Honing pays big for hydraulic valve leader HydraForce
Lincolnshire, IL - HydraForce, Inc., President and CEO Jim Brizzolara credits lessons learned on a Sunnen hone at his first job in 1966 in a hydraulic valve factory with helping him understand how precision bore finish and geometry can fine-tune the performance of a valve. Fast-forward to 2007, and Brizzolara has used that knowledge to help build the company he co-founded in 1985 into the world's leading supplier of cartridge valves and integrated circuit manifolds. With sales doubling in the last three years, HydraForce produces more than 7 million cartridge valves per year, releasing a new design to production almost every day.

Brizzolara considers honing one of the company's core competencies, using it to give HydraForce products a quality edge in a market where cartridge valves are often considered commodities. Superior quality at market prices is the chief reason HydraForce sales are up 20 percent in 2007, while the market as a whole is down. Another reason could be the company's five-year warranty. Brizzolara says honing is a key part of the technology equation behind the warranty, along with grinding, cleanliness, assembly methodology and a robust testing regimen.

HydraForce uses nearly 36 Sunnen conventional honing systems at its plants in the U.S. and U.K., where it has about 252,000 (23,225 square meters) of manufacturing space. The Sunnen machines produce the final size and geometry of valve cages and other critical parts to <0.00005-inch (0.00127 mm) accuracy, and customize the surface finish to specs tailored for various mating components used in the products. Brizzolara says precision size and surface finish on mating parts help eliminate leakage and ensure consistent performance on electrically actuated models down to just 65 percent of rated voltage, often encountered with low batteries on mobile equipment at the end of a shift. "Honing significantly reduces hysteresis, too," he adds. "By finishing both the upper and lower portions of the actuator, we hold the hysteresis to less than five to six percent."

When HydraForce was founded, Brizzolara says the partners made a strategic decision to differentiate their products with quality and delivery that were unheard of in the industry, using Deming methodologies and SPC. "We are one of a lucky few manufacturing companies in Lincolnshire, and every employee understands that if we want to do business from here, customers must have strong reasons to choose our product over a competitor's," he explains. Group-technology designs help control the number of unique parts, and a Kanban system that is closely coordinated with key single-source vendors in screw machining, heat treating, plating, etc., minimizes work in process. "Timeliness is one of our nine components of quality, because the customer can't experience the essential product quality until he has the parts in hand," he emphasizes.

Quality has been key to the company's ability to fund nearly all its own growth, while maintaining a targeted profit level. Quality of all incoming goods is audited to a defined Cpk level, and the company itself has QS-9000 and ISO 9001 registrations. "Our quality system has driven down our internal reject rate every year, so much so that we grew 42 percent one year in the nineties without adding staff in manufacturing," Brizzolara stresses. "Most people do not comprehend the huge impact improved quality has on a manufacturer's cost structure. The biggest real cost a company has is spending time and consuming assets to rework bad parts or make things that can't be shipped." In fact, quality-driven savings have helped the company maintain a very price-competitive position in the market.

HydraForce conducts many internal Kaizens every year, but concentrates on what goes on between processes, not on speeding up machines. This gets to the heart of Brizzolara's approach to honing, which differs in two distinct ways from most in the industry. HydraForce uses conventional honing, which takes a little more time, rather than single-pass honing, which is a quicker and less costly process. In addition, HydraForce performs two honing steps: typically a roughing operation on Sunnen ML-5000 Power Stroke (or earlier model) machines and a finishing operation on CGM and KGM 5000 Krossgrinding® machines. Cubic-boron-nitride abrasive is used for roughing. Finishing passes typically use diamond-plated tooling on the Krossgrinding machines, which can control hole size to accuracies of 0.000010 inch (0.00025 mm). If Rk and Rpk surface values are critical, aluminum oxide or silicon carbide abrasives may be used on the ML machine.

Although HydraForce uses stainless steel and other materials occasionally, most valve cages start as 12L14 or 12L15 steel that is carburized to about 60 Rc hardness after screw machining. "Any competitor can purchase screw machined components from suppliers just like ours, but what sets us apart is our knowledge of how the final fit and finish on mating parts can be optimized with honing," Brizzolara explains. "Conventional honing can really make a difference in how a product performs. It's not just the size control; it's the surface finish and crosshatch pattern you put on the bore of the cage. Our tight control of operations that produce the fit, finish and clearance of mating and sealing components results in more efficient hydraulic circuit. Most machinists are pleased with a bore tolerance of 0.002 inch (0.0508 mm), while we are holding less than 0.000050 inches (<0.00127 mm)."

Conventional honing gives a number of advantages for finishing valve cages, according to Brizzolara. The average L/D on a valve cage is 4-5:1, so a conventional honing mandrel - which contacts almost the full length of the bore while the part reciprocates - corrects any geometric error (straightness, cylindricity) from screw machining, or distortion from heat-treating or stress relief. A single-pass honing tool, on the other hand, is tapered, so the small area of the tool representing final size tends to follow path of the cage bore. "Single-pass honing produces a nice finish quickly, but the geometry of the part will tend to remain unchanged," Brizzolara explains. By determining the optimum point(s) at which to hone, HydraForce was able to reduce scrap rates due to geometric error on larger parts from 4-5 percent to 0.1 percent.

Conventional honing also produces a crosshatch pattern on the bore surface, while single-pass honing produces a helical pattern on the surface. "The crosshatch surface ensures a consistent full-length flow path for lubrication around the mating parts of the valve," Brizzolara says. "It's the same surfacing technology used in automotive cylinder bores, particularly in performance racing, and it definitely makes our product perform better at reduced current levels with DC power sources."

In addition to the crosshatch, HydraForce also measures and controls the surface roughness. "A superfine finish without crosshatch actually diminishes lubrication between mating parts, thus increasing friction," he explains. "We control Ra on our honing, and for certain parts will control Rk and Rpk (the mean height of the peaks protruding from the roughness core). We began to gain a better understanding of product performance by measuring Rk and Rpk. The valleys improve lubrication, but the peaks cause friction, which leads to sluggish operation." HydraForce customizes the surface roughness for optimum valve performance, based on the nature of the mating parts, i.e., O-ring, steel piston, etc.

Conventional honing also minimizes the need for deburring of the parts. Single-pass honing produces more burrs in the cross holes of the cage. "Single-pass honing mandrels tends to fold, tear and push more material, while conventional abrasive tools cut the material, though speeds, feeds and pressures need to be controlled to achieve the best result," Brizzolara says. "We document these parameters to develop best practices, stick to them, then try to improve them as we go on."

All of the honing machines at HydraForce have a two-fixture methodology for maximum in-cut time, allowing the operator to load one while the other is in the machine. Operators air gage every part after honing, and lot sizes run from a few hundred pieces to several thousand (honing is the only batch process in the plant due to cleaning requirements). Part diameters range from 0.097 inch (2.46 mm) to just over one inch (25.4 mm). Final product performance is 100 percent tested in one of 60 product-specific test cells.

Could Asian competition be a problem for this company? More likely it will be the reverse. HydraForce has been steadily building its sales through distributors in India and China for 20 years.

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