Divining Edge Quality by Reading the Burrs
The lesson came when 0.020-inch diameter pins coming off a screw machine and a subsequent deburring operation had failed inspection. When Gillespie viewed a photograph taken at 200X magnification, he found that the base of a burr too small to be seen by the naked eye had spilled over the edge. Although the deburring process had sheared off the top of the relatively thick burr, a slight deformation at the base had remained. The anvil of the gages used by the machinists and inspectors was resting on this tiny protrusion and showing the parts to be too big.
Simply leaving the parts inside the existing vibratory deburring process for a longer period of time was out of the question because it would have removed too much material and altered the parts' dimensions. Gillespie reckoned that making the burrs thinner so they would break cleanly would solve the problem. It was the design of experiments that he used to find the machining parameters for producing the optimum burr size that started him thinking about measuring burrs.
Although measuring the burrs themselves was straightforward, he discovered that communicating his ideas was not. Few engineers could agree on the definition of a burr, let alone decide on a standard method for describing them. A burr to one engineer is simply a sharp edge to another. In fact, German engineers use the same word for both burrs from metalworking and flashing from plastic molding, even though the two have different sizes, shapes and properties.
"In the automotive industry, a burr isn't a burr unless it tears up the mating part or interferes with fit," offers Gillespie, who eventually founded Deburring Technology International Inc. (Kansas City, MO) to educate engineers about the issues and problems in deburring. On the other hand, burrs that would pass inspection in an automaker's plant would be a disaster in the bearing industry and downright dangerous in medical devices.
Complicating the problem of defining burrs is the fact that they come in a myriad of types. They can range from a small raised edge to a traditional sliver that protrudes from the surface of the part. SomeArial, they have uniform height and thickness, but at other times, their size and shape vary. "Burrs can have waves, or oscillations, that are either periodic or aperiodic with some machining parameter," adds Dr. David Dornfeld, director of the Consortium on Deburring and Edge Finishing (CODEF) at the University of California, Berkeley. Some of his research papers also describe the formation of small secondary burrs as the primary burrs peel from the workpiece and break like chips.
The ambiguity about burrs can cause a great amount of consternation. For example, both Dornfeld and Gillespie have found themselves testifying as expert witnesses in lawsuits in which the definition and size of burrs were central issues. The increasing popularity of the Six Sigma philosophy among original equipment manufacturers also has forced suppliers to take in an interest in what most previously have considered unimportant. Now they must readjust their thinking to view burrs as a source of variation that can have adverse effects on product quality.
The all-too-common specification "no burrs allowed" no longer has value. "An electron microscope takes pictures at 500,000X magnification," notes Gillespie. "Do you suppose I could find a burr on a part?" Because the answer is, of course he can, he recommends that manufacturing and quality engineers learn to define and quantify burrs.
Where would one find such definitions and standards? Until 1998, the answer was nowhere. In that year, however, the Worldwide Burr Technology Committee, at its fifth biannual conference in San Francisco, adopted six standards for defining burrs. In the following year, Gillespie also offered a generic definition of burrs in his book, Deburring and Edge Finishing Handbook, published by the Society of Manufacturing Engineers (Dearborn, MI). To him, a burr is a plastically formed material produced by a cutting process.
The book also elaborates on the WBTC's standards and discusses writing practical specifications. "We give some suggested wording for universal methods of defining edge conditions so people can put them on drawings for everyone to understand," says Gillespie, one of the chief organizers of the ad hoc international committee of deburring experts. "If they don't want to use these standards, then maybe they can use them as models for developing their own."
One of the WBTC's six standards gives researchers details for measuring a burr's height, length, radius, and other properties. Another focuses on techniques useful on the shop floor, suggesting that engineers might specify that no burrs can be visible at certain magnifications, for example. Yet another of the standards builds on the ISO 9000 quality criteria to outline a hierarchical plan for a company. At the top level, the plan should state the company's basic philosophy on burrs and sharp edges. The middle levels should tell engineers how to specify allowable burrs in product design; how to specify, calibrate and maintain manufacturing processes; and how to inspect for burrs. The bottom level should outline a plan for training on burr and edge standards.
