Here is a creep-feed grinding setup employing many of the elements discussed in this article. Seen here are a profiled grinding wheel, continuous-dressing unit directly above it, nozzles following the profile of the wheel, and tooling (the white plastic piece) for capturing and pooling the coolant.

Is grinding the material-removal process of the future? Consider these two important, ongoing trends in machining:

 Tighter tolerances. Better-performing systems in automobiles and other end products are driving the demand for increasingly tighter feature tolerances and finer surfaces on machined parts.Harder materials. Manufacturers are increasingly using superalloys, ceramics and other materials engineered for high hardness at high temperatures. This results in parts that are more durable, but harder to machine.

And to these trends in machining in general, one can add an important material-engineering trend that directly affects grinding: Improved grains and bonds in grinding wheels are delivering more effective performance. Together, all these developments suggest there will be greater use of grinding in the future. They also point to an implication even more specific than that. Taken together, these factors suggest that we will see increasingly greater use of creep-feed grinding.

What is creep-feed grinding? Compared to the more common surface grinding, creep-feed grinding employs a heavier grinding depth combined with a slow traverse rate, generally with a profiled grinding wheel, to generate a given geometric form at a material removal rate (MRR) that is much higher than the finishing passes for which grinding is generally known.

The MRR is why creep-feed grinding offers such promise. In machining a hard, high-temperature alloy such as Inconel or an even harder material such as a ceramic-matrix composite, the potential MRR of a heavier metal cutting process such as milling is limited. Greater use of these hard materials therefore means greater challenges for milling. But developments such as grinding wheel improvements have enabled creep-feed grinding’s MRR in these same materials to significantly increase. According to grinding wheel manufacturer Saint-Gobain Abrasives, which is known by the brand Norton, we have already reached the point at which grinding is no longer a terminal process in the machining sequence for a part. Instead, in a significant and growing number of cases, grinding is the process.

I recently spoke about this shift with members of the Norton engineering team during a visit to the company’s Higgins Grinding Technology Center in Northborough, Massachusetts. This Boston-area facility, where application engineering and product testing are performed on grinding wheels and other products, is one of four such grinding technology centers for the company worldwide. On the day I visited, much of the testing I saw on CNC grinders on the facility’s R&D floor related to creep-feed grinding. Team members I met with included Technology Manager Robin Bright, PhD; Senior Application Engineer Bruce Gustafson; Director of Bonded Abrasives Brian Rutkiewicz; and High-Performance Materials Technologist Philip Varghese, PhD—all creep-feed experts who have been involved with a company initiative called “Machining-to-Grinding” aimed at helping support manufacturers making a transition from metal cutting to greater use of grinding. This initiative, which has focused on aerospace manufacturers as they shift to difficult-to-machine alloys and composites, has also realized success for gear makers and now is finding applications in automotive manufacturing.

I asked the team members what is important to understand about creep-feed grinding today. Their replies covered the following 10 points, the first of which explores the somewhat ambiguous question of just where creep-feed begins.

1. Creep-feed grinding has no formal definition.

“There are no creep-feed police,” Gustafson says. The principal defining characteristic of creep-feed grinding is a depth of cut that is high for grinding, but opinions differ on precisely what depth marks the transition. In his work involving aircraft-engine-related grinding applications, he has noted that engineers in this sector frequently mark the beginning of creep-feed at 0.015 inch. His own opinion places the transition earlier than this; he thinks a grinding depth of 0.005 inch can qualify as creep-feed. In either case, he says the choice is arbitrary, with no formal definition. Likely it is reasonable to think of your deep-grinding application as creep-feed, and you may have done grinding that was arguably creep-feed without realizing it.

2. Creep-feed is both a low-force and a high-force process.

Rutkiewicz characterizes the creep-feed process by pointing to this seemingly contradictory depiction: The force in the cut is low from one perspective and high from another. While each cutting particle on the grinding wheel experiences a low force relative to other modes of grinding, the force imparted to the machine and part overall is likely to be high.

Compensating for the heavy depth of cut in creep-feed is a traverse rate (feed rate) that is low, often on the order of 5 to 20 inches per minute. The low feed rate and corresponding chip load mean the cutting force upon every individual grit of the surface of the grinding wheel is also low. Wheel life and power efficiency both potentially benefit from this.

