Although possessing a very high strength-to-weight ratio, carbon-fiber-reinforced polymers (CFRP) have unique structures that make them difficult to machine cleanly and precisely. And the quest for better performance from CFRPs has meant new materials being introduced to the composite makeup, expanding the challenges for manufacturers, cutting tool suppliers, and machine tool suppliers.
Generally, composite structures consist of a relatively soft, tough matrix material layered with stronger, stiffer reinforcement fibers cured under heat and pressure. The base polymer matrix can be epoxy, phenolic, polyamide, or polyetheretherketone. Among the reinforcement fiber type are carbon, graphite, glass, ceramic, Kevlar, polyethylene, and even tungsten.
The most common method of fabricating CFRPs is the buildup of many layers of fiber-resin prepregs to achieve proper thickness and form. This fiber-resin is commonly applied from tape-like rolls, which may orient the fibers in either a unidirectional or a fabric-like weave pattern. The tape is laid in selected layers and orientations to achieve the desired strength and stiffness. Typically, this layup is then vacuum/bag-molded to create a laminate. Bulk resin impregnation, compression molding, filament winding, and pultrusion are other processes that may be used to complete a laminate structure.
Composites machining challenges
The expanding multitude of possible combinations of tough core materials with a variety of hard reinforcing materials makes choosing effective cutting tools difficult. Many other variables also affect mechanical performance of the composite and its machining properties. For example, using larger-diameter and a relatively higher volume of reinforcement fibers offers mechanical benefits in the final part, but they increase abrasiveness and greatly reduce tool life. Also, short fiber pieces contribute heavily to delaminating problems, as does single-direction orientation of the reinforcing fibers.
While machining ductile materials such as steel or aluminum is based on shearing and consistent chip formation, machining composites is a more complex mechanism involving fracture of fibers by compression or bending forces, along with shearing and cracking of the matrix material. As cutting tools wear, the appearance of spalling (surface chipping), uncut fibers or resin (bending but not fracturing), and delamination at the surface or internal wall of the hole all occur at an increasing rate. This causes costly inspection and repair delays.
In hole-making, delamination is one of the most serious quality issues. Typically, it occurs on the tool breakout at completion of the drilling cycle as the axial thrust force puts pressure on the lower surface laminations. It also happens at the part surface as the rotating drill torque is applied to the workpiece during entry. While there is a strong correlation between thrust and breakout delamination, Kennametal Inc. has found that variation in fiber position, voids, and other material variances are also contributors to the problem.
The quest for improvements must encompass not only tool geometry but also the base and coating material of the tool itself as well as the appropriate speeds and feeds to maximize hole quality and tool longevity. Objectives of geometry design include minimizing stresses leading to delamination, maintaining a sharp edge to cut fibers cleanly, and evacuating the dust created by the cutting action. Tests prove that a high helix angle, a severe clearance angle, and a high-rake gash angle for easier entry into the material are dictated.
Particular attention must be paid to the clearance angle behind the cutting edge. In one application, clearances of 10°, 20°, and 36° were tested. As the clearance increased, hole quality improved dramatically. This was attributed to a much smaller wear land created by the additional clearance and a limit on the buildup of torque and thrust from tool wear. Less thrust ensures cleaner cutting.
In low-thrust cutting of CFRP, initial edge preparation also is important. Kennametal has found that a sharp edge (a radius of up to 10 µm) before applying a coating material works best. It recommends a diamond coating thickness of 12 µm for maximum wear resistance and good cutting properties.
Selection of the point angle for the cutting tool is dependent on the problem one needs to control. A lower point angle (less than 90° included angle) delivers much better exit (breakout) hole quality, while a higher angle (greater than 90°) leads to much higher edge strength. The edge strength is needed to control chipping of the cutting edge, resulting in premature tool failure.
Among conventional tooling choices, polycrystalline diamond (PCD) veined drills and diamond coating of low-cobalt steel drills are the only practical choice for hole-making in composites. Diamond-coated drills deliver a 10-to-1 improvement in tool life over uncoated carbide and, in some cases, a 50% longevity increase over PCD drill technology. Based on a practical balance of hole entry and exit quality plus throughput, Kennametal suggests 400 surface feet per minute and 0.0015 in per revolution feed rate as starting process parameters.
The orbital alternative
Orbital drilling is a relatively new concept in hole-making of CFRP materials. Developed and patented by Novator AB, the orbital drill unit is a self-contained spindle that spins the drill on its own axis while rotating (orbiting) around its central axis. A range of hole sizes and fine adjustment is possible by using an adjustable offset feature that covers a range of 6 mm. Kennametal has developed a family of complementary high-performance tools with proprietary geometries, substrates, and coatings.
Because thrust and friction are the enemies when it comes to delamination, orbital drilling uses a multitooth cutter with a high helix angle, somewhat resembling an end mill, simultaneously rotated and fed into the composite material. The tool diameter is always smaller than the finished hole, so one tool can machine many finished diameters, providing inventory savings. The smaller tool size also promotes chip and fiber removal. Cutting is performed without coolant.
The cutting tool edges contact only the inside diameter of the hole intermittently, providing greatly reduced rotating friction compared to a conventional drill and controlling cutting temperature that can lead to melting of the matrix material. In addition, the multiflute design applies much less thrust pressure along the drilling axis than does a twist drill point. This feature allows delamination to be controlled.
Orbital drilling also addresses applications in which various materials are stacked together for drilling. A typical example of this might include a composite material combined with aluminum and titanium. In addition to effectively machining the composite layer, the multitooth cutter generates small, easy-to-handle chips because of the intermittent contact of cutting edge and material. The highly positive tool geometry also controls burrs that typically occur when drilling in aluminum. The cutters can be constructed to provide either through-holes only or incorporate a countersink operation in one drill cycle. And, orbital drilling is effective for repair operations including redrilling misaligned holes in material stacks.
Superior hole quality has been demonstrated in extensive Kennametal and Novator studies. In one study, the nominal hole diameter was measured over 1200 mm of CFRP drilling penetration. The variation of hole size covered a range of only 25 µm. In other tests focused on unidirectional and woven-fiber patterns, with countersunk as well as plain holes, in composites averaging 5.0 mm thick, more than 200 holes were machined before evidence of delamination or uncut fibers appeared.
The orbital drilling spindle unit may be mounted on machining centers for preprogrammed cycles when drilling composites and stacked materials. For semiautomatic drilling on the factory floor, the unit is mounted on templates to ensure accurate hole locations and to provide a stable platform for machining. When necessary, orbital drilling can be accomplished on curved or angled surfaces.
Recognizing that not every plant either has or will adopt orbital machining of CFRPs, Kennametal continues research to find better solutions for drilling stacks of dissimilar materials using conventional drilling tools. Issues involving burrs, stringy chips, and smearing of aluminum remain under study, as do considerations unique to titanium, including chip control, heat buildup, cutting edge strength, and the difficulty of using diamond tools on this material.
Karthik Sampath and Wang-Yang Ni, Senior Engineers, Kennametal Inc., wrote this article for SAE Magazines.