Machine Shop (Advanced Manufacturing Technology) Vol.7 Issue 1
By Ed Huntress
, Senior Editor
|New TurboHone® honing tools have stones mounted around their full periphery, allowing them to cut 3 - 6 times faster than conventional, single-stone tools|
Honing made a major breakthrough about a decade ago, when long-lived superabrasives were adopted for production honing. In one sudden advance, the process leaped from being a relatively fussy, labor-intensive finishing operation to a long-running, automated production process that could correct cylinder geometry and cut quickly enough to replace some intermediate boring and grinding operations.
Tools and machines advanced together to create the breakthrough. In the years since, there have been incremental advances in the process and the tools, but the important story recently has been the wider adoption of production honing around the world. Now another sharp advance, if a less dramatic one, is pushing production speeds to a higher level. This time it's a product of new tool designs, and an important refinement in tool configuration. More machine spindle power has accompanied the tooling advances to exploit their faster-cutting potential.
All of the important advances in production honing are related to the ability of the two superabrasives, cubic boron nitride (CBN) and diamond, to cut aggressively without intentional tool breakdown. Conventional abrasives require some wearing away of grains from their matrix, or of actual fracture of the grains, to keep the tool free of swarf buildup and to keep the grains sharp. The design of a particular stone, including its grit size and type and the binding matrix that holds it together, dictates a narrow range of forces that can be applied to the tool in cutting: enough to keep the grains fresh, but not so much that the stones wear too quickly. With superabrasives the tools can apply a wider range of force, and the interesting part of the curve is the high end, where these super-hard, very strong materials allow much more aggressive cutting.
Honing is usually thought of as a process for finishing precision hydraulic and pneumatic cylinders for machinery applications. But high-volume consumer applications are more practical now that it's a faster process -- such as this trumpet, the valves of which are being honed on a Sunnen ML-3500 with TurboHone® tooling
It's important to recognize that these new developments fall into one of two broad areas called "honing." Like other abrasive machining processes, there is some disagreement about where the process definition begins and where it ends. In this case we're not talking about the compliant type of honing that many of us identify with brake hones and glaze-breaking hones used for restoring automotive cylinders. Those traditional types of honing load the cutting stones against the work with spring action; they cut and refine the surface, but they basically follow the geometry that's already there. If you apply one of those hones to a rough, out-of-round cylinder, you get a nicely finished out-of-round cylinder, with little or no improvement in geometry.
The superabrasive tools fall into the other category, which some have called "hard" honing. There is little or no compliance of the tool or of the stones themselves to variations in bore geometry. The tool follows the direction of the bore that's already established by the previous cutting process, but there is a definite distance between one wall of the cylinder and the side or sides that are being cut. Poor application of this principle can still allow some departure from cylindricity, but, if the tool is designed for it, it forcefully tries to make a true cylinder out of whatever hole it's in. The newer tools are more effective at accomplishing this than some older designs.
|A three-spindle, eight-station Sunnen MHS equipped with three TurboHone® tools hones transmission gear bores, removing 0.016 in. of stock|
The principle has long been established, and, for decades, production honing tools for automotive and hydraulic-cylinder applications have employed it. The typical design has a steel body, one narrow stone that fits in a groove along the tool's length, and two guide shoes that bear against the sides of the cylinder facing away from the stone. With practically no compliance in the shoes, the stone is forced into the work at a fixed distance from them - in a triangular configuration, but one that imposes a cylindrical cutting path to the stone. Superabrasives made the process more aggressive, faster, and longer-lasting. The first generation of superabrasive honing tools simply replaced the softer, faster-wearing conventional stones with diamond or CBN grit, bound in a metal matrix. But the improvement was dramatic. Adjustments to the stone were much less frequent and the depths of cut were increased. With fewer adjustments, the process could be easily automated. And it was: in-process gaging provided a feedback to automatic, wedge-type adjustment. Production rates for a single stone ranged up into the thousands, even tens of thousands, of parts. Machine power increased to take advantage of the more aggressive cutting.
|The SingleStroke tools are simple in configuration: a tapered mandrel, a plated-grit sleeve, and a nut. The accuracy of the components is the key to accurate and reliable production honing|
Recently, Sunnen Hone, which pioneered several of the advances based on use of superabrasives, introduced an even more aggressive tool, called the TurboHone®. According to the company, it cuts three to six times faster than single-stone honing tools. Like the single-stone tools, it uses metal-matrix superabrasives. The TurboHone® is made in sizes of 0.150 in. to 1.250 in. diameter, with a suggestion that larger sizes can be had. It's based on a three-part design: the body, which is hollow and tubular; a tapered wedge, which can be adjusted manually or automatically, depending on application; and a set of stones. Up to six stones can be arranged on the periphery of the tool, which is the big departure from conventional designs. The tool doesn't bear against guide shoes. There are stones all around.
|With a subtle change in geometry, the new High-Helix tools (left) improve the swarf-clearing capability of the SingleStroke process. Unattended, high-speed honing requires efficient clearing of the tool|
It's designed for use with the company's ML 3500 honing machine, although it will work on Sunnen's MHS, EC, and KGM Series machines, as well as machines made by other manufacturers. The high productivity of the tool, plus the relatively high horsepower it can deliver to the cut, suggest that a powerful machine is needed to get the most out of it. And its automatic-adjustment feature begs for a machine that can take advantage of it, for long-run unattended machining. The body of the tool and the wedge are made to extremely precise tolerances, which makes the process possible, says Sunnen. However, it does have a fairly large range of adjustment: typically 0.030 in. to 0.050 in. on diameter. Sunnen wants to adjust the stones for wear and trueness itself, so it provides a service by which customers can return the whole tool for maintenance and a quick turnaround.
