The Big Picture of Micromachining

A niche market that shows room for growth.

As the capability of metal machining technology continues to improve the ability to machine with more exact precision is resulting in new generations of end products that can truly affect people’s lives.

Industries like aerospace, automotive and medical instruments have driven the demand for precision machining, where parts must meet tight tolerances and undergo enhanced testing and scrutiny to ensure the safety of passengers and patients. For even greater levels of accuracy—ultra-precision machining, or micromachining—hold tolerances of less than a micron, enabling the machining of miniature parts and ultra-fine surface finishes leading to new advanced biomedical applications.

Defining micromachining

It’s important to understand that micromachining is not limited to miniature parts. “For me micro machining is defined by high accuracy and/or high surface quality,” explains Toni Mangold, president of Kern Precision Inc., the North American division of German-based KERN Microtechnik, manufacturers of machining centers dedicated to high precision.

More precisely, according to Stephen Veldhuis, professor at McMaster University (Hamilton, Ontario) and director of the McMaster Manufacturing Research Institute (MMRI), ultra-precision typically involves features on the order of microns and/or surface finishes in the nanometers.

Veldhuis has been researching the area of ultra-precision machining for over a decade, studying the physics of these cutting processes at a microscopic level. “A more formal definition for ultra-precision machining involves the interplay between the chip thickness generated during cutting, the cutting edge radius of the tool and the grain size of the workpiece,” explains Veldhuis. “In many of these applications the tooling is a single crystal diamond, so the cutting edge radius is approaching 50 nanometers and the actual chip thicknesses are around 0.25 to 0.5 microns, or could go as high as 2 to 3 microns.”

To place this in perspective, a human hair measures around 100 microns in diameter, so we’re dealing with cutting depths not perceptible by the naked eye.

Process rethink

To understand the levels of complexity in micromachining, machine shop owners, process engineers and machinists need to embrace change. “I believe that it requires two main steps to conquer the world of micro machining,” says Mangold. “The traditional shop owner needs to be willing to re-think the processes he’s been working with, and the machinists who are working with the machines need to see and understand the advantage of working with micro machining equipment.”

Machining at this level requires machine tools specially equipped for the task. “Micro machining takes all of the issues in precision machining and multiplies them by 100,” says Veldhuis.

With a dedicated micromachining R&D center at its facility in Auburn Hills, Michigan, Makino offers up its iQ300 vertical machining center as an ultra-precision tool. “The iQ300’s actual positioning accuracy is +/- 400 nanometers, that’s +/- 0.4 microns, and the positioning repeatability of that machine is 100 nanometers, ” explains Mark Rentschler, director of marketing with Makino. “And our ability to provide positional accuracies is even greater than that. Our actual minimum input is 10 nanometers, so we can program and execute tolerancing to that degree.”

Veldhuis suggests that beyond tight servo loop control and positioning accuracy, an ultra-precision machine tool requires enhanced rigidity, thermal stability and the ability to maintain that stability over an extended period of time.

According to Rentschler, Makino’s leadership in terms of precision engineering is based on the company’s long-standing experience and manufacturing techniques. “A lot of that has to do with our long-term exposure to die and mold machining, where you need to have high-quality surface finish and the ability to manage those conditions over a long operating time.”

Holding temperature is critical when dealing with such fine tolerances. “From a technical perspective, the latest trends are thermal compensation of the entire machine in order to provide a stability of the process,” notes Mangold. “Most inaccuracies are caused by thermal impacts. Today the very first part produced needs to be in spec. This is very important for our customers in the mold and die industry since they usually only have to produce one part.”

Because thermal stability is essential, Veldhuis explains that long warm up cycles—up to three to four hours—which mimic real cutting is required prior to making any cuts. “You need to bring the machine tool to a normal operating thermal profile, and let it expand uniformly. Once it’s all uniform, then you can actually do your cut.”

