Photochemical Machining Produces "Stampings" at Low Cost
Photochemical Machining (PCM) answers the need for producing low-cost, flat, burr-free metal parts in both prototype and high volume quantities. It offers unique advantages in both parts manufacture and freedom of design.
Everyone knows what a metal stamping is. But often, parts which shouldn't be, or do not need to be stamped, are. You look at a part required to be made from metal. It's flat, or can be formed up from a flat part blanked in a press tool, in a steel rule die, or on a fabricator. "We'll stamp that". Why? The criteria dictating how a part should be made, in this case, a flat or formed-up flat metal part, include: flatness requirements, conditions of edges, and/or holes, tolerances which must be held on overall dimensions, hole and web size, spacings, material to be used, allowable burr, hardness requirements, quantities and more.
What steps are involved in preparation for stamping parts? Part design, of course. Perhaps machining one or more prototype(s) from the solid to prove them out. Then die design, diemaking, test blanking and/or forming, die correction or adjustment, or remanufacture from scratch if not correctable. Even with short-run stamping techniques, these steps can be prohibitively costly. What if the product the "stamping" is used in "doesn't fly"? You have some (relatively) expensive tooling on hand and no need for it.
While the process has its limits in meeting some of the criteria mentioned earlier, it offers some benefits most people are not aware of. It can produce flat, burr-free parts, in many types of metal, including hardened metals, without inducing stresses.
The "tooling" required is simple to make, costs very little, never wears out, requires no maintenance (only care in handling and storage). it takes up almost no space, can hold reasonably tight tolerances, can be designed to minimize scrap by "nesting" part forms into each other. It can produce parts one-up, two-up, hundreds-up, depending on part size.
The tooling can best be described as a drawing of the part. The drawing is made on a sheet of transparent plastic, which has an opaque coating. The part outline is scribed in the coating, leaving its shape as a "window" in the coating. If the part requires holes, or other inner forms, these, too, are scribed in the coating. The drawings are prepared manually on special drawing machines having highly accurate co-ordinate measuring devices. There are also machines which can be computer programmed to do the scribing work. When parts have "loose" tolerances, and are large and easy to dimension, they are often drawn full scale.
Small parts, or closely dimensioned parts, are drawn "blown up", in enlargements as large as 100 to 1. Considering that a line can be scribed with a width of as little as .002", and that the lines scribed can be held within an "envelope" of .005", with little difficulty, it is easy to see how accurately parts can be made by photochemical machining (photochemical etching). When the enlargement is photographically reduced to actual part size, the .005" "waver" of the scribed line reduces to .0001" when a 50X drawing is used.
Photochemical machining is a process directly related to photography, and using similar techniques for printing a "photo" of the part using its scribed negative (the drawing). Instead of printing paper, the image is exposed, through the windows on the drawing, on a sheet of metal which has been coated with a "resist", a light-sensitive material. The resist serves as a barrier to the etching solutions which will be used to literally "eat" the part out of the metal sheet. When the negative is laid over the sheet of metal, and a light source applied, all of the resist exposed to the light passing through the scribed windows is affected. Plates are often exposed on both sides, in register. The exposed plate is "developed" and the exposed resist is washed away. What remains, the part "image", is then hardened by baking. It becomes resistant to the etching solution. The developed plate is conveyorized through a corrosive spray solution in the etching equipment. The required speed of travel varies with the strength of the solution, the chemical and metallurgical properties of the metal sheet, and its thickness.
The bare-metal part outlines and holes are eaten away by the solution.
Rate of metal removal is .001" per minute in stainless steel, twice that on copper alloys. The final result is that the parts, with or without holes, are etched out of the plate, and collected on a screen, or, they can be left "tabbed" to the sheet for further processing. Sheets of webbed scrap are easily stacked for salvage value.
There is a limit to thickness (approximately 1.25") of metals which can be etched with satisfactory dimensional results for the parts. Where stamping conventionally produces edge and hole surface conditions of one-third shear, the remainder die-break, PCM produces a uniformly "coarse" finish. In addition, as the etching solution eats through the metal it creates a tapered "kerf", which widens as etching continues. This results in taper on edges and inner forms through the material thickness. This may not be significant on thin parts, but can be undesirable on thicker ones.
Regarding thicker materials, to minimize taper, the metal sheets are coated on both sides, the part drawing exposed against both sides "in register" and the etching solution then works on both sides at the same time.
The two-sided taper results in a "chisel point" on the edges and in holes. Let's look at the applications for PCM. When a relatively few parts are needed for development work, PCM can produce them economically and quickly. A one-up drawing (the tool) can be produced quickly.
When the parts it produces are checked, it there are any errors, or design modifications, the tool can be altered at little expense to make the corrections.
When the part proves satisfactory, and the next step is to produce in large quantities, the drawing can be duplicated to produce a sheet containing many parts.
It is common for some parts to be produced by PCM in the millions. This is especially true when material hardness prevents conventional stamping techniques from being applied. Also, if absolutely no burrs, at any point, are permitted on the part, PCM answers this need. Lack of induced stress is another reason for large volume production.
Stamping often induces work-hardening of edges of parts, or induces stresses, or introduces camber. PCM parts have the same flatness and freedom from stresses as the sheet of material from which they are being etched.
There are numerous examples of parts which can be made economically, if at all, only by the PCM process.
PCM also offers the capability of etching identification symbols on parts, as they are being produced.
Finally, consider printed circuit board manufacture, where electrically conductive mazes are required on substrates. It is easier to etch the mazes "in position" on the substrates than to produce them separately and affix them later.
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