work.

A single point cutting tool is the simplest tool type to calculate feed rate for, however with a milling machine or jointer where multi tipped/fluted cutting tools are involved then feed rate becomes dependent on the number of teeth on the cutter. The greater the number of cutting edges, the higher the feed rate permissible: for a cutting edge to work efficiently it must remove sufficient material to cut rather than rub, it also must do its fair share of work.

The ratio of the spindle speed and the feed rate controls how aggressive the cut is, and the nature of the swarf formed.

References

Fitting and Machining, Editor Ron Culley, 1988 TAFE Publications