by Adam Dimitroff
Selecting the right router for aerospace machining
Carbon fiber reinforced plastics, commonly referred to as CFRPs or composites, are an extremely strong and lightweight material made by layering a binding polymer and a composite material reinforced with carbon fibers. Their high strength-to-weight ratio and rigidity make them an ideal material for aircraft components. In many modern passenger aircraft, composites account for more than half of the total aircraft’s weight.
Due to their unique material structure, composites machine differently than other metals typically found in the machining industry. In metal cutting, the work material is softer than the tool and a chip is formed when the hard cutting edge of a tool shears the softer material. Composites, on the other hand, are machined by fracturing the carbon fibers with the impact of the cutting edge, reducing the material to a fine dust. Challenges associated with machining composites include their tendency to leave uncut fibers, their tendency for delamination and their abrasiveness, which causes rapid tool wear. Like any material, there are multiple styles of milling tools that can present advantages and disadvantages when machining these materials. To understand which router is best for your application, it’s important to understand the differences between router types.
Although technology varies slightly between different tool manufacturers, three common styles of composite routers prevail: the serrated router, (fig 1), the rougher router (fig 2) and the compression router (Fig 3). These tools are typically diamond coated to improve wear resistance in abrasive composite materials. Each style of router offers advantages and disadvantages when machining composites. You need to first identify the needs of your shop to maximize manufacturing efficiency.
When high removal rates are required, roughing style routers provide the best productivity. Out of the three styles of composite end mills, roughers are found to produce the least cutting forces and achieve the best stability at high spindle speeds and feed rates. The low cutting forces are ideal for low rigidity set ups or long overhangs. If high surface finish requirements are required, this router can be combined with a compression style or straight edge router for finishing.
When surface finish and preventing delamination are most important, the compression style router is the number one choice. This router combines a left and right hand helix cutting edge to exert a compressive load on the material. This eliminates the tendency for the outer layers of the material to delaminate. However, since both the left and right hand cutting edges need to be in contact with the work piece, only thicker laminates can be milled with this style of router. To pair with this, unlike the rougher and serrated router, we cannot use “flute management,” which is moving the router axially to use a different portion of the cutting edge as one portion wears. For this reason, this tool typically will provide the worst tool life of the three.
When trying to find a balance between surface finish and productivity, the serrated style router delivers the best of both worlds. It can be used as a rougher, but can also produce high quality surface finishes. This style of router is useful when machining complex workpiece geometries, and the usage of flute management allows for longer tool life. Serrated roughers tend to exhibit the best tool life of the three routers.
Due to the costly nature of composite materials, it’s important to ensure the proper tooling is chosen to get the job done in the most productive manner. Choosing the proper tooling can help reduce cycle time, increase tool life and reduce scrap, ultimately leading to reduced production costs. SMT
Adam Dimitroff is an applications engineer with OSG Canada.
