January 2012 / Volume 64 / Issue 1|
The great grinding divide
By Dr. Jeffrey Badger
| Bridging the gap between academic and shop floor grinding knowledge.
Grinding is a “strategic process” that occurs close to the end of the production chain after much labor has been put into a product. As such, it can determine the success or failure of a product.
Because of its seemingly random nature and negative rake angles, grinding is often viewed as a difficult process. But significant technical advances have been made in recent decades, shifting the perception of grinding from a random to an understandable chip-formation process in the same vein as turning, drilling and milling. Unfortunately, though, much of the fundamental knowledge gained has not found its way onto the shop floor.
Courtesy of United Grinding Technologies
For example, in 1951, grinding researcher L.P. Tarasov clarified the vague definition of “grinding burn”—oxidation burn, thermal softening, residual-tensile stresses and rehardening burn—in hardened steel by describing the metallurgical changes that occur with each and the approximate temperatures when they occur. Sixty years later, however, confusion still abounds about what constitutes grinding burn, and engineers often still rely on visual oxidation to judge whether a workpiece has suffered thermal damage. Considering that oxidation burn begins at around 250° C, whereas genuine thermal damage typically occurs at 600° to 1,000° C and may be present in the absence of visual oxidation burn, using this visual method can prove dangerous.
Figure 1 (below) shows oxidation burn after thread grinding (on the nonground surface in the flute-grinding region). Temperatures in this oxidized area were much lower than in the hot-spot region on the thread-ground surface—the clean surface. However, in the thread-ground region, the oxidation burn was ground away, whereas it was not on the nonground surface.
There is no way to determine if genuine thermal damage is present without using a more involved testing procedure, such as polishing, etching, X-ray diffraction or acid cooking. However, many operators and engineers believe that if the tool is clean, it isn’t burned, and if it’s brown and blue, it’s burned. That belief is false and risky.
The following are other key examples of fundamental grinding concepts that are often not well known by production engineers.
• Coolant velocity should match wheel velocity, with high flow rates not being necessary.
• Rotary dressing in the antidirectional mode dulls the wheel, whereas dressing in the unidirectional mode with a speed ratio (vdresser/vwheel) of 0.8 produces a sharp wheel.
• In single-point dressing, the dressing lead and overlap ratio are far more important in determining wheel sharpness than the dressing traverse speed.
• Diamond wheels go out of true if they are trued on an adaptor, taken off and remounted on the machine spindle.
Because these basic concepts haven’t made it to the shop floor, grinders are learning them the hard way—through trial and error.Applying Knowledge
I have visited grinding facilities in 29 countries and have seen low-tech grinding of drill bits and high-tech grinding of artificial-knee implants and turbine blades for jet engines. Regardless of the country or industry, many companies do not understand the basic concepts of grinding and, more importantly, do not have access to these concepts presented in an easily accessible, practical way.
I call this “The Great Grinding Divide” because there is a large gap between the knowledge held in academia and the knowledge held on the shop floor. One reason for this divide is that basic concepts have not been translated into a simple, useful format that can be quickly utilized by the grinding machine operator.
For example, as opposed to turning, which can be readily modeled in two dimensions, the 3-D nature of grinding makes calculating chip thickness difficult. Machine operators accustomed to the speeds-and-feeds diagrams used for turning are frustrated that such a relationship does not exist for grinding.
The most common equation for maximum chip thickness in grinding, hm, is some variant of:
where C is the cutting-point density, r is the shape factor, vw is the workpiece velocity, vs is the wheel velocity, ae is the DOC and de is the equivalent diameter.
However, measurement of C and r is rather subjective. More importantly, the equation is intimidating even to those with a higher education who work solely in machining.
Fortunately, the equation can be simplified to a speeds-and-feeds equation that uses only the machining parameters that can be varied—DOC, feed rate, wheel speed and wheel diameter—and called “aggressiveness.”
But this equation still uses variables that must be defined, along with units, and has typical aggressiveness values that are small, such as 1.8×10-5.
Therefore, the equation can be further simplified by multiplying the resulting value by a constant of 1 million to give reasonable values.
The issue of variables, nonetheless, still exists, along with the issue of units and unit conversions. Therefore, we can rewrite this equation in friendlier terms as:
With this equation, the machine operator can plug in the values from the CNC and generate an understandable number.
Typical aggressiveness values are from 3 to 60, with lower values for finishing and higher values for roughing. Moreover, each combination of wheel, workpiece and coolant will have an optimal aggressiveness value that will place it in the “sweet spot” of the operation, the place where the maximum chip thickness is large enough to form a chip and avoid excessive rubbing and high specific energies, but not so large as to cause excessive wheel wear.
