The automotive industry has trended toward greening itself for many years but never as rapidly as in the last decade. Although electric cars have gained market traction, the focus is still on making traditional automobiles more fuel efficient.

The Obama administration has worked on fuel efficiency standards with the goal of improving cars and light-duty trucks to a 54.5 mpg benchmark by 2025. The legislation governing this plan requires automakers to produce highly efficient gasoline engines.

Automated honing machines, like this SV-2400 Series precision vertical honing system from Sunnen, realize fuel efficiency in engine components. Source: Sunnen

For auto component manufacturers, machining parts to tight tolerances is a key to the efficient function of the final assembly for automakers. As a result, high-precision machining techniques and technology have been developed to produce emissions control systems and engine system components.
Key components in engines are the pistons that fire up and down in cylinders. The more attention paid to these cylinders, guaranteeing cylindricity and diameter, the better fuel efficiency will be. Additionally, tight connection between the cylinder and piston, or the “ring seal,” can also vastly improve fuel efficiency.
Typically, dimensional measurement of the cylinder and piston help guarantee tight tolerances, but using high-precision honing tools, part makers can better control the surface finish of these components and heighten performance standards. Essentially, honing creates crosshatch patterns across smooth metal surfaces that better retain oil lubrication. A honing operation involves inserting a hone or honing stone into a cylindrical diameter and spinning the tool at rapid speeds. The rough edges of the brushy hone cut into the interior surface of the cylinder and create the crosshatch patterns, shown below.

This metal surface has been honed, evidenced by the scratchy patterns. Source: Wikimedia Commons
As Rich Moelleberg of Sunnen explained to Modern Machine Shop (MMS), honing today requires precis

ion at the microscopic level to guarantee an accurate balance between “smooth” and “rough” to enable the proper amount of oil transfer while retaining an adequate amount of oil at the same time. This is why machinists prize the crosshatch pattern; although honing creates a smooth surface, it features enough valleys to retain a certain amount of oil lubrication.
Attaining this surface requires surface roughness measurement according to a variety of parameters. Average roughness is called Ra, but as Engine Builder Magazine explains, “A bore finish with tall peaks and deep valleys can have the same average roughness number as one with short peaks and shallow valleys.” So Engine Builder continues that high-precision honing relies on other roughness measurements:
Rpk is the peak height.
Rvk is the depth of the valleys.
Rk is the average core roughness depth based on the Rpk and Rvk measurements. A surface with a low Rk value will have long life characteristics.
Rmax is the highest peak-to-valley measurement taken from five samples.
Rz is the mean highest peak-to-valley measurement taken from five samples.
Further, the angles of the crosshatch pattern can greatly affect part performance and are dependent on the part function, cylinder diameter, and other operational and dimensional measurements.

Traditional honing operations were a combination of art and science, with operators often working by feel to find desirable approach angles and cut size. “It takes an experienced operator and a lot of work to get acceptable results with the older technology,” machinist Chris Webb explained to Fabricating and Metalworking. “Changeovers from one finish to another required belt changes on the machine and took time to set and adjust. All of this adds up to long cycle times on the hone, and reduced performance on the dyno.”
To achieve the modern requirements of accuracy and tight tolerances, machinists rely on computer control. Production Machining notes that computer-controlled hones are capable of machining to within 0.00001 in of a specified size with very little variability.

Watch the video below to see an automated honing machine in action.

The key to this operation is the closed-loop tool feed mechanism. This allows the tool to function free of operator intervention, with bore geometry, hole size, and surface all automatically controlled and monitored by the Windows-based system. The computer control uses air-gage probes to reduce variables in spindle placement, approach, and substrate positioning.
Further, honing methods such as diamond honing use automation of durable cutting tools to not only reduce process times and increase shop capacity, but also to increase the variety of operations available. While diamond, the hardest natural substance on earth, provides a fast and deep cut, tooling equipment must have more power and carry heavier loads. With diamond hone heads, high-torque honing machines can vary crosshatch angle at regulated spindle speeds to produce repeatable passes and reduce variance in a production run.
However, while it is fast and durable, diamond honing creates rough cuts with metal folding and other machining debris, which necessitates a final finishing step.

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