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Milling cutters are cutting tools used in milling machines or machining centres. They remove material by their movement within the machine (eg: a ball nose mill) or directly from the cutters shape (a form tool such as a Hobbing cutter).
[edit] Features of a milling cutter
Milling cutters come in several shapes and many sizes. There is also a choice of coatings, as well as rake angle and number of cutting surfaces.
- Shape: Several standard shapes of milling cutter are used in industry today, which are explained in more detail below.
- Flutes / teeth: The flutes of the milling bit are the deep helical grooves running up the cutter, while the sharp blade along the edge of the flute is known as the tooth. The tooth cuts the material, and chips of this material are pulled up the flute by the rotation of the cutter. There is almost always one tooth per flute, but some cutters have two teeth per flute.[1] Often, the words flute and tooth are used interchangeably. Milling cutters may have from one to many teeth, with 2, 3 and 4 being most common. Typically, the more teeth a cutter has, the more rapidly it can remove material. So, a 4-tooth cutter can remove material at twice the rate of a 2-tooth cutter.
- Helix angle: The flutes of a milling cutter are almost always helical. If the flutes were straight, the whole tooth would impact the material at once, causing vibration and reducing accuracy and surface quality. Setting the flutes at an angle allows the tooth to enter the material gradually, reducing vibration. Typically, finishing cutters have a higher rake angle (tighter helix) to give a better finish.
- Center cutting: Some milling cutters can drill straight down (plunge) through the material, while others cannot. This is because the teeth of some cutters do not go all the way to the centre of the end face. However, these cutters can cut downwards at an angle of 45 degrees or so.
- Roughing or Finishing: Different types of cutter are available for cutting away large amounts of material, leaving a poor surface finish (roughing), or removing a smaller amount of material, but leaving a good surface finish (finishing). A roughing cutter may have serrated teeth for breaking the chips of material into smaller pieces. These teeth leave a rough surface behind. A finishing cutter may have a large number (4 or more) teeth for removing material carefully. However, the large number of flutes leaves little room for efficient swarf removal, so they are less good for removing large amounts of material.
- Coatings: Tool coatings can have a great influence on the cutting process The right coating can increase cutting speed and tool life, and improve the surface finish. Polycrystalline Diamond (PCD) is an exceptionally hard coating used on cutters which must withstand high abrasive wear. A PCD coated tool may last up to 100 times longer than an uncoated tool. However the coating cannot be used at temperatures above 600 degrees C, or on ferrous metals. Tools for machining aluminium are sometimes given a coating of TiAlN. Aluminium is a relatively sticky metal, and can weld itself to the teeth of tools, causing them to appear blunt. However it tends not to stick to TiAlN, allowing the tool to be used for much longer in aluminium.
- Shank: The shank is the cylindrical (non-fluted) part of the tool which is used to hold and locate it in the tool holder. A shank may be perfectly round, and held by friction, or it may have a Weldon Flat, where a grub screw makes contact for increased torque without the tool slipping. The diametre may be different from the diametre of the cutting part of the tool, so that it can be held by a standard tool holder.
[edit] End mills
End mills (middle row in image) are those tools which have cutting teeth at one end, as well as on the sides. The words end mill are generally used to refer to flat bottomed cutters, but also include rounded cutters (referred to as ball nosed) and radiused cutters (referred to as bull nose, or torus). They are usually made from high speed steel(HSS) or carbide, and have one or more flutes. They are the most common tool used in a vertical mill.
[edit] Slot drills
Slot drills (top row in image) are generally two (occasionally three or four) fluted cutters that are designed to drill straight down into the material. This is possible because there is at least one tooth at the centre of the end face. They are so named for their use in cutting keyway slots. The words slot drill are usually assumed to mean a two fluted, flat bottomed end mill if no other information is given. Two fluted end mills are usually slot drills, three fluted sometimes aren't, and four fluted usually aren't.
[edit] Roughing end mills
Roughing end mills quickly remove large amounts of material. This kind of end mill utilizes a wavy tooth form cut on the periphery. These wavy teeth form many successive cutting edges producing many small chips, resulting in a relatively smooth surface finish. During cutting, multiple teeth are in contact with the workpiece reducing chatter and vibration.
[edit] Ball nose cutter
Ball nose cutters (lower row in image) are similar to slot drills, but the end of the cutters are hemispherical. They are ideal for machining 3 dimensional contoured shapes in machining centres, for example in molds and dies.
