Combinations of honing process equipment and tooling create highly finished surfaces
By Michael Tolinski
Honing could be considered an unfairly neglected material-removal technology, when you consider its importance in allowing millions of automobile engines to run cleanly, quietly, and efficiently. The widespread, practical use of internal combustion engines, both large and small, would essentially be impossible without carefully sized and surfaced cylinder bores and other components.
"There are three main reasons why you would hone," says Dennis Westhoff, global business development manager and former engineer and technical center head for Sunnen Products Company (St. Louis). First, there's size-control of the bore through the use of abrasive media for material removal, and then there’s the finessing of the bore's geometry. "Cylindricity, roundness, straightness, and taper all come into play with the geometry."
Surface finish is the third reason why honing is performed, says Westhoff. In a cylinder bore, the surface finish, typically in a crosshatch pattern, maximizes the lubricating effect of engine oil. "The combination of precise size, geometry, and surface finish minimizes blow-by at the ring/cylinder wall interface," and improves combustion efficiency. This facet of honing is particularly important in a world where emissions standards will only become stricter for automobiles and lawnmowers alike. And overall, honing improves the performance of "any device where a piston moves inside a cylinder or a part rotates on a shaft."
Therefore, to label honing a "material-removal process" may be oversimplifying. "That's kind of a misconception a lot of people have about honing," comments Westhoff. He says honing can be used to remove large amounts of material in some cases, as with oil-field parts where bores can be 40' (12-m) long, and honing is used to remove as much as one inch (25 mm) of material from a 14" (356-mm) ID. In mainstream engine applications, however, stock removal is only on the order of 0.005" (0.13 mm) for each honing step (usually two or more). "For a high-production situation, in order for the honing operation to be optimized, you want to take out just enough material to correct the bore geometry and create the proper surface finish." The trick is finding the right tooling and system.
Various honing methods suit various surface-finish needs, and they're generally split into "conventional honing" and "single-pass honing." Conventional honing rotates tools with abrasive-media inserts pressured against a bore surface to finish it in repeated strokes. Single-pass honing typically passes multiple diamond-surfaced tools of increasing diameter through a bore (in a single stroke for each tool), until the desired quality and diameter are achieved.
Single-pass bore finishing has many advantages over conventional honing, especially when finishing parts in large production batches, according to Robert Marvin of Engis Corp. (Wheeling, IL). "Conventional honing utilizes abrasive stones that must expand and contract during each cycle while the tool or part is continually reciprocated." He argues that single-pass tools, by being fixed in size, offer superior size control and repeatability.
"In regards to geometry, both techniques can produce submicron results if properly set." He says single-pass bore finishing can hold cylindricity (defined as roundness, straightness, and taper combined) to better than 0.0004 mm (under 0.5 µm). And because each tool passes through the bore only once, and tool wear rates are low, "production rates are almost always higher than conventional honing."
Single-pass honing works well for most medium-to-high production cases, but Marvin admits conventional honing offers more flexibility in low-production cases: "If someone needs to finish many sizes in batches of less than 100 pieces, single-pass tooling would be costrestrictive." Plus, single-pass tooling has difficulty finishing cylinder bores that require a specific crosshatch finish for oil distribution. "The typical finish pattern with the single-pass process is a faint swirl rather than a crosshatch." He adds, however, that new single-pass processes and tooling are making more crosshatch-like patterns possible.
Except for cases that require a true crosshatch finish, "there are certain applications when single-pass works very well," agrees Dennis Westhoff of Sunnen (which specializes in both single and multipass honing). But single-pass tools have one layer of abrasive diamond particles, and their sharp cutting points wear, eventually doing more "plowing and pushing" of material rather than cutting. "So the surface condition at that point isn't as acceptable or as cleanly cut as you have in a traditional [conventional] honing process, where the abrasive breaks down during the process and continually renews itself—a self-sharpening process."
