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21 - THEORY OF METAL MACHINING


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Machining
MACHINE REMOVES MATERIAL, shaping the part, bit by bit and chip by chip bringing it into the final shape by progressively removing chips from the material, producing a great variety of different and complex forms and shapes with great tolerance control and dimensional accuracy, yet the process takes a bit of time and produces waste material - the chips, a process usually used for metals, one of the most important manufacturing processes, a process we can divide into three categories: [1] Conventional machining - Turning, Drilling, Milling; [2] Abrasive processes - grinding, for example, [3] Nontraditional machining - mechanical energy processes, electrochemical machining, thermal energy processes, and chemical machining. >>> CAN: CONVENTIONAL, ABRASIVE, NONTRADITIONAL.
OVERVIEW OF MACHINING TECHNOLOGY
Machining operation removes chips from surface of part due to relative motion between the two, the tool moving or rotating at it's cutting speed and the material moving at a secondary motion called feed, the combination of the two motions added to the tool's penetration of the parts surface produces material removal and, ultimately, a finished shape. CUTTING SPEED FEEDS HUNGRY CHIPS
Types of machining operations
Most common types of machining operations are: [1] Turning - strong blade cuts turning part, [2] Drilling - long and hard drill penetrates soft and ready metal, [3] Milling - multiple edges remove many chips. >>> TiDM
Cutting tool
Cutting tool separates tool with sharp cutting edges, its hard teeth biting down upon the soft steel, a single or a multitude of teeth, teeth with certain nose radius, the cutting edge surrounded by two important surfaces: [1] Rake face - directs chips away at a rake angle, and [2] The Flank - provides clearance between tool an part, creating the clearance via a relief angle, the angle of the flank. TEETH BITE, RAKES REMOVE, FLANKS DISTANCE.
Categories of cutting tools
We can divide the cutting tools into two categories: (1) Single-point tools, used in operations such as turning and boring; and (2) Multiple-edge cutting tools, used in operations such as milling and drilling.
Cutting conditions
TOOL REMOVES MATERIAL WITH RELATIVE MOTION AND PENETRATION, moving at cutting speed $v$ and feed speed $f$, penetrating into the metal into the depth $d$ - the depth of cut, aided by the lubrication and cooling of the cutting fluid, fluid generously applied by the machine or the wage slave, thus removing material at the following rate: $$R_{MR}=vfd$$
Roughing and finishing
A roughing operation is used to remove large amounts of material rapidly and to produce a part geometry close to the desired shape. A finishing operation follows roughing and is used to achieve the final geometry and surface finish. ROUGH REMOVAL BEFORE GENTLE FINISHING.
Machine tools
A power-driven machine that positions and moves a tool relative to the work to accomplish machining or other metal shaping process.
THEORY OF CHIP FORMATION
Theory of chip formation in orthogonal cutting model simplifies chip formation into two dimensions rather than three.
Orthogonal cutting model
Orthogonal cutting model is useful in the analysis of metal machining because it simplifies the rather complex three-dimensional machining situation to two dimensions. In addition, the tooling in the orthogonal model has only two parameters (rake angle and relief angle), which is a simpler geometry than a single-point tool. SIMPLE WEDGE CUTS MATERIAL
Actual chip formation
Actual chip formation differs from the orthogonal model in following ways: [1] Chip formation occurs in a zone and not in a plane, [2] Secondary shear occurs between chip and cutting tool, [3] Chip type depends on machined material and conditions.
Chip types
The four chip types are (1) DISCONTINUOUS, in which the chip is formed into separated segments; (2) CONTINUOUS, in which the chip does not segment and is formed from a ductile metal; (3) CONTINUOUS WITH BUILT-UP EDGE, which is the same as (2) except that friction at the tool-chip interface causes adhesion of a small portion of work material to the tool rake face, and (4) SERRATED, which are semi-continuous in the sense that they possess a saw-tooth appearance that is produced by a cyclical chip formation of alternating high shear strain followed by low shear strain. >>> DiC BEeS: DISCONTINUOUS, CONTINUATIONS, BUILT-UP EDGE, SERRATED
Forces in metal cutting
In orthogonal cutting model the forces acting on the chip, forces that can't be directly measures, are: (1) Friction force - resists flow of cheep along rake of tool, (2) Normal force to friction - perpendicular to the friction force, (3) Shear force - causes deformation in shear plane , and (4) Normal force to shear - perpendicular to shear force. $$---$$ By placing a dinamometer on the cutting tool you can measure the cutting force and the thrust force, relating them to all the other forces.
Merchant equation
The real value of the Merchant equation is that it defines the general relationship between rake angle, tool–chip friction, and shear plane angle. The shear plane angle can be increased by (1) increasing the rake angle and (2) decreasing the friction angle (and coefficient of friction) between the tool and the chip. Rake angle can be increased by proper tool design, and friction angle can be reduced by using a lubricant cutting fluid >>> RAKE ANGLE AND FRICTION CONTROL THE SHEAR ANGLE.
POWER IN MACHINING
Power, energy per unit time, required in a cutting operation is equal to the cutting force multiplied by the cutting speed. Cutting power = Force x speed $$P_{c}=F_{c}v$$
Specific energy
Amount of energy required to remove a unit volume of the work material - cutting power divided by material removal rate. $$U=\frac{P_{c}}{R_{MR}} \left[ \frac{J}{mm^{3}} \right]$$
Size effect
The size effect refers to the fact that the specific energy increases as the cross-sectional area of the chip ($t_0\, \times \,w$ in orthogonal cutting or $f \,\times\, d$ in turning) decreases. >>> SMALL CHIPS STEAL MORE ENERGY
CUTTING TEMPERATURE
Since about 98% of energy consumed in machining converts into heat, keeping the temperature down is very important since it will: [1] Increase too life, [2] Reduce production of hot, safety hazard chips, [3] Prevent inaccuracies and deformities in worked part. Tool-chip interface temperature can be predicted analytically or measured experimentally by using the material and cutting tool as a thermocouple.