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Grinding Precision and Productivity Reach New Heights

Ed Sinkora
By Ed Sinkora Contributing Editor, SME Media

Maximizing productivity and high precision at the same time requires a delicate balance between material removal, heat removal, and wheel wear

A Toyoda cylindrical grinder in action shows the importance of coolant.

Grinding, like all machining processes, is generally thought of as a process of tradeoffs. To gain one attribute, you have to sacrifice another. However, that is not always true.

For example, one logical attempt to achieve good material removal rates as well as limited wheel wear is to fix diamond or CBN super-abrasives in a sintered metal bond.

As Andrew Osborn, a product engineer at United Grinding North America, Miamisburg, Ohio, explained, “metal bonded grinding wheels have always had very good dimensional stability, thermal stability, and thermal conductivity. They basically pull heat out of the grinding area.” But one big disadvantage to metal bond wheels is the difficulty of maintaining their shape throughout a production run. Another challenge is opening (i.e., dressing) the wheel when it becomes loaded with workpiece material.

Osborn said opening such a wheel typically required “sticking” it with aluminum oxide inside the machine (which had limited effectiveness and a cost in cycle time), or dressing it offline with a silicon-carbide wheel (another time-consuming hassle). And, if a radius or any point along the profile broke down, “basically the wheel was done. It was really hard to reform it,” he said. Together, these factors severely limited the adoption of such wheels. However, new technology from Studer (a member of the United Grinding Group) is changing this calculation.

Studer’s WireDress technology uses a thin, electrically charged wire to both shape and dress a metal bonded wheel—on the machine and at speed. And that’s not just a simple flat or a corner radius. It can maintain a complex form, including thread forms, with high accuracy, limited only to the size of the abrasive grain.

Osborn explained that unlike a dressing wheel, the radius of which will break down over time, the radius of the wire is used only once as it goes through the grinding wheel. “The radius never changes. It never breaks down,” he said. “So it’s very consistent on the forms you can dress. It’s extremely accurate because of that.”

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Today’s grinders combine high accuracy with high throughput.

Like any wire erosion process, the wire doesn’t actually make contact with the wheel, which Osborn explained has the added advantage of leaving the abrasive grains untouched. “So we’re not damaging the diamond or the CBN structure of the wheel,” he added. “We’re just eroding the metal bond around the diamonds, creating a really open and free cutting wheel… Typically with finer grit wheels, we now get more aggressive cutting action, up to about 30 percent higher than what we were seeing previously with the same wheel.”

When Studer first introduced WireDress four years ago, it required a large box that limited the size of the part the machine could handle. The company has now introduced a second-generation Wire-Dress system that fits entirely behind the workhead, so it doesn’t take up any table space on the machine.

Ryan Michels, regional sales manager at United Grinding North America, said all the grinding professionals he’s shown the technology to say it’s “the biggest advance they’ve seen in years, other than additive manufacturing.” Michels acknowledged that the option requires a considerable investment, but “for the right application with the right volume, it’s the way the go.” Suitable applications include carbide, carbide coatings, ceramics, or high-volume work where the combined material removal rate and durability of metal bond wheels add a lot of value.

Wheel Bond Cools Like Vitrified, Lasts Like Metal

Studer began its WireDress development with traditional metal bond wheels, which “are limited in their extreme hardness and density,” said Bruce Northrup, vice president and general manager of Meister Abrasives USA, North Kingstown, R.I. So, in addition to the disadvantages covered earlier, such a design also leaves no gaps in the bond for coolant and precludes other forms of customization. While not limited to machines with WireDress, Meister has introduced a conductive metal-ceramic hybrid wheel called Ceramet that “combines the wear resistance of a metal bond with the porosity of vitrified bonds,” said Northrup. “It looks and behaves very much like a vitrified bond in that it’s naturally porous, it carries coolant, and has low cutting force, but is much tougher than vitrified and will stand up to tough applications like grinding glass, or ceramics, or other materials that you would employ a metal bond for.” And unlike with a strictly metal bond, Meister is able to change all these properties, making the wheel harder or softer, or more or less porous, or vary the concentration of the diamond or CBN, to suit the application.

Although not suited to WireDress, new metal bond, single-layer, superabrasive products have found a role in the primary metals industry and in finished aerospace components, according to Phil Plainte, senior application engineer for Norton | Saint-Gobain Abrasives, Worcester, Mass. “One good example is the casting industry, where traditional conventional abrasive resin bond wheels are used to grind off gates and risers,” he explained. “Some of the aerospace materials are very difficult to grind, thus the wheel consumption can be quite high. The standard abrasive generates a significant amount of dust and odor. The wheel life can be relatively short and require frequent wheel changes. The short wheel life can cause programming problems when using robotic cells.”

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The Rollomatic NP5 pinch/peel grinder can hold part dimensions within 0.4 µm in a temperature-controlled environment.

