Fundamentals of Chip-Type Machining Processes

Chapter 20

Why machine parts? Create prototypes, models, molds, tool

and dies, one-offs, … Repair parts Create unique features

Sharp corners O-ring grooves Flat, smooth surface for mating two pieces Curved, mating surfaces (piston and cylinder)

Produce a part(s) dimensionally more accurate than castings

When it is not economical to cast

Types of Parts Machined Shafts Tube fittings Molds for plastic injection molding of

parts Fasteners (bolts, screws, nuts) IC engine parts Tool and dies for stamping Bearings, gears, sprockets Jewelry

Materials that can be machined

Metal Wood Plastics Ceramics Composites (cutting, drilling,

finishing) Stone (include jewelry) Concrete (cutting, drilling)

Basic Machining Processes

Shaping Turning Milling Drilling Sawing Broaching Grinding

Variables in the Machining Process

Independent Variables Tooling Workpiece material, condition,

temperature Machining Parameters Cutting Fluid Machine tool Fixturing

Variables in the Machining Process

Dependent Variables Type of chip produced Force and energy dissipated in the

cutting process Temperature rise in workpiece, chip,

tool Wear and failure of the tool Surface finish produced

Basic Machining Parameters

Speed (V) Primary motion provided by a

machine tool Relative motion between tool and

workpiece Usually absorbs most of the total

power required Units: Surface feet per minute (sfpm)

Basic Machining Parameters

Feed (fr) May proceed continuously or in steps Usually absorbs a small portion of

total power Unit: Inches per “something” – stroke,

revolution, etc.

Basic Machining Parameters

Depth of Cut (t) Third dimension Constant Units: inches (or mm)

Basic Machining Parameters Material Removal Rate

Volume of cutMRR =

Cutting Time

Chip Formation Localized shearing

process Material is

compressed and plastically deformed. large strains high strain rates work hardens; fails

by a shearing process

Shearing defined by the shear angle, Φ .

Chip Formation

Three basic types Discontinuous chips Continuous chips Continuous chips with a built-up edge

(BUE)

Chip Formation

Discontinuous chips Typically associated with brittle metals

like Cast Iron As feed is increased, some

compression takes place As the chip starts up the chip-tool

interference zone, increased stress occurs until the metal work-hardens to a maximum and fractures off the part.

Chip Formation

Discontinuous chips

Chip Formation

Conditions for Discontinuous chips Brittle work material Small rake angles on cutting tools Coarse machining feeds Low cutting speeds Major disadvantage—could result in

poor surface finish

Chip Formation

Continuous Chips Continuous “ribbon” of metal that

flows up the chip/tool zone. Usually considered the ideal

condition for efficient cutting action.

Chip Formation

Problems with Continuous Chips Increased frictional heating from

remaining in contact with the tooling longer.

Greater chance of worker injury Metal “ribbon” can become tangled in

tooling

Chip Formation

Conditions for Continuous Chips Ductile work Fine feeds Sharp cutting tools Larger rake angles Proper cutting speeds Proper coolants

Chip Formation Continuous chips

with a built-up edge (BUE) Same process as

continuous, but as the metal begins to flow up the chip-tool interface, small particles of the metal begin to adhere or weld themselves to the edge of the cutting tool.

Chip Formation Continuous chips

with a built-up edge (BUE) As the particles

continue to weld to the tool it effects the cutting action of the tool including the beginning of galling.

Chip Formation

Conditions for a built-up edge (BUE) Common in softer non-ferrous

metals and low carbon steel. Formation increases as the tool

begins to dull.

Chip Formation

Problems with a built-up edge (BUE) Welded edges break off and can

become embedded in work-piece Decreases tool life Can result in poor surface finishes

Chip Formation

Solution be a built-up edge (BUE) Reduce depth of cut Alter cutting speed Use positive rake tool Coolant Use different cutting tool materials

Chip Breakers

Two styles of chip breakers Groove Obstruction

Orthogonal Machining Orthogonal

Cutting is when the cutting edge of the tool is straight and perpendicular to the direction of motion.

Oblique Machining Most machining is performed with

oblique geometry.

Oblique Machining

Tool edge is set at an angle of inclination, i.

Effective rake angle is larger than normal rake angle, thus cutting force is lower.

Chip curls into a helical rather than a spiral, easily removed.

Oblique Machining Oblique machining

has 3 components FC – Primary cutting

force acts in the direction

of the cutting force vector.

Largest force (generally)

99% of the power

Oblique Machining

Ff – Feed force Acts in the direction of the tool feed. ~ 50% FC

Small power requirement Fr – radial force

acts perpendicular to the machined surface ~ 50% Ff

very small power requirement

Power

C

C

HP = F V

F VHP =

33000

Specific Horsepower

S

HPHP

MRR

Specific Horsepower correlates with shear stress.

Power

HPs is used for Estimate motor HP for a machining

process for a given material

Estimate Cutting Force (FC)

Determine maximum depth of cut (d)

Energy

30-40% of the total energy goes into friction.

60-70% of the total energy goes into the shear process

Table 20-3, U values for common metals

Shear Strain

Shear strain rates – 104 to 108 in/in-sec

Heat and Temperature

Three main sources of heat Shear front Tool/chip

interface Flank of the

tool

Heat Distribution

Heat and Temperature