Nontraditional Machining Processes

Transcription

1 Nontraditional Machining Processes The NTM processes can be divided into four basic categories: I. Chemical (Chemical reaction), II. Electrochemical (Electrolytic dissolution), III. Mechanical (Multipoint cutting or erosion), IV. Thermal (High temperatures in very localized regions melt and vaporize the material). Characteristics of NTM processes are: 1. Low metal-removal rates, 2. Very high specific powers (W/cm 3 /min), 3. Better accuracy, 4. Less surface damaging, 5. Can be employed for materials that are too hard. 1

2 Nontraditional Machining Processes I. Chemical Machining In chemical machining material is removed from selected areas of a workpiece by immersing it in a chemical reagent. Material is removed by microscopic electrochemical cell action as occurs in corrosion or chemical dissolution of a metal, which is called etching. 2

3 Chemical Machining Etching is material removal from the unprotected sections of the workpiece by means of microscopic electrochemical cell action as in corrosion or chemical dissolution of metal without the involvement of any external circuit. Chemical Machining is the targeted use of chemical etchants (acids and alkaline solutions) in the removal of material from metal parts surfaces. In order to prevent the removal of material from unwanted regions, a mask / resist on the surface of the workpiece that is resistant to the etchant used. 3

4 Chemical Machining Mixer Workpiece Workpiece holder Maskant Chemical etchant 4

5 Chemical Machining The basic chemical machining processes are: 1. Chemical Milling, used for pockets, contours and overall metal removal, 2. Chemical Blanking (etching through thin sheets), 3. Photochemical Machining (photosensitive resists are used for masks), 4. Gel Milling (uses reagent in jel form), 5. Chemical or Electrochemical Polishing (weak chemical reagents are used for polishing). 5

6 Chemical Milling Chemical Milling and Blanking Used for pockets, contours and overall metal removal. Shallow cavities are etched on the surface of a metal workpiece -most commonly, on large aerospace structures. (1) Raw part is cleaned/ degreased. (3) Maskant is removed from selected sections. (5) Maskant is removed completely and workpiece is washed. (2) Maskant is applied globally and cured. (4) Exposed sections of the workpiece are etched. Etchant 6

7 Chemical Blanking (1) Raw part is cleaned/ degreased. (3) Maskant is removed from selected sections. (5) Maskant is removed completely and workpiece is washed. (2) Maskant is applied globally and cured. (4) Exposed sections of the workpiece are etched. Some thin-walled part profiles 7

8 Nontraditional Machining Processes II. Electrochemical Machining (ECM) Suitable for electrically conductive materials. Tool should also be electrically conductive. Process of removing metal from a workpiece by a reverse-electroplating action. (In electroplating, workpiece which is the negative electrode (cathode), and plate metal which is the positive electrode (anode) are dipped in a solution which contains dissolved salts of the metal to be deposited.) 8

9 Electrochemical Machining (ECM) Tool Cathode (-) Tool holder and feed mechanism De-plated material Cavity created by deplating Forced electrolyte circulation Workpiece Tool Insulation Electrolyte supply Electrolyte Anode (+) The electrolytes are highly conductive solutions of inorganic salts usually NaCl (Sodium Chlorate), KCl, NaNO 3 (Sodium Nitrate), and are operated at about 30 to 55 C. Tools are usually made from copper, brass, titanium or sometimes stainless steel. 9

10 Electrochemical Machining Processes 1. Electrochemical Polishing Feed is halted, lower current density and slower electrolyte flow rates reduce MRR so that a fine surface finish is obtained. 2. Electrochemical Hole Drilling a. Drilling of very small diameter holes 0.1 to 0.75 mm in diameter with 50:1 depth to diameter ratios are accomplished in nickel and cobalt alloys. b. Drilling of medium sized holes sizes being 0.5 to 6 mm. 10

11 Electrochemical Machining Processes 3. Electrochemical Grinding (ECG) Used for shaping and sharpening carbide cutting tools, which cause high wear rates on expensive diamond wheels in normal grinding. Tool electrode is a rotating, metal bonded, diamond grit grinding wheel. Metal bond of the wheel is the cathode. 11

12 Electrochemical Grinding (ECG) Diamond particles are used as an insulator to preserve a gap between the cathode and the workpiece, to wipe away residues, and to cut chips if the wheel should contact the workpiece. MRR is competitive with conventional grinding. Less than 5 % of the material is removed by normal chip removal. Lack of heat damage, burrs and residual stresses are very beneficial. Used for fragile parts, surgical needles, tips of assembled turbine blades etc. 12

13 Nontraditional Machining Processes III. Mechanical NTM Processes Mechanical NTM Processes can be used for electrically conductive or nonconductive materials. 1. Ultrasonic Machining (USM) 2. Hydrodynamic Jet Machining 3. Abrasive Jet Machining ( AJM ) 4. Abrasive Waterjet Cutting (AWC) 13

14 1. Ultrasonic Machining (USM) Mechanical NTM Processes Employs an ultrasonically vibrating tool to impel the abrasive (carbide, ceramic) particles in a slurry against a workpiece. An inverse image of the tool in the workpiece is formed as the abrasive particles abrade (machine) the material. A transducer is used to impart high frequency vibration (100 khz) to the tool holder. High frequency oscillation Tool Slurry Workpiece 14

