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Fundamentals of Metal cutting and Machining Processes

Fundamentals of Metal cutting and Machining Processes. Lecture 6-7. Contents. THEORY OF METAL MACHINING MACHINING OPERATIONS AND MACHINING TOOLS CUTTING TOOL TECHNOLOGY. Material Removal Processes.

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Fundamentals of Metal cutting and Machining Processes

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  1. Fundamentals of Metal cutting and Machining Processes Lecture 6-7

  2. Contents • THEORY OF METAL MACHINING • MACHINING OPERATIONS AND MACHINING TOOLS • CUTTING TOOL TECHNOLOGY

  3. Material Removal Processes A family of shaping operations, the common feature of which is removal of material from a starting workpart so the remaining part has the desired geometry • Machining– material removal by a sharp cutting tool, e.g., turning, milling, drilling • Abrasive processes – material removal by hard, abrasive particles, e.g., grinding • Nontraditional processes - various energy forms other than sharp cutting tool to remove material

  4. Machining Cutting action involves shear deformation of work material to form a chip • As chip is removed, new surface is exposed (a) A cross‑sectional view of the machining process, (b) tool with negative rake angle; compare with positive rake angle in (a).

  5. Why Machining is Important • Variety of work materials can be machined • Most frequently used to cut metals • Variety of part shapes and special geometric features possible, such as: • Screw threads • Accurate round holes • Very straight edges and surfaces • Good dimensional accuracy and surface finish

  6. Disadvantages with Machining • Wasteful of material • Chips generated in machining are wasted material, at least in the unit operation • Time consuming • A machining operation generally takes more time to shape a given part than alternative shaping processes, such as casting, powder metallurgy, or forming

  7. Machining in Manufacturing Sequence • Generally performed after other manufacturing processes, such as casting, forging, and bar drawing • Other processes create the general shape of the starting workpart • Machining provides the final shape, dimensions, finish, and special geometric details that other processes cannot create

  8. Speed and Feed • Speed is rotational motion of spindle which allows the tools to produce cut into blank OR the relative movement between tool and w/p, which produces a cut • Feed is linear motion of tool which spreads cut on the blank OR the relative movement between tool and w/p, which spreadsthe cut

  9. Machining Operations • Most important machining operations: • Turning • Milling • Drilling • Other machining operations: • Shaping and planing • Broaching • Sawing

  10. Turning Single point cutting tool removes material from a rotating workpiece to form a cylindrical shape Three most common machining processes: (a) turning,

  11. Drilling Used to create a round hole, usually by means of a rotating tool (drill bit) with two cutting edges

  12. Milling Rotating multiple-cutting-edge tool is moved across work to cut a plane or straight surface • Two forms: peripheral milling and face milling (c) peripheral milling, and (d) face milling.

  13. Cutting Tool Classification • Single-Point Tools • One dominant cutting edge • Point is usually rounded to form a nose radius • Turning uses single point tools • Multiple Cutting Edge Tools • More than one cutting edge • Motion relative to work achieved by rotating • Drilling and milling use rotating multiple cutting edge tools

  14. Cutting Tools (a) A single‑point tool showing rake face, flank, and tool point; and (b) a helical milling cutter, representative of tools with multiple cutting edges.

  15. Cutting Conditions (parameters) in Machining • Three dimensions of a machining process: • Cutting speed v– primary motion • Feed f– secondary motion • Depth of cut d– penetration of tool into work piece • For certain operations, material removal rate can be computed as RMR = v f d where v= cutting speed; f= feed; d = depth of cut

  16. Cutting Conditions for Turning Speed, feed, and depth of cut in turning.

  17. Roughing vs. Finishing In production, several roughing cuts are usually taken on the part, followed by one or two finishing cuts • Roughing - removes large amounts of material from starting workpart • Creates shape close to desired geometry, but leaves some material for finish cutting • High feeds and depths, low speeds • Finishing - completes part geometry • Final dimensions, tolerances, and finish • Low feeds and depths, high cutting speeds

  18. Machine Tools A power‑driven machine that performs a machining operation, including grinding • Functions in machining: • Holds workpart • Positions tool relative to work • Provides power at speed, feed, and depth that have been set • The term is also applied to machines that perform metal forming operations

  19. Chip Thickness Ratio where r = chip thickness ratio; to = thickness of the chip prior to chip formation; and tc = chip thickness after separation • Chip thickness after cut is always greater than before, so chip ratio always less than 1.0

  20. Chip Formation More realistic view of chip formation, showing shear zone rather than shear plane. Also shown is the secondary shear zone resulting from tool‑chip friction.

