Trimming and Machining Operations

Composites are more prone to damage during trimming and machining than conventional metals. Composites contain strong and very abrasive fibers held together by a relatively weak and brittle matrix. During machining, they are prone to delaminations, cracking, fiber pullout, fiber fuzzing (aramid fibers), matrix chipping, and heat damage. It is important to minimize forces and heat generation during machining. During metallic machining, the chips help to remove much of the heat generated during the cutting operation. Due to the much lower thermal conductivity of the fibers (especially glass and aramid), heat build up can occur rapidly and degrade the matrix, resulting in matrix cracking and even delaminations. When machining composites, generally high speeds, low feed rates, and small depths of cuts are used to minimize damage.45

Most composite parts require peripheral edge trimming after cure. Edge trimming is usually done either manually with high speed cut-off saws or automatically with NC abrasive water jet machines. Lasers have been often proposed for trimming of cured composites, but the surfaces become charred due to the intense heat and are unacceptable for most structural applications.

Fig. 7.64. Hand Trimming46

Carbon fibers are very abrasive and quickly wear out conventional steel cutting blades; therefore, trimming operations should be conducted using either diamond-coated circular saw blades, carbide router bits, or diamond-coated router bits. A typical manual edge trimming operation, shown in Fig. 7.64, can be conducted with a high speed air motor (e.g., 20000rpm) with either a diamond-impregnated cut-off wheel or more typically a carbide router bit. Fiberglass laminate trim templates are often clamped to the part to insure that the true trim path is followed and provide edge support to help prevent delaminations. Typical feed rates are 10-14 in./min. Hand trimming is a dirty job. The operator should wear a respirator, have eye and ear protection, and wear heavy duty gloves. Many facilities have installed ventilated trim booths to help control the noise and fine dust generated by this operation. Being a hand operation, the quality of the cut is very dependent on the skill of the operator. Too fast a feed rate can cause excessive heat leading to matrix overheating and ply delaminations.

Abrasive water jet trimming has emerged as probably the most accepted method for trimming cured composites; however, these are large and expensive NC machine tools (Fig. 7.65). The advantages of abrasive water jet cutting are that consistent delamination-free edges are produced, and since the cutting path is NC controlled, the requirement for tooling is much simpler. Abrasive water jet cutting is primarily an erosion process, rather than a true cutting process, so there is very little force exerted on the part during trimming; therefore, only simple holding fixtures are required to support the part during cutting. In addition, no heat is generated during cutting, negating the concern for possible matrix degradation. Water pumped at low volume (1-2gal/min) enters the top of the head and is then mixed with garnet grit that is expelled through a 0.040 in.

Water

Water

Fig. 7.65. Abrasive Water Jet Trimming Machine Tool Source: The Boeing Company

diameter sapphire nozzle at 40 000-45 000 psi.47 In general, higher grit size numbers (smaller grain diameters) produce better surface finishes, with a typical grit size being #80. Once the abrasive slurry has penetrated the composite laminate, there is a catcher filled with steel balls that spin to dissipate the flow. Other than the expense of these tools, the other main disadvantage is the noise level generated during the process. It is not unusual for trimming operations to exceed 100 dB; therefore, ear protection is required, and many units are isolated within their own sound-proof rooms.

If edge sanding is required, die grinders at speeds of 4000-20 000 rpm can be used along with 80 grit aluminum oxide paper for roughing and 240-320 grit silicon carbide paper for finishing.

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