Machining Operations

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  • Topic: Milling machine, Machining, Force
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  • Published : April 8, 2013
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Machining Operations
In MACHINING, the shape, size, finish and accuracy are obtained by removing the excess material from the workpiece surface. Various surfaces are obtained as an interaction between a workpiece and a cutting tool with the help of a contrivance known as MACHINE TOOL.

Prof. S.K. Choudhury, Mechanical Engineering Department, IIT Kanpur

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Advantages and Disadvantages of Machining
Variety of work materials can be machined. o 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  Generally performed after other manufacturing processes, such as casting, forging, and bar drawing Disadvantages: Wasteful of material and time consuming Prof. S.K. Choudhury, Mechanical Engineering Department, IIT Kanpur 2

Basic Machining Parameters
Speed (V) [m/min] • Relates velocity of the cutting tool to the work piece (Primary motion). Feed (f) [mm/rev] • Movement (advancement) of the tool per revolution of the workpiece Depth of Cut (d) [mm] • Distance the tool has plunged into the surface

Prof. S.K. Choudhury, Mechanical Engineering Department, IIT Kanpur

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Chip Formation

Cutting action involves shear deformation of work material to form a chip. As chip is removed, new surface is exposed.

Prof. S.K. Choudhury, Mechanical Engineering Department, IIT Kanpur

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Cutting Tools & Types of Machining

A Typical Lathe Tool Wedge-Shaped tool

Orthogonal Cutting

Oblique Cutting
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Prof. S.K. Choudhury, Mechanical Engineering Department, IIT Kanpur

Tool Angles
Rake Angles (α) • Influence cutting forces, power and surface finish • Large α – lowers forces and improves surface finish – In general, power consumption ↓by ~ 1% for 1 degree change in α – Has adverse effect on tool strength because less metal is available to support the tool. – Greatly reduced capacity to conduct heat away from the cutting edge Prof. S.K. Choudhury, Mechanical Engineering Department, IIT Kanpur 6

Tool Angles
• 0 or negative rake angles employed on carbide, ceramic and similar “hard” tools – Increases tool forces, but keeps the tool in compression and provides added support to the cutting edge • Particularly important in making intermittent cuts and in absorbing impact during initial tool‐workpiece contact • Rake angles: 5 – 15 degrees for HSS; Lower for harder materials.

Prof. S.K. Choudhury, Mechanical Engineering Department, IIT Kanpur

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Tool Angles
Flank Angle • Minimizes rubbing of flank faces with the machined surface • Higher values of flank angle will reduce rubbing but also weaken the tool • Flank angles have no influence on cutting forces and power. So angles large enough to avoid rubbing is generally chosen • Angle: 5 – 12 degrees for HSS; higher for softer and lower for brittle material

Prof. S.K. Choudhury, Mechanical Engineering Department, IIT Kanpur

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Tool Angles
Cutting Edge Angles:
 Provided to clear the cutting edge from the machined surface  To Reduce tool chatter  Affects tool life as well as surface finish

Prof. S.K. Choudhury, Mechanical Engineering Department, IIT Kanpur

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Tool Parameter
Nose radius  Improves tool life and surface finish. • Large nose radius – Increases cutting forces and power – Causes chatter (self-excited vibration) Recommended value: 1 – 3 mm

Prof. S.K. Choudhury, Mechanical Engineering Department, IIT Kanpur

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Types of Chips
•Brittle work materials •Low cutting speeds •Large feed and depth of cut •Small rake angle •High tool-chip friction •Ductile work materials •High cutting speeds •Small feeds and depths •Large rake angle •Sharp cutting edge •Low tool-chip friction

•Ductile materials •Low-to-medium cutting speeds •Large feed •Small rake angle •Tool-chip friction causes portions of chip to adhere to rake face •Built up Edge (BUE)...
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