Down To The Micron, Up To The Stars

A souped-up CMM helps this shop machine to tolerances measured in single-digit microns. And the machine tool they're using is out of this world.

Article From: 4/1/1997 Modern Machine Shop, ,

Surrounded by cornfields in southwestern Ohio is a job shop machining aluminum reflector panels that will help unlock some of the deepest secrets of the universe. These panels will be assembled into precisely shaped antenna dishes which will be arrayed on a remote mountain top in Hawaii. This array of antennas will form a special kind of reflector telescope, one of the most advanced instruments ever built for observing the universe. Not even Hubble, the super-sensitive telescope orbiting the earth, will be able to probe as deeply into the far reaches of our galaxy and beyond as these antennas. With them, the astronomers and physicists working on this project expect to learn about the birth of stars, the architecture of galaxies, the makeup of quasars, and even find planets orbiting distant stars—stars not unlike our own sun, and perhaps, planets not unlike our own Earth.

But here on Earth, you will not find a machine tool quite like the one this shop is using to machine these panels. It defies easy classification. Even what to call it is not certain. It is a kind of "parabolic arc planer" but "semi-lathe" might also be an appropriate name.

Elegantly simple in design, this machine has no spindle, no toolchanger, no sheet metal enclosure or splash guards. The base of the machine is a 14-ton piece of metrology-grade granite. Yet the worktable platen rides on a cushion of air as it oscillates. The computer control is not CNC, but CPC—computer parametric control rather than computer numerical control.

A Bosma Machine

What this one-of-a-kind machine does have is the name Bosma across the bridge in big blue and red letters. Bosma is the family name of the company's founders, Marinus and Nelly Bosma, who started Bosma Machine and Tool Corporation in 1973. One of their sons, Lee Bosma, is now President/CEO and another son, Ben, is vice president of engineering. A third son, Dave, is vice president of manufacturing.

It was Ben Bosma who designed this most unusual machine and he holds the several patents that protect its innovative configuration. The machine is, however, a family creation. All of the fabrication, construction and assembly of this machine was carried on in the Bosma facility near Tipp City, just north of Dayton, Ohio. Some of its most critical design features were perfected based on suggestions from Marinus Bosma, drawing on his many years of practical shop experience.

The most remarkable thing about this machine is what it can do. It is machining the complex parabolic curves on every one of the 72 panels that make up a complete antenna dish, which is 6 meters in diameter. The contoured surface of each panel must be within 6 microns of the contour specified by the contractor. That's a tolerance across the entire surface of the panel in all directions. And the contour is not a simple, regular curve but a paraboloid described mathematically by a complex equation.

Six microns isn't very much. The average strand of human hair is about a hundred microns in diameter. In fact, the shop is holding tolerances well below this specification—these panels are being produced within a 3- to 4-micron tolerance band. Surface finish is 3 microns RMS or better. At these tolerances, the surface of each panel is like a mirror—but for reflecting radio waves, not light. To the eye, the panels do not appear highly polished.

An essential part of the machining strategy behind this remarkable achievement is a coordinate measuring machine (CMM). This CMM is the only other piece of equipment in the atmospherically controlled shop space occupied by the parabolic arc planer. The CMM provides the measurements for a closed-loop machining method that allows the parabolic planer to compensate and correct for the variables that would put machining out of tolerance. The loop, however, is not like most loops that are referred to in "closed-loop machining."

Out Of The Ordinary

Most of Bosma Machine and Tool is devoted to serving customers who are looking for complete manufacturing capability, from engineering to fabrication to machining to painting and finishing. The company sees workpieces in a wide range of sizes, but their specialty is high precision jobs involving larger workpieces—big gear boxes, machine bases, pump components, military and aerospace parts. The company even has a product line, mechanically operated concrete breakers for the construction industry, which it produces for a sister company. The shop's welding bay is spacious and well-equipped for massive but very accurate work.

The company's collection of large boring mills, VTLs, planers (the customary kind), and machining centers is impressive. Many of the shop's well maintained but older pieces of equipment carry names from builders that have disappeared or no longer make machines of that type or size. On the other hand, some of newest equipment from leading builders can also be found here. One of the first Magnum 800 high speed machining centers from Cincinnati Milacron was installed at Bosma not long ago, for example.

Once you see the shop's main plant and the kind of large fabricating and machining it is capable of, it isn't surprising that Bosma might design and build a large and truly unique machine tool of its own. This machine is so unusual and so different that it is not easy to describe.

One way to visualize its operation is to duplicate its motion for yourself. Sit down and put your left hand palm down on your left leg, with fingertips on the top of your knee. Without moving the elbow, swing your hand slowly from side to side making an arc about a foot wide. At the same time, point the index finger of your right hand directly over the knee. Bring your finger straight down until it begins scraping your knuckles on the left hand as it swings by.

Bosma's parabolic arc planer works somewhat in the same way. The wedge-shaped platen riding on the granite bed swings side to side like your left arm over your leg. The narrow end of the platen is hinged to one of the machine's columns, taking the place of your elbow. The cutting tool, a 0.5-inch button-shaped carbide insert coated with diamond, descends from a ram positioned by a ballscrew on the bridge of the machine. Your right index finger is like a rough copy of the cutting tool.

