Chasing material development

This is, in fact, the view, from one of the world leaders in tooling for aerospace and composite machining, Onsrud, based in Illinois and part of the LMT Group. "Secrecy and protectionism are paramount in the development and use of composite materials," explains Scott Feimster, LMT Onsrud's global manager. "This makes it difficult for the tooling industry to develop new solutions in conjunction with material research and development, and means we are largely playing 'catch-up', once production release is granted." According to Trevor Tolley, managing director of LMT UK (01676 523440), aerospace and specialist users of composites are presenting real challenges to the tooling industry. "This has led to Onsrud setting up a specialist aerospace team with a variety of disciplines to cover the demands of honeycomb milling, milling and routing, drilling and trimming of the ever growing multitude of different composite types. What is important for the UK is that we have direct access to these specialists to call upon application support." THE MAIN CHALLENGE According to Onsrud's engineers, the main challenge facing production, which is being pushed down the line towards the tooling supplier, is to avoid the creation of heat in any cutting process that involves composite materials. As machining is one of the final stages of production, components are close to the most expensive phase of their manufacture and, unlike metal parts, cannot be simply repaired or easily saved. As many composite parts now incorporate an integrated design to replace an assembly of lower cost smaller metal parts that could be repaired or replaced as separate items without destroying the assembly, this is no longer the case, so there is a high risk of scrapping very expensive components. The abrasive nature of the materials and the strength of individual fibres are major contributors to rapid tool wear, added cost and lower productivity. Additional problems when processing are created by the dust-like particles that act as a grinding media when they cannot be evacuated from a blind hole or a blind cut on a trimming operation. Maintaining a sharp edge is critical, but the strength of material fibres can approach, or even exceed, the base levels of a typical tool material. Under such circumstances, the tool's edge will blunt followed by tool failure and/or part damage. When drilling, the percentage of fibre fill can cause significant differences to the process, especially on tool entry and breakout. Side cutting loads bring a wholly different set of problems when milling and routing, with the added implications of dust and compaction. This is well demonstrated when working on unified tape, which has very strong fibres, resulting in high levels of material compaction. Other parts, such as those that have been filament wound, have similar fibre strength, but introduce added problems of voids in the material, leading to vibration and chatter during cutting and resulting in de-lamination. This not only applies to the upper and lower covering layers of the material, but also the hidden-from-view in-between layers. Tom Cornwell, Onsrud's applications specialist, explains that the aircraft sector mainly use carbon fibre reinforced plastic (CFRP), fibre glass (FG) and Aramid – the generic name for Kevlar. He suggests the resin systems and fibres used in these materials are unlikely to change with any significance over the next two or three years, with this largely due to the high levels of testing needed to meet safety standards. But developments there will be, and he believes the biggest challenge to tooling in the near future will come from materials with higher strength and wear resistance. The aerospace industry wants materials to be thinner and ever lighter in weight, but the downside is they will be more susceptible to damage in any trimming and drilling processes. Conflicting with this, on the production side ever greater savings in machining are being chased to lower the price of the finished product. Mr Cornwell describes a recent example of problems thrown up by thinner composites. Onsrud was involved in trials on a large high strength bulkhead for a business jet aircraft that had to be trimmed in a single pass, without any in-cycle tool change. Existing carbide tooling could not achieve any better than 25 per cent of the designated cutter path, due to the resonance created from the thin section of the part. Such was the problem that it would destroy a polycrystalline diamond (PCD) tool – normally the first choice for improved tool life. "Due to the process development within Onsrud, we had performed a similar trial using diamond film coated solid carbide routers (Onsrud 67.000), which had provided a viable solution," he offers. "When applied to this task, we were able to extend the production life of the cutters to between three and four components." TRIMMING AND ROUTING In trimming and routing, Mr Feimster outlines that it is important that tools are free-cutting to avoid any 'binding' and, in particular, the generation of heat in the component. Cutting tools must overcome the high abrasion factor of materials and compensate for any differences in the proportions of resin and adhesives. Another problem facing tooling is the varying concentrations and strengths of fibres in the material, which puts high demand on the ability of the tool to shear the fibres cleanly – a prime requisite of the cutting