PT Notes

Ultrasonic Machining

Ultrasonic machining (USM) is a mechanical material removal process used to erode holes and cavities in hard or brittle workpieces by using shaped tools, high frequency mechanical motion, and an abrasive slurry. A relatively soft tool is shaped as desired and vibrated against the workpiece while a mixture of fine abrasive and water flows between them. The friction of the abrasive particles gradually cuts the workpiece.

Materials such as hardened steel, carbides, rubies, quartz, diamonds, and glass can easily be machined by USM. Ultrasonic machining is able to effectively machine all materials harder than HRc 40, whether or not the material is an electrical conductor or an insulator

Electrochemical Machining (ECM)

· The physics - an electrode and workpiece (conductor) are placed in an electrolyte, and a potential voltage is applied. On the anode (+ve) side the metal molecules ionize (lose electrons) break free of the workpiece, and travel through the electrolyte to the electrode (a cathode; has a -ve charge; a surplus of electrons).

· NOTE: in EDM an arc was used to heat metal, here the metal dissolves chemically.

· Variation in the current density will result in work taking the electrodes shape.

· The electrode is fed with a constant velocity, and the electrolyte is fed through the tool. The tool is designed to eliminate deposition of the ionized metal on the electrode.

ELECTROCHEMICAL MACHINING (ECM)

INTRODUCTION:-

Electro chemical machining (ECM) is a method of removing metal by an electrochemical process. It is normally used for mass production and is used for working extremely hard materials or materials that are difficult to machine using conventional methods. Its use is limited to electrically conductive materials; however, this includes all metals. ECM can cut small or odd-shaped angles, intricate contours or cavities in extremely hard steel and exotic metals such as titanium, hastelloy, kovar, inconel and carbide.

ECM is often characterized as "reverse electroplating," and is similar in concept to electrical discharge machining in that a high current is passed between an electrode and the part, through an electrolyte material removal process having a negatively charged electrode (cathode), a conductive fluid (electrolyte), and a conductive work piece (anode); however, in ECM there is no tool wear. The ECM cutting tool is guided along the desired path very close to the work but it does not touch the piece. Unlike EDM however, no sparks are created. Very high metal removal rates are possible with ECM, along with no thermal or mechanical stresses being transferred to the part, and mirror surface finishes are possible. Electrochemical machining (ECM) also uses electrical energy to remove material. An electrolytic cell is created in an electrolyte medium, with the tool as the cathode and the workpiece as the anode. A high-amperage, low-voltage current is used to dissolve the metal and to remove it from the workpiece, which must be electrically conductive. ECM is essentially a deplating process that utilizes the principles of electrolysis. The ECM tool is positioned very close to the workpiece and a low voltage, high amperage DC current is passed between the two via an electrolyte. Material is removed from the workpiece and the flowing electrolyte solution washes the ions away. These ions form metal hydroxides which are removed from the electrolyte solution by centrifugal separation. Both the electrolyte and the metal sludge are then recycled.

Unlike traditional cutting methods, work piece hardness is not a factor, making ECM suitable for difficult-to-machine materials. Takes such forms as electrochemical grinding, electrochemical honing and electrochemical turning. The process schematic is such that a cathode (tool) is advanced into an anode (work piece). The pressurized electrolyte is injected at a set temperature to the area being cut. The feed rate is the exact same rate as the rate of liquefaction of the material. The area in-between the tool and the work piece vary within .003 in. and .030 in.

As far back as 1929, an experimental ECM process was developed by W.Gussef, although it took until 1959 for a commercial process to be established by the Anocut Engineering Company. Much research was done in the 1960s and 1970s, particularly in the gas turbine industry. The rise of EDM in the same period largely stopped research into ECM in the west, although work continued behind the Iron Curtain. The original problems of poor dimensional accuracy and environmentally polluting waste have largely been overcome, although the process remains a niche technique. The cutting heads on all Philips 'Philishave' shavers are made using ECM.The ECM process is most widely used to produce complicated shapes with good surface finish in difficult to machine materials, such as turbine blades. It is also widely and effectively used as a deburring process.

EQUIPMENTS:-

ECM machines come in both vertical and horizontal types. Depending on the work requirements these machines are built in many different sizes as well. The vertical machine is comprised of a base, column, table, and spindle head. The spindle head has a servo-mechanism that automatically advances the tool and controls the gap between the cathode (tool) and the workpiece.

TOOLS & GEOMETRY PRODUCED:-

Both external and internal geometries can be machined with an electrochemical machine. Copper is often used as the electrode material. Brass, graphite, and copper-tungsten are also often used because of the ability to be easily machined, they are conductive materials, and they will not corrode.