Conventional honing machines are becoming more friendly for high production, adds Westhoff. Robots for part loading and unloading, for example, are critical when the operator can't keep up with multispindle machines. And integrated in-process air-gaging at each spindle allows the machine to adjust itself during honing. Because honing abrasives gradually wear, "you need some mechanism to measure that wear, to bring that into control. Air-gaging gives you the ability to accept feedback and automatically compensate for that wear as it occurs during the cycle."
New machines also allow the "ultimate control of crosshatch." Older conventional honing machines don't provide a consistent crosshatch pattern angle in the bore throughout the machine's entire stroke, but this can be corrected via servo control of the spindle. This feature is offered on Sunnen's recently released SV-500 vertical CNC honing system, an all-electric machine that provides three-axis servo control of spindle rotation, stroke, and tool feed. The synchronized servo axes allow the operator to set the desired crosshatch angle through Windows-based control software, to eliminate the "flattening" of the crosshatch angle that can happen at stroke-reversal points.
Production honing often requires flexibility as well, regardless of tooling or method, notes Chris Sauer of Nagel Precision (Ann Arbor, MI). Companies may desire to hone different models of an engine family with minimal or no changeover, and some lines "are even designed to hone parts in batch lots of one," Sauer says. "Smaller, flexible machines can also be designed to accept virtually any engine block, which can be retooled for in a matter of hours," compared to the days or months of retooling time a transfer line system might require.
Automation can be a significant cost-investment factor for flexible honing systems, although, Sauer points out, "any transfer-type production line already has a significant amount of automation to move the parts from one machine to the other." Flexible systems typically employ an overhead gantry with pick-and-place loading, robotics, or a combination of both. And "if the customer is looking to reduce retooling costs in the future, gantry or robotic automation requires minimal reinvestment for another part type." By contrast: "A dedicated, roller-type conveyor, common in transfer-line applications, would most likely have to be totally replaced, taking up considerable time at significant cost."
Various types of high-volume applications can benefit from a flexible honing system. Pinion gear bore honing has become more sophisticated. "Transmission design has necessitated smoother, bearing-quality pinion bore surface finishes," says Sauer. Here, a Nagel Model 3 HS6-30 machine is used for removing approximately 0.080 mm of ID material in three honing passes. "The pinion honing machine is designed with palletized fixtures, which are readily exchangeable for different gear types," allowing some companies to hone as many as six different gear designs on the same machine.
For shops that don't have or can't justify dedicated honing machines, another flexible option is to hone with existing machining centers and lathes, says David Chobany, vice president of Bates Technologies (Indianapolis). The company's honing products and tools are adapted for use on common CNC machine tools, helping shops maximize the usage of their equipment and floor space. "For certain applications," he says, "the manufacturer can utilize existing or upgraded machinery by adding the honing operation without the capital expenditure required for a dedicated machine."
One approach is to "close the loop" between boring and honing in the same machine, using a boring tool, a honing tool with a standard toolholder adapter, and inprocess diameter-feedback air gaging. Chobany says this approach has been useful for diesel and small engine cylinders, air-compressor bodies, dies and molds, and long tubes and liners for the gas and oil industry.
To accommodate honing, certain capabilities are required in existing machines, explains Chobany. In general, a machine requires through-the-spindle coolant delivery, which is used to expand the honing abrasives against the part wall with constant coolant pressure, typically between 120 and 150 psi (827–1.03 MPa). Also, additional M-codes may be required on some machines to control the coolant and airgaging, and a stroke-control program is needed to control the coordinate location and number of strokes. "On some older-model CNC machine tools, the servodrive controlling the tool strokes may need to be upgraded," he adds.
The newest developments in honing address high-tech interests of both honing shops and their OEM customers. One thrust in honing development is simply to take steps—and costs—out of the boring/honing process, says Michael Schaefer, manager of process development for Gehring L.P. (Farmington Hills, MI). He points to a few recent innovations in honing that cut costs and/or increase engine performance.