The new single-layer metal bond wheels, like Norton’s MSL products, last much longer, reducing wheel changes. Plainte cited one example in which an MSL wheel achieved a cumulative G ratio of 253.1 grinding aerospace material at 12,000 sfpm with a Q’ of four, compared to a conventional wheel’s G ratio of 0.24 running at 20-26,000 sfpm with a Q’ of six to 12. Plainte added that if you ran the two wheels at the same material removal rate, the superabrasive wheel would still last roughly 100 times longer than the conventional abrasive. What’s more, these MSL wheels improve grinding accuracy and greatly reduce dust, odor, and grinding swarf.

Upgrades to Machine Design

The general manufacturing trend toward ever tighter tolerances has been good to grinding, and likewise grinding machine manufacturers are now chasing tolerances of mere microns in many situations. One way they’re hitting those numbers is by improving basic design features. Shane Farrant, national product manager for grinding machines at JTEKT Toyoda Americas Corp., Arlington Heights, Illinois, offered an excellent example. Toyoda achieves sub-micron static accuracies with what it calls a “floating plate ball nut, which is an EDM-cut piece of spring steel that absorbs the runout of the ball screw,” said Farrant. Intriguingly, he added that Toyoda does not rely on glass scales, “which have a tendency to become contaminated with swarf and require maintenance.”

Grinding generates a lot of heat (much more than machining) and it’s critically important to remove that heat in such a way that you prevent both damage to the part and changes to the machine geometry. Farrant said Toyoda has taken a number of steps to deal with thermal displacement, including “incorporating a steel plate separated by an air gap on top of the casting to help maintain the thermal stability of the casting during the grind. We also integrate a coolant channel underneath our wheel heads to prevent coolant from damming around the wheel head, which could potentially cause thermal growth or contraction of the wheel head.” He said Toyoda also designs casting to reduce the opportunity for swarf to accumulate so there is clean evacuation back to the coolant system.

Toyoda uses cast iron for the machine base, workhead, foot stock and wheel head, which according to Farrant allows all the machine and sub-assembly components to expand and contract at the same rate. “It has superior vibration dampening characteristics along with it, so that we’re getting the best of both worlds.” He added that Toyoda owns its own casting plant in Japan and allows castings to “weather, twist and stabilize before we even start the machining process prior to assembly.”

Conversely Studer has long touted its Granitan base, which is a type of polymer concrete consisting of natural hard stones of varying sizes mixed with a bonding agent and hardened. It’s inherently stable thermally and dampens vibration, but Osborn said this year Studer has enhanced the thermal stability by adding a radiator system through the machine base.

“We actually cycle process coolant through the machine base to make sure that the part, the machine and the coolant are always the same temperature,” he said. “That gives you a more accurate part when you take the part out from the shop air and put it into the machine, and vice versa.”

On a related front, Osborn said Studer has also recently emphasized the use of frequency controlled pumps on its coolant systems, enabling the preprogramming of specific flow rates for individual parts and operations. “It takes a lot of the manual adjustments of those things out of the operator’s hands,” he said. “Your coolant flow is programmed in the grinding programs, so when they switch jobs and pull up a part program that they ran a week ago, that same coolant flow and coolant supply comes up with that program.”

Farrant pointed to the special challenge of getting coolant to penetrate the air gap that surrounds wheels running at higher surface speeds (common when using superabrasives). The solution is higher-pressure pumps and better control of nozzle direction to create a coolant flow at speeds optimal for the wheel speed. Farrant said high-pressure coolant can also play a useful role in scrubbing the wheel to open it up, often using an alternate nozzle fed by a separate special duty pump. This can help reduce the frequency of dressing.

Eric Schwarzenbach, president of Rollomatic Inc., Mundelein, Illinois, reported that Rollomatic is now 3D printing nozzles for its large tool grinding machine, the 830XW. After experimenting with many nozzles and their positioning, the company determined it was most efficient to print nozzles optimized for the fluting and gashing operations rather than trying to bend copper pipes.

The bigger upgrades for Rollomatic have been the switch to linear motors on all of its tool grinders and the addition of hydrostatic ways on its Nano micro tool grinding machine and the 830XW large tool grinding machine. Schwarzenbach said hydrostatic guideways greatly increase the stability and vibration dampening of the machine, and because the hydrostatic oil is chilled to a consistent temperature it contributes to the overall stability of the process.

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The Norton Xtrimium wheel is a field-proven product that can efficiently grind a bevel gear from solid stock.

Plus “you have no maintenance on the rails,” he added. “There is no mechanical contact around the rails. No friction. There’s no stick slip. All this makes a contribution towards increased uptime, better stability, and the ability to grind faster. That’s forgotten sometimes, or underestimated.” And that ability to push the machine to grind faster extends to micro tools as well, because as Schwarzenbach explained, “the finish is of utmost importance on miniature end mills, and if you have chatter marks on the ball, the first thing the operator does is slow down.” He added that linear motors are another way of stabilizing the environment inside the machine.