What can't be imitated sitting in a chair is the machine's method of swinging the platen. A crank arm attached to a large belt-driven flywheel alternately pulls and pushes the platen as the flywheel rotates, giving it even, virtually vibration-free motion. The platen has three air bearings underneath—it does not contact the top of the granite plate. Riding on air, the oscillation of the platen is steady and smooth, almost hypnotic to watch. The clean scrape of the cutter across the aluminum panel and the quiet hum of the air compressor are all the noise this machine makes in operation.

With the panel fixtured at an angle above the surface of the platen, the cutting tool produces a gently concave contour as it passes across the surface. The precise parabolic shape of that contour is determined by the programmed infeed of the cutter during the pass.

After the panel swings past the cutter, the cutter retracts to clear the panel while it changes direction on the return stroke. Cutting is in only one direction. (If the platen could swing all the way around, then the machine's kinship to lathes and other turning equipment would be more obvious.) Before the cutter re-engages the panel, the bridge-mounted ram advances to make the next pass.

Keep in mind that all of these motions are measured in microns. Stepover of the cutter from pass to pass is 400 microns. Depth of cut is 25 microns. Cusp height is less than 0.0001 inch. Each panel requires a roughing and a finishing operation but the panels are batched for each operation.

A Wide But Very Closed Loop

What happens between the roughing and finishing operations is very unusual and interesting. It's where the CMM fits in. The CMM provides the inspection data the shop uses to derive cutter compensation for the finishing operation. That closes the loop. According to Ben Bosma, the integration of inspection data was part of the original concept for the special machine and an essential part of how it would achieve its goals. Both the strategic and the tactical aspects of this closed-loop machining are a departure from the norm.

In a nutshell, the strategy here is to measure each panel in the batch and map the contour of the entire surface, then take all of the "maps" and average them, creating a composite of the entire batch. Using curve-fitting software, this composite is analyzed to create a "best fit"—a mathematical formula that describes a smooth curve to which most of the data points in the composite contour would come very close. An algorithm for correcting each tool path is then created based on this curve. During the finishing operation, this algorithm is applied to each tool path, automatically compensating for deviations from the contour specified by the customer's mathematical definition.

"We batch the panels and average the results because we need to capture all of the conditions that were present during the rough machining," Ben Bosma explains. "Otherwise, we'd be compensating just for a few of these variables and ignoring others. Since some of the variables tend to cancel each other out, compensating selectively would actually cause the finishing passes to move farther away from the target, not closer."

Averaging the results from all panels in a batch takes into account all of the variables contributing to the error. "By using an algorithm based on this average to compensate for the individual deviation in each panel, we can close the loop effectively," Mr. Bosma contends.

Analog Beats Touch Trigger

The tactical issues implied by this radical strategy were challenging, to say the least. "If we were going to hold machining tolerances to a few microns, we knew we would have to achieve measurement accuracy ten times better," notes Mr. Bosma.

The CMM that provides this accuracy is a SIP Opus 7 (American SIP Corp., Elmsford, New York). This CMM, purchased used in superb mechanical condition but equipped with an obsolete control system running outdated software, was upgraded with an analog probe and an analog control system from Electronic Measuring Devices (EMD) of Flanders, New Jersey. EMD retained only the servo controllers and feedback cards, installing all-new computer/controller hardware and software. The new control is based on a Pentium PC.

According to Mr. Bosma, the new analog probe is a critical element in this scenario. Unlike a digital touch-trigger probe typically found on many CMMs and machine tool applications, an analog probe is constantly giving the computer information about the displacement of the stylus, whereas a touch-trigger probe only records data when a micro limit switch inside is tripped by contact with an object. With the EMD analog probe, the probe head itself operates like a miniature CMM reading X, Y and Z positions, sensing all force vectors acting on the stylus and simulating the characteristic of "feeling" the workpiece. EMD gives the displacement accuracy of this probe as 0.1µ (one tenth of a micron); and says the smallest displacement it can detect is one half of that figure (0.05µ or 2 millionths of an inch).

"Another nice feature of the EMD analog probe," notes Mr. Bosma, "is that you can control the amount of pressure you impart on the workpiece during the probing process. Our reflector panels are only 5 mm thick and start deflecting immediately upon contact with a probe." His shop inspects the panels with 20 grams of pressure.

EMD is best known for its Sceptre software for continuous contact scanning of contoured surfaces, which records as many as 3,000 points per second. However, the contoured surfaces of the reflector panels are inspected point to point across a grid of about one inch squares. Even so, that yields several thousand points per part. This data is preprocessed in the EMD software and formatted for transfer to MatchCad from MathSoft (Cambridge, Massachusetts), a popular PC software program that the shop uses to analyze this data.

This software generates "error maps". By reviewing these maps, it is easy to verify that a batch of panels all exhibit very similar deviations. This is important because it justifies using an average of these results as the basis for a general cutter compensation algorithm that is applied to all of the panels during the finishing operation.