PRINCIPAL & METAL REMOVAL ANAYLSIS:-

The physics - an electrode and work piece (conductor) are placed in an electrolyte, and a potential voltage is applied. On the anode (+ve) side the metal molecules ionize (lose electrons) break free of the work piece, and travel through the electrolyte to the electrode (a cathode; has a -ve charge; a surplus of electrons).

· NOTE: in EDM an arc was used to heat metal, here the metal dissolves chemically.

· Variation in the current density will result in work taking the electrodes shape.

· The electrode is fed with a constant velocity, and the electrolyte is fed through the tool. The tool is designed to eliminate deposition of the ionized metal on the electrode.

· Supply V = 8 to 20V, I = >1000A.

· Electrode gap is typically 0.1 to 0.2 mm.

· MRR is about 1600mm3/min. per 1000A, OR 3KWhr for 16000 mm3 (not very efficient, 30 times more than standard machining techniques).

· MRR is independent of material hardness.

· Good for low machinability, or complicated shapes.

· Very little tool wear,

· Forces are large with this method because of fluid pumping forces.

· Faraday's laws state that,

· The basic principle is shown below

· The chemical reaction between an electrode and the electrolyte leads to electrons being added, or removed from the electrode metal. This addition/subtraction leads to a voltage potential.

· To make a battery.

· To do electrolysis.

· e.g.

· Actual rates may vary from theory as other factors come into effect.

· The table below shows various materials and relevant properties,

· e.g.

· While the current required is related to the metal removed, the voltage required depends upon,

1. - electrode potential.

2. - the current flow in and about the electrodes will disturb the normal distribution of voltage. Extra potential is required to overcome the effects.

3. - Ion collect near electrodes and impede ion transfer from the electrode to the electrolyte, also adding a potential.

4. - Some solid film forms on the surface of the electrode, also increasing resistance.

5. - electrolyte resistance,

· The feed of the electrodes has the following effects

· The ECM process will erode material in a radial direction, so care must be made in tooling design.

· As current flows through the electrolyte, it is heated, and conductivity decreases.

· Surface finish is affected by,

1. - selective dissolution

2. - sporadic breakdown of the anodic film

3. - flow separation and formation of eddies

4. - evolution of hydrogen

· Typical electrolytes are,

· Summary of ECM characteristics,

1. - mechanics of material removal - electrolysis

2. - medium - conducting electrolyte

3. - tool material - Cu, brass, steel

4. - material/tool wear - infinite

5. - gap 50 to 300 μm

6. - maximum MRR 15*103 mm3/min

7. - specific power consumption 7W/mm3/min

8. - critical parameters - voltage, current, feed rate, electrolyte, electrolyte conductivity

9. - materials application - all conducting metals and alloys

10. - Shape application - blind complex cavities, curved surfaces, through cutting, large through cavities.

11. - Limitations - high specific energy consumption (about 150 times that required for conventional processes), not applicable with electrically non-conducting materials and jobs with very small dimensions, expensive machines.

12. - Surface finishes down to 25 μin.

· This technique has been combined with a metal grinding wheel in a process called Electrolytic drilling. The wheel does not touch the work, and gives a surface finish from 8 to 20 μin.

ADVANTAGES:-

1. The components are not subject to either thermal or mechanical stress.
2. There is no tool wear during electrochemical machining.
3. Non-rigid and open work pieces can be machined easily as there is no contact between the tool and work piece.
4. Complex geometrical shapes can be machined repeatedly and accurately
5. Electrochemical machining is a time saving process when compared with conventional machining
6. During drilling, deep holes can be made or several holes at once.
7. ECM deburring can debur difficult to access areas of parts.
8. Fragile parts which cannot take more loads and also brittle material which tend to develop cracks during machining can be machined easily through Electrochemical machining
9. Surface finishes of 25 µ in. can be achieved during Electrochemical machining

Electrochemical Grinding (ECG):-

Electrochemical grinding combines electrical and chemical energy for metal removal with an EDM finish. It is a non-abrasive process and, therefore, produces precise cuts that are free of heat, stress, burrs and mechanical distortions. It is a variation on electrochemical machining that uses a conductive, rotating abrasive wheel. The chemical solution is forced between the wheel and the work piece. The shape of the wheel determines the final shape.

Electrochemical deburring

Electrochemical deburring is another variation on electrochemical machining designed to remove burrs and impart small radii to corners. The process normally uses a specially shaped electrode to carefully control the process to a specific area. The process will work on material regardless of hardness. In the deburring process, the ECM uses techniques as described above to remove pieces of metal that are left over from the machining process, and to dull out sharp edges. This process is very fast and much more convenient than the conventional method of deburring by hand or nontraditional machining processes. It will tend to leave better surface finishing, and no metal deformation will occur because the tool piece doesn’t actually touch the metal.