- One example is "position honing," a tooling/machine-drive system developed in Europe over the last couple of years. By employing positioning honing, "we can replace finish boring and rough honing with one process," says Schaefer. Position honing reportedly can remove as much as 1 mm of material in 25–30 sec, while reducing overall abrasive costs about 30%, reducing tool rework costs, and eliminating one spindle step and one inspection step in processing. Moreover, asserts Schaefer: "By combining finish boring and rough honing, we get better quality and more consistency." He says it's of particular interest to small-engine manufacturers, including a plant in China where position honing will see its first production use this year.
- Another development is the company's "Life Hone" system, which speeds up material removal by moving the workpiece up and down rather than the hone spindle. For honing relatively small parts such as bushings, the system uses a linear motor to index the part at over 60 m/min, rather than the maximum 30 m/min of a typical honing tool stroke. It also uses higher tool rpm of 7000–8000, compared to standard honing's 2000 rpm or less; this results in better straightness and roundness of the bore, which are functions of speed, says Schaefer. Moreover, the honing is driven by linear or ballscrew drives rather than conventional hydraulics. "Everyone wants to get rid of conventional hydaulics because of noise, maintenance, and speed limitations," comments Schaefer. System machine-makers in Europe and a new diesel engine manufacturer in North America (which could not be named at this time) have adopted this manufacturing technology, he adds.
- Thermal spray honing is a process that addresses two different interests of the engine industry: small, lightweight aluminum engines, and the remanufacturing of worn-down engine cylinders. Based on patents the company acquired for a technology originally developed by General Motors, Gehring says the process replaces cast-iron cylinder sleeves with a 100–150 µm layer of low-carbon steel sprayed onto the cylinder inner surface, which is then finish-honed. The sprayed material possesses high density and low-porosity, and is actually oxidized in the process of being sprayed at mach-3 velocities, says Schaefer. And in comparison with cast-iron sleeves, the thermal spray process typically imposes much less utility and labor costs per cylinder.
- Finally, Schaefer points to a lesser-known honing process in North America which has been used since 2002 in over a million gasoline and diesel engines worldwide: laser structuring. Situated in the standard honing process between rough honing and finish honing, laser structuring uses a laser incorporated in a conventional honing spindle to burn microscopic pockets inside the engine cylinder.
Schaefer describes laser structuring as a technology that allows engine builders to further improve engine oil effectiveness. Creating a kind of enhanced crosshatching, the process produces a homogeneous "engineered surface structure" that reduces friction in the cylinder wall. This structure cuts emissions, and enhances gas mileage by over 6% when the entire cylinder surface is treated (according to tests cited by Gehring). The process can also be used for bushings and piston rings—"or wherever we need to optimize the lubrication and surface conditions," says Schaefer.
Material Matters with Honing
"Quite a bit of science goes into formulating abrasives to meet surface-finish requirements, depending on what type of material you're honing, bore geometry—thin or thick-walled processes, and material hardness," says Mike Murphy, Sunnen product manager for tooling and abrasives. Thus Sunnen and other abrasive suppliers offer multiple families of abrasives for honing an ever-growing number of workpiece materials, including new alloys and ceramics.
Honing abrasives include silicon carbide, aluminum oxide, diamond, and cubic boron nitride (CBN). David Chobany of Bates Technologies points out that in CNC honing environments, diamond or CBN "superabrasives" are the primary and preferred abrasives—especially when machining exotic or hard-to-machine materials such as Inconel, ceramics, and CGI (compacted graphite iron), and when finishing parts designed with interrupted cuts, tandem bores, or thin walls.
Nagel Precision's Chris Sauer illustrates the difficulties of these hardmachining materials. In comparing CGI to gray cast iron, for instance: "CGI requires heavier-duty honing spindles and stiffer mechanical expansion systems for the hone process."
And Sauer compares honing gray cast iron sleeves for typical aluminum engine blocks to honing liners made from hypereutectic Al-Si alloys used in high-end European production engines. Gray iron can be honed using standard practices (such as three-pass honing with superabrasives), "but the Al-Si liner requires very specialized ceramicbonded abrasives and unique hone tooling designed to expose the silicon particles found in the aluminum substrate on the cylinder running surface."
This article was first published in the June 2008 edition of Manufacturing Engineering magazine.