This is an example of taking a complicated concept and modifying it to provide a simple yet useful parameter. Machine operators can identify with the concept of aggressiveness, and there is no need to labor over complex variables and unknown units; it’s all given in a format that can be entered into a calculator.
This begs the question: Why haven’t similar concepts been translated into similar, easy-to-use techniques? And if they have been, is this information accessible to those on the shop floor?No Access to Basics
An even simpler formula that’s used in calculations in just about every grinding process is the specific material-removal rate, Q’, known as “Q-prime.”
Q’ = ae × vw
where ae is the DOC measured in mm and vw is the feed rate measured in mm/sec. The specific mrr is the total mrr per unit width of the grinding wheel.
Unfortunately, many companies are not aware of this calculation. For example, a company I visited in Europe was trying unsuccessfully to cylindrical grind hardened steel with a CBN wheel using a Q-prime value of 82. An ambitious Q-prime for that application is around 15. This company was trying to make it work with 82 and madly adjusting the dressing and cooling parameters and wheel speed to no avail.
Why was the engineer on the project trying to make a process work with parameters that were outside the practical realm of possibilities for this wheel? Because he was missing a piece of the puzzle. The engineer had a degree in mechanical engineering and was capable of high-level math, but he did not have access to the concept of specific mrr or reasonable values for cylindrical grinding. He was investing his energies in cooling and dressing when he should have been focused on grinding parameters. Once the concept was explained to him, he quickly adjusted the parameters to a more realistic Q-prime value.
One reason that companies lack people with grinding knowledge is these people have retired or were laid off during the cutbacks in the 1980s and in the era of “lean manufacturing,” which occurred mostly in the 1990s. Many of these people were not replaced, and their knowledge disappeared with them.
In 2002, I visited a multinational company working in grinding and was impressed by the workers’ high level of expertise and the company’s advanced research program, which included numerous test machines and measuring devices.
Six years later, the company asked me to give my basic 3-day grinding course and provide general technical advice about how to set up a test program. Not a single person from the 2002 group was still at the company, which had completely lost its technical expertise.
Some decision-makers appreciate worker training costs as a wise, long-term investment, and some don’t. For small shops with one grinding machine, it may take time for the knowledge gained in a grinding course to pay for itself. For companies with 50 grinding machines running three shifts, the return is quick. Those companies might consume $40,000 in grinding wheels a month, with millions spent on labor. Even learning to achieve a slight reduction in wheel consumption or cycle time will pay for itself almost immediately.
Advances in grinding technology and processes continue to be made every year. They promote better understanding of grinding in general. This is evident in many technical journals and at grinding conferences.
However, the information presented in these formats is often, understandably, at a level that is too high for the layman, and “translations” must be made to convert the information into easily accessible formats. Currently, no such translation exists.
Moreover, access to some of these conferences is restricted. One of my toolmaker customers wanted to attend the 2009 CIRP (International Academy for Production Engineering) conference in Boston, 60 miles from the company. A paper was being presented on the optimal cutting edge radius to achieve low cutting forces and long tool life, and how to impart this radius through loose abrasive media.
Upon contacting CIRP, the company owner was told he would have to pay the full fee of $600 to attend the one presentation and find a CIRP member to sponsor his attendance, which involved a lengthy application process. Not being connected with any CIRP members, reluctant to pay the full fee and not willing to invest the time required for the sponsorship process, he chose not to attend. Fortunately, I knew the speaker and arranged for dinner and drinks at a local oyster bar for all of us to discuss the matter. But what happens to others who don’t have that connection? Unfortunately, they end up staying home.Reinventing the Wheel
Another area where knowledge is lacking is in switching grinding wheel type. There are numerous requirements to successfully shift from aluminum oxide to CBN, including:
• CBN requires a higher wheel speed than Al2O3;
• Compared to Al2O3, a finer grit size is necessary with CBN to impart the same surface finish;
• Unlike Al2O3, single-point dressing is not practical with CBN;
• Dressing forces are higher with CBN than Al2O3 and may require a stiffer dressing spindle;
• The dressing depth of a diamond tool on CBN is typically 10 percent that of Al2O3;
• Wheel cooling is more important with CBN than Al2O3 for CBN to be economical;
• The performance advantage of oil vs. water-based coolant is more pronounced with CBN than Al2O3; and
• CBN is more prone to loading than Al2O3 and may require a high-pressure cleaning nozzle.
