Mechanical Knowledge

Milling Tools and Different Types of Milling Tools

This article is intended for entry-level readers who have a basic understanding of milling machines and their operation. This article is also useful to anyone interested in the metalworking industry who wants to gain knowledge about material removal in CNC machining.

From this article, you will know:
What are milling tools?
Features of a milling tools
What are the Types of Milling tools and Their Application ?

What are milling tools?

Milling tools are cutting tools typically used in milling machines or machining centers to perform milling operations. They remove material by their movement within the machine or directly from the cutter's shape.

 Milling Tools

Features of a milling tools

Milling cutters come in many shapes and sizes. There is also a choice of coatings, as well as a rake angle and number of cutting surfaces.

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 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 center of the end face. However, these cutters can cut downwards at an angle of 45 degrees or so.

Roughing or Finishing: Different types of cutters 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 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 (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 Titanium. 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 Titanium, 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 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.

 Milling Tools

What are the Types of Milling Tools and Their Application?

1. End Mill

End mills 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 or cemented carbide, and have one or more flutes. They are the most common tool used in a vertical mill. End mills are the most common milling cutters. End mills are available in a wide variety of lengths, diameters, and types.

Roughing end mills: They have serrations in the teeth to quickly remove large amounts of material without creating vibration. The serrations produce many small chips and a rough finish.

This type of cutter is used when you have to remove more amount of material from the workpiece. By using roughing end mills, we obtain a rough surface finishing. Roughing end mills are also famous as “rippa” cutters. They are more beneficial commercially and are used in various industrial applications.

 Milling Tools

Ball end mills: Ball cutters are also famous as ball nosed cutters. You can be easily identified as ball cutters as their end is hemispherical in shape. Ball cutters are used to decrease the stress concentration and are also known as ball end mills. Whenever there is a need of cutting three-dimensional shapes then, there is a use of ball cutters to perfectly cut those three-dimensional shapes. They produce a radius at the bottom of pockets and slots. Ball end mills are used for contour milling, shallow slotting, contour milling and pocketing applications.

 Milling Tools

Dovetail Cutter: It is one of the end mills. It has a unique name as it creates a dovetail shape while cutting.

 Milling Tools

2. Face Mills

A face mill is a cutter designed for facing as opposed to e.g., creating a pocket (end mills). 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.

 Milling Tools

You’ll use this tool to make a flat area on a block of material. This is typically done on the top of the stock to flatten it before other milling tools are used. A face mill contains one solid body with multiple cutter inserts that can be swapped as needed. The more cutters, the faster metal can be removed.

3. Fly Cutters

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 (e.g., rigidity, indexability of inserts without disturbing effective cutter diameter or tool length offset, depth-of-cut capability), 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.

 Milling Tools

Regular fly cutters (one tool bit, swept diameter usually less than 100 mm) 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.

Need to produce an amazing surface finish? Fly cutters can make it happen. This cutting tools moves along the surface of a material in a clockwise rotation, producing an ultra clean shine. The best thing about fly cutters is that they are very cheaper in price. In the fly cutter, there are some non-rotating cutters are present. Fly cutters with two non-rotating cutters are called as double fly cutters or fly bars. The fly bars are highly efficient and make the cutting process easier.

4. Hollow mill

Hollow milling cutters, more often called simply hollow mills, are essentially "inside-out endmills". They are shaped like a piece of pipe (but with thicker walls), 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 (such as a trunnion). 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.

 Milling Tools

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 (roughing), more blades are necessary.

Trepanning is 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.

5. 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 (cutting on one side only) without deflecting the cutter as would happen with a slitting saw or slot cutter (no side teeth). It makes the cutting on one side an easy and efficient process.

 Milling Tools

6. Thread mill

The thread milling cutter is another type of milling cutters mostly used in various industries. It operates the same as that of the end mill. It removes the material from the workpiece by moving helically on the metal surface. It is easily identified due to the helical motion that it possesses. These are very high in price.

 Milling Tools

7. Wood Ruff Cutter

Woodruff, or keyseat/keyway cutters are used to cut key slots into parts, for example shafts. The cutting tools have teeth perpendicular to the outside diameter to produce suitable slots for Woodruff keys.

 Milling Tools

8. Slab mill

Slab mills are not that common with modern machining centres. Rather, they are still used with manual milling machines to quickly machine large surfaces. That is also why slab milling is often called surface milling.

 Milling Tools

9. Hobbing Cutter

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 (blank size). A hobbing machine is a specialised milling machine.

 Milling Tools

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