The chips are extremely small, so the amount of heat generated is quite small, but if coolant is being used its temperature needs to be controlled. “Typically you use a very fine spray of mist to both lubricate the zone and also direct the chip away from the cutting zone,” says Veldhuis. “Chip control is important. A chip hitting the workpiece can damage it or can create vibration, or if you re-cut the chip at this scale it can lead to flaws.”

When it comes to micromachining, cutting by ear no longer applies, as the drone from the spindle exceeds any noise generated due to cutting. In the lab at McMaster, the researchers work on a Matsuura LX 1 with a 60,000-rpm spindle. “It’s a ceramic bearing spindle, very stable and rigid,” says Veldhuis, noting that high speed is necessary for micromachining.

“You need to achieve a certain speed between the cutting edge radius and the workpiece, so in that case you’re talking about a very high rpm, a high degree of dynamic rigidity to avoid vibration, and tight servo control of movements.”

Also essential is accurate tool and workpiece set up to avoid any vibration due to imbalance. The typical cutting tool, a miniature end mill, measures around 10 thou (0.25 mm). Veldhuis points out that because the chip loads are so small it doesn’t take much runout to really overload one side of the tool.

Tool locating is another critical practice, especially when you’re setting up offsets to make a cut of one-micron or less. “You have to be very careful with how you bring the cutting tool into the workpiece,” notes Veldhuis. “If you’re above the surface you achieve nothing, and if you’re too deep you break the tool. Your margin for error is very small.”

At McMaster they use vision-based systems to assist with tool set-up. Veldhuis suggests that operators new to the process need to experiment with the tools, try feeds and speeds, and get accustomed to the cutting conditions.

When it comes to metrology for ultra-precision machining, measuring surface finish requires optical solutions. “Because we’re producing such a small feature it’s hard to extract an Ra value from it,” says Veldhuis. “Normally you take a stylus and run it over the surface, but in these cases the geometry you’re producing is similar to the stylus, so it’s hard to measure.” The tools include non-contact laser interferometers to measure form accuracy, and white light interferometers for detailed surface finish information.

Applications

When it comes to industries requiring such fine machining parameters, any devices incorporating miniature electronics, high-quality optics or ultra-fine three-dimensional channels are candidates.

“Today we find the need for micromachining in larger parts in many different industries. With micro machining, shops are able to save certain steps of a process, like polishing or EDM’ing,” says Mangold. “For example, for the optical industry we can offer machined housings accurate enough that a manual adjustment of placed lenses is no longer needed.”

Yasda YMC 430

Yasda YMC 430

Optics is a large industry, from aerospace applications to photocopiers, precision optics with very tight form accuracy and surface finish requirements are in demand.

“A lot of success in the micromachining area has been the micro milling of bio-fluidic medical areas,” notes Rentschler. The medical field represents a real growth opportunity, echoes Veldhuis, who has worked with medical research departments at McMaster University.

He points to the emerging development of mobile point-of-care diagnostic devices—tools that would merge precision optical and microfluidic devices for advanced biomedical applications.

One device Veldhuis has experience with is a blood oxygenator developed for infants. This small lung-assist device, modular in design to accommodate a range of pre-term and term infant body weights, exposes flowing blood to air through a gas-permeable membrane. The blood flows through an array of micro-channels through capillary action and the gas exchange occurs across the membrane.

Veldhuis explains how micromachining is necessary to make the dies and molds for these products. “For the microfluidic channels, if you have too rough of a surface you won’t get the required capillary action for blood to flow,” he says.

This is one example of a potentially life-saving device that is being made possible thanks in part to the capabilities of ultra-precision machining.

“The need for high demanding parts is still growing,” suggests Mangold. “It is difficult [for machine shops] to survive by just producing standard parts, since these are often produced in countries with lower labor cost. I believe the market here has to concentrate on the highly specialized parts which cannot be manufactured overseas. Companies need to stay on the cutting edge of technology.”

DMG MORI ULTRASONIC 30 linear

DMG MORI ULTRASONIC 30 linear