Flashcards in Midterm Deck (34)
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1
Methods of Achieving Part Geometry
Additive
Subtractive
Mechanical Deformation
Material Solidification
Joining Sub-Components
2
Methods of Achieving Mechanical Properties
-Materials
-Cold-working
-Heat Treatment
-Surface Treatment
-Embedded components
-Fiber layout on composites
-Part Geometry
3
Machining(Metal Cutting)-Pros and Cons, and best for
Pros-
Accurate, Surface Finish
Flexibility for Geometry
Structural Integrity
CAD/CAM integration
CNC automation
Cons-
Single Material
Slow, Expensive for complexity
Wasteful
Major equipment advantage
Best For-
Finish Machining, and precision parts
Prototyping/small production
Complex geometries
4
CNC Machining
Use of digital controllers to compare machine positions with commanded positions, and a sampling rate of 1-10kHz
Maintains accuracy in presence of disturbances- machining force variations, ie.
Thermal expansion
5
CNC Lathe
Workpiece is mounted on rotary axis, and single edge tools moves radially and longitudinally relative to work piece.
Cuts rotational symmetry
6
CNC mill
Tool is attached to stationary spindle, workpiece is mounted on table that moves in x,y,and z axis (3 axis mill)
4-5 axis for change in workpiece orientation relative to tool. Can cut overhang features
7
Cutting speed
Workpiece rpm for lathe, tool rpm for milling
8
Depth of cut
Thickness of metal chip removed
9
Feed-Rate
Linear speed at which cutting tool is driven into workpiece
10
Cutting Fluid Use
Lubrication, Cooling, Chip Removal
11
Significance of Path/Process
Planning cuts, How to fixture workpiece , etc
12
Machining performance parameters
Geometrical part accuracy & surface finish
Tool integrity- wear, breakage, chatter
Material removal rate, and machining time
Avoidance of excessive heat generation
13
Mechanics of Metal Cutting
Machining involves SHEARING of thin metal strips(chips) from surface of workpiece
Shearing is Dominant form of material failure
14
Rake Angle
Factor in achieving constant engagement and constant velocity during cut.
If too large blade will slip and not cut
If too small blade digs in w/o cutting
Ideal angle results in consistent depth of cut, under action of consistent Force
15
Orthogonal Machining
Direction of tool motion is perpendicular to cutting edge
16
Oblique Machining
Cutting edge is at and angle not 90º to direction of tool motion
17
Up-Milling vs Down Milling
Up Milling- material is removed above surface of workpiece
Down-Milling- material is removed in direction of workpiece motions
18
Finishing Processes
Deburring
Grinding
Polishing
Knurling
Peeling
19
Types of Chips
1. Continuous chip- well lubricated cutting of ductile material at moderate speeds. Good Surface finish. May become entangled
2. Built up Edge- accumulation of particles on cutting edge, compromising part accuracy. May break off and damage surface finish. * if Stable may actually improve tool life
3. Segmented(serated) chips - exhibit a sawtooth pattern, may occur with metals of low thermal conductivity ( titanium ) poor heat removal causes localized weakening, also in high speed machining
4. Discontinuous Chips- Associated with large fluctuations in cutting forces and poor machined surface quality. May occur in cutting brittle materials, internal defects, low/high cutting speed, large depth of cut, insufficient tool holder rigidity/workpiece fixture.
20
Vibration of a Bit, f
F=sqrt(k/m)
K= stiffness
M= mass
21
Tool Chatter
Vibration of tool relative to workpiece, due to finite stiffness of tool, tool holder, or workpiece picture.
Avoid resonance frequencies
To avoid
-adjust cutting speed, and depth of cut
-minimize overhang of tool and workpiece
-improve rigidity of fixture
-design process to minimize variation of removal rate
22
Cutting Forces
Pc and Pt
Pc-cutting force, consumes power
Pt- thrust force, keeps engagement
Measured with a dynamometer mounted on a machine. Converts strain into voltages
Pn= normal force of chip on tool
Pf= friction force of chip on tool
[Pn,Pf]=[cos(a)-sin(a),sin(a)-cos(a)][Pc,Pt]
23
Power Required
Work = Pc * V
Cutting force x cutting speed
24
Specific Cutting energy
E = Pc*l/to*w*l
Pc- cutting force
I= length of chip
To= depth of cut]
W- width of chip
25
Cutting Fluids, Types and application
Mineral oil with chemical additives
Water based emulsion(synthetic fluids)
Applications:
Manual spray
Cont. Flooding
Special cutting tools with holes for coolant is fed
Mist application, can reach inexcessible regions of workpiece. Provides better visisbility
26
Tool wear and tool life
Tool wear, cratering on rake face, edge rounding, chipping, cracking, rubbing on flank face, catastrophic failure, breakage of the tool
27
Taylor Equation
T=K/V^n
T= Tool life (min)
V= cutting sped
K= constant for a given marching process
N= constant for a given tool material
N= 2 for ceramic tool
N= 3.33 for carbide tool
N= 10 for high speed steel
28
High Speed Machining
New trend of cutting speeds and feed rates 10-100x bigger than conventional practice, speeds of 1000-10,000 m/min, and spindles at 15,000-50,000 rpm
Benefits-
Reduced machining times
Improved surface finish
Reduced heat generation
Reduced mechanical stresses
Challenges-
Inertial forces of accelerating machine may exceed cutting forces
Dynamic instability, resonant vibrations caused by cutting edge engagement frequencies.
29
Non-Conventional Machining Processes
Electrical, chemical, laser, plasma, or other means of material removal instead of mechanical shearing
30
