Metal Cutting & Cutting Tool for Lathe

The process in which a thin layer of excess metal (chip) is removed by a wedge-shaped single-point or multipoint cutting tool with defined geometry from a work piece, through a process of extensive plastic deformation.

The cutting itself is a process of extensive plastic deformation to form a chip that is removed afterward. The basic mechanism of chip formation is essentially the same for all machining operations. Assuming that the cutting action is continuous, we can develop so-called continuous model of cutting process shown in the figure:

Depending on whether the stress and deformation in cutting occur in a plane (2-d case) or in the space (3-d case), we consider two principle types of cutting:

2.2.1 Orthogonal cutting the cutting edge is straight and is set in a position that is perpendicular to the direction of primary motion. This allows us to deal with stresses and strains that act in a plane.

2.2.2 Oblique cutting the cutting edge is set at an angle (the tool cutting edge inclination ?s). This is the case of three-dimensional stress and strain conditions.

Cutting tools are most important components in machining process and the efficiency of operation depends on the performance of tools. According to the number of active cutting edges engaged in cutting, we distinguish again two types of cutting:

2.3.1 Single-point cutting tool has only one major cutting edge. Examples: turning, shaping, boring.

2.3.2 Multipoint cutting tool has more than one major cutting edge Examples: drilling, milling, broaching, reaming. Abrasive machining is by definition a process of multipoint cutting.

Each machining operation is characterized by cutting conditions, which comprises a set of three elements:

Cutting velocity (V): the traveling velocity of the tool relative to the work piece (speed difference between the cutting tool and the surface of the work piece). It is measured in m/s or m/min.

Depth of cut (d): the axial projection of the length of the active cutting tool edge, measured in mm. In orthogonal cutting it is equal to the actual width of cut.

Feed (f): the relative velocity of the tool at which it is advanced along the workpiece in order to process the entire surface of the work piece. In orthogonal cutting it is equal to the thickness of and is measured in mm/revolution in turning, or mm/min in milling and drilling. Feed rate units depend on the motion of the tool and workpiece; when the workpiece rotates (e.g., in turning and boring), the units are almost always distance per spindle revolution [mm/rev]). When the workpiece does not rotate (e.g., in milling), the units are typically distance per time [mm/min]).

Machinability is a term indicating how the work material responds to the cutting process. In the most general case good machinability means that material is cut with good surface finish, long tool life, low force and power requirements, and low cost.

Most of the cutting tools have three angles, i.e. rake angle, clearance angle and setting angle. A general purpose hand chisel is shown in the figure below with all these three angles. These angles are provided to make the process of cutting easier in terms of reduced friction between work and the tool, easy disposal of cut material (chip), reduced cutting forces, low wear and more tool life.

The standard terminology is shown in the following figure. For single point cutting tools, the most important angles are the rake angles, end relief angle and side relief angle.

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With respect to direction of feed, single point cutting tools may be classified as either left hand or right hand, depending on their cutting edge on the specified side and will cut when moved from left to right or right to left.

In right hand cutting tool the side cutting edge is on the side of the thumb when the right hand is placed on the tool with the hand fingers pointing towards the tool nose. Right hand tool cuts from right to left.

In left hand cutting tool the side cutting edge is on the thumb side when the left hand is placed on the tool. Left hand cutting tools are designed to cut best when traveling from left to right.

A wide variety of cutting-tool materials are available. The selection of a proper material depends on such factors as the cutting operation involved, the machine to be used, the work piece material, production requirements, cost, and surface finish and accuracy desired.

Cutting tools are subjected to abrasion, high temperature and contact stresses. Therefore, major qualities (properties) required in a cutting tool are (See Table 1):
(1) hard
(2) hot hardness, ability of cutting tool to maintain sharp cutting edge even when turns red because of high heat during cutting
(3) resistance to mechanical impact and thermal shock,
(4) wear resistance, and
(5) chemical stability and inertness to the workpiece material being machined.
(6) Shaped so edge can penetrate work

Lathe toolbits generally made of five materials
a. High-speed steel
b. Cast alloys (such as stellite)
c. Cemented carbides
d. Ceramics
e. Cermets
f. More exotic finding wide use
g. Borazon and polycrystalline diamond

Understanding the different types of tool steels requires knowledge of the role of different alloying elements. These elements are added to:
(1) obtain greater hardness and wear resistance,
(2) obtain greater impact toughness,
(3) impart hot hardness to the steel such that its hardness is maintained at high cutting temperatures, and
(4) decrease distortion and warpage during heat treating.

a. May contain combinations of tungsten, chromium, vanadium, molybdenum, cobalt
b. Can take heavy cuts, withstand shock and maintain sharp cutting edge under red heat
c. Generally two types (general purpose)
Molybdenum-base (Group M)
Tungsten-base (Group T)
Cobalt added if more red hardness desired

Crater wear: consists of a concave section on the tool face formed by the action of the chip sliding on the surface.

Flank wear: occurs on the tool flank as a result of friction between the machined surface of the workpiece and the tool flank.

Corner wear (nose wear): occurs on the tool corner. Can be considered as a part of the wear land and respectively flank wear since there is no distinguished boundary between the corner wear and flank wear land.

There are three types of chips that are commonly produced in cutting
(i) discontinuous chips
(ii) continuous chips
(iii) continuous chips with built up edge

A discontinuous chip comes off as small chunks or particles. When we get this chip it may indicate
(1) brittle work material
(2) small or negative rake angles
(3) coarse feeds and low speeds

A continuous chip looks like a long ribbon with a smooth shining surface. This chip type may indicate
(1) ductile work materials
(2) large positive rake angles
(3) fine feeds and high speeds

Continuous chips with a built up edge still look like a long ribbon, but the surface is no longer smooth and shining.

Cutting fluids, frequently referred to as lubricants or coolants, comprise those liquids and gases which are applied to the cutting zone in order to facilitate the cutting operation. A cutting fluid is used:
(1) to keep the tool cool and prevent it from being heated to a temperature at which the hardness and resistance to abrasion are reduced;
(2) to keep the workpiece cool, thus preventing it from being machined in a warped shape to inaccurate final dimensions;
(3) through lubrication to reduce friction and power consumption, wear on the tool, and generation of heat;
(4) to provide a good finish on the workpiece;
(5) to aid in providing a satisfactory chip formation;
(6) to wash away the chips (this is particularly desirable in deep-hole drilling, hacksawing, milling, and grinding); and
(7) to prevent corrosion of the workpiece and machine tool.

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