Speeds and feeds

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The phrase speeds and feeds (or feeds and speeds) refers to two separate velocities in machine tool practice, cutting speed and feed rate. They are often considered as a pair because of their combined effect on the cutting process. Each, however, can also be considered and analyzed in its own right (as done in subsequent sections below). Cutting speed is the speed difference between the cutting tool and the surface of the workpiece it is operating on. (As discussed later, it is related to, but not the same as, spindle speed.) It is expressed in units of distance along the workpiece surface per time (typically surface feet per minute [sfm]). Feed rate is the velocity at which the cutter is fed, that is, advanced against the workpiece. It is expressed in units of distance per revolution for turning and boring (typically inches per revolution [ipr] or millimeters per revolution). It can be expressed thus for milling also, but it is often expressed in units of distance per time for milling (typically inches per minute [ipm] or millimeters per minute). Cutting speed and feed rate together determine the material removal rate, which is the volume of workpiece material (metal, wood, plastic, etc.) that can be removed per time unit.

If variables such as cutter geometry and the rigidity of the machine tool and its tooling setup could be ideally maximized (and reduced to negligible constants), then the amount of power (that is, kilowatts or horsepower) available to the spindle would determine the maximum speeds and feeds possible for any given workpiece material and cutter material. Of course, in reality those other variables are dynamic and not negligible; but there is still a correlation between power available and feeds and speeds employed.

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 (stationary) workpiece moving past a (rotating) cutter, 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).

Machinability rating

When calculating for other copper alloys the machine rating is arrived at by assuming the 100 rating of 600 SFM. For example, phosphorous bronze (grades A–D) has a machinability rating of 20. This means that phosphor bronze runs at 20% the speed of 600 SFM or 120 SFM. However, 165 SFM is generally accepted as the basic 100% rating for "grading steels".^[6]

Spindle speed

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:

• 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.
• 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 potentially 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 with a feature called Constant Surface Speed (CSS). 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, 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 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 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 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 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 cutting speed is known. As noted above, the cutting speed is the surface speed of the material as it passes the cutter, 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^[7] may be used to estimate this value.

Approximation

The exact RPM is not always needed, a close aproximation will work. This formula uses a constant of 4 to replace the 12/pi part of the calculation, so that the arithmetic can be quickly and simply performed. Speed is in feet per minute, Diameter is in inches.

eg: 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, it doesn't matter)

Accuracy

However for more accurate calculations, and at the expense of simplicity, this formula can be used:

and using the same example as above

where:

• RPM is the rotational speed of the cutter or workpiece.
• k is a constant, (320 for metric units, 4 for imperial unit units)
• Speed is the recommended cutting speed of the material (depending on k) in meters/minute or feet/min
• Diameter (again, depending on k) in millimeters or inches
• π is the constant pi (3.1416)
• Circumference of the workpiece measured in meters or feet
• conversion of inches to feet

Feed rate

Feed rate is the velocity at which the cutter is fed, that is, advanced against the workpiece. It is expressed in units of distance per revolution for turning and boring (typically inches per revolution [ipr] or millimeters per revolution). It can be expressed thus for milling also, but it is often expressed in units of distance per time for milling (typically inches per minute [ipm] or millimeters per minute), with considerations of how many teeth (or flutes) the cutter has then determining what that means for each tooth.

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.

When deciding what feed rate to use for a certain cutting operation, the calculation is fairly straightforward for single-point cutting tools, because all of the cutting work is done at one point (done by "one tooth", as it were). With a milling machine or jointer, where multi-tipped/multi-fluted cutting tools are involved, then the desirable feed rate becomes dependent on the number of teeth on the cutter, as well as the desired amount of material per tooth to cut (expressed as chip load). 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.

Formula to determine feed rate

This formula^[8] can be used to figure out the feed rate that the cutter travels into or around the work. This would apply to cutters on a milling machine, drill press and a number of other machine tools. This is not to be used on the lathe for turning operations, as the feedrate on a lathe is given as inches per revolution.

Where:

• FR = the calculated feed rate in inches per minute or mm per minute.
• RPM = is the calculated speed for the cutter.
• T = Number of teeth on the cutter.
• CL = The chip load or feed per tooth. This is the size of chip that each tooth of the cutter takes.

References

Further reading

• Groover, Mikell P. (2007). "Theory of Metal Machining". Fundamentals of Modern Manufacturing (3^rd ed.). John Wiley & Sons, Inc.. pp. 491–504. ISBN 0471744859. 

External links

• Speed and feed calculator