No matter which standard a company adopts, engineers must determine which dimensions to measure. Is the height of the burr from the surface sufficient, or must one also measure thickness at the root? "Many people measure height because it's the easiest thing to measure and they think that gives them a good answer," observes Gillespie. "Although there are cases when it does, there are many cases in which it's the wrong answer."
Where to take measurements also is an issue to be decided. "Don't take just one measurement on an edge," advises Gillespie. "You should be taking several from different spots." The goal is to find an average value that represents the overall picture. The irregularity of burrs can create wild swings in the data if one relies on only one measurement per part. Although taking measurements from the same spot on each part creates consistency, it could mask large variations nearby.
Unfortunately, there is no easy way for deciding which measurements to take and where to take them. A design of experiments must be conducted to determine the sensitivity of burrs to changes in the manufacturing process and the validity of assumptions about how well any measurements represent the burrs formed. Dornfeld reports that some of his students are developing software for conducting such designs of experiments.
"Once you've established the functional relationship between the problem with the part and the burr, whatever it looks like, devising a simple way to measure the burr to ensure it meets the specification becomes much easier," says Dornfeld. The methods for measuring burrs fall into three general categories: optical methods, conventional hand gages and profilometric techniques.
The most basic of the optical techniques is the microscope. A common way to measure burrs is to fit a binocular microscope with eyepieces that have reticules (fine scales) etched into the lenses or plated on them. "A lot of people don't stop to think that popping a $25 set of reticules into a microscope can turn it into a measuring device," says Gillespie.
He urges inspectors to measure burrs with stereozoom binocular microscopes that have through-the-lens lighting. The light from these instruments illuminates the entire depth of the burr and can penetrate deep cavities. Although external lighting can do the job for external features, it has difficulty illuminating burrs on internal features. Monocular microscopes are a poor choice for checking for burrs and measuring them because the design limits the depth perception necessary for evaluating small features.
Another microscope common in machine shops is the toolmaker's microscope. Scales along the verti-cal axis give these instruments the ability to measure burr height, as well as thickness. Height is the difference between the focal planes at the top and base of the burr, which an inspector reads from the scale along the vertical axis. When putting a workpiece under one of these microscopes is impractical, Dornfeld suggests that technicians in a quality laboratory could make a casting instead. "Encapsulating the burr in a polymer and slicing the polymer mold into four pieces would allow using any optical microscope to measure the subtleties of the burr," he says.
Boring in on burrs
Although a borescope has a fixed focus and can cause perception problems similar to those encountered with monocular microscopes, this optical device contains an integral lighting system, and skilled users can see several fields of view by deploying a variety of end pieces. The advantage of fussing with a borescope is that it can wend its way through tortuous cavities not possible to see with line-of-sight devices. Borescopes also can feed images to video screens that can provide 100X magnification and can store them on videotape for permanent documentation.
Perhaps the simplest and most robust of the optical devices is the optical comparator. Made for the shop floor, its structured lighting system shines parallel light rays on the part and projects a magnified silhouette of it on a screen. The trick to measuring burrs on this device is to use one with high enough magnification to show small burrs and to orient the parts so the inspector can see the entire plane of the dimension being measured. Typically, conventional comparators work best for specific parts and types of burrs. In fact, a number of special, portable comparators made to rest on the workpiece have appeared on the market from time to time, especially in the stamping industry.
Conventional contact gages such as the micrometers and height gages found in shops and quality laboratories also are capable of measuring burrs. The caveat is that the burrs must be strong enough to withstand the forces applied during the process. "In most cases, burrs from machining stainless steel and aluminum are relatively thick and fairly short, roughly 0.005 inch tall and 0.003 inch wide," says Gillespie. "So contact methods would be okay in most cases."
