ME 350 – Lecture 21 – Chapter 26

1. ME 350 – Lecture 21 – Chapter 26 NONTRADITIONAL MACHINING PROCESSES • Mechanical Energy Processes (USM, WJC, AJM) - high velocity stream of abrasives or fluid(or both) • Electrochemical Processes (ECM) - reverse of electroplating • Thermal Processes (EDM, Wire EDM, EBM, LBM, PAC) - vaporizing of a small area of work surface • Chemical Processes (CHM, Chemical Blanking, PCM) - chemical etching of areas unprotected by “maskant” Nontraditional machining is characterized by material removal that:

2. Nontraditional Processes Used When: • Material is either very hard, brittle or both; or material is very ductile: • Part geometry is complex or geometric requirements impossible with conventional methods: • Need to avoid surface damage or contamination that often accompanies conventional machining:

3. 1. Mechanical Energy Processes • Ultrasonic machining (USM) • Water jet cutting (WJC) • Abrasive jet machining (AJM)

4. 1a) Ultrasonic Machining (USM) 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

5. USM Applications • Used only on hard and brittle work materials: • 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

6. 1b) Water Jet Cutting (WJC) • Uses high pressure, high velocity stream of water directed at work surface for cutting

7. WJC Applications • Usually automated using CNC or industrial robots • Best used to cut narrow slits in flat stock such as: • 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

8. WJC Advantages • No crushing or burning of work surface • Minimum material loss • No environmental pollution • Ease of automation

9. 1c) Abrasive Jet Machining (AJM) 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.

10. 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

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

12. 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

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

14. Material Removal Rate of ECM • Based on Faraday's First Law: rate of metal dissolved is proportional to the current MRR = Aƒr = ηCI whereI = 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) • r= density of metal (kg/m3), • F = Faraday constant (Coulomb) • n = valency of the ion;

15. Equations for ECM (Cont’) • Resistance of Electrode: Gap, g Area, A r is the resistivity of the electrolyte fluid (Ohm∙m)

16. 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%, Determine how long it will take to cut the hole? 10mm 30mm Ideal CAl = 3.44×10-2 mm3/amp∙s - other ‘C’ values in Table 26.1

17. Solution:

18. 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

19. 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)

20. 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

21. Work Materials in EDM • Work materials must be: • Hardness and strength of work material are: • Material removal rate depends primarily on: • Applications: • Molds and dies for injection molding and forging • Machining of hard or exotic metals • Sheetmetal stamping dies.

22. 3b) Wire EDM • EDM uses small diameter wire as electrode to cut a narrow kerf in work – similar to a:

23. Material Removal Rate of EDM • Weller Equation (Empirical); Maximum rate: RMR = whereK = 664 (°C1.23∙mm3/amp∙s);I= discharge current; Tm = melt temp of work material • Actual material removal rate: MRR = vf∙h∙wkerf wherevf= feed rate;h= workpiece thickness;wkerf = kerf width While cutting, wire is continuously advanced between supply spool and take‑up spool to:

24. 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

25. 3c) Electron Beam Machining (EBM) • 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

26. 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

27. 3d) Laser Beam Machining (LBM) • 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

28. 3e) Plasma Arc Cutting (PAC) • 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

29. 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

30. 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

31. 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

32. 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, , Etch Factor: Fe= (see Table 26.2 pg 637)

33. 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).

34. 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

35. Quotes: • We are what we repeatedly do. Excellence, then, is not an act, but a habit. - Aristotle • If you want others to be happy, practice compassion. If you want to be happy, practice compassion. - Dalai Lama • When the heart grieves over what it has lost, the spirit rejoices over what it has left. - Sufi Epigram • A great pleasure in life is doing what people say you cannot do. - Walter Bagehot