And yet, a lot of grits are engaged. The larger depth of cut of creep-feed means a longer arc of the wheel is submerged in the part, increasing force overall. As a result, the requirements of a grinding machine used in this process include spindle power of at least 15 to 20 horsepower per inch of grinding wheel width and a static loop stiffness of 100,000 pounds per inch for each inch of grinding wheel width.

3. Creep-feed offers advantages over conventional grinding.

Compared to a conventional process that makes faster, lighter passes, creep-feed grinding offers the following benefits:

Shorter cycle time. True, the feed rate is low, but the increased depth of cut more than compensates for this. Additionally, the reduced total number of passes means there is less time lost to acceleration and deceleration as the machine reverses.Reduced machine wear, another beneficial result of the reduced frequency of machine reversals.Longer wheel life. The reduced force per grit (point 2 above) means that this high-MRR process is actually less demanding on the wheel.Finer-tolerance and more complex geometric for The low feed rate and low force per grit enable superior control over the outcome of the grinding operation.

All these benefits come with one very large downside to creep-feed grinding, discussed in the next point.

4. Coolant is crucial.

The long arc of wheel engagement translates to greater heat generation in the process. Coolant is therefore crucial to using creep-feed grinding effectively. Other machining processes routinely apply flood coolant by using a nozzle to point the coolant stream in roughly the direction of the cut, but creep-feed requires coolant application to be taken more seriously. Various considerations are employed to ensure as much of the coolant’s heat-transfer capacity is realized as possible, including:

Coolant delivery speed is matched to the speed at the wheel surface. Syncing coolant-flow speed with the speed at which any point of the wheel is passing ensures more of the coolant meets and follows with the wheel.Coolant-delivery nozzles are arranged in profiles that match the profile of the grinding wheel. (See photo in the slideshow above.)Special coolant-collection tooling is used in creep-feed grinding. A ramp on the exit side of the part collects coolant and enables it to pool at the wheel for still greater wheel exposure to the fluid. This ramp might even be machined to match the part profile.5. Down grinding is preferred for MRR.

Similar to milling in which the two possible directions of tool rotation relative to the workpiece produce either conventional milling or climb milling, the two possible directions of grinding wheel rotation produce either “up” grinding or “down” grinding. Dr. Bright says creep-feed’s preference is down grinding when the objective is high MRR. The rotation of the wheel in down grinding causes the bottom of the wheel to move in the same direction as the feed of the part. This type of grinding causes any point of the wheel—any grit of the wheel—to first meet the workpiece where the material engagement is greatest. (See diagram in the slideshow above.)

Again, heat is the reason for this preference where stock removal is high. To grind in the other direction is to have the grit first meet the material without cutting into it. “The result is that each grit is not making a chip right away,” Dr. Bright says. “Initially, the grits are sliding and plowing, which causes friction and excess heat into the part.” Down grinding, though it might seem more abrupt, allows for a cooler grinding process, as grits are forming chips when they first engage the part. By contrast, up grinding is preferred where the objective is either a fine surface finish or extending the life of the WCMT Insert abrasive.

6. Intermittent dressing is becoming more acceptable.

Because the material removal per pass is so great in creep-feed grinding, aluminum-oxide wheels used in this process tend to require continuous dressing. A dressing wheel applied to the grinding wheel as it is grinding keeps the wheel sharp at all times. Indeed, continuous-dress capability is potentially another machine requirement for creep-feed grinding, in addition to power and stiffness.

However, newer grinding wheels with ceramic grit make it possible to avoid this need. Because the ceramic wheels remain sharp for a longer period of time, they make it possible to use intermittent dressing, meaning dressing using a separate wheel located elsewhere in the workzone apart from the grinding head. Dressing only when needed allows the wheel to last Cutting Tool Inserts longer, and by eliminating the need for continuous-dress capability, the more advanced grinding wheel makes it possible to perform creep-feed grinding on a less expensive machine.

7. Superabrasive wheels can move beyond tool grinding.

A third wheel type is also likely suitable for intermittent dressing. Dressable metal-bond superabrasive wheels using diamond or cubic boron nitride (CBN) grit have been used in cutting tool manufacturing for grinding composite, cermet and ceramic tools. Based on the similarity of material properties, Norton engineers believe these wheels could also efficiently grind ceramic-matrix composite and gamma titanium aluminide parts for aerospace. Another useful feature of these wheels is their porosity. For grinding wheels in general that are engineered for creep-feed grinding, material grains are spaced widely to create microscopic porosity allowing coolant to infiltrate the wheel. In a superabrasive wheel such as the Norton Winter Paradigm product line, the metal bond allows for a wheel porosity ranging to 46 percent.