These requirements are well documented—but they are largely unknown by companies that decide to switch from Al2O3 to CBN. As a consequence, companies learn the hard way and make the same mistakes. They either eventually rectify those mistakes and successfully make the switch to CBN, wasting time and resources in the process, or revert to Al2O3 and decide CBN is not suitable for the application. These requirements could be gleaned from existing literature, but that would take time. A bulleted list showing the main points when switching to CBN would save production engineers time and headaches.
The primary reason CBN wheels have not been fully exploited is cost, as CBN is much more costly. The second reason is a lack of understanding about the grinding process.
The same can be said about “ceramic grit” (a.k.a., “seeded-gel,” “sol-gel,” microfracturing grits and the trade names Cubitron and Norton SG). These tough grits must be pushed hard to get them to microfracture. Unfortunately, many companies that try ceramic-grit wheels simply stick the wheel on, grind with the same parameters as a standard wheel and experience burn due to wheel dulling. Much of my work involves showing companies how to increase and optimize grinding aggressiveness values to get these abrasives to work the first time.
Again, basic knowledge is not in the hands of the grinding machine operator.Chatter Solutions
A prominent example of “The Great Grinding Divide” is grinding chatter. In the past 10 years, numerous papers have been published on grinding chatter in the “CIRP Annals” alone. We now have a high-level understanding of how forced and self-excited chatter develops, along with specifics for particular types of grinding. We also know strategies for reducing chatter.
Here’s an example of a complicated formula used in an academic paper on chatter.
In contrast, the formulas that would be more valuable to many companies—if they were aware of them—are:
The operator simply needs to make sure the ratio of wheel rotational velocity to workpiece rotational velocity avoids an “integer value.”
In 2009, I visited a company in Europe battling waviness in cylindrical parts. The operators spent weeks fighting it and, based on waviness measurements, knew wheel diameters existed that were “danger diameters.” However, they couldn’t piece together why these diameters were dangerous and why they shifted with a change in the wheel speed.
The science behind their dilemma was simple: They needed to avoid harmonics where the imperfect form on the workpiece imparted by the imperfectly round wheel did not catch the crest of the wave and repeat this imperfection into the workpiece—i.e., an integer value. They wanted this imperfect shape to be obliterated by an “irrational value.” This was true when dressing and grinding.
The solution was simple: Develop a basic Excel spreadsheet with the inputs of wheel diameter, wheel surface speed, workpiece diameter, workpiece surface speed, wheel runout and spark-out time. The outputs were the ratio of wheel velocity to workpiece velocity to see if an integer value was found, along with a rough estimate of the waviness based on wheel runout using superimposed sine waves on the workpiece. The company was then able to use this to avoid integer values (and fractional values).
Sophisticated chatter-avoidance models exist that involve finite-element analysis and thousands of lines of code created from thousands of man-hours of development. For some companies, these models are extremely valuable. However, 50 cells in Excel thrown together in an hour were enough to give the European company a rough-and-ready method of avoiding high waviness.Bridging the Divide
So what needs to be done to bridge the divide between the academic world and the practical world? Here are some suggestions.
• Academic organizations and journals should continue publishing excellent, peer-reviewed articles but with a “layman’s summary” of the main findings of the work, paying special attention to ensuring formulas are presented with units and typical values and in words rather than variables.
• Shop floor grinding machine operators and production supervisors must recognize that simply adopting new technology is not enough. They must take ownership of their grinding operations, which includes the slow, gradual and sometimes tedious acquisition of grinding knowledge.
• Company owners and managers must acknowledge that grinding is a “strategic process” and be willing to develop this core competency, which means spending time and money.
• Conferences geared toward people involved in practical production should be conducted, with each speaker required to answer “How can this help your production when you get back to the factory tomorrow?”
• Academic organizations should make their conferences easier to attend.
• Grinding education should be focused not only on new technologies, but also on the fundamentals and application of the process.
• Shops need to develop in-house grinding experts.
Now that grinding has become a science and not an art form, and with the loss of much grinding knowledge as baby boomers retire, a concerted effort is needed to improve the level of operators’ grinding knowledge. It is a slow and gradual process, but will more than pay for itself through lower grinding costs and improved part quality. CTEAbout the Author: Dr. Jeffrey Badger is an independent grinding consultant. His Web site is www.TheGrindingDoc.com. The author would like to thank Professor Brian Rowe, formerly of Liverpool John Moores University, for his input. Article was adapted from a paper presented at the Swedish Production Symposium 2011, held at Lund University. Dr. Badger will conduct his “Three-Day High Intensity Grinding Course” March 7-9 in Columbus, Ohio.
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