Making Your Mark
Properly done, part marking can tell the user your part's story wherever it goes
By James R. Koelsch
Marking parts for traceability is more than a buzzword at Borg Warner. For this manufacturer of mechanical and hydroelectric components, it's a crucial part of doing business in the automotive industry. Like other Tier One suppliers, Borg Warner relies on a number of marking technologies to make it possible to trace its products from cradle to grave.
The automobile industry is just one of a growing number of fields in which analysts need such information as product number, batch number, serial number, team identity, and manufacturing date for quality control and warranty studies. In some cases, technicians sometimes must be able to glean information from the parts themselves during certain assembly operations or repairs in the field. So, there has been increased interest in having each component carry the necessary information with it throughout its life.
The Borg Warner plant in Tulle, France needs these marks to be readable by both human beings and machines. For this reason, the plant marks some of the fuel-pump bodies, control valves, and other components that it produces in two ways; as alphanumeric characters and as data matrix codes, the so-called 2-D bar codes.
The data matrix is an improvement upon the conventional, linear bar codes that clerks scan at your local grocery or hardware store. Resembling square crossword puzzles, these compact squares are arrays of smaller black and white squares, and are capable of storing many more than the 18 characters found in conventional bar codes. A 3-mm-square code, for example, can hold as many as 50 characters, and larger blocks can hold as many as 350.
"When it comes to tracking discrete parts in manufacturing, the dominant technology used is bar coding," reports Carl Gerst, senior director and business unit manager for identification products at Cognex Corp. (Natick, MA), a producer of vision-based readers. "This symbology allows users to store information on virtually any component, subassembly, or finished good."
As the parts come off Borg Warner's production line, a micropercussion, or dot-peening, device from Technifor (Miribel Cedex, France, and Charlotte, NC) pecks one side of the metal parts to inscribe the necessary information as alphanumerical characters. Then, a Cognex vision system reads the characters and passes the information to an engraver, which cuts the bar code into the other side of the part. A Cognex In-Sight 5110 reader verifies both the content and quality of the mark.
After the next operations complete a series of hydraulic tests, a second reader identifies each piece, and tells the central computer system which ones have passed the tests. Sometimes, oil from the testers gets on the parts. It's not much, but it's enough to pose a challenge for this second reader, according to Olivier Skalinski, project leader at Alema Automation (Merignac, France), the systems integrator that installed the marking devices, readers, and software. His team had to do a little engineering to ensure that the reader could see and interpret the marks.
The team also configured the factory database and communications interfaces so that an automatic identification system in the packaging area could use the test results. A third bar-code reader at the entry to the packaging area reads the identifying code on each part, and Alema's software checks the database to see whether the part passed all of its tests. The system allows only good parts to enter.
Many marking methods exist and are used today, but the most popular ones seem to be dot-peen and laser marking, according to vendors of marking equipment. The rising interest in tracing parts demands that the marks be permanent; identifiers that will last throughout the parts' lives. Besides being capable of making permanent marks, the equipment also must offer the flexibility to accommodate the component mix, and the kinds of information that people need on parts.
Dot-peening and laser marking overcome the limitations of older techniques. Applying ink, for example, to paper labels or to the part can be as flexible, but such marks/labels are not always permanent. In metalworking, they often don't survive the manufacturing processes. Oil, water, and friction can wash or wear them away. Use in the field can do the same, either rendering these techniques useless as tracking mechanisms, or making them useful only for limited periods of time.
By selectively oxidizing the surface of a metal workpiece, electrochemical etching can create much more robust marks than printing. "The marker sandwiches a stencil between the part's surface and a pad soaked with electrolyte," explains Gerst at Cognex. "A low-voltage current does the rest." He says that the process works well on round surfaces and stress-sensitive parts, including certain components for jet engines, automobiles, and medical devices.
This technique has some drawbacks, however. "Etching is a messy process," says Tom Marquart, product manager, Telesis Technologies Inc. (Circleville, OH). "Users must create a stencil and deal with fluids. Serializing can be a problem, because you have to create a stencil for each increment." The chemical reaction, moreover, often can take a little time. For these reasons, he does not believe that etching is well-suited for high production.
Pneumatic or hydraulic roll or impact presses are much better suited to high production than etching. In fact, they can be much faster than even a laser or dot-peen marker. Once the press is set up, it can mark thousands of parts per hour—as long as the information in the mark doesn't change much. Changes require setup. The presses can stamp dates and serial numbers into parts, but this task requires a mechanical device that is prone to wear. "You have to replace the tools for striking the letters and numerals weekly, on average," notes Dave LaCosse of George T. Schmidt Inc. (Niles, IL).
