When the grinding gets tough it’s the bond and not necessarily the grain that matters
These days the most important theme in superabrasive grinding wheel development isn’t the abrasive, it’s the bond. The diamond or CBN grains do the cutting, but the bond plays a decisive role in exposing the grains to the workpiece and enabling coolant to remove heat. Without exposed grains the wheel won’t cut. And without porosity in the bond, coolant won’t be able to flush away material with the heat that would otherwise damage the workpiece, not to mention the wheel. So while no one seems to be touting new superabrasives, all the high-tech players are experimenting with new, open bonds.
The Master in ID grinding
Perhaps the clearest example of a recent improvement in bond technology is Meister’s 3D Abrasives Technology. You’ll excuse the pun, but Meister (North Kingstown, RI) was already a master at producing wheels for difficult ID grinding operations like fuel injector nozzle bores. That’s a tough application because the tolerance is tight and the bore is long relative to the diameter. That forces you to grind with a tiny wheel on a long quill, an inherently unstable configuration, making it very difficult to avoid tapering.
Meister’s Vice President and General Manager, Bruce Northrup, explains that the three “go to” options are problematic: “You could dress more aggressively to produce a sharper wheel, but that would probably result in a rougher surface finish, so you’d end up slowing the grind. Or you could use a softer wheel to get a better finish and taper, but the wheel would break down more rapidly. Or you could use a very fine mesh abrasive wheel. You’d get the desired finish and form, but fine grit wheels with conventional bonding have not been able to handle the advanced nozzle materials at full production rates without burning or loading.”
10 µm Grains in a 500 µm Matrix
Meister’s solution is a new vitrified product called 3D Abrasives Technology that incorporates diamond or CBN grits in the “most open, self-sharpening bonding matrices ever created.” Before 3D, Meister says, a grinding wheel manufacturer had to reduce a wheel’s porosity (its openness) when they used smaller grain sizes. So finer finishing wheels were necessarily less porous. With 3D, Meister can bond grains as small as 10 µm in a matrix with pores as large as 500 µm.
The goal, explains Northrup, is to “minimize the interaction of the bond” (as opposed to the abrasive) “with the workpiece. The less the bond rubs the workpiece, the less heat you’re generating and the less force you’re generating, which in turn means less deflection of the quill.” At the same time, Meister says they’ve figured out how to retain the exposed grains in the bond so the wheel stays sharp longer than you might expect.
To prove the benefits of their new technology Meister did controlled tests comparing their standard vitrified CBN ID wheel (a state-of-the-art industry benchmark) with a 3D wheel of identical CBN type, grit size, concentration, wheel hardness, and shank. They used the same machine and grinding process, including dressing parameters. The parts ground with the 3D wheel were seven times better in taper accuracy (0.22 µm vs. 1.44 µm) and seven times better in cylindricity (0.11 µm vs. 0.74 µm ), with nearly identical surface finishes.
It’s also important to note that Meister hasn’t yet experimented with 3D using different grinding parameters, trying (for example) to increase feed rates to see if they can gain cycle time improvements at acceptable taper or surface finish costs. But it’s clear that 3D Abrasives Technology will push current best practices in various applications in which hard materials must be ground with micro-precision accuracy and fine finishes.
Blasting Through Cranks at Nearly 20,000 sfm
Sticking with vitrified CBN wheels, Radiac (Oswego, IL) offers new bonds and wheel bodies in their Genis line. Marty Prokipchak, business development manager, says the Genis features a “higher strength bond, with a specific porosity that carries coolant into the grinding zone, optimizing wetting. This yields higher thermal resistance and profile retention, or form holding, to be able to get more grinds between dresses, extending the life of the wheel. A Genis wheel can also run up to 100 m/sec.”
The Genis targets cylindrical grinding in the automotive industry, a key application being camshaft grinding and crankshaft grinding. One customer testing the wheel grinding 52 Rockwell camshafts on a Schaudt Zeus found that the Genis increased life over a standard vitrified CBN product by 60%. In a test on a Junker machine grinding a 42CrMo4 crankshaft (54–58 Rockwell), the 700-mm diameter Genis running at 100 m/s (19,685 sfm) produced 5900 shafts versus 4200 (a 40% increase).
Radiac has also introduced a variant with a carbon fiber core (Genis CF) to decrease weight up to 80%. A lighter wheel is easier to mount of course, and less taxing on the machine’s spindle. Perhaps more importantly, Prokipchak says the carbon fiber core also dampens any vibration, reducing harmonics and potential chatter, thereby improving wheel life and part quality. A backing layer to the abrasive allows the Genis CF core to be reused up to three times to reduce the cost.
Another interesting option for such applications is the new Noritake (Mason, OH) vitrified CBN Mega-Life wheel. Also claiming a “high strength” bond and longer dressing cycles (making it “long-life”), the wheel has demonstrated impressive results in cam profile grinding, with material removal rates up to 282 mm3/mm of wheel width per second in FC250 chilled cast iron using a 350-mm diameter wheel running at 80 m/s. In a contour grind of SCM435 (48 Rockwell), a 380-mm diameter Mega-Life wheel had a stock removal rate of 183 mm3/s across the width of the wheel (traverse grinding) at a wheel speed of 120 m/s. That was a 3.5× improvement in the G-ratio versus the previous wheel.
Rough Grinding SIC Wafers with “Swiss Cheese”
Asahi Diamond America (West Chester, OH) also has a new porous bond for better chip evacuation. As its name suggests, AeroMetal features a metal bond for excellent grit retention and wear resistance, but it’s designed to be open. As Senior Sales Engineer Mark Smith says “the bond bridges are almost wiry looking, leaving huge voids. I call it Swiss cheese. It’s almost like milling rather than grinding.” And Asahi can adjust the porosity for the application.
