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A cutaway view of the Single Stroke Honing process with the tool inside the bore of a cast iron valve body

High-Precision, Single-Stroke Honing

April 28, 2014
Process achieves cylindricity/straightness under 2 microns in bores up to 10 times longer than diameter 200% improved precision Single-pass process Three vertical machine options 724 mm stroke, up to 105 mm/sec

SUNNEN PRODUCTS COMPANY introduced an improved process for finishing cast-iron hydraulic valve bodies, achieving cylindricity/straightness under 2 microns in bores up to 10 times longer than diameter. Developed in Europe to meet leak-free requirements for high-pressure hydraulic systems, the developer stated its Single Stroke Honing process is almost 200% more accurate than anything previously achievable for long, small-diameter, tandem bores.

"European mobile equipment hydraulic systems established this cylindricity specification to reduce internal leaks and achieve higher valve performance," according to Juerg Huber, managing director, Sunnen AG. "The precise fit between the sliding spool and valve body maintains the internal seal in this area. Precision cylindricity and straightness in this bore ensures uniform clearance between the moving parts from top to bottom in the valve, allowing free movement of the spool, without leakage around it."

Single Stroke Honing is a single-pass process that sizes and finishes a bore with a series of progressively larger superabrasive tools adjusted to a pre-set diameter. Single Stroke Honing tools rotate while passing through the bore one time, and then withdrawing. The machine's servo-controlled stroking system provides flexibility with adjustable speed and various feed profiles, such as pecking, short stroke, dwell, etc.

Column feed and spindle speed can be varied throughout a cycle using the machine control.

The Single Stroke Honing tool consists of a tapered arbor with an expandable diamond-plated abrasive sleeve mated to it. The external profile of the sleeve is designed for the application. Single-pass honing is ideally suited for solid bore parts with L/D (length/diameter) of 1:1 or less. Parts with much larger length to bore ratio can be honed under certain circumstances. Bores that have interruptions allow better chip flushing and reduce the load on the abrasive sleeve.  In the case of cast iron valve bodies, bore length-to-diameter ratios of 10 or greater can be single-pass honed with great success.

"Single-pass honing is a stable, high-production process that can achieve one micron cylindricity in a short bore – one with L/D less than one – assuming the part has a sufficiently rigid wall," explained Huber. "However, as the bore length increases, it becomes more difficult to achieve good straightness and cylindricity. Among other reasons, the honing tool's sizing land tends to follow the bore. On cast iron valve bodies with L/D of 10, the standard process has typically produced five microns cylindricity under optimum conditions. High-Precision Single Stroke honing takes this to two microns cylindricity or less. And with a tool life that can be 60,000 to 80,000 parts, the process is economical. A typical part suitable for this process might have a 16 mm bore, over 160 mm long, with 11 lands. This type of part can now be honed to less than two microns cylindricity, less than two microns straightness and one micron roundness."

The Single Stroke process uses a combination of proprietary tool processing, tool holding, workpiece fixturing and process parameters, without any penalty on cycle time.

Progressive Sizing, Finishing

The Sunnen VSS-2 Single Stroke Honing® machine, shown with the enclosure open.

Sunnen offers Single Stroke Honing on three different VSS-2 Series models that incorporate up to six spindles to progressively size and finish part bores. The machines are recommended for sizing of bores 3.9 - 50 mm (0.149 - 2.0 in.) diameter in stampings, hydraulic valve bodies, gears and sprockets, parking pawls, rocker arms, turbocharger housings and similar parts. Ideal materials include cast iron, powdered metals, ceramic, glass, graphite and other free-cutting materials.

The VSS-2 has the most accurate spindle alignment in the industry, combined with flexible, easy-to-use controls. Spindles are factory aligned independently for precision centering over the tooling plate to produce better bore geometry than possible when using an "average" alignment for all the spindles.

The VSS-2 uses a spindle carriage with a 724 mm (28.5 in.) stroke and up to 105 mm/sec (250 ipm) stroking speed for increased flexibility with a wide variety of parts and tooling combinations. The servo-powered stroke system provides process flexibility and is hand-wheel-controlled for quick setup.

The menu-driven touchscreen control allows the column feed and spindle speed to be easily varied throughout the cycle. Process flexibility is enhanced by the use of six available profiles, including pecking, short stroke and dwell, which are easily added to a setup. More than 100 setups can be stored. Additional I/O and memory are standard for integration with auxiliary equipment, such as robots or part loaders.

The VSS-2 Series 2 is available in three models – the 84 (eight-station, four-spindle), the 86 (eight-station, 6-spindle) and the 64 (six-station, four-spindle) – to meet various mid- to high- production needs. Spacing between spindles is 190 mm (7.48 in.) The 7.5 kW (10 hp) spindle drive provides a speed range of 100-2500 rpm.

Properly applied, Single Stroke Honing is a quick, cost-effective method to get a precise bore size, geometry and surface finish. Parts made of cast iron, powdered metals, ceramic, glass, graphite and other free cutting materials are ideal for the process. Single-pass bore sizing is also appropriate for splined bores or long L/D ratios if cross holes or other interruptions are present to allow chip flushing. Sunnen offers a range of single-pass plated-diamond tooling for the VSS-2 for precise, accurate and consistent bore sizing.