Combining milling and broaching for aluminum engine components

Automotive manufacturers and suppliers know it all too well: Face-machining of aluminum blocks and cylinder heads can be challenging, driven by cost pressures and very close quality parameters. Historically, the case could be made for milling vs. broaching when machining these parts. But a recent engineered alternative that combines both machining methods raises productivity and eliminates burr-formation problems.

Burrs top the list of aluminum-milling side effects. The cutting action in many aluminum facemills tends to have poor chip formation and direction. When the cutter reaches the end of cut and exits the milled face, it often leaves a burr that grows throughout time to an unacceptable degree, prematurely wearing down the insert. Insert changes and extensive cutting edge setting is the result, forcing frequent machine stoppages and tool-handling.

Add to that the general challenge for manufacturers to achieve cost-effective solutions in these types of operations. Advanced combustion systems, powered by multi-port engine layouts, enable higher operating temperatures and increased combustion pressure. All of that puts more mechanical stress on tops and heads, leading to more challenging material specifications on blocks and engine components. Alloys vary according to the designated fuel-type. The demands on the aluminum alloys drastically increase, affecting alloys, casting, heat-treatment, and subsequently machinability.

Broaching vs. milling

A well-established machining method practiced when precise results are needed, surface broaching is used frequently in one-stroke operations where the tool is pulled or pushed. In rotary broaching, the tool is rotated and pressed into the workpiece, often to produce a shape. The principle of broaching is built on progressive, light depths-of-cut for each tooth, where the teeth are arranged in set sequence. Generally, broaching is a very productive, smooth method with a minimum of tool setting for rough to fine cuts in one pass.

Milling is a far more flexible practice. Rotary, multi-edge tools that use advanced indexable-insert technology are available in many forms to perform a large variation of operations to produce flat or curved surfaces and various cavities such as slots and edges. Face milling is still a dominant type of milling, offering a uniform, axial cut while the rotating cutter is being fed radially across the part and is set to produce a certain chip thickness. Milling is a universal, productive method for roughing and finishing where some degree of tool setting is usually required.

A unique combination of these two machining methods was developed to specifically improve flat-surface machining of aluminum-alloy components, mainly cylinder tops or heads for the automotive industry. This new concept minimizes the setting of cutting edges, allows high machining rates, and enables high finishes to close tolerances without the typical pronounced aluminum-burr formation that normally requires a subsequent deburring operation. The need for dedicated broaching machines and broaching tools, often more expensive investments with longer delivery times than milling cutters, is eliminated. In addition, as tools don’t have to be reground, a time and cost factor is taken out of the equation.

CoroMill cutter

The CoroMill 5B90 cutter body follows the same principle as a broach. It rotates while machining, as a traditional milling cutter would, and the insert positioning allows each insert to remove the same amount of material axially. A set of inserts, typically about 10, has been positioned so each has a unique height which then distributes the axial depth-of-cut over all teeth in the cutter. In a typical milling application each insert might take a cutting depth of 0.030" (0.770mm). This depth is then divided equally among each insert. Each insert acts as a roughing edge, complemented by one edge which acts as a wiper to generate the surface finish and tolerances.

The CoroMill 5B90 cutter for face machining aluminum blocks is a relatively coarse-pitch cutter, needing fewer inserts, optimized for each application. Face-mounted polycrystalline diamond (PCD) inserts can be held in a steel-ring with fixed insert pockets, mounted on an aluminum cutter-body or a full steel body. PCD insert grades have shown tool life increases of up to 5x compared to carbide when machining alloys with silicone content of 8% to 18% that makes parts extremely abrasive on tools. Proper PCD inserts have good mechanical properties which make them less sensitive to impact loads in interrupted cuts.

Specific application factors, for which this solution was developed, focus on ideal cutter-diameter, adjoining protrusions on the part, machining allowance/depth-of-cut, and table-feed. Only one wiper/finishing insert is required, and individual insert pre-setting is not needed, despite being a high-precision cutter. Face-mounted inserts in this relatively coarse-pitch cutter provide accuracy, lower cutting forces, and a smooth cutting process.

With finishes and tolerances that are well within automotive industry standards, this unique approach of combining two well-known machining techniques offers an alternative approach to cost-efficiently producing aluminum parts in large batches.

Sandvik Coromant Co.

www.sandvik.coromant.com