Introduction to Lathe, Cutting Tool Nomenclature, Orthogonal cutting, Oblique cutting, Types of Chip, Tool Life & Cutting Fluids
Introduction to lathe:
Lathe is one of the oldest machine tools invented and came into existence from the Egyptian tree lathes. Egyptian tree lathes use a novel idea of rotating and machining a piece of work held between two adjacent trees. Lathe is a machine, which removes the material from a job and reduce the job to the required shape & size. The principal form of surface produced in a lathe is the cylindrical surface. This is achieved by rotating the workpiece, while the single point cutting tool removes the material by traversing in a direction parallel to the axis of rotation.
Working principle of lathe:
In a lathe, the workpiece is held in a chuck or between centres and rotated about its axis at a uniform speed. The cutting tool held in tool post is fed into the workpiece to the required depth and in desired direction. The workpiece is rotated [turned] and the cutting tool is moved relative to the workpiece. That is why, the lathes are also called “Turning machines”.
Types of lathe tools:
•Cutting tool used in lathe operations is single point cutting tool.
•Single point cutting tool means having only one cutting edge at end.
Lathe tool geometry:
The standard terminology is shown in the following figure.
For cutting tools, geometry depends mainly on the properties of the tool material and the work material. A cutting tool must posses a shape that is suited to the machining operation. The size of the tool is generally square or rectangular in cross-section. For single point tools, the most important angles are the rake angles and clearance angles.
Types of metal cutting:
The two basic methods of metal cutting are used in a single point tool are the orthogonal [2D] and oblique [3D].
Orthogonal cutting:
The cutting edge of the tool remains at 90° to the direction of feed [of the tool or the work ]. The direction of the chip flow velocity is normal to the cutting edge of the tool. Here only two components of forces are acting, Cutting Force and Thrust Force. So the metal cutting may be considered as a two dimensional cutting.
Oblique cutting:
The cutting edge of the tool remains inclined at an acute angle to the direction of tool feed or work feed. The direction of the chip flow velocity is at an angle with the normal to the cutting edge of the tool. Here three components of forces are acting: Cutting Force, Radial force and Thrust Force or feed force. So the metal cutting may be considered as a three dimensional cutting. The cutting edge being oblique, the shear force acts on a larger area and thus tool life is increased.
Chip formation:
The mechanism involving chip formation is already seen in the mechanism of metal cutting. Every Machining operation involves the formation of chips. The nature of chip formation differs from operation to operation, properties of work piece material and the cutting condition.
Types of chips:
There are three types of Chips produced in metal cutting,
•Continuous chip
•Discontinuous Chip
•Continuous Chip with Build-up Edge.
Continuous chip:
During cutting of ductile material, a continuous ribbon like chips is produced due to the pressure of the tool cutting edge in compression and shear. These type of chips are in the form of long coil and have the same thickness throughout the length. It gives the advantage of good surface finish, improving the tool life.
Discontinuous chip:
It consists of segments of chips that may be attached firmly or loosely to each other. It is formed due to brittle workpiece materials or contain hard inclusions e.g. graphite flakes. It may also be formed due to very low or very high cutting speeds. Handling of these chips is easier and it can be easily disposed off since they are having small lengths.
Continuous chip with built-up edge:
This chip consists of layers of material from the workpiece that gradually are deposited on the tool tip. As it grow larger, it eventually breaks apart from the tool tip. It affects surface finish, changes the geometry of the cutting edge and dulls.
Cutting tool materials:
Cutting tool is a device, used to remove the unwanted material from given workpiece. A cutting tool must have the following characteristics:
•Hardness: The tool material must be harder than the work piece material. Higher the hardness, easier it is for the tool to penetrate the work material.
•Toughness: Inspite of the tool being tough, it should have enough toughness to withstand the impact loads that come in the start of the cut to force fluctuations due to imperfections in the work material.
•Wear Resistance: The tool-chip and chip-work interface are exposed to severe conditions that adhesive and abrasion wear is very common. Wear resistance means the attainment of acceptable tool life before tools need to be replaced.
•Low friction: The coefficient of friction between the tool and chip should be low. This would lower wear rates and allow better chip flow.
•Thermal characteristics: Since a lot of heat is generated at the cutting zone, the tool material should have higher thermal conductivity to dissipate the heat in shortest possible time, otherwise the tool temperature would become high, reducing its life.
Tool life:
Tool life is defined as the time interval between two successive regrinds. Tool life represents the useful life of the tool expressed generally in time units from a start of a cut to some end point defined by a failure criterion. A tool that no longer performs the desired function is said to have failed and hence reached the end of its useful life.
Factors affecting tool life:
Tool life depends upon the following factors:
•Tool material.
•Hardness of the material.
•Type of material being cut.
•Type of the surface on the metal
•Profile of the cutting tool.
•Type of the machining operation being performed
•Microstructure of the material.
•Finish required on the workpiece.
Cutting fluids:
Cutting Fluids are used to:
•Reduce friction and wear, thus improving too] life and surface finish.
•Cool the cutting zone, thus improving tool life and reducing the temperature and thermal distortion.
•Reduce forces and energy consumption.
•Flush away the chips from cutting zone, thus prevent the chips from interfering cutting operations.
•Protect the machined surface from environmental corrosion.
Types of Cutting fluids:
•Oils: mineral, animal, vegetable, compounded, and synthetic oils. Used for low speed operations.
•Emulsions: mixture of oil and water and additives. Use for high speed operations.
•Semi-synthetics: chemical emulsions containing little mineral water, diluted in water, and additives.
•Synthetics: chemical with additives, diluted in water, and contain no oil.
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