ME364_cutting_mechanics

# ME364_cutting_mechanics - Valery Marinov Manufacturing...

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Mechanics of Machining 67 Valery Marinov, Manufacturing Technology 5.2 MECHANICS OF MACHINING Plastic deformation in cutting The cutting itself is a process of extensive plastic deformation to form a chip that is removed afterward. The basic mechanism of chip formation is essentially the same for all machining operations. Assuming that the cutting action is continuous, we can develop so-called continuous model of cutting process shown in the ±gure: Cutting speed V Cutting speed, V Direction of primary motion shear plane h ch h D b D % # ! "# ! \$ # tool face s e a r + f i c t o n workpiece chip cut tool (a) (b) (c) (a) Chip formation in metal cutting is accompanied by substantial shear and frictional deformations in the shear plane and along the tool face; (b) Schematic illustration of the two-dimensional cutting process (also called orthogonal cutting ), h D and b D are the thickness of cut and width of cut respectively, h ch is the chip thickness , γ o is the tool orthogonal rake angle, and Φ is the shear plane angle ; (c) Cutting with positive and negative rake angles. Note the change in the shear plane angle and chip thickness. Cutting is performed with a cutting tool moving at a cutting speed V in the direction of primary motion. The cutting tool is inclined at the rake angle γ o . The rake angle can be positive, zero, or negative, typically taking values from +15 o to -6 o . The rake angle in²uences signi±cantly the process of plastic deformation in cutting and therefore the chip thickness, cutting forces and temperatures. The tool is set to remove a cut with thickness h D and width b D . In the simplest model of orthogonal cutting shown in the ±gure, the plastic deformation takes place by shearing in a single shear plane inclined at the angle Φ ( shear plane angle ). The produced chip has a thickness of h ch ( chip thickness ), width b D and moves at speed V ch ( chip speed ). The chip thickness compression ratio Λ h is de±ned by Λ h = h ch /h D Chip thickness is always bigger than thickness of cut, therefore Λ h is always >1. The value of chip thickness compression ratio gives a valuable information about the rate of plastic deformation in the

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ME364_cutting_mechanics - Valery Marinov Manufacturing...

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