Why Chipbreaker and Diamond-cut Routers are Better for Machining Fiberglass and Carbon-fiber Composites
Carbon-fiber composites and fiberglass present a special class of problems for conventional cutters because the fibers themselves are so much harder, and difficult to cut, than the matrix that binds them together. Any cutting tool designed specifically to cut the fibers would have a flute geometry that is ill suited for simultaneously cutting the binder (many of which are thermoplastics that tend to melt and load up the flutes). Designing a flute that performs well in the binder, ends up with an edge that quickly dulls trying to shear through the carbon or glass fibers. Even with the high-performance carbides that have been developed, conventional cutters just do not perform that well in such composite materials.
One of the best analogies that I have heard is that cutting glass (or carbon fiber) composites is like trying to hand till a garden that is full of roots and stones. A conventional garden hoe, which was designed to turn relatively soft soil, simply skates off the top of any rock it encounters, or is stopped by a large enough root. A tool,like a mattock or pickaxe, designed to punch down below the rocks so that they can be dislodged or cut through most roots, is generally too narrow and too heavy to function as an efficient hoe. The "McLeod Hoe", which combines features of both is an implement often used work tricky soils, especially by wildland firefighters who use them to cut through and clear dense, matted undergrowth. With wide, flat tines leading out from the egde, the McLeod concentrates the force of the swing into a small number of points allow the tool to easily penetrate the ground to break up and loosen roots and rocks alike. (Really great tool for landscaping, too, unless you can avoid it.)
This is exactly the operating principal behind rotary cutters known as "chip-breaker" and "diamond-cut" router bits. The flutes of these tools, primarily designed to efficiently cut the binder and clear away the swarf, have edges that have been broken up into teeth that are strategically overlapped to insure that every position along the cutting edge is in the path of one, or more, teeth. This results in a remarkably smooth cut with no artifacts indicating the segmented nature of the cutter. In addition to significantly increasing the length of each cutting edge, these teeth also concentrate the cutting forces to such a degree that they easily shear and break apart any fiber that they encounter.
Even with their efficient tooth design, the side load that results from shearing the fibers is so large that the core of each tool must be much thicker than is normal for conventional end-mills. More flutes (as many as 6) are also added to further stiffen the cutting length as well as to increase the total amount of cutting edge. As a result, the flutes are much shallower and incapable of removing heavy swarf-loads. That is why the feed rates used with these cutters tend to be lower than one would expect with a tool of similar diameter and depth of cut (DOC). It is also why chipbreaker and diamond-cut bits routers are lousy tools for cutting most woods or conventional thermoplastics.
Diamond-Cut Router Flutes