ME 350 – Lecture 22 – Chapter 26

NONTRADITIONAL MACHINING PROCESSES1. Mechanical Energy Processes (USM, WJC, AJM)

2. Electrochemical Processes (ECM)

3. Thermal Processes (EDM, Wire EDM, EBM, LBM, PAC)

4. Chemical Processes (CHM, Chemical Blanking, PCM)

Nontraditional machining is characterized by material removal that:

ME 350 – Final Exam Update

Location:DCL 1320

Date:Friday May 13th, 2011

Time:1:30 pm – 4:30 pm

Nontraditional Processes Used When:

1. Material is either very hard, brittle or both; or

material is very ductile:

2. Part geometry is complex or geometric

requirements impossible with conventional

methods:

3. Need to avoid surface damage or contamination

that often accompanies conventional machining:

1. Mechanical Energy Processes

• Ultrasonic machining (USM)

• Water jet cutting (WJC)

• Abrasive jet machining (AJM)

1a) Ultrasonic Machining (USM & UW)

Abrasives in a slurry are driven at high velocity against work by a vibrating tool (low amplitude & high frequency)• Tool oscillation is perpendicular to work surface• Abrasives accomplish material removal• Tool is fed slowly into work• Shape of tool is formed into part

USM Applications

• Used only on hard and brittle work materials:

ceramics, glass, carbides, and hard metals.

• Shapes include non-round holes, holes along a

curved axis

• “Coining operations” - pattern on tool is

imparted to a flat work surface

• Produces virtually stress free shapes

• Holes as small as 0.076 mm have been made

• Uses high pressure, high velocity stream of water directed at work surface for cutting

1b) Water Jet Cutting (WJC)

WJC Applications

• Usually automated using CNC or industrial robots

• Best used to cut narrow slits in flat stock such as: plastic, textiles, composites, tile, and cardboard

• Not suitable for:

• When used on metals, you need to add to the water stream:

• Smallest kerf width about 0.4 mm for metals, and 0.1mm for plastics and non-metals.

• More info: http://www.waterjets.org/index.html

WJC Advantages

• No crushing or burning of work surface

• Minimum material loss

• No environmental pollution

• Ease of automation

High velocity gas stream containing abrasive particles (aka: sand blasting or bead blasting)

– Normally used as a finishing process rather than cutting process (e.g. gas sandpaper)

– Applications: deburring, cleaning, and polishing.

1c) Abrasive Jet Machining (AJM)

2. Electrochemical Machining Processes

• Electrical energy used in combination with chemical reactions to remove material

• Reverse of:

• Work material must be a:

• Feature dimensions down to about 10 μm

Courtesy of AEG-Elotherm-Germany

Material removal by anodic dissolution, using electrode (tool) in close proximity to work but separated by a rapidly flowing electrolyte

Electrochemical Machining (ECM)

ECM Operation

Material is deplated from anode workpiece ( pole) and transported to a cathode tool ( pole) in an electrolyte bath

• Electrolyte flows rapidly between two poles to carry off deplated material, so it does not:

• Electrode materials: Cu, brass, or stainless steel

• Tool shape is the:

– Tool size must allow for the gap

ECM Applications

• Die sinking - irregular shapes and contours for forging dies, plastic molds, and other tools

• Multiple hole drilling - many holes can be drilled simultaneously with ECM

• No burrs created – no residual stress

Trimmer et al, APL 2003Schuster et al, Science 2000

Material Removal Rate of ECM

• Based on Faraday's First Law: rate of metal dissolved is proportional to the current

MRR = Aƒr = ηCI

where I = current; A = frontal area of the electrode (mm2), ƒr = feed rate (mm/s), and η = efficiency coefficient

= specific removal rate with work material;

M = atomic weight of metal (kg/mol) density of metal (kg/m3), F = Faraday constant (Coulomb)n = valency of the ion;

FnMC

Equations for ECM (Cont’)

• Resistance of Electrode:

Gap, g

Area, A

ρ is the resistivity of the electrolyte fluid (Ohm∙m)

Example: ECM through a plate

• Aluminum plate, thickness t = 12 mm;

• Rectangular hole to be cut:

L = 30mm, W = 10mm

• Applied current: I = 1200 amps.

• Efficiency of 95%,

• MRR = Aƒr = ηCI

Determine how long it will take to cut the hole?

