January 2012 / Volume 64 / Issue 1|
Making a super switch
By Matt Huff, Stahli USA, and Ed Galen, Cinetic Landis-CITGO/Gardner Abrasives Operations
| A guide to converting from conventional to superabrasives in double-disc grinding.
Although the process is not well understood, anyone involved with superabrasives in double-disc grinding applications will tell you how beneficial and efficient they can be. That’s because superabrasives enable end users to grind more accurate parts, exert less stress within the part and typically provide higher yield rates by requiring fewer wheel adjustments, changes and sharpening operations.
However, to justify the conversion to superabrasive grinding wheels from conventional abrasive ones, end users must ensure the rewards outweigh the potential risks. Although diamond and CBN wheels can be extremely expensive, in some applications the conversion is justified. While a set of conventional abrasive wheels might cost $1,000, a comparable set of diamond wheels might range from $10,000 to $30,000, and a set of CBN ones could run from $15,000 to $40,000.
Courtesy of All images: Cinetic Landis-CITCO/Gardner Abrasives Operations
Initially, part manufacturers should review potential benefits to determine whether converting to superabrasive wheels is wise. The conversion is generally justified when grinding piston rings, rotors, stators and high-performance springs because, for example, CBN wheels can achieve the tolerance requirements at yields near 100 percent.
When grinding, the higher the G ratio, the better the wheel performs. The G ratio is the cubic volume of stock removed divided by the cubic volume of wheel wear. The G ratio changes from 1:1 to 10:1 for conventional abrasives to 100:1 to, in rare cases, 1,000:1 for superabrasives. An example of a rare case is high-speed grinding of highly conductive material, such as alumina ceramic. A realistic G ratio after converting from conventional abrasives to superabrasives would be around 300:1.
In cases where the G-ratio increase does not justify the conversion, the benefits of improved size control and the reduction in truing and dressing costs may be the justification. Truing and dressing costs are often significantly reduced because the truing and dressing frequency and volume of material to be removed is less when applying superabrasive wheels compared to conventional abrasive ones. In addition, converting to superabrasive double-disc wheels can reduce downtime because a conventional abrasive wheel is typically changed every couple of weeks, whereas a superabrasive wheel might be changed every couple of years.Know Your Machine
Engineering and design of the bond specification, superabrasive mesh size and concentration, and abrasive and its physical layout within the wheel have an enormous impact on the decision to convert to superabrasives. The potential for failure is amplified because of the large cost difference between abrasive types. The most important consideration is the condition of the double-disc grinding machine. The grinder must be able to achieve the precision needed for superabrasives. For example, a small amount of play in the spindle bearings is a minor nuisance for a conventional abrasive wheel but is unacceptable when applying a superabrasive wheel.
A quick check to see if a double-disc grinder can handle a superabrasive wheel involves opening the spindles and placing a dial indicator on one of the wheels. Then apply pressure to that wheel. If there is more than 0.005 " of spindle play, major problems will occur when using a superabrasive wheel. The ideal grinder has less than 0.001 " of spindle play.
In addition, the grinder must handle a cutting speed of at least 5,000 sfm, and any part of the wheel, including the ID, must be above 2,500 sfm. Normal, conventional abrasive dressing setups can be easily converted to diamond dressing tools from aluminum-oxide or silicon-carbide sharpening sticks. The only exception is when sharpening a vitrified-bond CBN wheel. Then, the best choice is a rotary diamond dresser.Material Consideration
After determining that the double-disc grinder can handle superabrasive wheels, the next consideration is wheel specifications. The first step is identifying the exact composition, hardness and size of the materials to be ground. Other key considerations are the material-removal rate, part parallelism, flatness and surface finish requirements. Some materials are more readily ground with superabrasives. Superabrasives cut all materials, but, to be able to justify their use based on G ratio alone, the work material must be hard enough to obtain the full benefits.
