that puts them outside quality limits. Also, when stacked with a metal such as titanium or aluminum, the tool has to have the right capability of penetration. There are no chips and conventional surface finish is usually not an indicator of hole quality. Hole quality is normally based on the degree of any separation of the bottom layer in the material (delamination) as well as any residual, frayed fibers in the hole (splintering). These are not directly detectable, and hole quality often dete- riorates before there are signs that the tool is worn out. The principal tool solutions today are based on diamond- coated cemented carbide drills and diamond-veined drills. Polycrystalline diamond (PCD) is the hardest of tool materi- als and therefore the most wear-resistant and well-suited for machining CFRPs and stacked materials. A drill with carbide as the core tool material with a PCD edge is an ideal tool for composite holemaking today.
The carbide tool can be strengthened through its geometry as well as through the drill shank, in this way providing the best
cutting action while maximizing clearance and material evacu- ation. Drills based on carbide are especially suitable for many unstable operations, when hand tools are used and thrust may be uneven from operators and when there are clearance variations between drill and drill bushing. They are also ideal for many power-fed operations and in machines involving single passes in stacked materials. The best result is achieved when the two tool materials are combined in a tool making use of the fact that carbide and PCD have different advantages and limita- tions as tool materials. Carbide is very strong but wears quickly in abrasive materials; PCD is very wear-resistant but brittle. The modern standard, semi-standard and engineered drills with a diamond-coating are available in different geom- etries and grades for different material and machining condi- tions. These drills have also been developed to drill holes opti- mally in CFRPs that vary from fiber-rich to resin-rich as well as being all around alternatives and suited for stacked materials. Two standard drills offer a choice to optimize operations in