Converting Machining Applications to Grinding
New technology can enable a broader use of abrasive machining by manufacturers
By Patrick Baliva
OEM and MTG Manager
Corporate Application Engineering North America
We have all seen and experienced the recent trends. Spiraling costs, global competition, and the green movement are just a few of the many pressures manufacturers are now facing in North America. Lean manufacturing, six sigma, and an increased focus on cost reduction, safety, quality, and service have become required endeavors for any corporation looking to become more competitive.
Many manufacturers are being forced to become more innovative on how best to process parts and reduce total manufacturing costs. Additionally, changes in part requirements have also brought new challenges. High-heat-resistant materials, improved strength-to-weight ratios, tighter tolerances, and finer, more-controlled surface finishes have challenged traditional methods of machining. These stresses have created a new opportunity for suppliers to bring themselves to a higher level by offering total cost solutions and new approaches. These are not based on existing applications on the same component, but are developed by looking across the spectrum of similar or like applications, and developing new processes.
Machining to Grinding (MTG) is the conversion of a high-scrap, high-cost machining operation to a stable, low-cost, and high-quality grinding application. Factors that allow the MTG process to be possible include advances in abrasive tools, higher cutting efficiency on difficult-to-machine materials, lean manufacturing, and the higher quality capability of grinding relative to machining.
Conventional analysis says machining provides much faster metal removal rate (MRR), and requires lower specific cutting energy for the same amount of work when compared to grinding. Advances in abrasive cutting tools and the shift in material properties have allowed grinding to come more in line with machining when comparing these two measurements. In traditional grinding operations, material removal rates were normally under 1 in.3/min (16.4 cm3/min) per width of wheel (in.3/min/inch). A new generation of grains, bonds, abrasive manufacturing and application technology has allowed these removal rates to skyrocket, exceeding 15–20 in.3/min/inch (97–129 cm3/min/cm). These are somewhat conservative figures. Numbers in the ranges of 50 in.3/min (819 cm3/min) have been achieved, but power requirements become cost prohibitive at this MRR. Further, these increased rates typically do not sacrifice quality level relative to machining. They still hold what industry regards as grinding tolerances (±0.0001" or 0.003 mm) quite easily. The same advances that lead to higher MRR also cause the specific cutting energy to drop significantly when compared to traditional grinding operations. This change can simply be attributed to sharper grains in more efficient cutting wheels, paired with optimal application technology.
It's debatable which materials are better suited for grinding versus machining. To create some structure around the argument, engineers at our company's Higgins Grinding and Technology Center (HGTC) have created a set of grindability curves based on MRR versus specific cutting energy on many materials. After creating the grindability curves, standard machinability curves were then referenced. These two sets of curves were then overlaid on a chart of materials (plotted by elongation and hardness). By doing this we were able to create zones of "hard to machine, easy to grind", "hard to grind, easy to machine", "hard to grind, hard to machine," etc. relative to various material types. When we first look at a difficult machining application, we can find which "zone" the material falls into. If the material being processed falls into the "easy to grind, hard to machine" category, we know we can likely develop a process where grinding achieves the end result more efficiently than machining. When materials fall into the "tie" regions or "easy to machine, hard to grind" category, we know we have to look beyond the material factor to what else may influence the efficiency of the entire process, and ask if grinding will eliminate inefficiencies.
In some cases, grinding may not be more efficient than machining, but when we look at the entire process, grinding can eliminate or reduce steps in the current process.
Years back, when throughput was challenged, manufacturers had to find a quicker way to remove hardened material, but also had to end up with finer ground finishes and tighter tolerances in the end product. Grinding technologies were not far enough along and antiquated grinding equipment could not create the circumstances in which a high-performance wheel could be implemented, even if one were available. Consequently, hard turning was implemented between hardening and grinding. While hard turning solved the throughput problem, it created new issues, including extremely high tooling costs, the need for new machines, and movement in components after hard machining due to tensile stresses.