Computer Parametric Control

Returning to the arc planer to see the loop as it closes, Mr. Bosma makes a point of emphasizing that the machine has a computer parametric control rather than computer numerical control. The distinction is this: a conventional CNC machines by moving point to point according to a set of predetermined position commands (G codes), whereas a CPC machines by solving equations in real-time to generate position commands. In this case, the equation defines a 2D curve.

The advantage of this parametric approach is that additional equations can be applied to cutter location data. For example, a value representing the thermal expansion of aluminum, the workpiece material, is factored into the basic control algorithm. Another equation corrects for deviations in the workpieces as derived from inspection data, as we have just seen in this instance.

One other equation deserves attention: it corrects for inherent inaccuracies in the machine itself, such as errors in the positioning system. This equation was derived in a manner somewhat similar to the concept applied in analyzing inspection data for cutter compensation.

After the machine was assembled, numerous test cuts were performed on a set of sample reflector panels. Inspection data from these test cuts was used to create error tables. But rather than build these tables into the control software to be applied as a look-up function to compensate for error, curve-fitting software was used to find a polynomial equation that best fit the data in the error tables. This technique is commonplace for dealing with data from experiments in organic chemistry. Mr. Bosma used the same type of software many chemists use, in this case, TableCurve 2D from Jandel Scientific (San Rafael, California).

This equation approximates the random error events caused by inevitable but extremely small errors in the machine's structure and positioning scales. "The curve for this error table doesn't stray more than one hundredth of a micron from the furthest actual data point, which is less than the noise in the data tables to begin with," Mr. Bosma notes. "So we were confident we had a good correction curve. The result is a near perfect positioning of the cutter to the parabolic curve defined by our customer's equation."

As you might expect, a CPC has a lot of number-crunching to do to drive a machine tool. The control system on the Bosma machine consists of a 386DX processor with 8 MB of RAM, running Windows 3.1. The source code is written in VisualBasic from Microsoft (Redmond, Washington). Essentially, all of the computer processing power and speed are concentrated on solving the control algorithms, which it does handily because no other tasks are assigned.

A more detailed account of the thinking and the math behind this strategy, in Ben Bosma's own words, can be found in the sidebar at right.

Of Time And Space

Bosma Machine and Tool has been making these aluminum reflector panels for some time now. To them, the technology involved is no longer a novelty, a daring departure from what most shops are familiar with. Although the thrill of seeing new and heretofore untested ideas proven practical and effective may have worn off, the challenge of maintaining and even improving a process that is accurate down to the micron seems to keep the personnel who do it on a day-to-day basis rather energized.

For example Chris Rogers, Jeff Miller, and Rick Blevins, who have been assigned to this project, follow setup procedures with an intense concentration and almost ritualistic uniformity. What seems like an obsession with doing everything exactly the same way each time actually reflects a very lively awareness of a peculiar paradox that exists in their isolated part of the Bosma manufacturing complex.

When you are working with microns, everything you do makes a difference. You easily sense how interconnected every activity is with the seemingly random flux of variables in the shop environment. Yet out of this chaos arises almost incredible orderliness, logic and control, which is ultimately embodied in metal workpieces.

For millennia, humankind has gazed at the night sky, randomly decorated with the sparkle of a million stars. The beauty and mystery continue to fascinate us. Slowly, we have come to perceive the patterns and processes still at work in the formation of the universe. Our knowledge has been growing by leaps and bounds, but sometimes because we are learning more about metal and machining—and microns, one by one.

Hawaiian Eye...On The Universe

An optical telescope detects light, which is radiant energy of certain wavelengths. Radiation in other wavelengths can also be detected with the proper instruments. The Smithsonian Submillimeter Array on the summit of Mauna Ke'a, Hawaii, is designed to detect radiation in the 1.3 to 0.3 mm wavelength region, a band lying between infrared and radio wavelengths and popularly called "submillimeter waves."

The SMA in Hawaii consists of eight movable antennas that can be repositioned on concrete pads lying on the mountain top. Bosma Machine and Tool is machining the panels for the dishes of these antennas. Each dish is 6 meters in diameter. Using a signal correlator to combine and integrate the radiation received by each of the eight antennas, the SMA will produce images of astronomical objects with a resolution comparable to the best optical telescopes and more than ten times that of any existing single-dish submillimeter telescope.

The submillimeter region of the electromagnetic spectrum has been called "the last frontier" of ground-based observational astronomy. It has remained largely unexplored because the technology to produce the precisely shaped antennas and highly sensitive receivers needed for its detection did not exist before. When operational later this year, the SMA will probe the murky dust clouds of the Milky Way where stars are born, peer into the hearts of distant exploding galaxies, and study cool faint objects of our own solar system, including comets and planets.

The Smithsonian SMA is operated under the auspices of the Smithsonian Astrophysical Observatory (SAO), a research bureau of the Smithsonian Institution. The SAO has headquarters in Cambridge, Massachusetts, near Harvard University.

—from a brochure issued by the SAO.

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