November 2007 Edition

HONING

Excellence Leads to Customers

Giving customers more than they need has given a gear maker a leading edge over its competitors

"Excellence Without Exception" is the philosophy of Forest City Gear, Roscoe, IL. It has served it well in acquiring and retaining business. The company's strategic directive is to always give the customer a higher level of quality than specified, either in customer interactions or part creation.

"The goal is to do this without adding cost or time to a job," Fred Young, company president, said. "We do everything we can to distinguish our product from competitors', and we try to do it inexpensively."

For bore-type gears, the company found automated honing gave customers tighter control of bore size, roundness, straightness, and finish. Forest City found customers noticed the difference in smoother, quieter, more efficient drives. This attention to quality led to repeat customers.

Gearing-up Production

Forest City's principle products are fine-and medium-pitch custom gears, such as internal, spline, sprocket, helical, spur and worms/worm gears. The company works to quality levels as high as AGMA 15 [DiN 2-3]. Part runs spur, from one-offs to several hundred thousand. Maximum O.D. on most parts is 20", except for worms at 5" and worm gears at 16".

Typical materials include 12L14, 1215, 4140, 8620, 9310, and various stainless grades, as well as aluminum, bronze, brass, Inconel, Hastelloy, titanium, plastics, wood fiber, and powdered metal.

About 30 percent of the company's work is aerospace-related, five to 10 percent medical, five percent military, and the rest in industrial or instrument work. Customers include Boeing, Airbus, Cessna, and Beechcraft. The gears are used in the space shuttle, the space station, Martian rover vehicles, and the Abrams tank.

According to Young, many of these applications demand high process capability where honing provides critical advantages in control and consistency.

The company used honing since it started. It tried hard turning, but found it more difficult to control quality, especially for microfinishes.

"ID grinding works for gears with larger bores – greater than 0.75" – and low L/D ratios of 0.5:1, but our work includes smaller diameters and relatively deep bores," Young said. "When we reach an L/D of 2:1, honing has the advantage in speed of material removal. At more than 5:1 we might start to see deflection on a grinding spindle, exacerbating taper issues. We still outsource parts for ID grinding, such as those with a blind hole where a counterbore leaves no relief for a honing tool."

Honing vs. Grinding

Precision ID grinding machines are more expensive than a hone of comparable capability, according to Young. Grinder accuracy depends on the machine's positioning capability. For a hone, accuracy is more dependent on tooling.

With a grinder, periodic checks, calibration, and refurbishing are needed to ensure tight positioning tolerances.

Honing tools are simple and rigid. When they wear, they are replaced. Unlike a grinding wheel on the end of an arbor, the honing tool isn't subject to bending forces. The tradeoff is that honing tools have limited diameter ranges, so a shop must stock more honing tools than grinding wheels.

"If a part comes off a hone just a little too small, you can re-run it, while that's almost impossible with ID grinding," Young said. "Turning and grinding can't produce honing's characteristic crosshatch pattern on the bore surface. That's a desirable feature for maintaining an oil film for rotating gears."

When working to high Cpk requirements, Forest City Gear finds the high resolution of the tool feed system and consistent nature of honing give it an advantage in holding a dimensional sweet spot. A high Cpk requirement means the band of variability around target values must be reduced.

"When the target is 1.33 Cpk, we shoot for about 60 percent of the print tolerance; at 1.67 Cpk, the target is 40 percent of tolerance," Young said. "A tolerance of five tenths on bore size, thus, shrinks to three tenths or less when six sigma quality requirements are imposed. Various holemaking processes, such as boring, drilling and reaming are capable of holding good tolerances, but when a high Cpk requirement is imposed, honing has the advantage in control and consistency. With automated honing, we can easily control tolerances to 50 millionths of an inch. In fact, we have run capability studies where we've hit double-digit Cpk levels when honing for bore size."

One of Forest City's core products, pump gears, start as flat, washer-type blanks made on a screw machine. These gears operate in a small, precision housing, so any perpendicularity error in a shaft-mounted gear causes wobble, loss of efficiency, noise, increased friction, and possible leakage.

Garbage In, Garbage Out

An adage of gear making is that a gear can be no more accurate than the blank from which it starts. On a bore-type gear, this means starting with parallel faces and a perpendicular, round bore with parallel walls, and no taper or belling. Forest City's minimum standard is 0.0005" for parallelism and perpendicularity, and the shop can work to tighter tolerances as gear quality dictates.

The pump gear blanks are double-disc ground for face parallelism and width, then rebored on an automated lathe to re-qualify the perpendicularity. Some stock is intentionally left in the bore so that final ID size and finish can be set on a hone.

Pump gear blanks are usually stack hobbed. They are grouped on an arbor in quantities based on four times the diameter of the bore, divided by the face width of the part, to determine the number of parts per load.

Lack of good parallelism and perpendicularity can introduce lead error when cutting the gear, or force a reduction in the number of blanks on the arbor, eroding production efficiency.

Pump gears often have a standard keyway or a blind-hole keyway added that must align with a tooth.

"When we cut that keyway, it throws up a tiny burr, so honing for final size allows us to clean up that burr, too," Young said. "This is where honing shines, letting us control final size down to a few microns. This kind of control is a real advantage when working to high Cpk requirements."

Depending on the requirements for a specific gear, Forest City hones gear bores at various points in the manufacturing process, working with three different systems from Sunnen Products, St. Louis. Parts are honed after hobbing, but on tight-tolerance gears, blanks might be honed before and after hobbing. Fixturing on the hones allows some degree of control and correction of perpendicularity, should that be needed. If parts are heat-treated, they are honed afterward to correct for the slight shrinkage in bore size.

"If there is a plating operation, we have found it is easier to hone a little plating out of the bore than it is to mask the part for plating," Young says.

Forest City Gear uses CBN and aluminum oxide honing tools in several configurations, depending on the bore. A tool life of about 250,000 parts is typical, depending on the material. Stock removal is usually 0.0020" to 0.0030" at cycle times of 15 seconds. In terms of finish, the hones can achieve a 16 micro-inches or better finish.

Keeping It Fresh

"Our newest system is a fully automated Sunnen SV-1005 vertical machine, with a rotary table and automated part handling," Young says. "Using Sunnen's Krossgrinding tools, this machine can control hole size to accuracies of 0.25µm, with minimal variability. The machine can even make corrections that are not intuitive for an operator. Switchable control features, such as 'correct for bore shape' let the operator select a 'problem' bore image, for example barrel or taper, and the machine will automatically correct the part."

Forest City has a continuous program to add new equipment with higher technology and productivity to go along with its Lean manufacturing philosophy, adding 25 to 40 percent per year of its gross revenue to keep current customers and gain new ones.

Based on ten different part numbers that Forest City Gear runs throughout a given year, the company estimates the advantages of the new Sunnen machine reduce costs for those parts by $50,000.

No matter what the part print specifies, all customers want quieter drives, smoother operation, greater efficiency and long life, Young said. "We strive to give customers a product that is noticeably better than what they would get from a competitor working to the same spec. Honing is one way we add this value without adding significant cost." Sunnen Products

Visit www.rsleads.com/711mn-204 for more information

What do you think?
Will the information in this article increase efficiency or save time, money, or effort? Let us know by e-mail from our website at www.ModernApplicationsNews.com or e-mail the editor at pnofel@nelsonpub.com.