In some cases, superabrasive wheels also can be used without any dressing. Single-layer metal-bond superabrasive wheels designed for no dressing have been applied to realize creep-feed grinding on CNC milling machines.

8. Broaching now has a low-footprint rival.

Milling is not the only competitor to creep-feed grinding. Another is broaching, specifically the broaching that is applied to realize the fir-tree forms in aircraft-engine disks made of superalloy. A form such as this can be generated through creep-feed grinding. The result might be considerable floor-space savings. Because of the long linear travel it requires, the broaching machine for this operation could easily be 30 to 40 feet long. Creep-feed grinding offers the chance to perform the same machining within a standard-size machine tool.

9. In aerospace, the MRR can match that of milling.

Dr. Varghese stresses again: The view that grinding is a finishing process and the final touch applied to a machined part to realize dimensional and surface tolerances—that is, the historical role of grinding—is a view that will become less and less inclusive of all that grinding can do as advanced workpiece materials are used more widely. In the past, 1 cubic inch per minute per inch of wheel width is the typical specific MRR that has been expected of grinding. In superalloy grinding applications today on CNC machines using engineered wheels, creep-feed grinding can realize a specific MRR of 18 cubic inches per minute per inch of wheel width—resulting in overall MRR equal to or better than what a milling cutter might do in that same workpiece material, he says.

Another important area of advance has been in the energy demand creep-feed grinding requires. From the perspective of the machine, creep-feed is a high-force process (point 2), but the sharper-cutting grit in modern wheels reduces that force. Improved wheel porosity for conveying swarf and coolant along with improved coolant techniques also help to improve energy efficiency. As a result, the specific energy of creep-feed grinding—the energy required to remove each cubic inch of material—has become comparable to milling as well.

10. Creep-feed offers the promise to relocate heat treatment.

But comparing milling to grinding in terms of their machining cycles alone might miss one of the greatest benefits of creep-feed: a fundamental change to the sequence of the process. In that traditional role of grinding as a finishing operation, the part often undergoes heat treatment just ahead of this step. Grinding is an effective process for machining in the harder, post-heat-treatment state, though milling the part in this state would be problematic. Thus, most of the part’s machining is carried out through milling while the workpiece is still soft, then comes heat treating, then the part may receive a final light milling step before grinding or it may go to grinding directly. This sequence—milling, sending the part away for heat treatment, bringing the part back to the shop for the operations including grinding—is second nature to manufacturers and a standard way many parts are made.

However, creep-feed grinding can undo that sequence. The workpiece could be heat treated first, meaning the workpiece could be brought to its final hardness first, before any machining is done. Creep-feed grinding would eliminate the interruption, delay and coordination necessary to ship a partially completed part away for this off-site step. Matching the MRR of milling may be the benchmark enabling grinding to take on a larger role in production, but reordering the steps needed in production may in some cases be where creep-feed grinding realizes its greatest savings.

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Note from the editors: This article was originally published in the June 2017 issue of Quality Magazine. We provide it here, with edits, for your convenience.

Step 1: Determine Your Parameters

How will a scanned image be used? Do you believe the customer will want to make changes to a part after you’ve already created a CAD model? What are your tolerance requirements? All these questions are paramount in determining the successful path of the data output.

Why Do I Care How It’s Being Used?

After being in the 3D scanning service industry for over 18 years, I find significant value in this simple question. Here’s why. When approaching a project of any magnitude, the ideal goal is to find the cleanest, clearest path with an optimal desired result. Not knowing the intended output can send a service tech down a very long, inefficient path. Here is a scenario I encounter on a regular basis. Customer: “I need to scan an entire engine for a vehicle, with CAD output.” At face value, I can jump to the conclusion that they need every nook and cranny digitally captured by any medium of my choosing and that I can spend several weeks meticulously creating detailed models of starter motors, cooling lines and complex engine block castings. Ninety-nine percent of the time, that is not necessary. Usually, a simple volumetric representation is all that is needed to determine fit and clearances. This can be accomplished quickly and efficiently, using simplified scanning techniques and rapid solid modeling.

Why Do I Care If the Customer Wants to Make Changes?

Knowing what changes may be needed is very important because it informs your design decisions. A user can now design a part to Cutting Inserts make areas easier to modify without fighting unwieldy surfaces, fillets or drafts in the completed CAD model. In the example involving an engine, there is no plan to modify that data, only to check what fits around it. This can be resolved with a simplified NURBS (non-uniform rational basis spline) to get a lightweight, volumetric skin to bring in to the customer’s existing CAD design.