Dot peening and laser marking, on the other hand, combine both speed and flexibility. Both are programmable using software that runs on proprietary or PC-based controllers. Such software usually has a variety of programming aids, including those that permit counting and making serial and date codes. Of course, users can store their programs in files that they can retrieve when they need them.
Dot-peen markers drive one or more hard pins into the work pneumatically or electromechanically, much as a jackhammer drives its tool into concrete. Upon impact, the steel, carbide, or diamond-tipped pins displace material, marking the surface with a dot. A two or three-axis programmable head moves the pin in the sequence of positions required to produce letters, numerals, or designs. "It's creating a series of dots, but they can be so fine that they look like a straight line to the naked eye," says LaCosse. These marks can be alphanumeric characters or data-matrix codes.
Laser markers focus intense monochromatic light onto a surface to heat the material quickly to melt, vaporize, or discolor it. The mirrors manipulating the beam can produce both round and square marks precisely and quickly. Lasers tend to produce cleaner, more aesthetically appealing marks than dot peening. They are also better at creating 2-D data matrix bar codes. "Because the process doesn't upset the material as much and provides better contrast, codes produced by laser are easier to read," explains LaCosse.
Although dot-peen markers can be very fast, they are not always fast enough to keep pace with some production lines. "Dot-peen markers commonly make 2–4 characters/second," says Telesis' Marquart. The faster the process goes, the coarser its mark. Lasers, on the other hand, are 4–10 times faster than dot-peen systems. For this reason, lasers tend to be the method of choice when a fine or aesthetically appealing mark is important, and when high speeds are required.
Dot-peen markers, however, are much less expensive than their laser counterparts. "You can get a basic dot-peening system for anywhere from $8000 to $20,000," says La-Cosse. "A laser typically starts between $45,000 and $100,000." He acknowledges that lower-cost lasers exist, but says that they are not as versatile for marking metals as the YAG (yttrium aluminum garnet) lasers that his company offers for this task.
The expense of laser marking goes beyond the cost of the laser itself. Operating costs tend to be higher, albeit not as much as they used to be. Today's laser markers tend to energize their YAG crystals with diode pumps, rather than krypton flash lamps; consequently they generate much less heat and need less maintenance. Yet lasers still need more maintenance than dot-peen markers, which typically require replacing the pin every so often.
Another expense for laser marking is the light-tight enclosures necessary to protect people from the laser beam. These barriers usually require some kind of manual or automatic door and, sometimes, other mechanisms for moving parts in and out of the enclosed work area. Despite these costs, the use of lasers for marking has been on the rise. "Their prices are dropping faster than those of pin markers," reports Marquart.
Expense is not the only tradeoff. Lasers also leave a relatively shallow mark that is typically only a few tenths deep–0.001" (0.03 mm) at the most. Paint and other coatings can cover these marks easily, and scrapes and corrosion in the field can render them unreadable over time. Juicing up the power to cut deeper will just melt the material in the neighboring squares, causing a phenomenon called overprinting. "The dots touch each other, and it becomes an unreadable blob," explains Tom Phipps, CEO, Columbia Marking Tools Inc. (Chesterfield, MI).
Although conventional dot-peening solves this problem by producing marks as deep as 0.003" (0.076-mm) or so deep, it suffers from limited resolution. Dot-peening creates a conical hole in the square space reserved for it. "Only 5 to 15% of the box is filled," says Phipps. Consequently, the vision system reading the code can confuse the dots with imperfections in the part's surface.
To get both depth and resolution, Columbia Marking Tools designed a series of patented miniature CNC silentscribing and dot-peening machines driven by servomotors and ballscrews. Called Square Dot, each machine drives a diamond-tipped pin into the material as a conventional machine would, but takes the additional step of moving the pin a tenth or so in each direction. The action pushes more material to the sides, and creates a square bottom that fills about 80% of the space. The mark is both easier to see on rough surfaces like cast iron and more resilient to scratches, corrosion, and other damage.
Despite the additional movement, the design of these CNC machines makes them much faster and more accurate than their conventional, belt-driven counterparts. Most beltdriven markers make 2–4 characters/sec, depending on the size of the mark and the make of machine. "Our slowest machine does four, and our fastest does 10," notes Phipps. And they can make the quarter-inch matrices with ±0.0002"(0.005-mm) accuracy, which means that the squares can be very close to each other, yet not touch.
The resolution is as high as a laser's mark, or within 2%, according to Phipps. "In some cases, the speed is the same as the laser, depending on the material and the specs of the mark," he adds. "If we're making a deep mark in cast iron, our square dot can go as fast as our laser, sometimes even faster." The opposite is true, however, for shallow marks. In that situation, lasers are faster by far.
