Carbon Fiber Reinforced Plastic

The most widely used advanced composite has been the high strength carbon fiber reinforced plastic composites (CFRP). The term 'advanced', however fails to properly characterize these new reinforced plastic materials. No two CFRP materials are exactly alike. Each composite can take on many different characteristics by changes in the matrix formulation, fiber type, content, orientation, build-up, and the method of forming these materials.

Understanding the construction of CFRP is helpful when it comes to machining operations. The reinforcing fiber most widely used in aircraft structures is a carbon fiber produced by thermal decomposition of polyacrylonitrile (PAN). The thermal decomposition converts the PAN fiber to a pure carbon fiber that is highly abrasive yet very strong.Carbon fibers are reported to have tensile strengths of about 800,000 psi and a modulus of 40 million psi. These high strength fibers are woven into thin sheets and combined with resins to form prepregs. The composite skin section of an aircraft is normally produced by placing multiple layers of this prepreg in molds and then using pressure and heat to cure the CFRP into a complex wing surface. The carbon fibers can also be chopped and blended with resins to form structural parts and then processed by compression molding or resin-transfer molding.

Drilling CFRP

Unlike the easily machined aluminum it most often replaces, the low melting point of the resin systems in CFRP restricts temperatures of any machining operation to no more than a few hundred degrees F. Keeping the cutting edge cool is made difficult by the low thermal conductivity of the resin matrix. Unlike aluminum chips, the cutting swath of advanced composites carry away very little of the heat generated by the machining process. Heat build-up in the cutting zone is very likely and must be avoided to prevent the resin from oxidizing and being degraded.

The widely different material properties of the fiber and resin also create changes. Slight dulling of tool cutting edges can cause separation and fiber pull out. Even modest cutting forces can separate layers causing delamination or leave residual stresses. The difficulty of producing quality holes with the abrasive nature of CFRP makes this an ideal application of Precorp's DiaEdge PCD tools.

Recommended speeds and feeds for drilling CFRP

Two common cases are found for drilling CFRP, a stack up of CFRP over aluminum and unbacked CFRP. The stack up with aluminum reduces the problem of delamination and fraying and allows for higher feed rates. However, gaps between the aluminum and the CFRP can create a serious problem of chip counter boring as the drill retracts through the CFRP. A popular design is Precorp's Series 83 DiaEdge Drill. This eight facet point design helps breakup the aluminum chip and prevents damage to the CFRP hole wall. Additionally, the hole size with this point is held within a few ten-thousandths of drill diameter.

Drilling CFRP without a metal backup requires a low thrust drill design for good hole quality. The Precorp Series 84 DiaEdge Drill is widely used in the aircraft industry. This drill yields over 3 times the number of holes as compared with a 118 degree veined PCD drill, six times that of an insert PCD drill, and 100 times that of a carbide drill. The relativity sharp point angle lowers the thrust of the drill and allows the cutting lip to cleanly trim fibers at the hole exit.

Drilling feed rates per revolution for CFRP need to be fairly light to prevent delamination. Excessive speeds will generate frictional heat and melt the resin. As with most machining operations, flood coolant is recommended when using PCD tools for the best tool life. However this is often impractical with CFRP and with sufficient speeds and a good vacuum system, drilling dry yields excellent results. For materials 1/4in to 3/8in thick, 400 SFPM is recommended for both wet or dry drilling.

The above information was taken from the flyer
'Drilling Advanced Composites with Diamond Veined Drills'
by John Bunting

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