The use of full-periphery honing tools has a precedent. While the first applications of superabrasives were going on, another honing process, which pushes the definition of "honing" in another direction, also got a large boost from superabrasive materials. These are very hard tools indeed. They're full cylinders, with a coating of abrasive around their entire periphery. Sunnen introduced tools with diamond or CBN grit plated onto a steel cylinder sleeve, which allowed small adjustments by expanding the sleeve itself. These led to the development of "single-stroke" honing, a breakthrough in production efficiency: one pass, up to several thousandths of an inch of stock removal, and cutting of the full cylinder at one time. Again, the tools would cut thousands of parts before replacement, even though the single-layer of plated-on abrasive didn't allow for re-dressing.
|High-production SingleStroke honing machines often employ a dial-index configuration, with multiple honing spindles for sequential roughing and finishing operations|
Although these tools are not compliant in the radial direction, either the workpiece or the tool has to float on its axis, to follow the direction of the existing bore. This should clarify which part of the cylinder's geometry is being refined and which is not; the axis of the cylinder is left alone. Thus, production machines for honing small parts and large ones have some provision for floating. It isn't a simple case of reaming with abrasive tools.
Sunnen has made some refinements in these tools, and another recent introduction: their High-Helix mandrels, which feature a fast-pitch helical pattern in the abrasive, for clearing swarf more effectively. They're available for finished hole sizes ranging from roughly 0.120 in. to 3 in., in sleeve lengths of up to 11 in. These tools typically make a cut that removes 0.003 in. from a stainless cylinder; aggressive and deep-cutting, they need to remove cut material as quickly as possible to avoid loading the cutting grit with swarf.
The first point of interest in honing is still surface finish. With the opening up of applications for rough-honing, correcting geometry rather than finishing, production honing now is often a two-step process, in which a separate, finer hone does the final finishing. Production machines for these applications have multiple spindles that work in sequence. The smaller machines used with the full-cylinder hones, which are limited to smaller parts than the hones with separate stones, sometimes index parts around a rotary arrangement of honing spindles, like a dial-index chipmaking machine.
|Configurations of SingleStroke tools vary to meet the application. The common characteristics are superabrasive grit in an unyielding tool, which forcefully and aggressively removes stock while correcting bore geometry at the same time|
Overall, the objectives of honing are becoming more complex and refined, which superabrasive honing is well equipped to satisfy. Besides improving geometry, honing in many mechanical cylinder applications is expected to provide a pattern of grooves for retaining oil, and a flatted, or "plateau" surface for bearing. These two objectives are highly quantified today, with the availability of advanced surface-finish-measurement equipment and a wider set of parameters for specifying and measuring surface finish. In addition to Ra, or roughness average, honed surfaces are often specified in terms of Rk, or core roughness; Rvk, or valley depth; Rpk, or peak height; and a bearing-area specification, which can be of several types. A honing tool that holds its shape and sharpness contributes to holding such demanding specifications and repeating them. Superabrasive honing is now essential to many quality-oriented processes in automotive and other applications. Small marine engines, hydraulics, and even cylinders for the small engines that drive weed cutters and leaf blowers are now specified in multi-dimensional surface specifications of valleys and plateaus. The broader application of these complex surface specifications has been more prevalent in Europe than in the USA, but globalization of quality is driving it all around the world.
Superabrasive honing now dominates Sunnen's production-honing business, and other makers of honing machines and tools are competing in the same market, making their own versions of these high-productivity, high-accuracy, high-metal-removal systems. Automation is widespread, including loading and unloading, automatic tool-diameter compensation, and in-process gaging. All of it is made possible by superabrasive honing, which, as Sunnen's new tools illustrate, is still a work in progress.
While productivity provides the major force for development, the other capabilities of high-tech honing have not yet been fully mined, as one process under development at Sunnen indicates. With geometry under good control and with the ability to repeat precise dimensions through part runs counted into the thousands, the company is pursuing another step of refinement. They're honing automotive engine blocks with high-temperature coolant, which is being run through a heat generator that replicates the actual operating temperature of the block. Not content to make a cylinder that's true while sitting on a measuring machine at room temperature, they're seeking a cylinder that may be out of whack while sitting on a shelf, but that's true while measured at the distorting temperatures at which an engine actually operates. It probably will require a new tool.