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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

Electrical Discharge Machining

Tool and metal workpiece in di-electric fluid.

High voltage that pulsates at 100-1000Hz, which cause pulse and a high localized heating on workpiece, melts small chip of workpiece which is carried away by di-electric fluid flow

Tools also gradually erode and must be replaced. Tools may be copper, brass, or tungsten, or graphite. MRR and Finish depend on current, and pulse frequency.
Best results with low current and high frequencies