mems 0040 machining

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MEMS 0040 Materials and Manufacturing

Machining

©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

• Focus here on conventional machining where a sharp hard tool is used to cutaway material. (not EM beams or particulate streams)

• Definition of: chip, rake face, rake angle, cutting edge and relief angle, shear

plane, angle of shear plane, depth of cut and feed for a simple orthogonal cut.

Orthogonal cutting

depth of cut




Machining

Topical Subject: CNC machining

• Computer Numerical Controlled

Machining.

• Automation of machining processes by

merging.

• Computer Aided Design (CAD) and

Computer Aided Manufacturing (CAM) to

link shape to a sequence of processes

and then to machine commands.

• Often uses multiple traditional machining

techniques (e.g. cutting, turning, milling,

drilling) in one “cell”.

• Very close matching to CAD for large

number of components.




Machining Advantages and

Disadvantages of Machining

1. Applied to the vast majority

of metals

2. Wide shaping flexibility

3. Dimensional accuracy

tolerances up to +/- 0.025mm.

4. Good surface finish:

Roughness down to 0.4 mm.

This is very important to

surface sensitive properties

such as fatigue.

5. Volume of material

removed is waste.

6. Usually takes a long time

to machine large shape or

volume changes.




Machining

Flexibility of Conventional

Machining




Machining


Machining




Machining


Machining

• Conventional machining is rarely removed by screening of processes but

can rank low due to cost and time! Depends on number of components.




Machining

• There is usually not just the one

motion during machining. The

cutting speed is the primary

motion of the tool but the feed is

the secondary motion (e.g. Drill)

and finally there is the depth of

cut.

• E.g. drill:

• primary spin motion of tool

• Secondary motion is feed along

tool axis.




Machining


• Machining Operations

Turning: cutting tool has one edge and

reduces diameter of a spinning work piece.

Tool moves perpendicular to the work

piece axis and feed is parallel to axis

Drilling: create round holes

and drill bit had two or more

cutting surfaces. Tool moves

parallel to its axis.




Machining


• Machining Operations

Peripheral milling Face milling

Milling: rotating tools have multiple cutting edges is moved along surface of the

work piece to give a flat surface. If tool spins on axis parallel to work surface it is

called peripheral milling. If the tool rotates on axis perpendicular to the work

surface it is called face milling.




Machining


Material Removal Rate in Machining All these operations involve moving the tool relative

to the work piece and this leads to depth of cut and

feed being important variables for the process.

The final variable is the speed :rotation if the tool

spins.

These can be used to calculate the material removal

rate RMR . Example of turning:

RMR (mm3 /s) = V (m/s) f(mm) d(mm)

Where V is the cutting speed, f is the feed (shouldbe in mm/rev for turning) and d is the depth of cut.

For an orthogonal cut

RMR = v to w

V




Machining

• Roughing removes a lot of material but tolerances and surface

finish are not good. Feeds of 0.4-1.25mm and depths of 2.5 to20mm.

• Finishing is used to obtain surface finish and tolerances. Feeds of

0.125 to 0.4mm and depths of 0.75 to 2mm.

• Cutting fluid is always used to cool and lubricate tool and work

piece.




Machining

• Chip Formation

• Orthogonal cut model has shown that material removal occurs by shear

on a plane that extends from the cutting edge of the tool through the

material to the surface of the piece. This is called the primary shear.

• Once the material has failed in shear the material being removed (chip)then moves up the rake face with friction occurring between the tool rake

face and the cut surface of the chip. This is called the secondary shear .

• The type of chip formed also depends on the mechanical properties of

the work piece and cutting speed.©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e




Machining

• Chip Formation

Discontinuous chip – occurs when

brittle material such as cast iron aremachined at low speed. The chip

segments due to fractures. The

discontinuous nature of the chip

formation causes high surface

roughness.

Continuous chip – when ductile

materials are cut at high speed withsmall depths and feeds, a long

continuous chip if formed with good

surface finish. If chips become too long

in turning may become tangled with

work piece or tool.





Machining

• Chip Formation

Continuous chip with built-upedge - ductile materials at low to

moderate speeds when secondary

friction causes chip material to stick

to rake face. This is called built up

edge.

