Wollaston process
The modern field of powder metallurgy dates to the early nineteenth century, when there was a strong interest in the metal platinum. Around 1815, Englishman William Wollaston developed a technique for preparing platinum powders, compacting them under high pressure, and baking (sintering) them at red heat. The Wollaston process marks the beginning of powder metallurgy as it is practices today.
(Fundamentals of Modern Manufacturing: Materials, Processes, and Systems, Mikell P. Groover, p.346)
Coolidge process
In 1908, William Coolidge developed a procedure that made production of tungsten incandescent lamp filaments feasible. In this process, fine powders of tungsten oxide were reduced to metallic powders, pressed into compacts, presintered, hot-forged into rounds, sintered and finally drawn into filament wire. The Coolidge process is still used today to make filaments for incandescent light bulbs.
(Fundamentals of Modern Manufacturing: Materials, Processes, and Systems, Mikell P. Groover, p.346)
Shaped tube electrolytic machining (STEM)
A second process, known as the shaped-tube electrolytic machining (STEM) process, was also created in response to unique challenges presented in the jet engine industry. Like the electrostream process, STEM is also capable of gang drilling small holes in difficult-to-machine materials However, the STEM process is generally not capable of drilling holes smaller than about 0.02 in. STEM is capable of making shaped holes with aspect ratios as high as 300:l. Holes up to 24 in. m depth have been drilled. Like the electrostream process, it uses an acidic electrolyte to minimize clogging due to sludge buildup. The major differences between the STEM process and the electrostream process are the reduced voltage levels (5 to 10 V dc) and the special electrodes, which are long, straight, metallic tubes coated with an insulator. The insulator helps to eliminate taper by constraining the electrolytic action between the bottom of the tool and the workpiece. Titanium is often used for its ability to resist acids. The electrolyte is pressure-fed through the tube and returns through the gap (0.001 to 0.002 m.) between the insulated tube wall and the hole wall. Electrolyte concentrations may include up to 10% sulfuric acid. Lower concentrations may be used to increase tool life.
(MATERIALS AND PROCESSES IN MANUFACTURING 10th edition, J. Temple Black, Ernest Paul DeGarmo, Ronald A. Kohser, p.507)
Photochemical Machining
The specific steps that are involved when photochemical machining (PCM) is performed with the use of photoresists. These are as follows:
1. Clean the workpiece.
2. Coat the workpiece with a photoresist, usually by hot-roller lamination of dry-film photoresists, on both sides, although liquid photoresists may also be applied by dipping, flowing. rolling, or electrophoresis (i.e., migration of charged molecules in the presence of an electric field). For liquid photoresists, the coating is heated in an oven to remove solvents.
3. Prepare the artwork. A drawing of the workpiece is made on a computer-aided design (CAD) system.
4. Develop the phototool. The CAD file is used to derive a photographic negative workpiece. Several methods may be used. Typically, the CAD drawing is downloaded to a laser-imaging system that exposes the desired Image directly onto photograph (e.g., silver halide) film. In the past, oversized artwork was used to Increase the curacy of the phototool through photographic reduction of the artwork.
5. Expose the photoresist. Bring the phototool in contact with the workpiece, a vacuum frame to ensure good contact, and expose the workpiece to Intense violet (UV) light
6. Develop the photoresist. Exposure of the photoresist to intense UV light alters the chemistry of the photoresist, making it more resistant to dissolution in certain solvents. By placing the exposed maskant in the proper solvent, the unexposed areas of the resist are removed, exposing the underlying material for etching. All residue is rinsed away
7. Spray the workpiece with (or immerse it in) the reagent
8. Remove the remaining maskant.
PCM has been widely used for the production of small, complex parts, such as printed circuit boards, and very thin parts that are too small or too thin to be blanked or milled by ordinary sheet metal forming or machining operations, respectively Refinements to the PCM process are used in the microeletronics fabrication.
(MATERIALS AND PROCESSES IN MANUFACTURING 10th edition, J. Temple Black, Ernest Paul DeGarmo, Ronald A. Kohser, p.489)
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