Pretreatment for spray-in cylinder liners

Precision cylinder roughing, using a single-edge boring bar with polycrystalline diamond-coated (PCD) inserts offers high-quality processing with multiple benefits. In engine blocks, where traditional grey cast iron is being replaced with aluminum and other light-metal castings, less weight consumes less fuel, resulting in fewer CO2 emissions. Grey cast iron cylinder liners are still used in aluminum engine blocks due to superior sliding qualities, but they are increasingly being replaced with coatings that offer reduced wall thickness and better wear behavior.

Roughing tool

Getting cast-iron coatings to stick to aluminum cylinders requires pre-treatment of the substrate, and classic methods are water or corundum blasting. An alternative method lies in machining the surfaces with a roughing tool. The crankcase can remain on the machining center after cylinders are bored, so cylinder pre-treatment can be integrated into the production line.

“We rely on a stable, single-edge boring bar,” says Burkhard Krauss, automotive component manager for machining solutions at Walter. “Single-edge means that it is equipped with just one cutting insert. This is made of PCD, is very delicate, and has many small individual edges, which create a profile with indentations on the cylinder surface.”

Like a threading tool, this tool cuts dovetail-like profile grooves through the entire crankcase. The width of the groove ranges from 120µm to 180µm, and the depth ranges from 60µm to 130µm. The indentations enable the coating metal to interlock with them.

One is easier than two

One cutting insert is easier to adjust than two-, three-, or four-edged tools. The diameter needs adjusted only once, and the cutting insert does the rest of the work. The individual PCD teeth have varying heights and shapes, cutting the profile in steps. The cutting insert reproduces the entire profile, comparable to the cutting edge of a tap. In contrast, with multi-edge solutions, the profile is divided across the various cutting inserts, which need to be adjusted to each other radially and axially.

The base of the cutting insert is a carbide body with a PCD plate soldered to it. The teeth are created by electrical discharge machining (EDM) with wire electrodes. The thin wire presents a challenge, as a pushing force acts on it, while its diameter is limited by the contours. Quality assurance with a high-powered microscope compares the target with the actual contours. The quality of the crankcase is checked in a similar way, with a camera monitoring the condition of the PCD insert after machining the last cylinder bore.

This mechanical pre-machining is used for coatings on high-end luxury cars with the hope that medium and small cars eventually will have coated crankcases.

Walter USA LLC

www.walter-tools.com/us