For years the flexible, ball-style hone has been machinists’ first choice for applications that require honing of cylindrical metal surfaces, such as engine piston bores and drilled through-holes. Now, the tool is being recognized for its effectiveness on two-dimensional or planar, surfaces — in a slightly different configuration but with the same filaments and abrasive globules.
This change is creating possibilities for applications in which hardened metals interact or mate in surface-to-surface friction with other metals or materials. Examples include sliding guideways, flywheels, brake rotors, and others.
“The same type of surface we have promoted and produced for years for internal cylinders also has validity for flat surfaces,” said Jonathan Borden, national sales manager of Brush Research Manufacturing, the company that developed the Flex-Hone Tool. Known widely as a tool used for deburring, plateau honing, and deglazing, the Flex-Hone is a highly versatile device used for a variety of machining operations. Somewhat resembling a spinning bottlebrush, it is characterized by abrasive globules that are permanently mounted to flexible filaments that are attached to a center shaft. While extremely flexible, it is a low-cost tool for sophisticated surfacing, deburring, edge-blending, and cleaning.
For example, in automotive parts production honing tools are used to create an effective surface finish on engine cylinders. Engine cylinders have surface-to-surface contact with pistons sliding against the wall; to reduce power losses from friction, engine cylinders are honed so that piston rings can seat properly, as well as to allow for lubricant flow and oil retention on the walls of the cylinder.
According to Borden, the principles behind cylinder honing apply also to flat, or planar, surfaces.
Borden explained that the process had been put the test recently for sliding guideways, a critical structural component of CNC machines. These guideways provide the smooth, linear motion of the machine tools along its predetermined path. It is therefore fundamental to the accuracy and precision that can be achieved with this type of equipment.
The goal of the study was to establish if Cubic Boron Nitride (CBN) milling tools followed by a secondary planar honing operation could replace the traditional grinding process used to manufacture the hardened cast iron guideways. The experiment and subsequent conclusions were outlined last year by Kory Chang and Masakazu Soshi of the Advanced Research for Manufacturing Systems (ARMS) Laboratory in the Dept. of Mechanical and Aerospace Engineering at the University of California, Davis, in “Optimization of Planar Honing Process for Surface Finish of Machine Tool Sliding Guideways", published in the Journal of Manufacturing Science and Engineering.
As the foundation for linear motion, the surface finish on sliding guideways is a critical factor: A poor surface finish can shorten CNC tool life, increase power consumption, and cause errors in tool position, resulting in out of tolerance workpieces.
However, the traditional grinding process for manufacturing sliding guideways involves removing, cleaning, and then repositioning parts several times. The hardened surfaces, which measure as high as 50-60 on the Rockwell C Scale, also require grinding because milling cutters typically wear out too quickly.
An alternative approach using cubic boron nitride (CBN) milling tools along with a secondary planar honing operation was proposed in the design experiment. CBN is one of the hardest known materials for machining of steels and cast iron. Unfortunately, the milling process does not create a consistent surface finish. For that, planar honing would be used within the same machining center.
According to the study, the BRM Flex-Hone for Rotors was selected as the tool for the research. The planar honing tool provides a low-temperature abrading process that exposes the undisturbed base metal. In contrast to a “bottlebrush” design used to hone cylinders, the planar honing tool uses the same type of filaments with abrasive globules mounted to a disc. The tool can be held in a collet, chuck, or similar holding device. To achieve an optimal finish, a 400- and 600-grit silicon carbide Flex-Hone for Rotors was selected after it was determined that a fine grain size was all that was required to flatten and create the plateau peaks on the cast iron sliding guideways.
The Flex-Hone tool, along with factors such as pressure, spindle speed, feed rate and number of strokes, was expected to produce a surface finish in the range of 0.3–0.6 lm and 0.2–0.3 lm Ra.
The study concluded, “analysis showed that the ideal plateau finish desired on the hardened cast iron was supported by both measured real parameters and the Abbott–Firestone curve.” Sliding guideways offer only one example of the type of surfaces that could be improved with planar honing. “Other applications that have already benefitted from this type of hone include automotive and motorcycle disc brake rotors, fly wheels, and clutch plates,” Borden said.
Jeff Elliott is a Torrance, Calif.-based technical writer. He has researched and written about industrial technologies and issues for the past 20 years.