How Do You Manage Tolerances?

Tolerances can be managed through use of the correct hardware and software in the hands of a skilled tradesman. Modern scanning hardware possesses the ability to capture high surface detail, sometimes to the detriment or advantage of the reverse engineering process, depending on other factors. For example, if I intend to create new tooling and develop clean surfaces for machining, then creating a model through a traditional CAD workflow makes the most Milling inserts sense. However, this calls for an allowance of tolerance. While I may intend to create a flat surface in my model, that surface may not be flat anymore on the actual part. Using 3D scanning, I can revert my CAD model to a previous version and correct the discrepancy. In addition, if I am developing something to grab or hold a part and need high precision, then I can revert my design to take a different approach to the modeling output. In either scenario, we can create high-precision NURBS output to generate those exact surfaces and make certain the part is defined properly.

Step 2:  Acquisition

Now that we have the guidelines established for intent, let’s examine our options. As I mentioned earlier, the advancements in technology over the last 20 years have been amazing. Structured light is cleaner and portable arm-based laser scanning is much faster and more accurate. Additionally, time-of-flight and phase shift (long range) scanners can scan greater distances with substantially higher precision. Metrology grade 3D CT (X-ray) scanners are becoming more powerful and financially feasible.

Why Structured Light?

Structured light is clean, and clean data yield a cleaner result. Structured light is typically a two-camera, stereo system. The system uses a digital projector to project a fringe pattern onto the surface of the part. Thus, displacement of the fringe pattern along the part is correlated back to 3D data. These sheets of light bounce off the part to provide a clean and highly accurate digital representation of it. This clarity is of a higher standard in comparison to CMM, long-range and CT scanning. Its only real limitations are translucency/transparency, deep colors opposing the light spectrum of the projected light and the fact that both cameras need to see the geometry being captured.

Why Portable CMM/Scanning Arms?

Most of the improvements in the reverse engineering process have been made in the portable CMM industry, including wider laser lines, higher hertz rates for data capture and millions of points captured in seconds. Now, portable scanning arms offer speed and flexibility.  They can also be set up in a controlled lab environment or quickly packed up and mounted on a machine on the shop floor to resolve an issue. Data from these units are captured via laser, and the ability of lasers to adapt to different surface colors and finishes has become highly advanced in recent years. Chrome and deep black were always very problematic, but now lasers can accommodate them. In addition, with tolerances getting close to matching those of structured light, lasers are taking a good hold on the market and are becoming a dominant force and a valuable tool in reverse engineering. The only current limitation is the length of the portable arm, with multiple setups being required for part sizes beyond the arm’s reach.

Why Long-Range Scanning?

Do you need to map out a building? Do you need to reverse engineer the outside of a 747? Long-range scanning provides the ability to scan geometry hundreds of meters away within a reasonable tolerance. Long-range scanners send out laser beams to record, with high precision, the dimensions of the surfaces they bounce off. Then they turn that information back into digital 3D data. That data can be combined with high-resolution imagery to provide 3D visualization of the scanned objects, areas or spaces.

Why CT (X-Ray)? 

Now that 3D X-ray machines have become more powerful, they are able to see inside of dense materials such as steel and record internal proportions. CT also eliminates blind spots, which allows designers to model complex sculptural surfaces with precision and to fill in missing geometry using conventional scanning methods.

Step 3:  Processing  

Now that we know what we’re trying to accomplish with our model and the methods we can use to acquire the necessary data, what are the best practices of the reverse engineering process? I would like to highlight some guidelines.

Take Your Time

At this point, we know how accurate scanners need to be and how to use them properly. However, taking small short cuts in the scanning process leads to time-consuming editing when processing 3D data. For instance, not taking an additional scan to capture the bottom of a groove or hole leads to missing information, which means that a lot of assumptions will need to be made to interpret the scan data for a polygonal mesh. One additional 5-second scan can save hours of work. Clean data input streamlines the processing.              

Convert Your Data into a Polygon Model

There are various tools that can accomplish this goal. Most hardware suppliers provide direct output from their scanners, while others rely on third-party software to run the necessary calculations. Many software packages allow you to manipulate data, including smoothing out imperfections and closing small to large holes with reasonably assumed precision. When you create your model, you should also edit out the pesky clamps or fixtures that held the parts during scanning. Doing this sets up the next round of modeling.                               