Recently, with new investments in grinding machinery and advancements in wheel technology, manufacturers have had their eyes opened to a new world of grinding performance. Their traditional finish-grinding applications tripled and quadrupled MRR, while holding tighter tolerances. Manufacturers were not able to grind faster when compared directly to hard turning, but when you compare the total process, there are many benefits.
In today's new grinding process, you can totally eliminate hard turning, saving downtime, machine overhead, labor, reduction in WIP, setup time, and the tensile stress hard turning puts into the component. A single fixturing after hardening also allows tighter tolerances than can be achieved with multiple setups. Total process time decreases significantly.
A second option we have utilized when looking at lean manufacturing is grinding from solid in the soft state. The old process involved machining a component in a soft state prior to heat-treat. After heat-treat, the component was finish-ground to dimensional and surface tolerance. The old process can be improved in certain circumstances by grinding the soft component prior to heat-treat. Using high-porosity, premium-grain wheels, we grind the component to a tighter rough tolerance. Doing so offers several advantages and yields several benefits.
As mentioned above, grinding does not put the level of tensile stresses into a component that are created by machining. A common problem with gears and other deepslot case-hardened components is that they tend to move and flex in heat-treat because of tensile stresses produced in the part during the machining process. Grinding avoids this situation. In fact, in some cases, when using a plated CBN wheel, we are able to put compressive stresses into the component. By reducing or eliminating tensile stresses prior to heat-treat we are able to reduce scrap and improve Cpk. Furthermore, no one will argue the assertion that grinding can hold closer tolerances than machining. So by holding a closer size tolerance, we are able to reduce the depth of case hardening in heat-treat, greatly reducing takt time and energy costs.
And now let's address two of the first questions that typically get asked about grinding versus machining: tooling costs (be it only 3% of typical process cost), and cycle time. Depending on material factors, grinding can significantly reduce tooling costs. Tough materials, inclusions, interrupted cuts, and varying hardness across a surface are all things that challenge machining operations, but can be easily handled by grinding. Broken wheels are very rare in grinding, but the same cannot be said for cutting tools. Mean time to failure is also another area where grinding excels versus machining, saving inventory, downtime, tool life, and maintenance costs.
When we look at cycle times, we go back to our grindability charts—in some cases grinding is faster than machining, and in some cases it is not. It depends on the application. This unknown should not prevent you from investigating the value of grinding in an application, however, because a test grind can tell you exactly what to expect from this process.
Through-hardened components are another grinding challenge. Unlike cutting tools, grinding wheels typically perform better on harder materials. Using wheels capable of reaching very high MRRs, we can create an economic and technical case to totally eliminate soft machining, as well as the hardening process. We bring in raw, through-hardened material, and grind the component from solid, eliminating two process steps. Benefits include decreased cycle time, energy savings from heattreat elimination, quality improvement due to material movement in heat-treat, and a significantly leaner process.
The benefits of higher quality produced by grinding are fairly easy to explain. While there are people who will argue this point, it's pretty well known that grinding permits tighter specifications across the board when compared to machining.
When we talk about surface damage created by heat, white layer is the most common defect. While both grinding and hard cutting generate heat, aspects of grinding such as very small cutting edges, low wheel wear, and constant resharpening capability (dressing) can all create a grinding process that results in no layer or a very controllable white layer. Machining, on the other hand, is performed with a cutting edge that constantly wears until it fails. In all machining operations, as the cutting point wears, more surface heat is generated and, therefore, white layer becomes deeper and deeper.
When we discuss surface roughness, grinding is typically a winner hands down. At our company, we consider machining to have a common surface finish of 0.6 µm (Ra), and under optimal conditions machining can achieve a surface finish of 0.1 µm (Ra). This performance is determined by the tool's nose radius, feed, speed, and DOC. Wiper inserts can be used, but at the cost of shortened tool life.
