What is an End Mill?
An end mill is a cutting tool most commonly used in a vertical milling machine. They are called end mills because both the side and the end of the tool are sharpened. End mills resemble spiral drill bits in many ways but unlike drill bits end mills are designed to take heavy side loads and can cut both straight down into the material (plunge), or along the side or top in a back and forth manner (facing). There are many varieties of end mills which can be distinguished by the number of flutes, shape, and material.
End Mill Construction
End mills have two or more cutting edges and a corresponding number of flutes. A flute is the groove in both spiral drill bits and milling cutters. It runs parallel with the cutting edge and serves as a path for shavings to exit the work area. The "helix angle" of the flute is an important characteristic of the end mill; it is a measure of how steep the flute is. Helix angle is measured from the cutter axis. The actual value can vary from 0 degrees (straight-tooth, no spiral at all) to around 45 degrees (fast-helix) for some special purpose cutters. Standard helix angles are closer to 30 degrees.
The most common material for end mills is high speed steel (HSS), although carbide tooling is becoming more and more popular because of longer cutter life and superior performance.
Types of End Mills
Solid End Mills
Solid end mills are the most common type of mill cutter used in the small shop. The most common type of solid end mill has an end that is perpendicular to the axis of the cutter, the other types of solid end mills are described in their own section. The most common type of end mills have either two or four flutes though it is possible to have more. The number of flutes on an end mill strongly effect it's usage. Two flute end mills have larger flutes. This means that there is a larger path for debris and they can therefore remove more material. Two flute end mills are however not as strong as the four flute variety. Two flute mills also are more apt to chatter* because it is not uncommon to have only a single cutting edge in contact with the material at any one time. As one edge comes into contact with the material the other edge looses contact. This constant making and breaking of contact can cause resonance in the machining setup which can cause anything from a poor finish to a broken tool.
Because of these properties two flute end mills are usually reserved for soft materials such as aluminum and plastics where the low load and high material removal rate of the tool can be taken advantage of. Four flute end mills are used extensively with steel and harder materials because of the higher load capacity and better surface finish.
Another property which is important when considering an end mill is whether it is "center cutting". Center cutting means that an end mill has a cutting edge/edges that extends/extend to the center of the mills axis. Center cutting end mills can be plunged directly into the material much like a twist drill. Non center cutting end mills cannot do do this unless a pilot hole already exists. In other words center cutting end mills are capable of creating holes in materials, non center cutting end mills are only capable of enlarging existing holes. Two flute end mills are inherently center cutting, four flute end mills are not, but center cutting varieties are available. Center cutting four flute end mills can be identified by looking at the end of the tool. On this type there will be two (no more, no less) opposing edges that meet at the center of the tool. If your end mill does not have this feature, then you have a non center cutting end mill.
Ball End Mills
The end of a ball end mill is hemispherical, the radius of the hemisphere equal to the radius of the end mill body. Ball end mills are used for cutting half round channels as well as in
die making to produce
complex internal curves. Ball end mills are usually solid and two fluted.
Tapered End Mills
Tapered end mill resemble standard end mills except that the tool itself is tapered (thicker at the top than at the bottom). Tapered end mills are also used in die making when a particular angled edge cut is desired. Tapered end mills come in a range of tapers, usually in 1 degree increments between 1 and 10 degrees.
Roughing End Mills
Roughing end mills, as their name implies, are used for roughing, or removing large amounts of material quickly. They are not usually used as a final pass. Roughing end mills resemble a standard end mill that has had threads cut on its cutting edges. These "threads" are actually all separate cutting edges. They serve to break the material into smaller pieces to facilitate removal. Roughing end mills are usually of the four flute variety.
Shell End Mills
Shell end mills are so named because they are look like a solid end mill that has had it's center removed, leaving only a shell. Shell end mills are usually much larger than solid end mills and therefore designed for much larger jobs. Shell end mills also have more teeth than their solid counterparts and as such produce a cleaner finish with less chatter.
* From 2
Chatter: Vibration of workpiece, machine, tool, or a combination of the three due to looseness or a weakness in one or more of these areas. Chatter may be found in either grinding or machine operations and is usually noted as vibratory sound and seen on the workpiece as wave marks.
Olivo, C. Thomas. Machine Tool Technology and Manufacturing Processes. Albany: C. Thomas Olivo Associates. 1987
Kibbe, Richard R., et al. Machine Tool Practices 5th ed. Englewood Cliffs: Prentice Hall. 1995