Counteracting carbon complexity

The key to success in this area, say the two partners, is the application of proven solutions to a wide range of challenges, which include the machining of both composite and composite/metallic stack materials using handheld, power feed and fixed bed CNC machines. This is where the combination of Precorp’s specialist technology and Sandvik Coromant’s tooling and Application Center expertise provide aerospace manufacturers with support for composite machining applications. Successfully applied recent solutions include cemented carbide, diamond coated and PCD vein tools adapted for hole, edge and surface machining applications. A helping hand Producing holes in CFRP using handheld machines is common practice in the aerospace sector and as structures like the fuselage and wing spars are extremely large and it’s often simply impractical to use a fixed bed machine tool. Tools for handheld machines need special qualities to resist deflection and the presence of non-axial forces. Here the CoroDrill 452 range based on H10F uncoated carbide substrate offers more potential when drilling, reaming and countersinking CFRP, both unidirectional and woven types such as M21E and BMS8-276. Drills such as CoroDrill 452.1-C are stocked as standard from 2.5 to 12.7mm, offer low thrust force due to an innovative split point geometry design and feature a smooth drill exit which incorporates a left-hand helix on a right-hand drill. Sharp cutting edges produce high quality holes offering tolerances of ±0.025mm using drill bushing. This can be improved further to ±0.01mm using the CoroDrill 452.R-C range of reamers. CoroDrill 452.C-C countersink tools offer a choice of 100° and 130° chamfer angles. When drilling stack materials, more variables are introduced to the process due to the presence of aluminium or titanium layers. Meeting this challenge, CoroDrill 452.1-CM utilises a double margin flute design and self-centring drill point to enable optimised accuracy. When drilling and reaming CFRP/aluminium stacks, cutting speeds are typically around 60m/min, although this should be reduced to around 15m/min when producing holes in composite/titanium stacks. Staying power Where possible, power feed drilling is preferred to hand drilling as the drill is locked into a template that establishes both hole location and maintains the drill perpendicular to the workpiece. In addition, these units can be programmed to drill at a given speed and feed, and some offer peck cycles. Using power feed machines on CFRP materials, solutions such as the 85PT series PCD vein drill offer a shielded diamond design that is optimised for stable drilling processes. These are available made to order in diameters from 3-16mm and through-coolant is also offered. Alternatively, 8F85 series drills (based on H10F or N20C, CVD diamond coated) feature a carbide drill point that generates good hole exit quality in demanding unidirectional and woven CFRP. The eight facet point geometry reduces common problems associated with these materials, such as splintering and delamination. Users can expect to achieve cutting speeds in the order of 60m/min at feed rates of 0.06-0.15mm/rev, depending on application. When drilling composite/metallic stacks using power feed machines, 86PTA is recommended for CFRP/aluminium materials, while 86PTB is offered for CFRP/titanium stacks. Alternatively, the 40DH series is a carbide tool with innovative flute design for optimised chip evacuation, while for improved roundness accuracy and stability in larger holes up to 25.4mm diameter, the PD – D2WM series is recommended. Both PCD and carbide reamers can be made to order with flute design dependent upon material and application. Drilling and reaming CFRP aluminium stacks using power feed machines can typically be performed at 60m/min with a feed rate of 0.13mm/rev, however this should be reduced to around 18m/min and 0.03mm/rev when hole making in CFRP/titanium stacks. In control Where drilling using CNC machine tools is possible, greater accuracy, security and productivity can generally be achieved due to enhanced machine dynamics and rigidity. The CoroDrill 859V vein PCD drill is designed purposely for the automated drilling of unidirectional CFRP materials such as M21E as well as epoxy and BMI resins. The double angle, sharp vein drill geometry helps improve quality and reduce delamination. Alternatively, the 85 series comprises all-round drills with four facet point geometry suitable for various CFRP materials. It is available as carbide with diamond coating or vein PCD in diameters from 3-16mm. For unidirectional CFRP and composites with high epoxy resin content, including BMI resins, CoroDrill 856 is recommended. When CNC hole making in composite/metallic stacks using CNC machine tools, 86A and 86B series tools are the choice for reduced burr and fibre breakout. Another option is the carbide 40DH series drills which offer stable tool life in high speed/feed applications and good chip evacuation due to their 40° helix. Edging closer to perfection Cutting tools are often put to the test when edge milling CFRPs. The heterogeneous combination of fibre and matrix resin means that a cutter encounters varying resistance from the inter-layered hard abrasive fibres and softer resin, which puts stress on the tool drive mechanism and leads to rapid tool wear. The secret to solving this problem is having a range of tools offering different geometries and grades to create application specific solutions. The CoroMill Plura S215 is such a range, offering different geometries, such as a compression helix design for thicker CFRP or when equal geometry is needed for both sides of a composite sheet. There is also a low helix design of +5° when the bottom surface is the most important and -5° when the top side is most important and down-forces are critical. The tools come in different grades, such as 1610 and 1630 to maintain sharp edges, or N20C diamond coated for longer tool life but where ultra-sharp edges are not top priority. There is also a PCD brazed version for long tool life and optimum surface finish. For edge milling of CFRP when the machine spindle is not in line with the component (for larger cut-off operations) then CoroMill 329 is preferred. This standard product comes in diameters of 100-160mm and widths of 2-5mm. Surface advantages Surface machining of composites brings a new set of criteria. Finish, accuracy and high integrity of primary surface structures are highly important to aerospace manufacturers. Cutting tool solutions for flat and curved surface machining of CFRP is served by the CoroMill 590 light cutting indexable insert face mill. Available in diameters above 40mm, CoroMill 590 inserts come in PCD and uncoated H10 grades with high precision tip seats for enhanced accuracy. Finally, Sandvik also suggests that machine shops should look at CoroMill 390 for sculptured surfaces using sturtz milling techniques. Sturtz milling can replace machining with ball end mills in many areas. This innovative machining method involves tilting the tool relative to the component surface to create an elliptical cutter path. The resulting shallow ellipse allows greater step-over to be used without exceeding the maximum permissible cusp height. www.sandvik.coromant.com/uk