Crossing the Finish Line: Design & Engineering

Numerous approaches exist when mass finishing machined metal parts, depending on the application. This article covers two of them: honing and superfinishing.

When looking to improve the form of a bore, including the bore geometry and finish, part manufacturers can employ honing, said Dennis Westhoff, business development manager for St. Louis-based Sunnen Products Co., which is celebrating its 100th anniversary this year. “Everything we make related to honing is to generate the perfect cylinder.”

Sunnen’s honing offerings include an extensive line of machines, tools and abrasives. The machines include horizontal and vertical models. Traditionally, horizontal machines, or tube hones, are for honing long components with long bores, such as landing gears, rifle barrels and hydraulic cylinders. Westhoff explained. “In many cases, it’s easy to load and unload out of a horizontal machine with an overhead crane arrangement.”

In addition, smaller horizontal machines, which can be found in many toolrooms and job shops, are available to hone a variety of small parts with varying bore sizes, he noted.

In large-diameter bores, however, the honing tools are heavy, creating a situation where the effects of gravity can shift the centerline of the bore, Westhoff said. A counterrotation fixture for the workpiece can offset that shift by rotating the workpiece during honing to maintain the required centerline position.

All of Sunnen’s honing machines are designed for wet honing, mostly with oil (pictured) but also using water-miscible fluids. Image courtesy of Sunnen Products

If the parts are appropriate, a vertical machine provides a solution to avoid a centerline shift, Westhoff added. Compared to a horizontal platform, a vertical arrangement is “more production worthy” because multiple vertical spindles can be arranged side by side to perform roughing, semi-finishing and finishing, for example. “You basically can stack three machines, in essence three vertical spindles next to each other, to perform those three operations.”

Regardless of the orientation or application, he noted that all the company’s honing machines are designed for wet honing, mostly with oil but also using water-miscible fluids. “I’ve been doing this for over 27 years and maybe have honed a few pieces by hand — dry — over the years.”

It’s common knowledge that honing is used to generate a crosshatch pattern on a bore’s ID for oil retention. When the part is for some type of internal combustion engine, the specification for crosshatch pattern generally includes an angle for the pattern and surface finish requirement, Westhoff said. “There are probably way more applications where there is no defined crosshatch.”

In addition, some honing applications require a crosshatch pattern that is flattened so shallow as to be undetectable in the bore, he noted.

Bore Refinement

Nagel Precision Inc. is another builder of honing machine tools. Nagel builds conventional stroke honing machines and “precidor” honing systems that it developed. Precidor honing provides form and dimensional accuracy down to 0.001 mm (0.00004″), which is achieved using a precidor tool with an adjustable honing mandrel fitted with a cutting layer designed for the machining task, the company reported. The complete cutting process is made in one stroke, but more strokes are possible in special cases.

For conventional honing, Jason Collins, project manager for the machine builder and cutting tool manufacturer in Ann Arbor, Michigan, estimates about 85% to 90% of production honing is done vertically, such as for finishing engine blocks, cylinders and large and odd-shaped gears. Horizontal honing is typically performed for finishing the ID on standard gears, such as piston and transmission gears.

Unlike the massive transfer systems that were common in the past where the whole line shut down when an issue arose, Collins pointed out that a vertical honing cell, for instance, is modular with redundant spindles, or bases, for roughing, semi-finishing and finishing. “The good thing is that if one of those bases happens to go down for maintenance or needs repair, you just bypass it so you can continue running production using the remaining ones.”

A connecting rod tool/crank bore being honed. Image courtesy of Nagel Precision.

Like other honing machine builders, he said all of Nagel’s equipment is for wet honing, whether it is a water-, oil- or synthetic oil-based fluid.

In addition, Collins estimated that 75% to 80% of the company’s honing machines are equipped with a robot for loading and unloading. With ongoing challenges to hire and retain workers, one operator can be placed at the beginning of a line and another at the end and “the robot will take care of the rest.”

Engineered Surfaces

In addition to honing equipment, Nagel builds superfinishing machines for finishing ODs and cup wheel superfinishers for finishing flat, concave and convex surfaces. The superfinishing process essentially smooths a course or rough surface to an extremely fine finish, as fine as 0.02 Ra µm (1 Ra µin.), according to Collins.

“It’s basically a mirror finish with no visible or detectable scratches on the surface,” he said.

Although most superfinishing specifications call for no visible lines, Collins noted that some applications do require a fine grooved surface or crosshatch pattern for sealing and fluid retention, respectively. To remove all lines, the abrasive in a superfinisher oscillates at a frequency as high as 70 hertz — “almost a blur to the naked eye.”