A Solution for Gear Bore Accuracy

A new generation of machines from Sunnen has become the "ten millionths" solution for bore accuracy, providing OEMs with increased product performance.

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Did you hear about the amazing new technology that enables gear makers to achieve bore accuracies measured in ten-millionths and even “fix” bore geometries and achieve high process capabilities (Cpk), all in automated, high-throughput production? To tell the truth, this mystery technology isn’t really new. Conventional honing has been hiding in plain sight for about 70 years, quietly and continuously being improved, refined, and automated to make it accessible and capable for 21st century part requirements.

Like most in industry you may have misconceptions about it as a manual process, but today’s honing is increasingly an automated, multi-spindle process. This new generation of machines has become the “ten millionths” solution for bore accuracy, enabling OEMs to meet a host of increased product performance requirements for lower noise, longer life, less vibration, and reduced tolerances. Overlaying the need for precision is the requirement for high process capability, an area where servo-controlled honing shines.

When holes produced satisfactorily on lathes or with other processes like I.D. grinding suddenly have to meet a process capability of 1.67 or 2.0 Cpk, these operations may fall short. That kind of capability requires a process that’s easy to “dial in” with high precision, and very stable once it’s established. A computer-controlled hone can easily get within  0.1 µm ( 0.000004 inches) of a specified size, and with the resolution on the tool-feed systems of today’s machines, the variability is small.

CUT CYCLE TIME, ABRASIVE USE
These new honing machines become even more versatile with a patented multi-feed system for tool control. The new multi-feed technology gives users a choice of tool-feed modes to achieve the shortest cycle times, lowest part cost,  longest abrasive life and best bore geometry possible. Multi-feed combines a new controlled-force tool-feed with a proven controlled-rate feed system. The two different tool-feed modes allow the user to optimize the process based on the workpiece geometry, material, and tool type/size.

Controlled-force honing works like cruise control to ensure the optimum cutting load on the honing abrasive throughout a cycle. Depending on the application, controlled-force honing cuts cycle times by as much as 50 percent, lengthens abrasive life for lower consumable cost and allows finer control of surface finish parameters than ever before possible. Controlled-force technology eliminates glazing of the abrasive due to too little force and maintains a steady, free-cutting, self-dressing condition for maximum metal removal in the shortest possible cycle time. Figure 1

During the development of controlled-force honing it was noted that more-durable abrasives could often be used, resulting in more parts per set of abrasives and therefore a lower cost per part.  Small variations in incoming parts caused by upstream machining, heat treating, or plating can throw a stable honing process off balance. Controlled-force honing overcomes this by always maintaining optimum feed force on the honing abrasive, eliminating wasteful “air cutting,” glazing, or tool damage. The beauty is that if conditions allow—with a batch of parts requiring less stock removal, for example—the honing cycle will be shortened significantly and automatically. Controlled-force’s ability to control the cutting load within a very fine range also allows much tighter control of final surface finish parameters.

Controlled-force honing is an enhancement of the controlled-rate feed system for tool wear compensation, which is already capable of adjusting tool size in increments as fine as 0.1 µm (0.000004 inches). Controlled-rate tool feeding is typically used with plated-diamond CGT honing tools, which use a sleeve of abrasive for full contact with a bore surface. CGT tools are frequently used on cast iron and powder metal gears, segmented bores, or parts with multiple lands, ports, keyways, or crossholes in the bore.

FAST SETUP, HIGH PRECISION
New tooling recently introduced marries the precision performance of expensive custom tools with the low cost and quick-change flexibility needed for mid-volume production runs. Using a standard collet connection, the new KRQ honing mandrel requires no indicating for parallel or conical runout, speeding setup and job changeover. Figure 2 Its low cost and fast setup capability enable shops to inventory a wide range of tool sizes for efficient processing of short production runs, while its ability to work with the new controlled-force honing technology enables shops to obtain the ultra-precise bore-size control that typically requires custom tools. The KRQ tool is designed for optimum performance with vertical honing systems, but it can also be used with horizontal machines for dramatic reductions in setup time. It is available in diameters of 5.0 to 19 mm, in size increments of 0.10 mm. The new mandrel works with diamond and CBN superabrasives and utilizes long-lived carbide shoes.

“GREEN” HONING OIL
The market will also see the introduction of “green” planet-friendly industrial consumables with the introduction of a long-lived general-purpose honing fluid based on a renewable vegetable formulation that makes sustainable metalworking affordable and practical. Offering a breakthrough in oxidation stability, this new oil is formulated without additives that many find objectionable such as sulfur, chorine, and fluorine. It is an ideal solution in honing applications where additives cannot be used, such as nuclear and some aerospace parts, or where company policies discourage use of fluids with such additives. This new vegetable formulation is acceptable in all parts of the world, and to all religions and cultures. It is suitable for use with conventional vitrified honing abrasives and metal-bond superabrasives. Figure 3

ADVANTAGES OF HONING
Conventional honing is unique in its ability to consistently produce a specific surface finish with a selectable crosshatch pattern. Makers of gears, hydraulic valves, and fuel injectors use a wide range of surface finish parameters—Rpk, Rk, Rvk, MR1, MR2, Ra, Rsk, Rz, R3z, etc.—to improve the performance of their products. Honing before the hobbing operation, for example, improves a gear’s centerline-to-tooth accuracy, resulting in quieter, longer-lasting drives. The opposing helical patterns of the crosshatch also prevent gears from “walking” and needle bearings binding on a shaft, while controlling the lubricating film of oil between mating surfaces. Unlike the honing process, “single point” unidirectional rotary machining processes leave a faint “threaded” finish, which can lead to lubricating films being pushed out of the bore. Figure 4

Today’s computer-controlled honing systems can correct a multitude of errors in bore geometry, too, such as barrel, taper, and centerline bow while optimizing lubrication with the crosshatch finish. Machine controls, software, and servo systems now take the “art” out of honing that’s always been associated with the manual process. Even the crosshatch angle can now be dialed in at the control and then held constant from top to bottom in the bore, thanks to servo control of spindle rotation and stroking motion. An air gage-equipped machine can sense a misshaped bore such as barreled or tapered, and the machine will automatically correct the part. Integrated with other servo accessories such as pick-and-place or robot-handling systems, a single machine can now process a million or more parts per year running unattended.

Automotive, marine, and air-race winners since the 1930s have depended on the performance edge they get from precisely honed cylinder bores. Now computer-controlled and automated, that same technology is ready to give hundreds of other products a performance advantage as product designers strive to get more from every component.