For example, steels softer than 35 HRC generally load up superabrasive wheels and are not good candidates for conversion. Although aluminum can be effectively cut with diamond cutting tools, diamond wheels are not suitable for grinding it because aluminum chips also quickly load the wheels.
Conversely, materials harder than 35 HRC are excellent candidates for superabrasive wheels. For these materials, the improvement in G ratio alone usually justifies the high upfront cost of superabrasive wheels.
Guidelines for converting from the conventional grain to superabrasive crystal are application-dependent. In general, the majority of double-disc grinding applications that can be successfully converted will use CBN rather than diamond. However, certain materials are suitable for converting to diamond, including ceramic and nonferrous materials such as tungsten carbide, because they do not contain sufficient levels of carbon. Ferrous materials, however, chemically react with diamond, which is a crystalline form of carbon, causing diamond to revert to the graphitic form of carbon.
When converting from a conventional grit size, users must go at least one—and possibly two—grit sizes finer for superabrasives to achieve surface finish requirements because a superabrasive crystal penetrates deeper than conventional wheels. For example, if the conventional specification is 120-grit Al2O3, the appropriate superabrasive crystal should be at least 150-mesh diamond or CBN. Applying the same size superabrasive grit would impart a much rougher surface finish.
A resin-bond conventional abrasive wheel specification is usually converted to a resin- or hybrid-bond superabrasive wheel unless the application has shown success in the past with a vitrified bond. Superabrasive bonds are similar to conventional ones in terms of formulation. They both include a number of different ingredients, such as phenolic resins, polyimide resins, melt-phase metal alloys and glass frits, which serve as binders, grain spacers, lubricants and wear-resistant agents. The bond conversion should be aided by a specialist in superabrasive bond formulation.
Superabrasive crystal concentration does not have much correlation with a conventional abrasive. When converting, it is best to start with a common concentration of crystal, specifically “50 con,” which is equal to a 12.5 volume percentage of diamond or CBN. That’s the case unless a specification has been proven on a similar part.Measuring Limitations
Users must also consider wheel dimensions. The overall thickness of the super-abrasive wheel is important because some double-disc grinding machines have capacity limitations. This can be controlled by designing the core to fit the appropriate machine dimensions.
For example, the standard useable thickness of a conventional abrasive wheel varies from 2 " to 3 " and has a density around 2 g/cc. The standard useable thickness of a superabrasive segment or button is generally 0.25 " to 1 ". This difference has to be made up with use of a flange, or “core.”
Steel has a density of around 7.85 g/cc. Therefore, the weight of the extra 1 " to 2 " of distance in the core would be too heavy for some double-disc grinders. Aluminum is much lighter, which can help overcome core weight limitations.
The preferred design is a superabrasive wheel that bolts directly onto the machine collar. This minimizes runout and improves wheel performance. If the machine can handle the weight and the application requires a much more rigid core, steel is the optimal choice.
Converting from conventional abrasives to superabrasives for double-disc grinding is enticing, to say the least. Typically, a successful conversion provides tighter tolerance control, less scrap and higher throughput. However, it is crucial to determine that your shop can actually reap those benefits before superabrasive wheels are tested. An experienced grinding application engineer can help determine if your shop’s jobs are good candidates for conversion, if your grinders can handle superabrasive wheels, and whether new grinding equipment is required. Doing your homework will help ensure that the conversion process is as painless as possible. CTEAbout the Authors: Matt Huff is engineering manager with Stähli USA, Lake Zurich, Ill., and Ed Galen is business unit manager for Cinetic Landis-CITCO/Gardner Abrasives Operations, South Beloit, Ill. For more information about Stähli USA’s lapping, flat honing and polishing machines, call (847) 719-0360 or visit www.stahliusa.com. For more information about Cinetic Landis-CITCO/Gardner Abrasives Operations’ grinding wheels, call (440) 709-0736 or visit www.fivesgroup.com/tools. This article was adapted from a paper presented at the Industrial Diamond Association’s Intertech 2011 conference, which was held in Chicago.
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