Rotary engraving with a cutting tool enjoys a measure of popularity, as well. Not only is this method of marking programmable, it's also capable of making deep marks. Another reason that machine shops like laser engraving is that, if they don't spend a lot of time marking, they need not buy a dedicated engraving machine. Instead, they can put a carbide engraving tool in any of their machining or turning centers.
In these cases, "this is often the preferred method of engraving because the part is in the mill or multiaxis lathe anyway," says Lance Nelson, president, 2L Inc. (Hudson, MA). Not only does engraving the part on machines already on the shop floor save a capital investment in dedicated markers, doing so also can contribute to leaner manufacturing by eliminating an extra step for handling and fixtures.
The technique comes with its own set of problems, however. For example, it has not tolerated small deviations very well. Because the depths of cut are typically only 0.005" (0.127 mm), the mark might show up in some places, but not in others, if the operator places a part in the vise improperly, if the surface to be engraved varies too much, or both. "In a production environment, it's important that the parts are all sized identically or located on the table in the same way," explains Nelson. Castings often pose problems, because they are notorious for having rather wide tolerances.
Another reason that rotary engraving production applications tolerate deviation poorly is that the solid-carbide engraving tools tend to be thin and break easily. Programmers, therefore, favor approaching the work and feeding into the cut slowly to lessen the chance that inconsistencies in the workpiece will break the tool. In some cases, users feel that the relatively slow cutting speeds will divert the machines too long from their primary task of producing parts.
Nelson believes that the give in his company's spring-loaded engraving tool solves these problems. Floating over peaks and valleys, the tool compensates automatically for uneven surfaces. The spring absorbs any impact with high spots, and applies pressure to keep the tool engaged in low spots. Working with a tip made from the latest generation of submicron-grade carbide, the spring also accommodates hard spots and difficult-to-machine aerospace alloys. Not only does this capability mean faster feed rates in the cut, it also allows dramatically faster approaches and entrances into the work.
Consider the advantage that these qualities give a tool engraving 25 characters that are 0.150" (3.8-mm) tall and 0.005" (0.13-mm) deep into a stainless casting. Cutting feed rates for a rigid tool might be 6 ipm (152 mm/min) and plunging rates to enter the work might be 3 ipm (76 mm/min). Such a job will take 194 sec, according to Nelson. Because the spring-loaded tool can cut at 13 ipm (330 mm/min) and plunge at 250 ipm (6350 mm/min), it can complete the job in only 47 sec, which is 76% less time than the rigid tool requires.
Bar Codes on the Battlefield
Tanks are like any other vehicle. They eventually break. So it's not really all that out of the ordinary for the military's repair depot in Baghdad to receive a call to come and repair a tank that broke down while patrolling the nearby countryside. When they do, technicians rush to the vehicle to replace the critical component, but every so often they find that the part on hand isn't the right version.
Preventing such scenarios on the battlefield is one of the reasons that the US Department of Defense wants certain parts in its inventories to be marked with ISO 15434-compliant 2-D data-matrix codes. These codes contain an item unique identification (IUID) number that identifies each item, not simply a particular product. Right now, the policy applies to items that cost $5000 or more, are mission critical, or fall into a few other categories.
The military technician's problem outside Baghdad is similar to the one that automobile dealers occasionally face back home when an automaker makes an engineering change in the middle of a model year. When you take your car in for repair, the parts department might receive the wrong version of the part, causing a delay in your repair until the right version arrives.
As bad as it is when it happens to you, the problem is much worse in the military. Unlike most cars, a particular model of tank and other hardware can remain in production for decades. Meanwhile, not only will the model undergo several redesigns that make subsequent units slightly different from their predecessors, but individual tanks in the arsenal will also receive upgrades from time to time. "You might have a 20-year-old tank with a two-month-old circuit board," says Tom Phipps, CEO, Columbia Marking Tools Inc. (Chesterfield, MI).
The realities of the battlefield complicate matters. People, for example, are reassigned rapidly, often causing a lack of thought continuity on jobs. Consequently, the repair depot has been known to dispatch technicians as many as seven times to the same tank before happening upon the right part to make the repair. Each trip is more than an inconvenience. It is a risk, because the enemy knows that someone will be coming to recover the multimillion-dollar asset.
The codes should reduce that risk. "The military's wide-area network will be able to locate that exact part, the exact engineeringrevision level," says Phipps. "The technicians can make one trip versus seven to repair the vehicle." He adds that the automakers are doing the same for you and your mechanic.
This article was first published in the October 2007 edition of Manufacturing Engineering magazine.