Even so, he warns inspectors and quality engineers to beware of any effect that these devices might have on the burrs being measured. Given the propensity of contact devices to bend burrs, experts often advise against using them for precise studies. They also warn inspectors and quality engineers of the wear that occurs on the anvils and contacts from the abrasion caused by the deforming material.
A number of researchers and manufacturers are using dial indicators to measure the edges of laminates and sheet metal. Dornfeld reports that one aircraft manufacturer with which he works frequently uses a simple dial indicator mounted on a tripod that rests on sheet metal or plates. As the anvil at the end of the dial rests on top of the burr, it actuates the dial. "If the dial says that the burr is less than the specification, then the part is good," he says. "If it's greater, then it's not." He notes, however, that the technique does not detect many subtleties, such as subsurface damage caused during burr formation.
Gillespie recounts how one researcher was able to construct a similar dial indicator gage for measuring a stack of five sheet metal parts. "The overall height of five parts was the total of the part thickness plus the burr heights," he says. He points out that the technique assumes a flat surface and no bowing induced by the preceding stamping operation. If the assumption is good, then the technique reduces the time to measure the parts and yields an average for a larger sample in less time.
Although the limitation of dial indicators is that they measure height only, using them with other gages in the quality control laboratory can be an effective way to measure both the height and thickness of burrs. For example, an inspector can measure the height of burrs with a dial indicator and then determine their thickness by focusing a toolmaker's microscope on the flat surface and moving it laterally.
Among the profilometric devices effective for measuring burrs, mechanical stylus profilometers can measure the heights of small burrs with relative accuracy if the burrs are stout enough not to bend in the process. They also can gage thickness on burrs not bent already and not too large for the stylus to negotiate. Inspectors must take care not to damage the instrument or aggravate it enough to affect its calibration, however.
A safer profilometric technique is laser mapping, because the beam applies no pressure on the burr and the burrs have no detrimental effects on the equipment. Moreover, lasers can detect variations on the order of nanometers. Unfortunately, cost has been a big obstacle to the widespread use of lasers. "The typical shop isn't going to invest that kind of money in burrs," says Gillespie. So their use for measuring burrs has been limited to either research laboratories or high-technology facilities already using lasers.
For facilities choosing to invest in laser measurement, phase measuring interferometry and vertical scanning interference microscopy can produce 3-D images of part surfaces or 2-D plots of key summary data. Both will provide images of burrs along the edges of parts and can document the topography along the entire length of an edge rather than at only one point. Phase-measuring interferometry can detect variations as fine as 0.1 nanometer, and vertical scanning interference microscopy can measure burrs several millimeters tall.
At least two mechanisms give lasers nanometer accuracy: confocal measurement and interferometry. "In the confocal method, the beam is focused on a surface, and the reflected light is caused to converge to a pinhole," explains Gillespie. "The degree of convergence determines the resolution of height measurement." For example, a laser with a 670-nanometer wavelength can produce a 7-micron spot, making height resolution 0.2 micron and the measuring range + or - 1 millimeter. Despite the accuracy, measuring the actual profile of the burr is difficult because of the diffusion that occurs at the tip of the burr.
Besides having experience with the optical, contact and profilometric techniques, Gillespie also knows of research into measuring burrs on machining centers. Adapted from technology for determining surface finish, the technique exploits changes in capacitance. When the analog feedback signals resemble a predetermined characteristic signature, the controller knows that no burrs exist. On the other hand, signals deviating from the signature denote a burr. The device measures edges at approximately 4 inches per minute on a machining center.
Dornfeld is excited by this and other research into deburring. "Deburring is so fundamental to quality," he observes. "Sometimes 30% of the cost of producing a part is spent getting rid of the burrs."
By simply being able to quantify burrs, one of his research teams was able to design a set of experiments that saved one automaker about $50,000 per machine per year by tripling tool life. With hundreds of these machines in the factory, the automaker is now a convert that believes in the power of measuring burrs to divine the ability of a process to make quality