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Several types of endmills

An endmill is a type of milling cutter, a cutting tool used in industrial milling applications. It is distinguished from the drill bit, in its application, geometry, and manufacture. While a drill bit can only cut in the axial direction, a milling bit can generally cut in all directions, though some cannot cut axially.

Endmills are used in milling applications such as profile milling, tracer milling, face milling, and plunging.

Contents

Types

Several broad categories of end- and face-milling tools exist, such as center-cutting versus non-center-cutting (whether the mill can take plunging cuts); and categorization by number of flutes; by helix angle; by material; and by coating material. Each category may be further divided by specific application and special geometry.

It is becoming increasingly common for traditional solid endmills to be replaced by more cost-effective inserted cutting tools (which, though more expensive initially, reduce tool-change times and allow for the easy replacement of worn or broken cutting edges rather than the entire tool).

Endmills are sold in both imperial and metric shank and cutting diameters. In the USA, metric is readily available, but not commonly used by machine shops; in Canada, due to the country's proximity to the US, much the same is true. In Asia and Europe, while imperial is readily available, metric diameters are standard.

Processes

In a milling operation, the workpiece is moved around the stationary cutting tool, the tool is moved across the stationary material, or some combination of the two. In any case, material is removed from the workpiece by the rotating tool. The tool is mounted to a chuck or collet and the workpiece is held in place by some sort of vise or other workholding device such as a strap clamp. Vises are good for a horizontal hold while strap clamps are used for vertical. Vertical milling machines, in which the workpiece is moved through two horizontal axes and the cutting tool is moved vertically, are common. Other machines exist which can rotate the workpiece around multiple axes, as well as machines which can rotate the cutting tool's axis of motion with respect to vertical. Feed rate and spindle speed for a milling operation can be calculated to optimize for tool wear and surface finish, and depend several variables, such as tool size, material, and geometry, use of coolant, workpiece material, width and depth of cut, and type of milling operation. Cutting tool manufactures typically supply such information along with the cutting tools. New cutting geometries as well as coatings are constantly being developed to increase the cutting speed as well as improve surface finish on all types of materials. Programming software is changing the way features are machined into parts. The types of features which used to require a specially ground form tool are now being created using new surfacing and multi-axis technology. However, in some instances it is more cost effective to have a form tool made for large production runs.

Geometry and tools

A variety of grooves, slots, and pockets in the workpiece may be produced from a variety of tool bits. Common tool bit types are: square end cutters, ball end cutters, t-slot cutters, and shell mills. Square end cutters can mill square slots, pockets, and edges. Ball end cutters mill radiused slots or fillets. T-slot cutters mill exactly that: t-shaped slots. Shell end cutters are used for large flat surfaces and for angle cuts. There are variations of these tool types as well.

There are four critical angles of each cutting tool: end cutting edge angle, axial relief angle, radial relief angle, and radial rake angle. See graph for common values.

Depending on the material being milled, and what task should be performed, different tool types and geometry may be used. For instance, when milling a material like aluminium, it may be advantageous to use a tool with very deep, polished flutes and a very sharp cutting edge. When machining a tough material such as stainless steel, however, shallow flutes and a squared-off cutting edge will optimize material removal and tool life.

A wide variety of materials are used to produce the cutting tools. Carbide inserts are the most common because they are good for high production milling. High speed steel is commonly used when a special tool shape is needed, not usually used for high production processes. Ceramics inserts are typically used in high speed machining with high production. Diamond inserts are typically used on products that require tight tolerances, typically consisting of high surface qualities (nonferrous or nonmetallic materials). In the early 1990s, use of coatings to reduce wear and friction (among other things) became more common. Most of these coatings are referred to by their chemical composition, such as:

  • TiN (a basic yellowish coating that has fallen out of wide use)
  • TiCN (a popular bluish-grey coating)
  • TiAlN (an extremely popular dark purple coating)
  • TiAlCrN (PVD coating).
  • PCD veins. Though not a coating some endmills are manufactured with a 'vein' of polycrystaline diamond. The vein is formed in a high temperature-high pressure environment. The vein is formed in a blank and then the material is ground out along the vein to form the cutting edge. The tools can be very costly, however can last many times longer than other tooling.

Advances in endmill coatings are being made, however, with coatings such as Amorphous Diamond and nanocomposite PVD coatings beginning to be seen at high-end shops (as of 2004).

Tolerances and surface finish

In mass production, endmilling machines are able of holding a tolerance of +0.001 in or -0.001 in. In precision work, tolerances improve to plus or minus 0.0005 in. Surface finishes can range from 16 to 500 microinches, but normally range from 32 to 125 microinches.

Factors affecting process results

  • Surface finishes and the tolerances produced are dependent on a few variables:
  • tool geometry and sharpness
  • cutter and feed rate
  • rigidity of tool workpiece and machine
  • alignment of machine components and fixtures
  • cutting fluid

Power requirements

Unit power is based on the horsepower required to remove one cubic inch of material per minute. Generally, the power required is proportional to the hardness of the material being machined. More power is required to remove one cubic inch per minute of mild steel or stainless steel than is required for plastic or aluminum.

Manufacture

Endmills are typically made on CNC (Computer Numeric Control) machines under high-pressure lubricants such as water, water-soluble oil, and high-flashpoint oil. Grinding inside the machine is accomplished with abrasive wheels mounted on a spindle (and in some cases, multiple spindles). Depending on what material is being ground, these wheels are made with industrial diamond (when grinding tungsten carbide), cubic boron nitride (when grinding cobalt steel), and other materials (when grinding, for instance, ceramics), set in a bond (sometimes copper).

References

  • Robert H. Todd, Dell K. Allen, Leo Alting, "Manufacturing Processes Reference Guide", Industrial Press Inc., New York, 1994 pg 49-53
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