Speeds and feeds
The phrase speeds and feeds or feeds and speeds refers to two separate parameters 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.
Cutting speed is the speed difference between the cutting tool and the surface of the workpiece it is operating on. It is expressed in units of distance across the workpiece surface per unit of time, typically surface feet per minute or meters per minute. Feed rate is the relative velocity at which the cutter is advanced along the workpiece; its vector is perpendicular to the vector of cutting speed. Feed rate units depend on the motion of the tool and workpiece; when the workpiece rotates, the units are almost always distance per spindle revolution. When the workpiece does not rotate, the units are typically distance per time, although distance per revolution or per cutter tooth are also sometimes used.
If variables such as cutter geometry and the rigidity of the machine tool and its tooling setup could be ideally maximized, then only a lack of power available to the spindle would prevent the use of the maximum possible speeds and feeds 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. In practice, lack of rigidity is usually the limiting constraint.
Outside of the context of machine tooling, "speeds and feeds" can be used colloquially to refer to the technical details of a product or process.
Cutting speed
Cutting speed may be defined as the rate at the workpiece surface, irrespective of the machining operation used. A cutting speed for mild steel of 100 ft/min is the same whether it is the speed of the cutter passing over the workpiece, such as 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 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 can be calculated from this speed. Factors affecting the calculation of cutting speed are:
- The material being machined
- The material the cutter is made from
- The economical life of the cutter
- Metal removal rate
- Full and constant flow of cutting fluid
- Rigidity of the machine and tooling setup
- Continuity of cut
- Condition of material
| Material type | Meters per min | Surface feet per min |
| Steel | 18–50 | 60–100 |
| Mild Steel | 3–38 | 10–125 |
| Mild Steel | 6–7 | 20–25 |
| Cast Iron | 1–2 | 6–8 |
| Alloy Steels | 3–20 | 12–65 |
| Carbon Steels | 4–51 | 0–70 |
| Free Cutting Steels | 35–69 | 115–225 |
| Stainless Steels | 23–40 | 30–75 |
| Bronzes | 24–45 | 10–80 |
| Leaded Steel | 91 | 30 |
| Aluminium | 122-305 | 400-1000 |
| Brass | 90–210 | 300–700 |
| Machinable Wax | 6 | 20 |
| Acetal Copolymer | 11 | 35 |
| Polyethylene | 12 | 40 |
| Acrylic | 15 | 50 |
| Wood | 183–305 | 600–1000 |
Machinability rating
The machinability rating of a material attempts to quantify the machinability of various materials. It is expressed as a percentage or a normalized value. The American Iron and Steel Institute determined machinability ratings for a wide variety of materials by running turning tests at 180 surface feet per minute. It then arbitrarily assigned 160 Brinell B1112 steel a machinability rating of 100%. The machinability rating is determined by measuring the weighed averages of the normal cutting speed, surface finish, and tool life for each material. Note that a material with a machinability rating less than 100% would be more difficult to machine than B1112 and material and a value more than 100% would be easier.Machinability ratings can be used in conjunction with the Taylor tool life equation, in order to determine cutting speeds or tool life. It is known that B1112 has a tool life of 60 minutes at a cutting speed of 100 sfpm. If a material has a machinability rating of 70%, it can be determined, with the above knowns, that in order to maintain the same tool life, the cutting speed must be 70 sfpm.
When calculating for copper alloys, the machine rating is arrived at by assuming the 100 rating of 600 SFM. For example, phosphorus bronze 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".
Formula
Cutting Speed = /1000 m/min
Where
D=Diameter of Workpiece in meter or millimeter
N=Spindle Speed in rpm
Spindle speed
The spindle speed is the rotational frequency of the spindle of the machine, measured in revolutions per minute. The preferred speed is determined by working backward from the desired surface speed and incorporating the diameter.The spindle may hold the:
- Material
- 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.
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. 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 . 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, 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 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.