Lecture 2 Machining Processes.pdf

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Mechanical Engineering
Alokesh Pramanik

Lecture 2: Machining Processes Wednesday, 23 October 2013 11:26 AM Overview • Introduction • Basic Machining Processes:Turning, Drilling, Milling • Fundamentals of Machining Introduction • Machining: Broad term used to describe removal of unwanted material from a workpiece in the form of chips • Cutting: Material removalby a sharp cutting tool, eg. Turning, milling and drilling • Abrasive Process:Material removedby hard, abrasive particles, eg. Grinding, honing and lapping • Non-traditional Process:Various energy forms other than sharp cutting tool removematerial, eg. Electric discharge machining (EDM),water-jet machining and laser beam machining Machining • Various work materials can be machined, most commonlymetals • A wide variety of part shapes and special geometric features are possible, eg. Screw threads, accurate round holes, very straight edges and surfaces • Good dimensional accuracy and surface finish. • Wasteful of material:chips generated in machining are wasted material, at least in the unit operation • Time consuming: generally takes moretime to shape a given part than alternativeshaping processes, eg. Casting, powder metallurgy, forming • Generally performed after other manufacturing processes such as casting, forging and bar drawing • Other processes create the shape of the starting workpiece,Machining provides the final shape, dimensions, finish and special geometricdetails • Rotational:cylindrical or disk-like shaped • Non-rotational (Prismatic):block-like or plate-like • Generating: part geometrydetermined by feed trajectoryof cutting tool (eg. Milling, tapering, using a lathe) • Forming: part geometrycreated by shape of cutting tool (eg. Drilling, broaching, tapping, form turning) • Roughing: removelarge amounts of material as rapidly as possible in order to produce a shape close to the desired form, leaving somematerial on for a subsequent finishing operation (large depth cut at high feed rate) • Finishing: Completethe part and achieve the final dimension, tolerances and surface finish (small depth cut at slow feed rate) Turning • Single point cutting tool removesmaterial from a rotating workpiece to generate a cylinder • Performedon a lathe • Facing: tool is fed radially inward • Contour Turning: Instead of feeding tool parallel to axis, tool follows a contour that is not straight, thus creating a contoured shape • Chamfering: Cutting edge, cuts an angle on the corner of the cylinder, forming a chamfer • Cut-off: Tool is fed radially into work at some location to cut off end of part • Threading: Pointed form tool is fed linearly across surface of rotating workpiece parallel to axis of rotation at large feed rate • Holding Tools:solid tool (HSS), brazed insert (cementedcarbides), mechanically clamped insert (cementedcarbides and ceramics) • Tool Shapes: Round, square, rhombus (2x 80⁰), hexagon (3x 80⁰), triangle (equilateral), • Tool Shapes: Round, square, rhombus (2x 80⁰), hexagon (3x 80⁰), triangle (equilateral), rhombus (2x 55⁰), rhombus (2x 35⁰) • Centres: Work is mounted between centres using a 'dog' • Three-Jaw chuck: Self-centring, can hold hexagonal bars, quick and easy to use; cannot hold square bars, run-out/off-centrecannot be easily fixed, cannot hold irregularly shaped work, cannot turn off-centre • Four-jaw independent chuck: Work can be centred to high precision, can hand square/rectangular bars, can turn work off-centre, slightly more grip on round stock; slower/fiddlierto mount work, cannot hold hexagonal bar stock • Collet: Used to hold smooth cold-rolled bar stock or machined workpieces moreaccurately than chucks. Built to hold a particular bar stock and can be round, square or hexagonal • Face-Plate: Used to support irregularly shaped work that cannot be gripped easily in collets or chucks. Work is either bolted or clamped onto the faceplate or using an auxiliary attachment. Drilling • Creates a round hole in a workpiece (boring enlarges an existing hole) • Cutt
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