Lathe Operations: Facing

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Facing Operations Facing is the process of removing metal from the end of a workpiece to produce a flat surface. Most often, the workpiece is cylindrical, but using a 4-jaw chuck you can face rectangular or odd-shaped work to form cubes and other non-cylindrical shapes.

When a lathe cutting tool removes metal it applies considerable tangential (i.e. lateral or sideways) force to the workpiece. To safely perform a facing operation the end of the workpiece must be positioned close to the jaws of the chuck. The workpiece should not extend more than 2-3 times its diameter from the chuck jaws unless a steady rest is used to support the free end. Cutting Speeds

If you read many books on machining you will find a lot of information about the correct cutting speed for the movement of the cutting tool in relation to the workpiece. You must consider the rotational speed of the workpiece and the movement of the tool relative to the workpiece. Basically, the softer the metal the faster the cutting. D…

Crystal Structures for Metallic Elements

The crystals of most metals have highly symmetrical structure. The most common types of lattices are



(a) Body Centred Cubic Structure (BCC)



(b) Face Centred Cubic Structure (FCC)



(c) Hexagonal Close Packed Structure (HCP)



Body Centred Cubic Structure (BCC) - In this type of structure, atoms are located at the corners of the cube and one atom at its centre. This type of unit cell is found in metals like lithium, sodium, potassium, barium, vanadium etc.



Face Centred Cubic Structure (FCC) - In this type of structure, atoms are located at the corners of the cube and one atom at the centre of each face. This type of unit cell is found in metals-like copper, silver, gold, aluminium, lead etc.



Hexagon Close Packed Structure (HCP) - In this type of structure, an atom is there at each of the twelve corners of the hexagonal cell, one atom at the centre of each of two hexagonal faces and three atoms in the body of the cell. This type of unit cell is found in metals like zinc, magnesium, lithium, beryllium etc.



Importance of Crystal Structure in Metallic Elements



Crystal structure is important because it contributes to the properties of a material. For example, it is easier for planes of atoms to slide by each other if those planes are closely packed. Therefore, lattice structures with closely packed planes allow more plastic deformation than those that are not closely packed. A Face-Centered Cubic (FCC) crystal structure will exhibit more ductility (deform more readily under load before breaking) than a Body-Centered Cubic (BCC) structure. The BCC lattice, although cubic, is not closely packed and forms strong metals. Alpha-iron and tungsten have the BCC form. The FCC lattice is both cubic and closely packed and forms more ductile materials. Gamma-iron, silver, gold, and lead have FCC structures. Finally, HCP lattices are closely packed, but not cubic. HCP metals like cobalt and zinc are not as ductile as the FCC metals.

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