ROUTING: Routing Composites

Why are composites such difficult materials to cut? What can be used to cut them in a cost-effective method? Can coatings help tool life? These are several of the most asked questions today. This article will explain some of the major factors and how to address them.

The terms Carbon and Graphite are used interchangeably in today’s conversations to describe Graphite composite parts, even though Graphite is Carbon fiber processed to a higher strength. In this discussion they will be considered the same because they are cut with the same types of tools.

The fibers themselves can be in the forms of: small pieces (chopped), a mesh or scrim, fibers in a unified direction, or woven cloth and a mix of any or all of the above. To further expand things, the use of two or more types of fibers may be used in the above combinations. The finished parts have many layers of these materials because layers are very thin. A cure layer of composites is approximately .008” thick. These means that a part 1/8” (.125”) thick will have 15 or more layers. So we have a very strong base material that is highly concentrated and uniformly layered. All of these increase the part’s strength dramatically.

The supporting matrixes are usually some type of catalyst/ resin forms. They are thermoset types and they are processed in two ways: first, those that are cured at room temperatures and pressures (most as resins introduced as a wet-layup), and, second, those that are cured at elevated temperatures and pressures (most as resins already pre-impregnated into material). The higher temperatures and higher pressures increase the composite’s strength greatly. In most cases the pressures are applied by the use of a pressurized oven (autoclave) or by a press.

These parts are further divided into divisions: solid and filled. The different types of composites in either unified tape or woven goods are not segregated because both, after curing into a finished part, are uniform and consistent, and therefore shear in much the same manner. These can occur in the same parts, for example aircraft interior panels, like overhead storage doors, can have a solid edge to attach the hinges and the center is filled with honeycomb core. This combination of systems allows for a lightweight, strong panel with a strong edge.

One of the major characteristics of these two processes is fiber to resin content ratio. In room temperature/pressure parts there is more resin than fibers, 60%/40% is normal. In the elevated temperature/pressure types the ratio is closer to 40%/60%, resin to fiber. Since the fibers are the highest strength values and there are more of them in this type, it presents a challenge in tool designs and materials. As stated previously, these composites are very strong. The fibers are between 300 and 500 kpsi tensile strength for fiberglass and aramid. The graphite fibers range from 350 to 900+ kpsi in some forms, most about 500 kpsi.

The first of these systems lend themselves to lower strength applications such as boats, auto body panels, spas, etc.

The second is for the higher strength needs of parts for aircraft interior panels, aircraft structural parts, racecar frames/bodies, circuit boards, electrical insulators, etc. These are the parts that are addressed in this article.

Recap high strength composite parts characteristics:

• High strength fibers
• Strong resins
• Combined and cured together in densely packed parts
• Can be combined with lighter core materials
• Many uniform layers

Modern day cutting tools materials for composites are Solid Carbide (SC) (Uncoated and Coated), Polycrystalline Diamond (PCD) (brazed or mounted to a body), and Natural Diamonds (DG) (in grit form, bonded to the tool body by electro-plated or fire-brazed methods). They are also offered in the forms of straight flutes, spirals, burrs and combinations of the all the above. The basic grade for this carbide is C-2.

The SC tools in most applications offer the best cost-effective tool material so this article will address its use in these operations.

Solid carbide itself is a composite of Tungsten Carbide wear particles and Cobalt binders. The solid carbides in micrograin or submicron grain sizes used for tools today have a traverse rupture strength of 350 to 450 kpsi. These values are close to the tensile strength of the fibers themselves.

For many years the fiberglass routers (FGR), better known as burrs, have been used in the cutting of composites (See Figure 1).

An examination of the geometry of the FGR’s will show they are comprised of up and down spiral geometry. These spirals generate many cutting points that shear the fiber in the composites. The resins are carried along with the fiber chips. These tools have an uneven count of spirals, up verses down, to make the points overlap so that they do not make grooves in the finished part edge. These tools have performed very well in the past. They act like chip-breakers and reduce the amount of cutting forces needed to cut the fibers. There are improvements on this design by:

Un-ruffer style combining a burr geometry with up/down compression spiral flutes to act as a finishing cut on the fibers (See Figure 2).

CG style tools for the up/down shearing actions combined with a strong flute geometry for fiberglass and graphite composites (See Figure 3).

FMR style tools for the up/down forces combined increased shearing action of the flutes required for cutting the more pliable aramid composites (See Figure 4).

The Up/Down flutes of the tools sends the force toward the center of the parts, not to the surfaces. This action reduces the delamination or separation of the plies of the composites. The modifications of flute edges and the chip-breaker action of the opposing flute make the shearing action work to cut the fiber easier and to clear the chips from the cut.

The Un-ruffer is used in sandwich panel applications where the thin skin on the outside of the panels supports a core of lighter materials. The cores can be honeycomb core, foam or balsa wood. The burr portion of the tools cut small chips from the surfaces and the core with the up/down flutes make a finishing pass on the skins. This combination greatly reduces the risk of delaminating the skins. It should be noted that the honeycomb core surface will still be rough in some areas because of its thin wall. It will fold over rather than shear in a condition known as flagging.

The CG tools are a highly modified version of the up/down compression fluted spirals that address the high strength of the fiberglass and graphite fibers. As in the above examples, the cutting forces are directed to the center of the part. The reason they are an improvement over the FGR’s is because of the higher strengths of the newer carbides available and grinding equipment. This is higher strength, because of the more uniform grain structure than the previous versions of C-2 carbide. An additional factor is the more uniform mix of the individual components of the carbide itself. Solid carbide processes and equipment are highly controlled today, adding to the uniformity of the product.

The FMR tools follow the CG design in the up/down geometry. The fibers of the aramid parts are weaker than the graphite fibers, so the flutes need not be as strong. This allows for adjustments to be made to the sharpness of the flutes by rebalancing the flute’s rake angles to cross-sectional area ratios. Since the chips are usually larger than the fiberglass and graphite chips, additional adjustments to the helix angles can be made for the chip evacuations.

The use of coatings is not a panacea. Coatings can be used to increase surface hardness, protect the carbide matrix, and/or add lubricity to the flute surface. The process adds a covering to the cutting edge creating a small radius which reduces its sharpness. This can increase the amount of force required for shearing the fibers.

As to protecting the cobalt binders, the cement holding the carbide together, most coatings do an excellent job. By coating the tool surface, the microscopic grooves created by the grinding process are flattened slightly and the coating is slicker than the carbide allowing the chips to flow easier across the face. This can reduce the amount of force required to cut the composites.

As to feed and speed parameters for composite materials there some basic rules. Consider the tools to be a single edge cutter. In theory these tools will have one edge on a larger cutting diameter than all the rest. It will lead the others, microscopically, in the cutting action. This will also allow easier calculations of the chip loads by dividing the feed rate by the spindle speed, i.e. Chip Load = Feed Rate (IPM) / Spindle Speed (RPM).

Solid carbide tools in one of the four styles are one of the most cost-effective methods of cutting today’s high strength composites. The raw materials are very uniform and have increased strength. The modern grinding equipment produces a more accurate and consistent geometry. This, combined with the some usage of coatings, offers the best solutions for cutting composite panels.

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