Lightly touched: Choosing the best marking method for delicate workpieces

Back in my shop days, most workpieces were marked by using hardened steel stamps and a 2-lb. hammer. WHAM!

That heavy-handed approach didn’t cut it when the shop began supplying thin-walled aluminum brake sleeves to Boeing. We ordered a bunch of rubber stamps and gently rolled the part number in permanent ink around the sleeve’s circumference. Quite often the ink would smear and we’d have to clean it with alcohol before trying again.

Courtesy of Gravotech

This compact Gravotech CO₂ laser marks almost any material at up to 2.54 m/sec (8.33 sfm).

Later, we got an order from Sundstrand for tight-tolerance 4140 steel bearing housings. The print called for electrochemical etching of the contract and part number. We invested $500 in a benchtop machine, a supply of electrodes and some weird-smelling chemicals. Using a typewriter to generate the characters on a sheet of transfer paper, we’d then cut out the stencil with scissors, fit it to the electrode and apply DC current, one workpiece at a time. What a hassle.

Hands-Free Marking

Manual electrochemical etching and rubber and steel stamps are still in use. These tried and true part-marking methods are simple and affordable, and, except for steel stamps, gentle enough for even flimsy workpieces. The problem is those manual processes are slow. The good news is there are better ways to mark parts, especially ones with thin walls or other delicate features.

Perhaps the most flexible is laser marking, used to mark everything from saw blades to circuit boards, and pacemaker housings to bearing races. Best of all, laser marking, contrary to what some might think, is simple to perform: Set up the workpiece in the machine, load the program and press cycle start. A few seconds or minutes later, the part’s been marked with the required number, phrase, bar code or graphic design. Laser marking is noncontact, so clamps are typically unnecessary, although in some cases a multipart fixture might be used if production volumes warrant.

Andrew Wellons, laser marketing manager for marking provider Gravotech Inc., Duluth, Ga., said 4 hours of training is often enough to perform basic marking and machine setup on any of the company’s Technifor or Gravograph laser marking machines. “We offer user-friendly, Windows-based software with a WYSIWYG graphic preview, and clear tools and functions to set up the marking processes,” he said. “It’s very easy.”

Courtesy of Direct Color Systems

A gage for a pressure dial is marked via UV LED inkjet printing.

Wellons explained that, when purchased as a turnkey solution, any of Gravotech’s machines meet Class 1 safety regulations from the Food and Drug Administration’s Center for Devices and Radiological Health (CDRH). This primarily means there’s guarding in place to prevent leakage of harmful light from CO₂, YAG and fiber lasers. Most marking machines operate on 110v power and offer low power consumption. That doesn’t mean you should put one of these machines in your living room and mark parts while watching “Dancing with the Stars.” Lasers mark by ablating, or burning, a small amount of the metal, plastic, glass or wood workpiece material. If you’re going to mark parts on your coffee table, you’d better keep the windows open or opt for a fume extractor.

Wherever used, lasers are the way to go for just about any marking application, according to Brian Bittner, sales manager at Beamer Laser Marking Systems, Flushing, Mich., a division of Arch Global Precision, Livonia, Mich. He said: “In the case of delicate parts, the material, font size and desired penetration depth all come into play, but I’d typically start with a low power setting—maybe a couple of watts—and a fast repetition rate: many of today’s lasers are capable of very high frequencies, in some cases 200kHz (pulses per second) or beyond. This allows a light dusting of the workpiece surface with multiple passes and avoids any potential heating or warpage.”

Smoke and Mirrors

In the marking realm, laser beams are guided by ultrafast, ultraprecise galvo mirrors, which reflect and direct the beam into the desired part-marking path. Because these mirrors are fixed, there’s a limitation when the marking angle becomes too steep or when attempting to mark cylindrical or spherical surfaces. In these cases, it’s like trying to write your name on a beach ball. That’s where 3-axis lasers come in.

“In addition to X and Y galvos, 3-axis marking uses a Z-axis mirror,” Bittner explained. “Not only does this allow for a much larger marking field—up to 36″ square, compared to 7 ” square on one of Beamer’s standard machines—it also provides greater flexibility in beam movement. If you need to mark a cylinder, for example, there’s usually no need for a rotary table to turn the part. The beam is directed around the side of the workpiece, covering approximately 120° of the cylinder, depending on its size.”

There’s a caveat to laser markers, however: They’re not cheap. The ballpark price for a Beamer marking system is $40,000. Budget-conscious buyers can find a base model machine at half that price, but if you need a conveyor, custom enclosure and auto door, the price tag on one of Beamer’s machines could hit the low eighties.

If you can’t justify even a low-cost base model, don’t worry. Bittner said there are plenty of contract marking houses doing this kind of work. One is Wichita, Kan.-based marking service and supply company Automation-Plus Inc. Douglas Wohlford, vice president, said the company is happy to laser mark your parts with one of its in-house marking machines, but added that ink is a good alternative.

Power of the Printed Word

Suppose you want to put a company logo on machined parts. Laser marking will do a fine job of producing crisp lines, legible text and even gray-scale images, but in many cases only a printer will do. In addition to its laser marking services, Automation-Plus manufactures direct part marking systems (DPMS). Wohlford said the systems are based on continuous inkjet (CIJ) printers, the same technology that puts the expiration date on your canned peas and soda bottles.

Courtesy of Schmidt Marking Systems

This high-resolution spider and racetrack design was engraved on this plastic part using a fiber laser from Schmidt Marking Systems.

Courtesy of Dapra Marking Systems

Dot-peen marking, a low-pressure process, can permanently mark 0.035″-thick (0.889 mm) stainless steel tubing. A unit from Dapra Marking Systems, Bloomfield, Conn., was used to mark the part.