Weight Reduction Through Chemical Milling
Chemical milling is the process of removing large amounts of metal by means of chemical etching. All metal types and shapes are candidates for chemical milling, especially contoured parts which are not easily machined.
Chemical milling originated as a way to rework a spacecraft part by lightening the component in an etching bath. Since those days, the process has evolved to be the tool it is today: a highly controlled, cost- effective method of manufacturing aerospace and aircraft parts. Chemical milling can be found in every airplane in the commercial and military markets, both in the airframes and engines.
The best applications for chemical milling involve reduction of weight in contoured parts. Even convoluted or complex shapes, such as formed extrusions, can be uniformly etched on all or selected surfaces. Whereas conventional machining is used to create planes having relationship to the x, y, and z axes of the machine, chemical milling removes material with relationship to the surface of the workpiece before etching. Therefore, chemical milling can maintain wall thicknesses to exacting tolerances.
Using large tanks (6,000 to 40,000 gallon capacity) many parts can be processed simultaneously. The wing skins for the Saab 340 commuter plane (35 feet length) can be processed in six piece loads. Small access doors and cover plates can be etched in 100 piece loads, making chemical milling an extremely cost effective alternative to machining.
The chemical milling process is really a very simple process, although it is loaded with subtle control requirements that need constant attention. Aerochem's many years of experience have rendered the process routine and high quality parts are produced reliably and within schedule restraints, day in and day out.
The parts to be etched are first cleaned and masked with a synthetic rubber paint which protects the part from being etched where no etching is desired. Using a template to apply the pattern, a careful operator will scribe the features to be chem milled using an "exacto" knife. Areas to be etched are then peeled and parts are submerged in the etchant. Removal of material is controlled by etchant, chemical concentration, temperature, and time in the tank. Using an ultrasonic thickness tester, the operator monitors thickness of the workpiece periodically until blueprint thickness is achieved.
The etched parts are stripped clean and touched up if necessary by hand polishing and deburring. Finished parts are quality control inspected for conformance to blueprint and specifications.
The chemical milling process can achieve thickness tolerances of up to .001" in thickness, surface finishes of 25 RMS to 150 RMS, depending on material and etchant type. Typical fine location tolerances are .030". Cost of the chemical milling process can be reduced dramatically with tolerance relief in the thickness category. Thickness variation improvement can also be achieved.
Aerochem can handle raw material purchases, forming and heat treating, machining, routing, surface coatings, and like processes. By doing so, the customer can place one purchase order for many processes and avoid costly and time consuming outside processes which are difficult to control. Small parts, such as shims or cover plates, or large fuselage skins can all be produced quickly and cost effectively.
There are virtually no metallic materials used in the aircraft business that cannot be chemically milled by Aerochem. A million-dollar laboratory facility and a staff of chemists and technicians formulate new etchants for new alloys. Aluminum, aluminum lithium, stainless steels, nickel steels, cobalt steels and titanium's can be etched with good results.
Tooling for the chemical milling process is usually comprised of sheet aluminum featuring the blueprint pattern cut into the metal or a laminated fiberglass. The pattern is registered to the part by use of tool holes and locating pins. Typical template costs range from $500 to $1,500. While exceptions can range well into the thousands, normal tool costs are low in comparison to machine tools and no set up charges are imposed for most work.
The obvious opportunities for chemical milling are:
As designers are discovering new ways to benefit from the chemical milling process, the market for chemical milling is expanding. New commercial airliners are flying farther, and fighter, with the held of this process. Military aircraft are featuring new airframes and engines which demand chemical milling to achieve high performance. In the next 20 years, Aerochem stands ready to meet the challenge, to be ready to serve its customers as they are increasingly involved in the industry of the future.
- Where doublers and triplers are used to create multiple elevations or thicknesses. A one-piece chem mill part can replace an assembly with a unitized part.
- In formed parts where complex surface contours make machining impractical.
- In tapers of all lengths, from 1/2" to 300" in2.
- Where uniform removals are required from all surfaces.
- Where parts are overweight.
- Where exotic metals make machining difficult.
- In formed parts that experience thinning and thickening which needs to be made uniform.
- Where undesirable surfaces must be removed (alphacase, oxidation, recast remelt heat affect zone, or decarburized layers).
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