mfe midterm 2 - CHP 21 Basic machining geometry Removing...

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CHP 21 Basic machining geometry - Removing unwanted material from a workpiece in the form of chips Feed - Amount of material removed per revolution or per pass of the tool over the workpiece Cutting speed - Primary cutting motion, related the velocity of the cutting tool relative to the workpiece DOC - Distance the tool is plunged into the surface MRR - Used to estimate the horsepower needed to perform a cut Cutting time - Depends upon the length of cut L divided by the rate of traverse of the cutting tool past the rotating workpiece Different machining processes - Turning, sawing, grinding, milling, shaping, planning, broaching, drilling Chips - Continuous, discontinuous, continuous with built up edge Different workpiece materials - Chatter - Mechanism by which a process dissipates energy; speed influences it; a self-excited vibration that is caused by the closed loop force displacement response if the machining process - To avoid it a constant chip thickness in a steady cutting force and the elimination of the feedback mechanism responsible Heat and temperature - The higher the temperature the great the tool wear CHP 22 Cutting tool requirements - High hardness - High hardness temperature, hot hardness - Resistance to abrasion, wear, chipping of the cutting edge - High toughness - Strength to resist bulk deformation - Good chemical stability - Adequate thermal properties - High elastic modulus - Consistent tool life - Correct geometry and surface finish Ranked tools in order of hot hardness and toughness - Toughness: high speed steels, carbides, coated hss, coated carbides, ceramics, cbn, diamond (decreasing) - Hot hardness same order (increasing) Coatings
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- Improving life and performance of tools Rake angle - Affects the ability of the tool to shear the work material and form the chip; can be positive or negative Relief angle - Mainly affect the tool life and the surface quality of the workpiece Nose radius - Has major influences on the surface finish; increasing it reduces tool wear and improves surface finish Tool failure - Slow death mechanism: gradual tool wear on the flank of the tool below the cutting edge or wear on the rake face of the tool - Sudden death mechanism: rapid, usually unpredictable and often catastrophic failure resulting from abrupt, premature death of a tool Flank wears - At lower speeds and temps the tool most commonly wears here; wear on the flank Crater wear - The depth of the crater is used to determine tool failure; wear on the rake surface Taylor tool life equation - VT^N=C - V=cutting speed - T= tool life - N= constant (tool material) - C= constant (tool material, work material, etc.) Reconditioning a tool - Resharpening to original tool geometry specifications - Grind cutting edges and surfaces to a fine finish - Remove all burrs on resharpened cutting edges - Avoid resharpening practices that overheat and burn or melt the tool surfaces - Cost is 1/5 the cost of purchasing a new tool Machining economics - Machining costs, tool costs, tool changing costs and handling costs Machinability -
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