The
spindle speed is the rotational frequency of the spindle of the machine, measured in revolutions per minute (RPM). The preferred speed is determined by working backward from the desired surface speed (sfm or m/min) and incorporating the diameter (of workpiece or cutter). The spindle may hold the: • Material (as in a
lathe chuck) •
Drill bit in a
drill •
Milling cutter in a milling machine • Router bit in a
wood router • Shaper cutter or knife in a
wood shaper or spindle moulder •
Grinding wheel on a
grinding machine. Excessive spindle speed will cause premature tool wear, breakages, and can cause tool chatter, all of which can lead to potentially dangerous conditions. Using the correct spindle speed for the material and tools will greatly enhance 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. However, facing, forming, parting off, and recess operations on a lathe or screw machine 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, producing constant surface speed (CSS). Mechanical arrangements to effect CSS have existed for centuries, but they were never applied commonly to machine tool control. In the pre-
CNC era, the ideal of CSS was ignored for most work. For unusual work that demanded it, special pains were taken to achieve it. The introduction of CNC-controlled lathes has provided a practical, everyday solution via automated CSS Machining Process Monitoring and Control. By means of the machine's software and
variable speed electric motors, the lathe can increase the RPM of the spindle as the cutter gets closer to the center of the part. 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 in woodworking than metalworking. Most woodworking machines, including
power saws such as
circular saws and
band saws,
jointers, and
thickness planers, 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
black 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 (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 or 25 millimeters in diameter, can be dangerous and can lead 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
hardens very easily under
cold working, 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 tool's 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. The spindle speeds may be calculated for all machining operations once the SFM or MPM is known. In most cases, we are dealing with a cylindrical object such as a milling cutter or a workpiece turning in a lathe so we need to determine the speed at the periphery of this round object. This speed at the periphery (of a point on the circumference, moving past a stationary point) will depend on the rotational speed (RPM) and diameter of the object. One analogy would be a
skateboard rider and a
bicycle rider travelling side by side along the road. For a given surface speed (the speed of this pair along the road) the rotational speed (RPM) of their wheels (large for the skater and small for the bicycle rider) will be different. This rotational speed (RPM) is what we are calculating, given a fixed surface speed (speed along the road) and known values for their wheel sizes (cutter or workpiece). The following formulae may be used to estimate this value.
Approximation The exact RPM is not always needed, a close approximation will work. For instance, a machinist may want to take the value of {\pi } to be 3 if performing calculations by hand. :RPM = {Cutting Speed\times 12 \over \pi \times Diameter} e.g. for a cutting speed of 100 ft/min (a plain HSS steel cutter on mild steel) and diameter of 10 inches (the cutter or the work piece) :RPM = {Cutting Speed\times 12 \over \pi \times Diameter} = {12 \times 100 ft/min \over 3 \times 10 inches} = {40 revs/min} and, for an example using metric values, where the cutting speed is 30 m/min and a diameter of 10 mm (0.01 m), :RPM = {Speed \over \pi \times Diameter} = { 1000 \times 30 m/min \over 3 \times 10 mm} = {1000 revs/min}
Accuracy However, for more accurate calculations, and at the expense of simplicity, this formula can be used: :RPM = {Speed \over Circumference}={Speed \over \pi \times Diameter} and using the same example :RPM = {100 ft/min \over \pi \times 10 \, inches \left ( \frac{1 ft}{12 \, inches} \right )} = {100 \over 2.62} = 38.2 revs/min and using the same example as above :RPM = {30 m/min \over \pi \times 10 \, mm \left ( \frac{1 m}{1000 \, mm} \right )} = {1000*30 \over \pi*10} = 955 revs/min where: •
RPM is the rotational speed of the cutter or workpiece. •
Speed is the recommended cutting speed of the material in meters/minute or feet/min •
Diameter in millimeters or inches. == Feed rate ==