The wheel can be delivered with either diamond or CBN abrasives in grain sizes from 40 to 1000 µm. AeroMetal diamond wheels target carbide, ceramic, sapphire, quartz, and similar applications, while the CBN variant is best used for rough grinding of hardened steel and cast iron.
In one test, an AeroMetal axial face grinding diamond wheel (30-mm diameter, 3-mm bond width, 80-µm grit), ground a circular pocket (60-mm diameter × 0.7-mm deep) into a block of polycrystalline silicon carbide (SiC). The wheel rotated at 2500 rpm, moving helically at a 600 mm/min feed rate, under coolant. At a depth of cut of 0.01 mm per helical pass the wheel achieved a surface finish of 1.4 µm with wheel wear of 0.06 mm. At a 0.03-mm depth of cut the surface finish was 2.2 µm with 0.08 mm of wheel wear.
A Resin Bonded Wheel You Can Bend
Asahi has also just introduced Bright Star, an elastic resin bond wheel for super finishing and polishing. The more porous BRS5 version is so elastic you can bend the wheel in your hands, though you’ll want to keep it on your grinder where it can achieve a mirror-like finish in carbide and other hard materials.
In one test on carbide with a surface grinder, a 175-m diameter, 1000 grit, 1A1 Bright Star wheel operating at 1540 rpm at a feed rate of 2 m/min and 0.001 mm/pass achieved an Ra of 0.012 µm and Rmax of 0.10 µm, versus 0.08 µm and 0.17 µm, respectively, for a standard resin bond wheel with the same specs. Running the same test on ceramic (Al2O3), the Bright Star achieved an Ra of 0.03 µm and Rmax of 0.41 µm versus 0.1 μm and 0.87 μm, respectively, for a standard resin bond. In another test, a Bright Star 1500 grit wheel ground carbide inserts with a surface finish of Ra 0.01 μm and Rmax of 0.01 μm.
Asahi says the wheel should be perfect for applications like semiconductors, glass, optics, carbide tools, and medical devices. Bright Star can be shaped and trued like any resin bond wheel, so that would add to the possibilities. Smith says his first success has been with D2 steel in a mold shop, where the Bright Star wheel replaced labor intensive hand polishing using loose abrasives.
Dressing with 2 µm accuracy
In-process wheel dressing or truing necessarily changes the profile of the wheel. And unless you’re using a flat wheel and can rely on an acoustic sensor or probe to locate the precise location of the wheel in space, how do you know exactly what you’re doing to your wheel profile when you dress? The solution is to dress with a very precise roll, with a certified diameter and radius, and to ask your machine control to compensate for dressing cycles based on those values. But how “precise” is “precise?” Some automotive and aerospace bearing races, high-precision gears, fuel injection armatures and needles, and other components, have form tolerances as tight as 2 µm.
That’s why Meister has made major investments to bring their micro-precision cDD Diamond Dressers to a certified radius form accuracy of two microns, (±1 µm). Northrup says that’s an “order of magnitude improvement in the accuracy and precision for these unique dressers.”
Another advantage to Meister’s cDD dressers is that they excel both at truing and sharpening CBN wheels, thanks to a unique structure in which high-quality CVD diamond inserts are strategically embedded within Meister’s hDD porous metal-ceramic hybrid bond. Northrup says this combination gives the roll “the wear characteristics of a metal bond with the porosity of a vitrified wheel. That’s important because the need to provide porosity for chip removal and coolant flow is just as important in dressing as it is in part grinding. At the same time, the CVD inserts hold the edge. In effect, the inserts true your wheel and knock the dull abrasives out, and the diamond grains come behind and tickle away the bonding, opening the wheel.
“We find that if you dress your diamond or CBN wheel with a porous dressing tool, you get a sharper wheel. So not only are you truing the wheel, you’re leaving it in a much better condition to cut. That makes the wheel last longer, you can improve your cycle time, a whole host of advantages. A typical CVD dresser has a steel or brass body and all that does is rub your wheel.” Furthermore, he says, “these are the only dressers in the world that offer both porosity and this level of accuracy.”
“Conventional” Wheels Still Have a Place
So called “conventional” AlO abrasives are still widely used, because they’re not only much less expensive than superabrasive wheels, they are more capable than they’ve ever been. For example, Radiac’s CSS Ultra uses a ceramic AlO grain in a higher strength vitrified bond for a wide range of cylindrical OD and thread grinding applications. For example, Prokipchak says they’ve observed a 50% improvement over traditional AlO wheels in grinding a 58–62 Rockwell crankshaft main bearing. Another case study shows thread grinding on taps (in M6 tool steel) using a Reishauer machine. The CSS Ultra wheel reduced dressing by 30% and achieved a 15% savings in cycle time.”
Prokipchak also makes the point that because superabrasives are so much more expensive than AlO wheels, you “need to push the wheel to the max to get a satisfactory payback, and that requires a very robust machine.”
“A 16 × 1 × 5″ [405 × 25 × 125 mm] Strato Ultra wheel AlO might cost about $110. A 16 × 1 × 5 vitrified CBN wheel would be up at around $8000–$8500. Maybe you’d use a smaller CBN wheel for the same job, but you get the idea. If you have a high-volume part and you can dedicate machine time to that one setup, vitrified CBN is the way to go. But if you have to switch between numerous parts with various forms on that machine, vit CBN is probably not the optimum solution because you’d need a different vit CBN wheel for each shape you need to grind. It’s not cost effective to change the form in a vit CBN wheel.”
This article was first published in the November 2016 edition of Manufacturing Engineering magazine.