Serrated chip – These chips are semi

continuous and shear becomes much

localized within shear bands in thematerial. This chip leads to a saw tooth

appearance due to alternate high shear

and low shear behavior. Found in

difficult to machine materials such as

titanium and nickel alloys when they are

machined at high speed.©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e




Machining

Analysis of Orthogonal Machining


• The friction force between the tool

and the material on the rake face

is F and the associated normal

force is N. This defines the

coefficient of friction:

m = F/N

• The resultant force R can then be

defined at the friction angle b to

the direction of N.

• Therefore

m = tan b




Machining



• R must be balanced by a force R’ in the

work piece

• R’ can be related to (i) the shear force Fs

on the plane between the chip and the

work piece and the (ii) normal force Fn.• Therefore the shear stress on the plane

can be defined:

t = Fs/As

Where the area As is :

As= to w / sinf

Where f is the shear plane angle to the machined surface, to is the

depth of cut and w is the width of the machined surface.




Machining



These forces are not directly measurable and so

they need to be calculated from the cutting force

Fc which is the force measured in the direction

of cutting at the cutting speed and the thrust

force Ft perpendicular to the surface and is

controlled by the depth of cut. Using the aboveexpressions for t and As and the fact that:

Fs = Fccosf –Ftsinf And so

t = (Fccosf – Ftsinf )/ (tow/sinf)




Machining



The Merchant equation assumes that the

fracture will occur along the direction of

highest shear and therefore only one

angle of f is possible.

In this direction the derivative of the shear

stress t relative to f will be zero (peak

value) and this leads to:

f = 45 + (a/2) - (b/2)

Where a is the rake angle and b is the

friction angle.




Machining



Can decrease As using shear plane angle f which can be increased by:

(i) increasing the rake angle a (design of tool and machining

practice) .

(ii) decreasing friction angle b by using lubricant.

This reduces the shear force, and power required to cut.

Larger aSmaller a




Machining

Machining Energy and Power


• The power to perform machining can be computed from:

P c = F c v

• where P c = cutting power; F c = cutting force; and v = cutting speed.

• In U.S. customary units, power is traditional expressed as horsepower

(dividing ft-lb/min by 33,000)

• where HP c = cutting horsepower, hp 00033,

v F

HP c c




Machining



• Gross power to operate the machine tool P g or HP g is given by:

or E

P P c g E

HP HP c

g

• E is mechanical efficiency of machine tool (usually ~90%).

• Useful to convert power into power per unit volume rate of metal

removal .

• Called unit power , P u

or unit horsepower , Hpu

or

MR

c

U R

P P =

MR

c

U R

P P =




Machining



Unit power is also known as the specific energy U

Units for specific energy are typically N-m/mm3 or J/mm3 (in-lb/in3)

w vt

v F

R

P

P U o

c

MR

c

u ===




Machining

Machining Energy and Power: correlates with material hardness


Assumes sharp tool and depth of cut to = 0.25 mm (chip thickness).




Machining



Reducing high cutting power requirements:

1. Reduce cutting speed V

2. Reduce depth of cut or feed to reduce cutting force Fc

3. Use a more machineable material (e.g. free machining steels: may

cost up to 20% more but contain higher lead, sulfur or phosphorous

impurities that create soft particulates and dry lubricants that aid in

chip formation and breakage.

4. Use effective lubricant to reduce friction angle b.

5. Use higher rake anglea.

Or use cutting tool with more power




Machining

Cutting Temperature


• Approximately 98% of the energy in machining is converted into heat.

• This can cause temperatures to be very high at the tool-chip.

• High cutting temperatures:

1. Reduce tool life

2. Produce hot chips that pose safety hazards to the machine operator

3. Can cause inaccuracies in part dimensions due to thermal expansion of

work material




Machining

Cutting Temperature


• Experimental methods can be

used to measure temperatures in

machining. Most frequently used

technique is the tool-chip

thermocouple

• Using this method the

speed-temperature relationship is

usually:

T = K v m

where T = measured tool-chip

interface temperature, and v =

cutting speed

mems 0040 machining

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