May 2011 / Volume 63 / Issue 5|
By Bill Kennedy, Contributing Editor
| Superabrasive wheels add science to the art of polishing.
A major advantage of grinding vs. other metal-removal methods is its ability to impart extremely fine surface finishes. Now, advances in grinding techniques and wheel technology are pushing that capability into the realm of what traditionally has been considered polishing, using slurries of loose abrasives. The loose abrasive polishing process is said to be slower and less repeatable than grinding.
Courtesy of Walter Grinders
Mike Hitchiner, OEM technology manager for Saint-Gobain Abrasives, Worcester, Mass., considers the overlap between grinding and polishing to be in the 1 to 4 µin. Ra range of surface finishes. “The gray area between grinding and polishing is where the competition is.”
What level of finish actually constitutes polishing is a matter of opinion. Glen Rosier, applications engineer/business development for Abrasive Technology Inc., Lewis Center, Ohio, said: “Polishing is defined differently by each and every customer. When someone refers to polishing, I always ask what Ra they are looking for. A lot of people consider a polish to be a 5, 7 or 9, while my idea is a bright polish, which is somewhere around 2 µin. Ra or better. Whenever a customer says he needs a certain finish, I ask him to send me a part. I have it measured with a surface profilometer so I know what he is really talking about.”
The appearance of a polished surface may not accurately reflect actual surface roughness. “We did a grinding test for a customer who was trying to generate what he called a bright polish,” Rosier said. “He took a competitor’s wheel and ran a part, then he took our wheel and ran a part, and he kept repeating that the other part looked better than ours.”
Rosier said the competitor-ground part actually did look smoother, and suggested measuring the surface finish on a profilometer. The competitor-ground part measured 3.5 to 4 µin. Ra, while the part polished with the Abrasive Technology wheel was 1.8 to 2.2 µin. Ra. “The other wheel was just polishing the peaks of unevenness off the surface, where our wheel actually generated a superior finish,” Rosier said. He attributed the results to differences between the two wheels’ blend of bond and superabrasive grit.
Courtesy of Abrasive Technology
Polishing applications using superabrasive wheels exist in nearly every industry segment. One example is carbide dies used to form aluminum cans. The dies essentially are plungers made of carbide with a high percentage of cobalt to enhance toughness. Polishing the dies is challenging, Rosier noted, because the combination of relatively soft cobalt with hard carbide promotes frequent wheel loading, requiring it to be touched up frequently with dressing sticks or truing wheels.
For such parts, polishing typically involves rough, semifinish and finish grinding with 180-, 600- and 1,200- to 2,000-grit wheels, respectively. Each step removes less material from the plunger surface. Final finishes are in the range of 1 to 2 µin. Ra.It Takes a System
While creating a polished surface with a grinding wheel requires tight control of all grinding system elements, “many times the customer believes the diamond wheel alone is going to solve every problem that he has,” Rosier said. For example, job shops often wish to use the grinding equipment they have, rather than employing what might be best for a certain job. In some cases, “If we evaluate a situation and the customer doesn’t have sufficient equipment to do what he is trying to accomplish, we may suggest that he is not going to have success with his existing setup,” he said.
In applications where off-the-shelf abrasive and bond combinations aren’t productive, a custom wheel can be appropriate. “We may have to adjust the bonds for specific conditions,” Rosier said. “Bonds for polishing in many situations are a variety of phenolic- or polyimide-type materials. They are simply easier to work with, and there are more additives, such as nondiamond polishing materials, available to enhance the grinding characteristics, compared to vitrified bonds. The plastic bonds are more pliable in general and more easily grind and remove material.”
Courtesy of Saint-Gobain
Saint-Gobain’s Hitchiner agreed that, to maximize polish quality, a wheel must be carefully matched with the grinding machine. For example, Saint-Gobain has application specialists who review customer processes and work with the company’s R&D group to create custom grinding packages. In addition to wheel configurations, those packages can include special quills and spindles.
A key factor is the precision of the work drive. “When I look at a machine, I always look straight away at the dimensional and stiffness specs compared with the required part tolerances,” Hitchiner said. “For example, if the roundness tolerance of a fuel injector valve seat is 0.25µm, you should be sure that your workhead rotational accuracy, which is the primary driver for part accuracy, is on the order of 0.1µm.”
Courtesy of Abrasive Technology
Finish ID polishing of fuel injection components illustrates the increasing interrelation of polish process elements. According to Hitchiner, a fuel injector component made of 70-HRC nitrided steel typically would be ground with a 3mm-dia. CBN wheel applied at 100,000 to 150,000 rpm. While wheel grit sizes formerly were from 325 to 400 mesh, tighter finish requirements now dictate use of grit in the 600- to 800-mesh area. The wheels typically feature vitrified bonds so they can be dressed on the machine and thereby maximize the potential for automation.