Surface roughness and MRR in machining have an inverse relationship. When we look at the shape of the surface, a machined surface will have a well-defined peak-and-valley shape, while grinding will exhibit a random-shaped surface. This grinding finish is created by the multiple and randomly distributed cutting edges on a grinding wheel. The sizes of these cutting edges are, of course, predicated by the grain size in the grinding wheel. In grinding, we can commonly impart surface finishes of 0.1 µm (Ra), and under optimal conditions 2 nm (ELID for SKD61).
Deburring operations are extremely time consuming, so much so that companies are making capital investments in mass finishing and robotic deburring to reduce costs. When compared to machining, grinding leaves almost no burr. There have been situations where the deburring costs alone created a situation where grinding was a better solution. This is typically true when the part has to come offline prior to the next value-added operation for deburring.
Size Tolerances, Quality Measurements (Cpk etc.)
Again, this is an area where few will question that grinding can outperform machining. To put data behind this assertion, compare some common and optimum-size parameters. Hard machining, on round parts, can hold a ±0.0005" specification on size, which for some applications is certainly good enough. Of course, grinding is the standard when it comes to holding tighter tolerances, so anywhere we see tighter tolerances grinding will be an obvious solution. This being said, with new wheel and machine tool technologies, grinding can now compete on some applications with the removal rates of machining. When we look at all of the above benefits, therefore, grinding can be a solution on what would be considered a wide-open tolerance band for grinding. By grinding rather than machining on these parts, consistency is greatly improved and standard deviation is greatly reduced, providing an overall improved Cpk.
Of course, with any process there are pros and cons. It would not be prudent to look at replacing a process with another without understanding both sides. Grinding has some disadvantages versus machining, including swarf disposal versus chip recycling, training operators on a new process, and sometimes coolant.
Swarf disposal is certainly an issue if a manufacturer is machining very highvalue material from which machining chips are easily recycled back into their process. However, sometimes washing, collection, and sorting costs can washout potential savings. When we look at grinding swarf, it's very similar to the consistency of steel wool, sometimes mixed with abrasive grains and bond, depending on wheel technology. One of the more efficient ways to manage this is to use a pucking machine, which can press chips into a hockey-puck-sized cylinder. These can then be sold to recyclers, be it at a reduced rate versus machining chips.
Training operators is viewed as a drawback versus machining because grinding is viewed as a "black art" or very difficult to do. This assumption really isn't true. A better description is that grinding is different from machining. All in all, when we look at the tangible measurements in grinding—such as wear, life, tooling cost, quality—they are much easier to understand and measure when compared to those associated with the tooling used in machining. This is one reason why grinding is able to hold better quality variance numbers—it's simply more consistent. On the operator-training side, OEMs and abrasive suppliers typically supply very good training programs, which can bring operators up to speed quickly.
Coolant can be both a positive and a negative. In cases where oil is used in the machining process (typical for broaching), grinding coolant can be a huge positive due to the fact that it is water-based when we use conventional or vitrified CBN abrasives. Environmentally friendly, water-based coolants are, of course, safer, more hygienic, and generally easier to manage than oil-based fluids. Typically, grinding coolant, if kept up by regular maintenance, must be changed about once per year. Almost any integrator handling any type of machining coolants can also offer grinding-coolant maintenance services.
We've outlined parameters and process components that create a good environment for grinding when compared to machining. In the correct circumstances the results can be impressive. Not all machining operations can be replaced by grinding. Probably 5% of machining operations are candidates for grinding. But the 5% that are likely candidates for grinding make up for 70% of the processes that cause plant engineering, quality and management headaches. When the current process is challenged, we tend to look at changing something about that specific process as a solution. When you are reviewing your troublesome machining operations, I suggest that you investigate an entirely new process that may include grinding. In the cases where grinding fits, the rewards of switching can be plentiful.
This article was first published in the February 2009 edition of Manufacturing Engineering magazine.