[edit] Types of milling cutter
[edit] Using a milling cutter
[edit] Chip Formation
Although there are many different types of milling cutter, understanding chip formation is fundamental to the use of any of them. As the milling cutter rotates, the material to be cut is fed into it, and each tooth of the cutter cuts away small chip of material. Achieving the correct size of chip is of critical importance. The size of this chip depends on several variables.
- Surface cutting speed (Vc): This is the speed at which each tooth cuts through the material as the tool spins. This is measured either in metres per minute in metric countries, or surface feet per minute (SFM) in America. Typical values for cutting speed are 10m/min to 60m/min for some steels, and 100m/min and 600m/min for aluminium. This should not be confused with the feed rate.
- Spindle speed (S): This is the rotation speed of the tool, and is measured in revolutions per minute (rpm). Typical values are from hundreds of rpm, up to tens of thousands of rpm.
- Diametre of the tool (D):
- Feed per tooth (Fz): This is the distance the material is fed into the cutter as each tooth rotates. This value is the size of the deepest cut the tooth will make.
- Feed rate (F): This is the speed at which the material is fed into the cutter. Typical values are from 20mm/min to 5000mm/min.
- Depth of cut: This is how deep the tool is under the surface of the material being cut (not shown on the diagram). This will be the height of the chip produced. Typically, the depth of cut will be less than or equal to the diametre of the cutting tool.
The machinist needs three values: S, F and Depth when deciding how to cut a new material with a new tool. However, he will probably be given values of Vc and Fz from the tool manufacturer. S and F can be calculated from them:
| Spindle Speed | Feed rate |
|---|---|
 |  |
| Looking at the formula for the spindle speed, S, it can be seen that larger tools require lower spindle speeds, while small tools may be able to go at high speeds. | The formula for the feed rate, F shows that increasing S or z gives a higher feed rate. Therefore, machinists may choose a tool with the highest number of teeth that can still cope with the swarf load. |
[edit] Conventional milling vs climb milling
A milling cutter can cut in two directions, sometimes known as climb or conventional.
- Conventional milling: (left) The depth of the cut starts at zero thickness, and increases up to the maximum. The cut is so light at the beginning that the tool does not cut, but slides across the surface of the material, until sufficient pressure is built up and the tooth suddenly bites and begins to cut. This deforms the material (at point A on the diagram, left), work hardening it, and dulling the tool. The sliding and biting behaviour leaves a poor finish on the material.
- Climb milling: (above) Each tooth engages the material at a definite point, and the width of the cut starts at the maximum and decreases to zero. The chips are disposed behind the cutter, leading to easier swarf removal. The tooth does not rub on the material, and so tool life may be longer. However, climb milling can apply larger loads to the machine, and so is not recommended for older milling machines, or machines which are not in good condition. This type of milling is used predominantly on mills with a backlash eliminator.
[edit] Swarf removal
Another important quality of the milling cutter to consider is its ability to deal with the swarf generated by the cutting process. If the swarf is not removed as fast as it is produced, the flutes will clog and prevent the tool cutting efficiently, causing vibration, tool wear and overheating. Several factors affect swarf removal, including the depth and angle of the flutes, the size and shape of the chips, the flow of coolant, and the surrounding material. It may be difficult to predict, but a good machinist will watch out for swarf build up, and adjust the milling conditions if it is observed.
[edit] Selecting a milling cutter
Selecting a milling cutter is not a simple task. There are many variables, opinions and lore to consider, but essentially the machinist is trying to choose a tool which will cut the material to the required specification for the least cost. The cost of the job is a combination of the price of the tool, the time taken by the milling machine, and the time taken by the machinist. Often, for jobs of a large number of parts, and days of machining time, the cost of the tool is lowest of the three costs.
- Material: High speed steel (HSS) are the cheapest, and shortest lived cutters. Carbide tools are more expensive, but last longer, and so prove more economical in the long run.
- Diameter: Larger tools can remove material faster than small ones, therefore the largest possible cutter is usuall chosen, that will fit in the job.
- Flutes: More flutes allows a higher feed rate, because there is less material removed per flute. But because the core diameter increases, there is less room for swarf. So a balance has to be chosen.
- Coating:
- Helix angle:
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