process. Also, the growth in stacked materials, such as those containing titanium and aluminium, pose further challenges for development of totally different drilling solutions. Aerospace design teams, in particular, are also increasing their demands. While production engineers want to produce holes on a machine in one shot without any lost time due to tool changes to centre, drill and then ream a hole to size, there is a trend by part designers to impose tighter total tolerances on holes - often within 0.10 mm. As Mr Cornwell comments, this has created a shift to combination tools – called 'dreamers' – that centre drill, drill and ream. Onsrud is offers that 5-axis machining is being more widely used for complex geometry and larger composite parts, and this is adding to a combination of different processes. Initially, water jet and lasers are used to rough cut, due to the speed achieved, lower stress imparted to the material, and a significant cost advantage over round rotating cutting tools. However, these processes cannot achieve the desired level of edge finish, which is where Onsrud's routing tools and the application of 5-axis machining come into their own. Tools that have long cutting edge ratios of 3 to 4 times the diameter of the tool are best reserved for finish machining, to overcome the compound loads generated by 5-axis machining strategies around changing profiles, and especially from compound contours. Mr Cornwell explains that in most 3-axis machining programs, tools experience a bending moment and a uniform torsional load, due to the constant cutting depth. However, in 5-axis mode tools progress along a compound contour or profile and into corners. During the pass of the cutter, the edge of the part being machined will move up and down the contact area of the cutting edge as it progresses, varying bending moment and torsion. In addition, when the machine feed slows or comes to a halt at a point of surface transition, care has to be taken to prevent a rubbing action, which will allow heat to soak into the part and create delamination. RECENT DEVELOPMENTS Among recent tooling developments is Onsrud's 66-900 Series High Performance Composite Router (HPCR) for hand-fed and CNC applications. HPCR breaks new ground by overcoming heat generation problems created by conventional diamond tooth pattern routing tools more commonly referred to as 'burrs'. Image: LMT Onsrud's 66-900 breaks new ground With deep cutting flutes, HPCR has an advantage, enabling chip flow to be increased, which means cut material quickly and effectively evacuates from the cutting zone. As a result, heat is rapidly dissipated from both the workpiece and cutting tool, giving the benefit of higher speeds and feeds, coupled with extended in-cut life for the tool. When used in hand-controlled applications, cutting forces are reduced for the operator, due to the tool geometry. SERF – which stands for Sinusoidal Edge Rougher and Finisher is another recent introduction for roughing and finishing composite materials reinforced with glass and carbon fibre. The new tool range overcomes problems normally associated with multi-layer structured materials and prevents fibre breakaway. Because it cuts cooler, the effects of temperature on the resin used in the material adhesive are reduced. Tools between 6 and 12 mm diameter can be run at up to 18,000 rpm at feed rates of 3 m/min, due to the optimised geometry in the design that reduces mechanical loading on both materials and tool for extended 'in-cut' life and high surface finish. Image: The Sinusoidal Edge Rougher and Finisher is for roughing and finishing composite materials reinforced with glass and carbon fibre Special parabolic groove geometry that reduces thrust forces has also been developed by Onsrud, which provides more manageable chip formation, while providing better centring and allowing higher penetration rates to be maintained when drilling both carbon fibre and carbon graphite materials. Reflecting back to its development expertise of the highly successful compression router for machining wood laminates in the 1990s, LMT Onsrud has now incorporated similar tooling expertise into its 66-800 Series of 6-flute Compression Routers. These tools are based on a solid carbide router with a diamond film coating. Image: LMT Onsrud's compression router, based on a wood laminate machining design The 66-800 combines upper-cut and down-cut spirals, which overcome any problems of delamination when machining. The upper-cut spiral exerts an upward pressure on the bottom of the material, while the down-cut spiral pressurises the top of the material, which compresses the panel being processed. In addition, to prevent any escape of resin from the material, heat is dissipated by the open flute geometry and positive rake angle ground into the tool. AlTiN (aluminium-titanium-nitride) coating or diamond film coatings can be supplied to improve performance. The development by Onsrud of the Parabolic Flute Form Drill also overcomes problems associated with delamination, flaking, splintering and whiskering, and gives improved chip flow and abrasion resistance due to the multi-layer, nano-crystalline diamond coating. While just six microns in thickness, the coating creates a very low coefficient of friction between 0.15 and 0.2. Cutting trials have demonstrated an ideal speed of 75 m/min at feed rates of 0.075 mm/rev to obtain the best surface finish. First published in Machinery, November 2010