Chemical Machining:-

Chemical Machining aides in the manufacture of light gauge metal parts. The photo etching process (also called chemical etching and chemical milling) allows people to produce intricate metal components with close tolerances that are impossible to duplicate by other production methods. It is also known as chemical milling.

Applications -

Chemical Machining is utilized in the manufacturing of encoders, masks, filters, lead frames, flat springs, strain gauges, laminations, chip carriers, step covers, fuel cell plates, heat sinks, shutter blades, electron grids, fluidic circuit plates, reticles, drive bands, haptics, and shims.

LASER BEAM MACHINING

INTRODUCTION:- Laser beam machining (LBM) is a technique used to join multiple pieces of metal through the use of a laser. The beam provides a concentrated heat source, allowing for narrow, deep welds and high welding rates. The process is frequently used in high volume applications, such as in the automotive industry. Gas lasers use high-voltage, low-current power sources to supply the energy needed to excite the gas mixture used as a lasing medium. These lasers can operate in both continuous and pulsed mode, and the wavelength of the laser beam is 10.6 μm. Fiber optic cable absorbs and is destroyed by this wavelength, so a rigid lens and mirror delivery system is used. Power outputs for gas lasers can be much higher than solid-state lasers, reaching 25 kW

OPERATION:-

Like electron beam welding (EBW), laser beam welding has high power density (on the order of 1 Megawatt/cm² (MW)) resulting in small heat-affected zones and high heating and cooling rates. The spot size of the laser can vary between 0.2 mm and 13 mm, though only smaller sizes are used for welding. The depth of penetration is proportional to the amount of power supplied, but is also dependent on the location of the focal point: penetration is maximized when the focal point is slightly below the surface of the work piece.

A continuous or pulsed laser beam may be used depending upon the application. Milliseconds long pulses are used to weld thin materials such as razor blades while continuous laser systems are employed for deep welds.

LBW is a versatile process, capable of welding carbon steels, HSLA steels, stainless steel, aluminum, and titanium. Due to high cooling rates, cracking is a concern when welding high-carbon steels. The weld quality is high, similar to that of electron beam welding. The speed of welding is proportional to the amount of power supplied but also depends on the type and thickness of the work pieces. The high power capability of gas lasers make them especially suitable for high volume applications. LBW is particularly dominant in the automotive industry.

Some of the advantages of LBW in comparison to EBW are as follows: the laser beam can be transmitted through air rather than requiring a vacuum, the process is easily automated with robotic machinery, x-rays are not generated, and LBW result in higher quality welds.

A derivative of LBW, laser-hybrid welding, combines the laser of LBW with an arc welding method such as gas metal arc welding. This combination allows for greater positioning flexibility, since GMAW supplies molten metal to fill the joint, and due to the use of a laser, increases the welding speed over what is normally possible with GMAW. Weld quality tends to be higher as well, since the potential for undercutting is reduced.

EQUIPMENT:-

• The two types of lasers commonly used in are solid-state lasers and gas lasers (especially carbon dioxide lasers and Nd:YAG lasers).
• The first type uses one of several solid media, including synthetic ruby and chromium in aluminum oxide, neodymium in glass (Nd:glass), and the most common type, crystal composed of yttrium aluminum garnet doped with neodymium (Nd:YAG).
• Gas lasers use mixtures of gases like helium, nitrogen, and carbon dioxide (CO2 laser) as a medium.

· Regardless of type, however, when the medium is excited, it emits photons and forms the laser beam.

ADVANTAGES:-

(i) Due to rapid rate of operation, the (heat) distortion to the base metal is minimum.
(ii) The narrowness and accuracy of the cut together with relatively shallow HAZ with negligible surface cracking are major advantages of this process.
(iii) Even most complicated profiles can be cut.
(iv ) Very hard material like silicon carbide, friable material like glass, sticky material like confectionery etc., can be cut by laser.
(v) Laser is a faster process than sawing or nibbling and is very efficient.

LIMITATIONS:-
The limit on thickness of section (6 mm in plain carbon steel; for other metals it is still lesser) and the fact that work piece must be moved relative to the rather cumbersome equipment restrict the use of gas-jet laser cutting.

Applications of Laser Beam Cutting –

(i) Laser can cut in locations in a (metal) structure very difficult to reach by any other method.

(ii) Laser can cut both metals and non-metals, (e.g., plastics, leather, etc.).

(iii) Laser has been employed for making die boards in block-board which are used in the manufacture of packages, cartons and gaskets.

(iv)Asbestos products cut readily and clearly by the laser.