Milling cutter
Milling cutters are cutting tools typically used in milling machines or machining centres to perform milling operations. They remove material by their movement within the machine or directly from the cutter's shape.
Features
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 cutters 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. Often, the words flute and tooth are used interchangeably. Milling cutters may have from one to many teeth, with two, three and four 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 two-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 to give a better finish.
- Center cutting: Some milling cutters can drill straight down 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, or removing a smaller amount of material, but leaving a good surface finish. 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 teeth for removing material carefully. However, the large number of flutes leaves little room for efficient swarf removal, so they are less appropriate for removing large amounts of material.
- Coatings: The right tool coatings can have a great influence on the cutting process by increasing cutting speed and tool life, and improving the surface finish. Polycrystalline diamond is an exceptionally hard coating used on cutters that 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 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 set screw, also known as a grub screw, makes contact for increased torque without the tool slipping. The diameter may be different from the diameter of the cutting part of the tool, so that it can be held by a standard tool holder.§ The length of the shank might also be available in different sizes, with relatively short shanks called "stub", long, extra long and extra extra long.
Types
End mill
End mills are those tools that 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 and radiused cutters. They are usually made from high speed steel or cemented carbide, and have one or more flutes. They are the most common tool used in a vertical mill.Roughing end mill
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 act as many successive cutting edges producing many small chips. This results in a relatively rough surface finish, but the swarf takes the form of short thin sections and is more manageable than a thicker more ribbon-like section, resulting in smaller chips that are easier to clear. During cutting, multiple teeth are in simultaneous contact with the workpiece, reducing chatter and vibration. Rapid stock removal with heavy milling cuts is sometimes called hogging. Roughing end mills are also sometimes known as "rippa" or "ripper" cutters.Ball cutter
Ball nose cutters or ball end mills 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 moulds and dies. They are sometimes called ball mills in shop-floor slang, despite the fact that that term also has another meaning. They are also used to add a radius between perpendicular faces to reduce stress concentrations.A bull nose cutter mills a slot with a corner radius, intermediate between an end mill and ball cutter; for example, it may be a 20 mm diameter cutter with a 2 mm radius corner. The silhouette is essentially a rectangle with its corners truncated.
Slab mill
Slab mills are used either by themselves or in gang milling operations on manual horizontal or universal milling machines to machine large broad surfaces quickly. They have been superseded by the use of cemented carbide-tipped face mills which are then used in vertical mills or machining centres.Side-and-face cutter
The side-and-face cutter is designed with cutting teeth on its side as well as its circumference. They are made in varying diameters and widths depending on the application. The teeth on the side allow the cutter to make unbalanced cuts without deflecting the cutter as would happen with a slitting saw or slot cutter.Cutters of this form factor were the earliest milling cutters developed. From the 1810s to at least the 1880s they were the most common form of milling cutter, whereas today that distinction probably goes to end mills. Traditionally, HSS side and face cutters are used to mill slots and grooves.
Involute gear cutter
There are 8 cutters that will cut gears from 12 teeth through to a rack.Hob
These cutters are a type of form tool and are used in hobbing machines to generate gears. A cross-section of the cutter's tooth will generate the required shape on the workpiece, once set to the appropriate conditions. A hobbing machine is a specialised milling machine.Thread mill
Whereas a hob engages the work much as a mating gear would, a thread milling cutter operates much like an endmill, traveling around the work in a helical interpolation.Face mill
A face mill is a cutter designed for facing as opposed to e.g., creating a pocket. The cutting edges of face mills are always located along its sides. As such it must always cut in a horizontal direction at a given depth coming from outside the stock. Multiple teeth distribute the chip load, and since the teeth are normally disposable carbide inserts, this combination allows for very large and efficient face milling.Fly cutter
A fly cutter is composed of a body into which one or two tool bits are inserted. As the entire unit rotates, the tool bits take broad, shallow facing cuts. Fly cutters are analogous to face mills in that their purpose is face milling and their individual cutters are replaceable. Face mills are more ideal in various respects, but tend to be expensive, whereas fly cutters are very inexpensive.Most fly cutters simply have a cylindrical center body that holds one tool bit. It is usually a standard left-hand turning tool that is held at an angle of 30 to 60 degrees. Fly cutters with two tool bits have no "official" name but are often called double fly cutters, double-end fly cutters, or fly bars. The latter name reflects that they often take the form of a bar of steel with a tool bit fastened on each end. Often these bits will be mounted at right angles to the bar's main axis, and the cutting geometry is supplied by using a standard right-hand turning tool.
Regular fly cutters are widely sold in machinists' tooling catalogs. Fly bars are rarely sold commercially; they are usually made by the user. Fly bars are perhaps a bit more dangerous to use than endmills and regular fly cutters because of their larger swing. As one machinist put it, running a fly bar is like "running a lawn mower without the deck", that is, the exposed swinging cutter is a rather large opportunity to take in nearby hand tools, rags, fingers, and so on. However, given that a machinist can never be careless with impunity around rotating cutters or workpieces, this just means using the same care as always except with slightly higher stakes. Well-made fly bars in conscientious hands give years of trouble-free, cost-effective service for the facing off of large polygonal workpieces such as die/mold blocks.
Woodruff cutter
Woodruff cutters are used to cut the keyway for a Woodruff key.Hollow mill
Hollow milling cutters, more often called simply hollow mills, are essentially "inside-out endmills". They are shaped like a piece of pipe, with their cutting edges on the inside surface. They were originally used on turret lathes and screw machines as an alternative to turning with a box tool, or on milling machines or drill presses to finish a cylindrical boss. Hollow mills can be used on modern CNC lathes and Swiss style machines. An advantage to using an indexable adjustable hollow mill on a Swiss-style machine is replacing multiple tools. By performing multiple operations in a single pass, the machine does not need as can accommodate other tools in the tool zone and improves productivity.More advanced hollow mills use indexable carbide inserts for cutting, although traditional high speed steel and carbide-tipped blades are still used.
Hollow milling has an advantage over other ways of cutting because it can perform multiple operations. A hollow mill can reduce the diameter of a part and also perform facing, centering, and chamfering in a single pass.
Hollow mills offer an advantage over single point tooling. Multiple blades allow the feed rate to double and can hold a closer concentricity. The number of blades can be as many as 8 or as few as 3. For significant diameter removal, more blades are necessary.
Trepanning is also possible with a hollow mill. Special form blades can be used on a hollow mill for trepanning diameters, forms, and ring grooves.
Interpolation is also not necessary when using a hollow mill; this can result in a significant reduction of production time.
Both convex and concave spherical radii are possible with a hollow mill. The multiple blades of a hollow mill allow this radius to be produced while holding a tight tolerance.
A common use of a hollow mill is preparing for threading. The hollow mill can create a consistent pre-thread diameter quickly, improving productivity.
An adjustable hollow mill is a valuable tool for even a small machine shop to have because the blades can be changed out for an almost infinite number of possible geometries.