  21. Four Basic Types of Chip in Machining • Discontinuous chip • Continuous chip • Continuous chip with Built-up Edge (BUE) • Serrated chip Type of chip depends on material type and cutting conditions

  22. Discontinuous Chip • Brittle work materials • Low cutting speeds • Large feed and depth of cut • High tool‑chip friction

  23. Continuous Chip • Ductile work materials • High cutting speeds • Small feeds and depths • Sharp cutting edge • Low tool‑chip friction

  24. Continuous with BUE • Ductile materials • Low‑to‑medium cutting speeds • Tool-chip friction causes portions of chip to adhere to rake face • BUE forms, then breaks off, cyclically

  25. Serrated Chip • Semicontinuous - saw-tooth appearance • Cyclical chip forms with alternating high shear strain then low shear strain • Associated with difficult-to-machine metals at high cutting speeds

  26. Orthogonal Cutting • Cutting tool is considered as a wedge • The cutting edge is perpendicular to cutting speed Shear plane angle can be calculated using this relation: r: chip thickness ratio = to/tc

  27. Orthogonal Cutting- Shear Strain

  28. Example 21.1 Φ α= 10 deg 1. Shear plane angle: Φ ; ; 2. Shear strain:

  29. Cutting Forces Fc: Cutting force acting in direction of cutting speed Ft: thurst force acting perpendicular to Fc. Ft increases with increase in chip thickness b4 cut * Fc & Ft both increase as shear strength of material increases These force can be measured using dynamometer F: Friction force b/w chip and rake face N: Normal to friction force F Fs: Shear force applied by w/p on chip Fn: Normal to shear force Fs These force can not be measured directly. These need to be calculated using force diagram

  30. Approximation of Turning by Orthogonal Cutting Not Included

  31. Power and Energy Relationships • A machining operation requires power • The power to perform machining can be computed from: Pc = Fc v where Pc = cutting power; Fc = cutting force; and v = cutting speed

  32. Cutting Temperature • Approximately 98% of the energy in machining is converted into heat • This can cause temperatures to be very high at the tool‑chip interface • The remaining energy (about 2%) is retained as elastic energy in the chip • Tool-Chip thermocouple is used for measuring temperatures in machining • One wire is linked to tool • 2nd wire is linked to chip • Voltage difference is measured and then converted into current and temp using appropriate relations

  33. Cutting Temperatures are Important High cutting temperatures • Reduce tool life • Produce hot chips that pose safety hazards to the machine operator • Can cause inaccuracies in part dimensions due to thermal expansion of work material

  34. B - MACHINING OPERATIONS AND MACHINE TOOLS • Turning and Related Operations • Drilling and Related Operations • Milling • Machining Centers and Turning Centers • Other Machining Operations • High Speed Machining

  35. Machining A material removal process in which a sharp cutting tool is used to mechanically cut away material so that the desired part geometry remains • Most common application: to shape metal parts • Most versatile of all manufacturing processes in its capability to produce a diversity of part geometries and geometric features with high precision and accuracy • Casting can also produce a variety of shapes, but it lacks the precision and accuracy of machining

  36. Classification of Machined Parts • Rotational - cylindrical or disk‑like shape • Nonrotational (also called prismatic) - block‑like or plate‑like Machined parts are classified as: (a) rotational, or (b) nonrotational, shown here by block and flat parts.

  37. Machining Operations and Part Geometry Each machining operation produces a part geometry due to two factors: • Relative motions between tool and workpart • Generating – part geometry determined by feed trajectory of cutting tool • Shape of the cutting tool • Forming – part geometry is created by the shape of the cutting tool

  38. Generating Shape Generating shape: (a) straight turning, (b) taper turning, (c) contour turning, (d) plain milling, (e) profile milling.

  39. Forming to Create Shape Forming to create shape: (a) form turning, (b) drilling, and (c) broaching.

  40. Forming and Generating Combination of forming and generating to create shape: (a) thread cutting on a lathe, and (b) slot milling.

  41. Turning A cutting operation in which single point cutting tool removes material from a rotating work-piece to generate a cylinder • Performed on a machine tool called a lathe • Variations of turning performed on a lathe: • Facing • Contour turning • Chamfering • Threading

  42. A Turning Operation Close-up view of a turning operation on steel using a titanium nitride coated carbide cutting insert

  43. Cutting Conditions in Turning Rotational speed N (rev/min): Cutting speed at cylinder surface v (m/min) Final diameter of part: Feed (mm/rev): f Feed rate (mm/min): fr Time to machine: L: Length of cut/part Alternatively, Material Removal rate: v (m/min); f (m/rev); d (m). Neglect rotational xtic; v (m3/min)

  44. Operations Related to Turning: Facing Tool is fed radially inward - An operation of reducing length/thickness of stock

  45. Operations Related to Turning: Taper Turning • Instead of feeding tool parallel to axis of rotation, tool is fed at an angle thus creating tapered rotational shape

  46. Operations Related to Turning: Contour Turning • Instead of feeding tool parallel to axis of rotation, tool follows a contourthat is other than straight, thus creating a contoured shape

  47. Operations Related to Turning: Form Turning • The tool has a certain shape that is imparted on the w/p by feeding the tooling radially

  48. Operations Related to Turning: Chamfering • Cutting edge cuts an angle on the corner of the cylinder, forming a "chamfer" • How is the tool motion?

  49. Operations Related to Turning: Cut Off • Tool is fed radially into rotating work at some location to cut off end of part

  50. Operations Related to Turning: Threading • Pointed form tool is fed linearly across surface of rotating workpart parallel to axis of rotation at a large feed rate, thus creating threads

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