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Precision Surface Finishing Process is Tailor-Made for Parts with Complex Geometry or Critical Specs

A UH-60 helicopter gear, used in a helicopter tail rotor gear box, after MMP was applied
Photo courtesy of MicroTek Finishing

A proprietary process with applications in aerospace and medical enlists millions of microscopic, mill-like cutters that flow across the surfaces of parts to smooth roughness

Traditional polishing methods normally rely on abrasive materials, electrical currents, or harsh chemicals to remove roughness. Moreover, these processes don't always respect the part's geometry or sharp edges.

"These other processes cannot be used in a lot of places, like watch cases or cutting tools," says Tim Bell, vice president of business development at MicroTek Finishing, a Hamilton, Ohio company that specializes in a highly-specialized finishing technology called the Micro-Machining Process (MMP). "Electropolishing is a great technology and has been around for a long time, and it has its place. MMP, however, is an automated, mechanical process that follows the profile of the part in a free-form fashion. This allows us to respect the fine details of a part, and only remove the roughness on the areas that we focus on.

"The other effect of traditional polishing is that most of these processes displace material; they don't remove it," Bell continues. "When you hand polish a part, you actually move around more material than you remove. So quite often, you'll roll over the peaks and valleys of the roughness. In addition, our process is automated and mechanical, so there's no manual labor involved. The parts are fixtured and then bolted into specialized tanks, and then the machines run automatically." 

According to MicroTek (www.microtekfinishing.com), the Micro-Machining Process is the only surface finishing technology of its kind in the world. MicroTek has the exclusive license in North America. "So many times, people say 'We have something new,' but it's usually just a new twist on an old technology," Bell remarks. "Our company only uses MMP; we don't do any hand finishing and we don't tumble anything. This is the first time I've been involved in something that's truly new and unique. Our doors have only been open about two and half years, but our growth has been exponential and our market segment has expanded very rapidly."

Bell says that about 60 percent of the company's work involves gas turbine engine components, 20 percent is devoted to medical parts (implants or surgical instruments), and the remaining 20 percent is divided among cutting tools, injection molds, punches and dies, and luxury goods. The specialized finishing company just received its ISO 13485 medical certification, and will be applying for its AS9100 aerospace certification this year.

"One new area of application for our process is high-load transmission gears for helicopters and race cars," says Bell. "It has been reported to us that these polished gears will run cooler, transmit higher loads, and last longer."

Process is Well Suited for Custom, High-Precision Parts

MicroTek's MMP is used mostly for custom-made, high-precision parts with tight tolerances. The process is said to mimic a machining process, using very small milling-like cutters [that operate] in a treatment tank. Roughness is viewed as ranges of frequency stacked on top of each other. Every machining process used today, such as milling, turning, grinding, and EDM, is said to leave behind a fingerprint.

"We measure that fingerprint with a profilometer, and end up with a set of peaks and valleys," says Bell. "We can then break it into orders of magnitude that are standard. The form of the part is a very low frequency--you have a wavy profile on top that's imparted on the surface from a moving machine, and on top of this, you have the primary wave created by the action of the tool. On top of this, you have the secondary wave, which is the highest frequency roughness, which is usually the tool being transferred to the part. Once we know all of these frequencies, we extrapolate the high frequencies off of the part to create a range of the peaks and valleys on the part. So now we know the height of the roughness and its pitch."

MicroTek reportedly has access to over 300 micro tools--called micro milling cutters--to use for different applications or specialized polishing challenges. The size and shape of the milling cutter will match the frequency of the roughness that needs to be removed. "First we fixture the parts rigidly in the machine, and then fill the tank with millions of that size of micro tool," says Bell. "Then we will impart mechanical energy into the tank that will cause the micro tools to rotate in three axes of freedom, allowing them to flow across the surface of the parts.  As soon as they find the frequency they match, they lock into it and start cutting--only into that frequency."

Once the process is run a predetermined amount of time, says Bell, process operators know that all of that frequency of roughness will be gone. After the first stage, operators stop the machine, clean it out, and identify the next frequency to be removed. They choose a micro tool that matches that size and frequency, and then they refill the machine and start another process.

"What's unique about this is that we can accept a wide range of input from our roughness profiles," Bell insists. "A customer can send us a part that's been milled, turned, and ground, and we can adjust all of the frequencies and set up a range of MMP treatments that will remove each one of these areas."

A 40-Year Journey to Perfect the Process

The evolution of the MMP process is a fascinating slice of finishing technology history. Bell mentioned that the technology is based on a white paper written in the 1960s, but the problem with the white paper was that there was no technology to make the micro tools. "It took 30 years to figure out how to make the micro tools and create the MMP machine," Bell commented. "Once that was done, they were able to go into the laboratory, where they struggled for another 10 years until Dr. Laurent Cataldo came along and industrialized the technology, and started a company in Switzerland. Ten years ago, Dr. Cataldo was teaching material science at the University of Geneva. He was approached by the investment group that owned the technology at the time, and was asked to head the R&D. He found someone to buy out the investment group to protect it, so here we are 10 years later."

Nevertheless, educating clients about MMP can sometimes be challenging. Bell says the biggest mistake is to try to compare the process to older technology. "I've been in manufacturing for over 25 years, so it's very hard for me to take something 'new' as being new," Bell revealed.  "It [MMP] really is a paradigm shift from the old ways of doing it. Our customers who have learned how to accept this are the best application engineers of our technology. They understand it better than I do."

The little cutters are very specific microscopic materials that have never been used in the removal of metal. "This is the first time in history that these materials have been used," Bell points out. "It is the proprietary component. The three things that we never reveal without someone buying a sub-license to the technology are the micro-tool materials, how they are made, and their size and shape."