30mm10mm

Ideal CAl = 3.44×10-2 mm3/amp∙s

- other ‘C’ values in Table 26.1

3. Thermal Energy Processes - Overview

• Very high temperatures, but only:

– Material is removed by:

• Problems and concerns:

– Redeposition of vaporized metal

– Surface damage and metallurgical damage to the

new work surface

– In some cases, resulting finish is so poor that

subsequent processing is required

3. Thermal Energy Processes

• Electric discharge machining (EDM)

• Electric discharge wire cutting (Wire EDM)

• Electron beam machining (EBM)

• Laser beam machining (LBM)

• Plasma arc cutting or machining (PAC)

3a) Electric Discharge Machining (EDM)

• One of the most widely used nontraditional processes• Shape of finished work is inverse of tool shape • Sparks occur across a small gap between tool and work• Holes as small as 0.3mm can be made with feature

sizes (radius etc.) down to ~2μm

Work Materials in EDM

• Work materials must be:

• Hardness and strength of work material are: not factors

• Material removal rate depends primarily on: melting point of work material

• Applications:– Molds and dies for injection molding and forging

– Machining of hard or exotic metals

– Sheetmetal stamping dies.

• EDM uses small diameter wire as electrode to cut a narrow kerf in work – similar to a: bandsaw

3b) Wire EDM

Material Removal Rate of EDM

• Weller Equation (Empirical);

Maximum rate: RMR =

where K = 664 (°C1.23∙mm3/amp∙s); I

= discharge current; Tm = melt temp

of work material

• Actual material removal rate:

MRR = vf ∙h∙wkerf

where vf = feed rate; h = workpiece

thickness; wkerf = kerf width

23.1mTKI

While cutting, wire is

continuously advanced

between supply spool

and take‑up spool to:

Wire EDM Applications

• Ideal for stamp and die components– Since kerf is so narrow, it is

often possible to fabricate punch and die in a single cut

• Other tools and parts with intricate outline shapes, such as lathe form tools, extrusion dies, and flat templates

• Part loaded inside a

vacuum chamber

• Beam is focused through

electromagnetic lens,

reducing diameter to as

small as 0.025 mm

• Material is vaporized in a

very localized area

3c) Electron Beam Machining (EBM)

EBM Applications

• Ideal for micromachining– Drilling small diameter holes ‑ down to 0.05 mm

(0.002 in)

– Cutting slots only about 0.025 mm (0.001 in.) wide

• Drilling holes with very high depth‑to‑diameter ratios

– Ratios greater than 100:1

• Disadvantage: slow and expensive

• Generally used for: drilling, slitting, slotting, scribing, and marking operations

• Holes can be made down to 0.025 mm

• Generally used on thin stock material

3d) Laser Beam Machining (LBM)

• Uses plasma stream at very high temperatures to cut metal 10,000°C to 14,000°C

• Plasma arc generated between electrode in torch and workpiece

• The plasma flows through water‑cooled nozzle that constricts and directs plasma stream to desired location

3e) Plasma Arc Cutting (PAC)

Applications of PAC

• Most applications of PAC involve cutting of metal sheets and plates

• Hole piercing and cutting along a defined path

• Can be operated by hand‑held torch or automated by CNC

• Can cut any:

• Hole sizes generally larger than 2 mm

4. Chemical Machining (CHM)

CHM Process:• Cleaning ‑ to insure uniform etching • Masking ‑ a maskant (resist, chemically resistant to etchant)

is applied to portions of work surface not to be etched• Patterning of maskant• Etching ‑ part is immersed in etchant which chemically

attacks those portions of work surface that are not masked• Demasking ‑ maskant is removed

Maskant - Photographic Resist Method

• Masking materials contain photosensitive chemicals

• Maskant is applied to work surface (dip coated, spin coated, or roller coated) and exposed to light through a negative image of areas to be etched – These areas are then removed using photographic

developing techniques

– Remaining areas are vulnerable to etching

• Applications:– Small parts on thin stock produced in high quantities

– Integrated circuits and printed circuit cards

Material Removal Rate in CHM

• Generally indicated as penetration rates, i.e. mm/min.

• Penetration rate unaffected by exposed surface area

• Etching occurs downward and under the maskant

• In general, d ≤ u ≤ 2d, Etch Factor: Fe=

(see Table 26.2 pg 637)ud

Chemical Blanking

• Uses CHM to cut very thin

sheetmetal parts ‑ down to

0.025 mm thick and/or for

intricate cutting patterns

• Conventional punch and

die does not work because

stamping forces damage

the thin sheetmetal, or

tooling cost is prohibitive

Parts made by chemical blanking (photo courtesy of Buckbee-Mears St. Paul).

CHM Possible Part Geometry Features

• Very small holes

• Holes that are not round

• Narrow slots in slabs and plates

• Micromachining

• Shallow pockets and surface details in flat parts

• Special contoured shapes for mold and die applications