Cutting tool (machining)

In the context of machining, a cutting tool (or cutter) is any tool that is used to remove material from the workpiece by means of shear deformation. Cutting may be accomplished by single-point or multipoint tools. Single-point tools are used in turning, shaping, plaining and similar operations, and remove material by means of one cutting edge. Milling and drilling tools are often multipoint tools. Grinding tools are also multipoint tools. Each grain of abrasive functions as a microscopic single-point cutting edge (although of high negative rake angle), and shears a tiny chip.

Cutting tools must be made of a material harder than the material which is to be cut, and the tool must be able to withstand the heat generated in the metal-cutting process. Also, the tool must have a specific geometry, with clearance angles designed so that the cutting edge can contact the workpiece without the rest of the tool dragging on the workpiece surface. The angle of the cutting face is also important, as is the flute width, number of flutes or teeth, and margin size. In order to have a long working life, all of the above must be optimized, plus the speeds and feeds at which the tool is run.

Types

Linear cutting tools include tool bits (single-point cutting tools) and broaches. Rotary cutting tools include drill bits, countersinks and counterbores, taps and dies, milling cutters, and reamers. Other cutting tools, such as bandsaw blades and fly cutters, combine aspects of linear and rotary motion. Also hack saws and Bench saws.

Cutting tool inserts

Cutting tools are often designed with inserts or replaceable tips (tipped tools). In these, the cutting edge consists of a separate piece of material, either brazed, welded or clamped on to the tool body. Common materials for tips include tungsten carbide, polycrystalline diamond, and cubic boron nitride.^[1] Tools using inserts include milling cutters (endmills, fly cutters), tool bits, and saw blades.

Materials

To produce quality p, a cutting tool must have three characteristics:

• Hardness — hardness and strength at high temperatures.
• Toughness — toughness, so that tools don’t chip or fracture.
• Wear resistance — having acceptable tool life before needing to be replaced.^[2]

Cutting tool materials can be divided into two main categories: stable and unstable.

Unstable materials (usually steels) are substances that start at a relatively low hardness point and are then heat treated to promote the growth of hard particles (usually carbides) inside the original matrix, which increases the overall hardness of the material at the expense of some its original toughness. Since heat is the mechanism to alter the structure of the substance and at the same time the cutting action produces a lot of heat, such substances are inherently unstable under machining conditions.

Stable materials (usually tungsten carbide) are substances that remain relatively stable under the heat produced by most machining conditions, as they don't attain their hardness through heat. They wear down due to abrasion, but generally don't change their properties much during use.

Most stable materials are hard enough to break before flexing, which makes them very fragile. To avoid chipping at the cutting edge, most tools made of such materials are finished with a sightly blunt edge, which results in higher cutting forces due to an increased shear area. Fragility combined with high cutting forces results in most stable materials being unsuitable for use in anything but large, heavy and stiff machinery.

Unstable materials, being generally softer and thus tougher, generally can stand a bit of flexing without breaking, which makes them much more suitable for unfavorable machining conditions, such as those encountered in hand tools and light machinery.

Tool material	Properties
Carbon steel	Unstable. Very inexpensive. Extremely sensitive to heat. Considered obsolete today although it is still found in non-intensive applications such as hand operated tools (e.g. reamers and taps). Hardness up to about HRC 65. Sharp cutting edges possible.
High speed steel (HSS)	Unstable. Inexpensive. Retains hardness at moderate temperatures. The most common cutting tool material used today. Used extensively on drill bits and taps. Hardness up to about HRC 67. Sharp cutting edges possible.
HSS cobalt	Unstable. Moderately expensive. The high cobalt versions of high speed steel are very resistant to heat and thus excellent for machining abrasive and/or work hardening materials such as titanium and stainless steel. Used extensively on milling cutters and drill bits. Hardness up to about HRC 70. Sharp cutting edges possible.
Cast cobalt alloys	Stable. Expensive. Somewhat fragile. Despite its stability it doesn't allow for high machining speed due to low hardness. Not used much. Hardness up to about HRC 65. Sharp cutting edges possible.
Cemented carbide	Stable. Moderately expensive. The most common material used in the industry today. It is offered in several "grades" containing different proportions of tungsten carbide and binder (usually cobalt). High resistance to abrasion. High solubility in iron requires the additions of tantalum carbide and niobium carbide for steel usage. Its main use is in turning tool bits although it is very common in milling cutters and saw blades. Hardness up to about HRC 90. Sharp edges generally not recommended.
Ceramics	Stable. Moderately inexpensive. Chemically inert and extremely resistant to heat, ceramics are usually desirable in high speed applications, the only drawback being their high fragility. Ceramics are considered unpredictable under unfavorable conditions. The most common ceramic materials are based on alumina (aluminium oxide), silicon nitride and silicon carbide. Used almost exclusively on turning tool bits. Hardness up to about HRC 93. Sharp cutting edges and positive rake angles are to be avoided.
Cermets	Stable. Moderately expensive. Another cemented material based on titanium carbide (TiC). Binder is usually nickel. It provides higher abrasion resistance compared to tungsten carbide at the expense of some toughness. It is far more chemically inert than it too. Extremely high resistance to abrasion. Used primarily on turning tool bits although research is being carried on producing other cutting tools. Hardness up to about HRC 93. Sharp edges generally not recommended.
Cubic boron nitride (CBN)	Stable. Expensive. Being the second hardest substance known, it is also the second most fragile. It offers extremely high resistance to abrasion at the expense of much toughness. It is generally used in a machining process called "hard machining", which involves running the tool or the part fast enough to melt it before it touches the edge, softening it considerably. Used almost exclusively on turning tool bits. Hardness higher than HRC 95. Sharp edges generally not recommended.
Diamond	Stable. Very Expensive. The hardest substance known to date. Superior resistance to abrasion but also high chemical affinity to iron which results in being unsuitable for steel machining. It is used where abrasive materials would wear anything else. Extremely fragile. Used almost exclusively on turning tool bits although it can be used as a coating on many kinds of tools. Sharp edges generally not recommended.

Construction

Cutting tools that rotate often have the following features:

Flute 

A recessed portion of the tool's cross-section that conveys chips away from a cutting edge as the tool rotates. In the common twist drill, two flutes are usually provided, one for each cutting edge. Taps and end mills may have up to six or more cutting edges and flutes.

References

1. ^ Stephenson, David A.; Agapiou, John S. (1997), Metal cutting theory and practice, Marcel Dekker, p. 164, ISBN 9780824795795, http://books.google.com/books?id=19AdVu1O4fQC&pg=PA164. 
2. ^ Schneider, George, "Chapter 1 - Cutting Tool Materials," American Machinist, October, 2009