According to Bell, millions of these microscopic materials flow across the surface of the parts that are being processed. "The tool size and shape matches the frequency of the roughness being removed," says Bell. "If you look at the roughness of a part, it looks like a lot of peaks and valleys. These peaks and valleys can be easily quantified as a frequency. Also, there are no acids or other harsh chemicals being used, no electrical currents or lasers, and there is no 'alien' technology, which has been suggested."

Other advantages to using the precision polishing process are that small amounts of material can be removed, and almost any material, except diamonds, can be processed. "The first advantage is the total cost of processing a part," Bell emphasizes. "With MMP, we can remove other polishing steps, like tumbling and blasting. [With regard to technical surface development] We can leave behind specific frequencies for fluid retention, for coating adhesion, and anti-microbial growth. There are a lot of ways that surface typography can bring value to a part, and we can engineer that value. It's not just polishing the parts to make them shiny; the typography that we leave behind is very specific."

The MMP technology can be used to process ceramics, plastics, and parts that have coatings, such as PVD (physical vapor deposition) coatings, thermal barrier coatings, metal matrix, and many other materials where there is no other way to finish them. Most of these coatings are two to five microns thick, and sub-microns can be removed, so the process can handle a coating without removing it. Bell says that there is no other polishing technology that can do this.

"The controllability, repeatability, and high quality are also advantages of this process," says Bell. "The quality is very consistent and very repeatable. Our customers know that batch-to-batch, month-to-month, and year-to-year, the results will always be the same."

Testing is always the first phase of part processing to find out the level of incoming roughness. "We need to know the variability of the roughness," says Bell. "It's very easy for clients to say 'You're going to get the parts and there will be a 24 micro-inch Ra (roughness average).' We say 'OK,' but when we check the parts, we find they are anywhere from a 12 to a 32 Ra. We will need to adapt to this wide range of roughness. So we will create a treatment based on the range of roughness. No matter what they send us, the output should be the same."

The Micro-Machining Process is a very exacting process to reduce part roughness and create a mirror-like finish. Its high level of precision reportedly exceeds what traditional polishing processes can deliver.

"To achieve mirror-like finishes, traditional polishing processes can only do it on components that have an axis, like cylindrical parts that can be put in a machine and spun to polish the outside or inside diameter," Bell explains. "Or, the part needs to be flat so they can lap it. This is one reason we are so successful in the gas turbine engine industry. We can work on the blades and vanes, which are very complex 3-dimensional airfoils. There's no other method to achieve mirror-like finishes on parts with complex geometry. Electropolishing can, but it's very limited in what it can do. Someone could twist a blade to a 45-degree helix, and we can still process it. This is because ours is a flow process."

According to MicroTek, the polishing technology can achieve roughness levels of less than 1 micro-inch Ra (0.0254 micrometer roughness average). "We are trying to achieve smoothness with our polishing, but it's still a level of roughness to us," says Bell. "The level of roughness is subjective, depending on the customer's applications. If you go to an optics manufacturer, or someone who makes hard drives for computers, their level of smoothness or roughness is on the angstrom measurement scale. Traditional manufacturers of machined components will be on the micro inch scale. It is a physical measurement, and an aesthetic thing. It may be aesthetics for the luxury industry, but for a bearing or a wheel, there is a dimensional measurement."

Demand from Markets--such as Medical--that Require Perfection

Parts and products that require extreme precision--such as those for tool and die work, aerospace, aviation, medical, and luxury goods--constitute the main markets for the Micro-Machining Process, which can also be used for metal injection molded (MIM) and direct metal laser sintered (DMLS) parts. "Precision parts are very expensive to manufacture," Bell said. "Traditional polishing methods cannot respect the high level of quality that's put into these parts. It normally takes three or four meetings with a customer before they understand how this process works. For example, if you have a batch of punches and tried to hand polish all of them, each one would come out different. With MMP, every one of them will be identical, and there is only a very small amount of material removal."

For medical parts and components, the ability to finish or polish titanium to less than a 2 micro-inch Ra (0.0508 micrometer) makes MMP desirable. The MMP process doesn't embed materials into the surface of the parts and doesn't leave behind residues or change the chemistry of the material. However, MicroTek still ultrasonically cleans everything that it processes.

"This is the threshold for a lot of medical components that is not easily achieved with other technologies," Bell comments. "A hand polisher can do it, but he can't respect the shape of a component."

One customer-reported issue is with metal-on-metal implants and material shedding. If two metal parts are rubbing together, the roughness peaks or the rough spots themselves can break off. "With our technology, the removal goes down to the form, so there's nothing to come off--you don't have material shedding," says Bell. "Fluid dynamics another customer-reported impact associated with roughness, and is an issue often seen with aerospace or aviation parts. If all of the fan blades inside an engine are rough, it can't efficiently move air through the engine. It's called the boundary layer effect, which means that the rougher the surface, the more air that gets stuck."

The MMP finishing process is one of the technologies assisting in the increased efficiency of aviation fan blades. "We have been told that by 2015, all airlines will need to reduce their fuel consumption by 15%, as mandated by the global aerospace world," Bell points out. "We are quickly being requested to help them achieve this number. Our growth is very fast right now because of this mandate."

According to MicroTek's website, MMP is scalable from one part to millions of parts, and turnarounds can be achieved in two to four days for many projects. These attributes make it attractive to OEMs with high-precision parts and components with complex geometry.

"The OEMs today, especially those that need high-precision parts, are going toward lean manufacturing," says Bell. "They want a leaner, cleaner production flow, whether that means one piece or smaller piece batches. What makes MMP so attractive is that we can tailor the flow-through to meet their pipeline requirements. So we don't have to let parts pile up in a big bundle before we send them. If they only want 25 parts at a time, we only have to run 25 at a time because our process is batch based."