Midterm

Question 1

Methods of Achieving Part Geometry

Answer 1

Additive
Subtractive
Mechanical Deformation
Material Solidification
Joining Sub-Components

Question 2

Methods of Achieving Mechanical Properties

Answer 2

• Materials
• Cold-working
• Heat Treatment
• Surface Treatment
• Embedded components
• Fiber layout on composites
• Part Geometry

Question 3

Machining(Metal Cutting)-Pros and Cons, and best for

Answer 3

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

Question 4

CNC Machining

Answer 4

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

Question 5

CNC Lathe

Answer 5

Workpiece is mounted on rotary axis, and single edge tools moves radially and longitudinally relative to work piece.

Cuts rotational symmetry

Question 6

CNC mill

Answer 6

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

Question 7

Cutting speed

Answer 7

Workpiece rpm for lathe, tool rpm for milling

Question 8

Depth of cut

Answer 8

Thickness of metal chip removed

Question 9

Feed-Rate

Answer 9

Linear speed at which cutting tool is driven into workpiece

Question 10

Cutting Fluid Use

Answer 10

Lubrication, Cooling, Chip Removal

Question 11

Significance of Path/Process

Answer 11

Planning cuts, How to fixture workpiece , etc

Question 12

Machining performance parameters

Answer 12

Geometrical part accuracy & surface finish

Tool integrity- wear, breakage, chatter

Material removal rate, and machining time

Avoidance of excessive heat generation

Question 13

Mechanics of Metal Cutting

Answer 13

Machining involves SHEARING of thin metal strips(chips) from surface of workpiece

Shearing is Dominant form of material failure

Question 14

Rake Angle

Answer 14

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

Question 15

Orthogonal Machining

Answer 15

Direction of tool motion is perpendicular to cutting edge

Question 16

Oblique Machining

Answer 16

Cutting edge is at and angle not 90º to direction of tool motion

Question 17

Up-Milling vs Down Milling

Answer 17

Up Milling- material is removed above surface of workpiece

Down-Milling- material is removed in direction of workpiece motions

Question 18

Finishing Processes

Answer 18

Deburring
Grinding 
Polishing 
Knurling
Peeling

Question 19

Types of Chips

Answer 19

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.

Question 20

Vibration of a Bit, f

Answer 20

F=sqrt(k/m)
K= stiffness
M= mass

Question 21

Tool Chatter

Answer 21

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

Question 22

Cutting Forces

Answer 22

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]

Question 23

Power Required

Answer 23

Work = Pc * V

Cutting force x cutting speed

Question 24

Specific Cutting energy

Answer 24

E = Pcl/tow*l

Pc- cutting force
I= length of chip
To= depth of cut]
W- width of chip

Question 25

Cutting Fluids, Types and application

Answer 25

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

Question 26

Tool wear and tool life

Answer 26

Tool wear, cratering on rake face, edge rounding, chipping, cracking, rubbing on flank face, catastrophic failure, breakage of the tool

Question 27

Taylor Equation

Answer 27

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 ```

Question 28

High Speed Machining

Answer 28

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.

Question 29

Non-Conventional Machining Processes

Answer 29

Electrical, chemical, laser, plasma, or other means of material removal instead of mechanical shearing

Question 30

Electrical Discharge Machining

Answer 30

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

Question 31

Wire EDM

Answer 31

Small wire acts as positive node in conventional EDM, used for sheet metal cutting, high energy usage.

Question 32

ElectroChemical Machining

Answer 32

Converse to electro plating, cathode (-) tool, and anode(+) workpiece. Material removed from by electrolyte flow before it can plate onto tool. Very energy intensive Disposal of wasted electrolyte is a problem MMR- proportional to current between tool and WP, and federate of tool. Tool shape defines part geometry. Little tool wear

Question 33

Metal Casting

Answer 33

Involves pouring molten metal into mold cavity with despised part shape, after cooling and solidification, part is removed. ``` What to consider- Choice of mold material Design of mold Allowance for part shrinkage Internal structure ```

Question 34

Reynolds Number

Answer 34

``` Dimensionless # Re=VDp/u V- flow velocity D- Channel Width P- density u- viscosity ``` Typically 2000