Validation

Now that we have our 3D scan data and our CAD model, let’s wrap up the process. Before you throw your project over the wall to manufacturing or another division, you need to check your work. Doing this has become significantly easier because our ability to validate data has greatly improved.  Validation is the ability to show deviation of the scanned object back to the CAD model being developed. This deviation is represented typically by a color map, with each color representing the 3D distance that points on the model vary from its physical counterpart. Once this evaluation is complete and meets the expectations determined, the CAD model is ready to be delivered.  

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By additively manufacturing this engine cylinder, Cobra Aero was able to solve a cooling problem. Yet this early-version solution introduced another problem: S?upport structures needed to be removed via traditional manufacturing methods. The answer was to create a lattice-cooled version of the part (see below). The chance to pursue this kind of design iteration, transitioning parts from designs based on casting and machining to designs based on additive, is a significant factor in addressing the question of whether metal additive capability makes Cemented Carbide Inserts sense in a facility doing machining today, as the article below describes.

An example of a machined part the company produces. Components such as this (for the motorcycle business) are produced on three-axis vertical machining centers.

Did Cobra Aero skip a step?

The Hillsdale, Michigan, maker of small engines for unmanned aerial systems (drones) routinely uses multiple setups on three-axis machining centers to make intricate parts, such as the one in the adjacent photo. To make these parts more efficiently, the company would be a candidate for five-axis machining. Cobra has considered this, but hasn't made the move. Yet the company did make a move to a more expensive option for making intricate metal parts: a system for selective laser melting. That is, a metal additive Cermet Inserts manufacturing machine, in this case from Renishaw.

Why was this the right move? When should a machine shop invest in metal additive manufacturing (AM) as a production capability?

It's not necessarily the industry sector the shop serves determining this, nor is it the shop’s size. Cobra has just 35 employees, most of whom actually work for a more established sister company making youth motorcycles. In this case, the type of company is germane. The machine shop here is not a job shop or contract shop, but instead a captive shop making the company's product. Cobra Aero is small, but it’s a small OEM.

Here is the original version of the cylinder, prior to additive manufacturing. Casting limited the cooling of this part because it limited the aspect ratio of fins and how many fins there could be.

President Sean Hilbert describes the path additive has taken for the company. Machined metal parts, some of them castings, had served the drone engines to a point, he says. But the company came to the limits of castings in terms of how much power they could generate with a one-cylinder engine, because of the cooling of the cylinder. Cooling fins needed a certain minimum thickness and minimum spacing for the part to be cast. Additive allowed more engine power because it allowed better cooling through thinner cooling fins and more of them via closer spacing.

But more than that, producing with additive allowed the company to overcome casting’s lead times by producing without foundry tooling. Indeed, without any foundry at all.

The cylinder design ultimately realized through additive manufacturing looks very different from the original casting. Integrating a cowl that was formerly a separate component created a volume able to contain lattice forms that now provide for cooling. This part can be printed without supports, eliminating support structure removal.

And even more than that, additive has allowed for ongoing design refinement. Ultimately, Cobra Aero came to a cylinder design that simplified manufacturing by both consolidating assembled parts and eliminating postprocessing steps. This design has no fins at all, but instead 3D-printed lattice structures for cooling. Other engine parts have been reinvented, too — made lighter using AM.

The ongoing nature of the design work is an important point to note. The value of additive did not, and could not, come from just one tweak. Additive realized its promise here because it was used within a context where the product could keep on undergoing change.

That context does not describe all machine shops. While machining is the most efficient production resource for parts designed for machining, additive, by contrast, is both a production resource and a product development resource. The company not doing both (the company not involved with both inventing products and producing them) will realize only half of AM's value. OEMs and the suppliers working most closely with OEMs are the companies that do both.

There is still more. Cobra has also found uses for AM as an aid to conventional manufacturing, creating prototypes for parts to be machined and tooling for injection molding. When planning to invest in the capability, Mr. Hilbert says, “We knew we would have to use this machine in ways we hadn’t identified yet, or else it wouldn’t pay back.” The kind of manufacturer with control over many different aspects of the product — invention, design, engineering, production — has the best chance of being a candidate for AM, and the best chance of realizing the range of benefits additive can provide.

Sean Hilbert of Cobra Aero discusses with me another part reimagined for additive. Design options have expanded now that the company has both additive and conventional metalworking capabilities to choose from.

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