For injectors, Hitchiner said: “20 years ago we were looking at roundness tolerances in microns and finishes around 0.25µm Ra. Now with the demand to control fuel flow for economy, we’re finding roundnesses down to more like a quarter of a micron, and surface finishes around 0.05µm Ra. The finish becomes one element of the overall tolerance. You might have 100nm of actual roundness runout, maybe 100nm of surface finish effect and 50nm runout in the 3-jaw chuck that is holding the part.”Tighter Ranges
Finish requirements for polished surfaces now include not only lower Ra measurements, but may also specify a range of acceptability. Troy Heuermann, business development manager, 3M Abrasive Systems Div., St. Paul, Minn., said in the past a seal surface might have had a single finish requirement of 15 µin. Ra or finer. “Now we are seeing requirements specifying a maximum and a minimum finish, between 4 and 8 µin. Ra, for example.” The lower limit is set because too smooth a finish will lack the capability to retain lubricant on the seal surface.
Ever-tightening finish requirements sometimes present engineering and measurement challenges. “I’ve seen specifications that say we need a 1 to 2 µin. Ra finish, but there are other aspects of the specification that make it impossible to measure,” Heuermann said.
Courtesy of Form Tool Technology
Traditionally, polishing often involved applying a loose abrasive material, suspended in a compound or slurry, with a rigid disc or soft pad. “Those lapping processes can produce a mirror finish, but they are slow and part conformity also becomes an issue,” Rosier said. “Today, with tight tolerance requirements on everything, it is much more practical to use fixed abrasive in a diamond wheel to gain superior finishes. The process is more repeatable and size is easier to control.”
In certain applications, Heuermann noted, polishing with loose abrasives in a slurry can produce subsurface damage. With hard workpiece materials such as glass or ceramics, the loose grains don’t cut a chip but simply roll on the surface of the material and wear it away. High pressure is generated at the points of the grains, and instead of cutting the workpiece material the grains may crush it and generate microcracks in the surface. Superabrasive grit fixed on a wheel or disc pad, on the other hand, makes tiny chips and can reduce or eliminate subsurface damage.
“For some years now there has been a move away from the free lapping of flat parts to fine grinding involving planetary kinematics (simultaneous circumferential and radial relative movement of the part and wheel),” Hitchiner said. “That essentially performs the lapping process but it is 10 times faster with grinding.”Go With the Chip Flow
Polishes generated with superabrasives can provide benefits in machining operations. “The cutting tool industry is now talking about finish not just in terms of a specified finish on machined parts, but also on the tools themselves,” Heuermann said. “From discussions with and information from cutting tool manufacturers, we are hearing that if you can produce a better finish in the flute area of a drill or endmill, the chip material will flow easier with less friction and less heat. That is especially the case on materials like composites, aluminum, titanium, other aerospace materials and even plastics.” Better chip flow contributes to better surface finish in the part as well as longer tool life. (See “Hole in Four … or More,” by Jim Destefani in the January issue of CTE for a discussion of drilling composites that includes a reference to polished drill flutes.)
Courtesy of 3M
Heuermann said grinding with fine-grain diamond wheels is one method to remove grinding lines generated when forming tool flutes and produce a polished surface. “The process has to remove enough material to eliminate the lines without affecting the shape of the tool,” he said. “It is a bit of challenge.”
However, polishing with fine-grain diamond wheels can also produce part quality problems. According to Heuermann, traditional wheels use a mixture of resins and fine-grade diamond in a solid matrix. The combination can be very dense, and, in polishing operations involving minimal material removal, the wheel may rub rather than polish, creating heat. To avoid burning the tool surface, users slow the wheel speed and other grinding parameters.
Tool flutes also can be polished manually offline, using a variety of relatively low-tech methods, such as brushes or leather wheels soaked in diamond slurry. These manual processes can be hard to control and can lead to ergonomic issues for operators.
Both grinding with fine-grain diamond wheels and manual polishing processes are slow. As a result, Heuermann said toolmakers don’t offer a polish on many tools, and when they do the cost of reduced productivity is passed on to customers.
To increase productivity, 3M has developed a diamond polishing wheel to primarily finish the flutes of drills and endmills. Marketed under 3M’s Trizact abrasive brand name, the wheels combine the attributes of resin-bond diamond and nonwoven abrasive wheels, Heuermann noted. The wheels’ makeup allows for efficient polishing while maintaining enough rigidity to be trued and dressed to consistently follow complex tool geometry. The 10µm-size diamond mineral is dispersed throughout the entire wheel, so fresh diamond is constantly exposed as the wheel wears.
The wheels are for CNC grinders and enable polishing of flutes prior to finishing the cutting edge, in one setup. Regarding the surface finish generated by the wheels, Heuermann said it is difficult to measure the finish inside the flute of a tool, but when testing the wheel on part ODs, finishes were below 2 µin. Ra.
Traditionally, polishing has been almost an art, and talented technicians still can produce excellent polishes manually. New technology in abrasive wheels and grinding machines “are taking some of the art out of it, but also are taking out dangers of the manual operation and worker fatigue,” Heuermann said. “By enabling end users to dial in an engineered process, we are trying to put some more science into polishing.” CTE
About the Author: Bill Kennedy, based in Latrobe, Pa., is a contributing editor for CTE. He has an extensive background as a technical writer. Contact him at (724) 537-6182 or at firstname.lastname@example.org.
When are diamonds like lobsters?
ContributorsAbrasive Technology Inc.
American Superabrasives Corp.
Anco Industrial Diamond Corp.
3M Abrasive Systems Div.
(866) 279-1288, x1258
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