Cutters used in Machining

In machining, a cutting tool (or cutter) is a tool used to remove material from the work piece by means of shear deformation.

On the basis of point of contact, a cutter can be – single point cutting tool and multi – point cutting tool. Single point cutting tool is used in turning, shaping, plaining whereas multipoint cutting tools are used in milling, drilling.

In single point cutting tool, the metal is removed by one cutting edge.

In multi point cutting tool the multiple edges are used to remove the material.


But what about grinding wheel? Is it a single point or multiple cutting tool? In grinding, each grain of abrasive functions as a microscopic single-point cutting edge (although of high negative rake angle), and shears a tiny chip. One most important thing to remember is the 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.

Cutting tools can also be classified as per their cutting motions – linear and rotary. Linear cutting tools include tool bits (single-point cutting tools) and broaches. Rotary cutting tools include drill bits, countersinks and counter bores, taps and dies, milling cutters, and reamers. Other cutting tools, such as band saw blades and fly cutters, combine aspects of linear and rotary motion.

Cutting tools are often designed with inserts or replaceable tips (tipped tools). In these, the cutting edge consists of a separate piece of material, brazed, welded or clamped on to the tool body. Common materials for tips include tungsten carbide, polycrystalline diamond, and cubic boron nitride (CBN). Tools using inserts include milling cutters (end mills, fly cutters).
The desirable properties of tool are: Hardness (hardness and strength at high temperatures), Toughness (toughness, so that tools don’t chip or fracture) and Wear resistance.

Then we have stable and unstable materials:
Materials (usually steels, are unstable) have low hardness and high toughness, when heat treated promotes 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 the cutting action produces a lot of heat, such substances are inherently unstable under machining conditions. It is advisable to use coolant / cutting fluid while machining with these materials to prevent the burning/excessive hardening of cutters. They 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.

Whereas, materials (tungsten carbide, are stable) remain relatively stable under heat produced by most machining conditions, as they don’t attain their hardness through heat, wear down due to abrasion. They generally don’t change their properties much during use. Such 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 makes them suitable for use in large, heavy and stiff machinery.