15 The tool materials are usually brass, carbide, or mild or tool steel. Abrasive particles (grit materials), most commonly are Boron Carbide, Aluminum Oxide and Silicon Carbide. The process can cut virtually any material, conductive or nonconductive, metallic, ceramic or composite. The tool wears continuously. Ultrasonic Machining (USM) Wear ratios (material removed versus tool wear) of 1:1 to 100:1 are possible. There is an overcut during the machining by about twice the size of the abrasive grit. Surface roughness is controlled by the size of the grit. 15

16 Ultrasonic Machining (USM) USM is primarily targeted for the machining of brittle materials (dielectric or conductive): Boron carbide, ceramics, glass, titanium carbides, etc. The tool must be selected to be highly wear resistant, such as low-carbon steels. The abrasives (25-60 m in diameter) in the slurry (water-based, 20-60% solid volume) include boron carbide, boron nitride, silicon carbide, aluminum oxide and diamond. Surface finish in USM can be an order of magnitude better than that achievable through milling. 16

17 2. Water Jet Cutting (WJC) (Hydrodynamic Jet Machining) Uses a fine, high-pressure, high velocity stream of water (water jet). Velocity: Mach 2 (680 m/s); up to 900m/s (~Mach 3) Pressure: 70 to 415 MPa. Mechanical NTM Processes Mainly used to cut soft, non-metallics like plastics, paperboard, asbestos, leather, rubber, fiberglass, textiles, composites, floor tile, carpet, leather, and cardboard 17

18 Hydrodynamic Jet Machining Cutting rates vary from 75 m/min up to 1750 m/min. Feed rates are between 5 mm/s to 500 mm/s depending on material and its thickness. Advantages: no crushing or burning of work surface, minimum material loss, no environmental pollution, and ease of automation. CNC machines or industrial robots can be used to cut along a desired complex shaped trajectory. Jet flows through small nozzle (made of sapphire, ruby or diamond) opening of diameter (0.1 to 0.4 mm). A long-chain polymer is added to the water to make the jet coherent (not come out of the nozzle as a mist). 18

19 Hydrodynamic Jet Machining Abrasives can also be utilized 19

20 3. Abrasive Jet Machining (AJM ) Mechanical NTM Processes AJM is mainly a finishing process that removes material with the abrasive action of gas jet (air, nitrogen or carbon-dioxide) loaded with abrasive powder particles (silicon carbide, aluminum oxide or glass). Can also be used to cut slits, grooves or shapes or holes. Works well in heat-sensitive, brittle, thin or hard materials. 4. Abrasive Waterjet Cutting (AWC) Abrasives are added to a waterjet to improve the cutting efficiency. 20

21 Thermal NTM Processes High temperatures in very localized regions melt and vaporize the material. 1. Electrodischarge Machining (EDM) 2. Electron Beam Machining (EBM) 3. Laser Beam Machining (LBM) 21

22 1. Electrodischarge Machining (EDM) Thermal NTM Processes Suitable for electrically conductive materials. Tool (electrode) should also be electrically conductive. EDM removes metal by the eroding action of small electrical sparks. Cathode (-) Tool (Electrode) Material removed from workpiece Cavity created by discharge Discharge Flow of dielectric fluid Workpiece Anode (+) 22

23 Electrodischarge Machining (EDM) In operation, both the electrode (tool) and the workpiece are immersed in a dielectric fluid (non-conductive fluid). Thousands of sparks per second are generated and each spark produces a tiny crater by vaporization, thus eroding the shape of the tool into the workpiece. 23

24 Electrodischarge Machining (EDM) Crater formed by a single spark on a ground surface. (Ground : Made by grinding.) 24

25 Electrodischarge Machining (EDM) 25

26 Electrodischarge Machining (EDM) The dielectric fluid (hydrocarbon oils, kerosene and deionized water) is an insulator between the tool and the workpiece, a coolant and a flushing medium for the removal of chips. 26

27 Electrodischarge Machining (EDM) Materials of any hardness can be cut as long as the material can conduct electricity. Absence of almost all mechanical forces makes it possible to machine fragile parts. A primary disadvantage of EDM is tool wear. It is common to utilize several identical-geometry cutting tools during the machining of one profile. Tool (electrode) materials can be, graphite, copper, brass, tungsten, steel, aluminum, molybdenum, nickel, etc. 27

28 Electrodischarge Machining (EDM) EDM, primarily, exists commercially in the form of die-sinking machines (EDM) (Ram EDM / Sinker EDM) and wire-cutting machines. Dielectric fluid 28

29 Wire-EDM (W-EDM) Special form of EDM is wire EDM, wherein the electrode is a continuously moving conductive wire made from copper, brass, tungsten or molybdenum. The process is widely used for the manufacture of punches, dies and stripper plates. 29

30 Wire-EDM (W-EDM) Wire-EDM ed parts. 30

31 3. Laser Beam Machining (LBM) Thermal NTM Processes LBM is a thermal material removal process that utilizes a high-energy, coherent light beam (laser) to melt and vaporize materials (metallic and non-metallic). 31

32 Laser Beam Machining (LBM) Can drill mm diameter holes in 2.5 mm thick material in seconds with a depth to diameter ratio of 10/1. High-energy solid state or gas lasers are used. Not a mass metal-removal process. Recast layer and heat-affected zone can be detrimental to material properties. Can be used as well for welding, cutting, trimming, blanking and heat-treating by varying power density, beam intensity, focus and duration. 32

33 Laser Beam Drilling 5-DOF Laser drilling machine 33