To Gage or Not To Gage

In-process gaging is another logical aid to maintaining tight tolerances with high productivity, and United Grinding’s Osborn said gaging is getting better. One such example is the Marposs T25, an off-the-shelf probe but one with 0.5 µm repeatability. In a common approach, the machine would probe a ground diameter and use that information to calculate the actual diameter of the grinding wheel, adjusting the appropriate axis position (generally X) going forward. Toyoda’s Farrant said cam and crankshaft grinding are perfect examples that require this approach. “In a typical crankshaft application, you could be controlling six or seven axes at a single time. We have machines with twin wheel heads that are chasing two crank pins at the same time, moving back and forth.” As he explained, these complicated setups rely on the machine’s accuracy augmented by touch probes or touch pins that measure the diameter of the CBN wheels. “We use the probes to make sure that our axis position is regularly updated.”

Farrant added that roughly 70 percent of Toyoda’s installations are at some turnkey level in which it contracts to meet stringent run-off criteria and statistical analysis, but gaging isn’t always needed. “In many cases, the print tolerance says plus or minus a 10th but we need to hold the accuracies much tighter to get a 1.67 Cpk. We can generally decide whether or not the machine can hit the required numbers without extra gaging at the start of the project.” Farrant also said that if the lot sizes are small, “we generally try to steer the customer away from an in-process gage and explain that it will take more time to set up the gage than to run the parts. We say, ‘throw a mic on them, measure them, and make an offset if required.’”

Schwarzenbach of Rollomatic offered useful perspective on both sides of the equation. On one hand he agreed that controlling wheel wear drives the need for gaging in many cases, adding that controlling for temperature fluctuations is another motivator. On the other hand, he argued, you can’t achieve the highest possible accuracies based on in-process feedback. To do that you need an extremely accurate machine and complete control over the machining environment.

Rollomatic also uses Marposs gages in its pinch/peel cylindrical grinders and here Schwarzenbach explained that the software that analyzes the data is more important than the gage itself. “If you’re using a simple moving average, you often get an overcorrection. Your adjustment goes up, and then it goes minus the next time, and it jumps around too much. We’re analyzing data points over a number of parts. We’ve found that using a series of subsets is a better, more stable way to let the gage make corrections.”

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Toyoda achieves sub-micron static accuracies with a floating plate ball nut that absorbs the runout of the ball screw.

Schwarzenbach said Rollomatic’s OD grinders have ground pins consistently within 0.4 µm. But “if you really want to grind below one micron,” you can’t rely on the gage. “You need to make sure that you have a temperature-controlled environment and the oil has to be very, very well-managed for temperature.”

On its cutter grinders, Rollomatic has just introduced a Blum laser to measure the OD, runout, and index. Why measure index? Schwarzenbach explained that wheel wear on the fluting wheel will naturally cause the core to grow, but because much of the helical grind is also done by the side of the wheel, that wear will cause a change in the index. He reported that the system can keep the OD within a bandwidth of under 2-3 µm as long as the ambient and coolant temperatures are reasonably stable.

New Wheel Technology for Auto Parts

Northrup of Meister Abrasives observed that auto manufacturers are now faced with the prospect of having to do a rough and finish grind on fuel injection nozzle bores and seats in order to meet the tolerances and required finishes for the next generation of internal combustion engines. As a result, Meister adapted technology it developed for grinding silicon carbide wafers to angstrom level surface finishes for the semiconductor industry. The company calls it VM for “vitrified micron” because it evenly distributes fine-mesh superabrasive grains in a very open and porous bond structure that in a traditional bond system would be too dense to grind effectively.

Northrup said “we’re able to maintain the free cutting behavior that everyone likes about a vitrified product, and achieve extremely smooth surface finishes, at removal rates you would normally expect from a much coarser-grit wheel. Rough and finish grinding is now possible in one step.”

As an example, Northrup said a typical wheel for a fuel injection seat application would use a 500 grit (FEPA 40), whereas the VM wheel can achieve the same or better productivity at 1,000 grit (FEPA 16). “We’re able to achieve all the productivity they need,” said Northrup. “In fact, we’re reducing grinding forces with a 1,000-mesh wheel instead of 500 mesh, which you can imagine produces a much smoother finish, and with low cutting forces allows [shops] to improve the geometry and the tolerances dramatically as well.”

Another interesting new Meister vitrified product is High-Precision Lubrication (HPL). Northrup said it does not replace the need for coolant, but “adds additional lubricity into the cut to allow you to push the limits of what you can do with a vitrified CBN wheel. So, for example, it’s allowed us to employ more premium, tougher CBN crystals.” He explained that these tougher CBN crystals are helpful when grinding hard tool steels “but [that] in higher concentrations would perhaps burn, or cause chatter, or metal load in standard vitrified bondings.” Another good application would be for a bore grinding application “where you don’t have a lot of coolant to begin with, you don’t want quill deflections, and you want a nice sharp, cool-cutting, friable wheel. We can take that wheel and just swap the bond with this HPL and reduce the onset of burn.”

For its part, Norton has introduced a new ceramic grain that allows for lower specific grinding energy and a higher MRR and G ratio. Plainte said it has “added a performance kick to grinding gears from solid, a process that eliminates the traditional rough operations (hobbing and gashing) associated with rough cutting the gear teeth from a blank. This eliminates the need for hobbing and gashing machines and the complementary tooling.” In one test against the previous generation wheel, the new Xtrimium wheel increased feed rate from 70 to 150 ipm (the machine max) with a 30 percent reduction in power draw.

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