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Milling and drilling the tough materials

News International-French

22 May 2013

The goal of aircraft weight reduction is driving the current increased use of advanced exotic materials such as composites, titaniums and Inconels in the aerospace industry. While materials such as these are lighter, yet stronger than those typically used, they each present their own sets of challenges when it comes to machining, more specifically milling and drilling.

For milling and drilling operations, aerospace manufacturers often use solid carbide and/or solid high-speed-steel tools. During those machining operations, these manufacturers must achieve the highest levels of quality possible, often accomplished through carefully monitored and maintained process security. There are concerns about cost per part, but in most instances, producing perfect parts is a much higher priority, and increasing productivity tends to be secondary.

Aerospace manufacturers strive for process security and consistency through predictable performance of machines and tooling. In the case of tooling, these manufacturers must have milling cutters and drills that deliver practically the same exact amounts of tool life from one tool to the next. And even when they do know precisely how long a tool will last, aerospace manufactures often schedule machines to exchange tools well before they are completely worn.

Thanks to machine and cutting tool technologies, materials such as composites, titaniums and Inconels have advanced from a stage of being almost impossible to machine to a point today where aerospace manufacturers machine them with confidence and efficiency. One tooling technology that gives better process control and consistency is advanced specialised solid rotary mills and drills. These tools have been developed specifically for overcoming the machining challenges presented by these materials. Through the incorporation of various innovative coatings and geometries used in tandem with advanced machining techniques and strategies, these specialised tools will not only provide process security, but will also increase production speed and output.

The market for machining Carbon Fibre Reinforced Plastics (CFRP) materials is surging within the aerospace industry. However, the materials are difficult to machine because they are very abrasive and tough on milling tools. Plus, delamination must be prevented from occurring while machining. These challenges can be overcome with hard, sharp solid-carbide milling cutters that employ special surface coatings.

Two types of coating processes commonly used are Physical Vapour Deposition (PVD) and Chemical Vapour Deposition (CVD), along with an advanced cutting material Polycrystalline Diamond (PCD). PVD coatings involve a physical process and include aluminium nitride, chromium nitride and titanium nitride coatings with hardnesses of approximately 3 000 Vickers. The Diamond coatings that are imparted by the chemical process CVD are about three times harder, resulting in a Vickers rating of 10 000. PCD tools incorporate solid PCD-plates that are brazed to solid-carbide cutter shanks.

From a geometry standpoint, effective cutters for composites incorporate low helix angles to reduce axial forces on the laminate layers of the material to prevent delamination. Additionally, cutters with both a left and right helix are also effective geometries for composite materials. These types of cutters, often known as compression routers, direct and compress cutting forces toward the centres of workpiece thicknesses – in the case of side milling – to keep the laminate layers intact. Plus these types of cutter geometries make for much freer cutting of composites.

While compression cutters are a common approach, some cutting tool companies, such as Seco, have developed compression cutters with new different geometries, such as a double helix. Seco, for instance, developed two such double-helix routers. One is a multi-flute tool with smooth cutting edges.

As far as machining techniques are concerned, cutting parameters for composites are often dependent on the various materials themselves. Typical speeds for solid-carbide cutters for composites are about 150 m/min, and feed rates are around .07 mm. Also fibre content and fibre orientation have a significant influence on the machining process governing cutting speeds and feeds and the optimum tool path.

For aerospace applications, drilled holes in composites must be perfectly clean and without ragged or frayed fibres that can interfere with and compromise subsequent assembly operations.

Two common challenges of drilling composites are delamination and uncut fibres, especially on the backside or drill exit side of workpieces. When drilling, tool forces push down on the material and, as the drill nears the exit side, excessive force can cause the drill to push through, as opposed to cut through, the last portion of the hole. The result is composite fibres that are ripped and ragged instead of cleanly cut, causing material delamination.

To overcome these challenges, tooling companies strive to decrease drill feed forces against the material through the use of different point angles and helix angles on drills. It should be noted that some drill geometries generate lower feed forces and perform better than others.

For example, a 140-degree point angle – the most common for solid carbide drills – will work quite well for several holes when drilling composites. Unfortunately, as soon as the tool dulls at all, it loses its effectiveness. With C1 diamond coated solid carbide drill for composites, Seco imparts a geometry with two point angles – a 130-degree angle in the centre and 60-degree angle on the chamfer of the drill. In operation, the drill’s centre point exits the end of the hole first, cutting away some of the hole’s material. So when the 60-degree portion exits, the feed forces of the drill through the material are drastically reduced. Thus, there is less delamination and fewer, if any, uncut fibres.

In addition to two fluted, diamond coated drills, Seco has developed a unique 3-fluted PCD-tipped drill geometry for composites. Applied with the same cutting conditions as standard composite drills, this new PCD drill geometry provides much better results because three edges are cutting as opposed to only two.
The drills have sharper cutting edges and generate less feed force per revolution, especially when exiting a hole. Additionally, with a full PCD tip, as opposed to diamond coated, the drill can provide up to four times more tool life in many instances.

To effectively machine challenging aerospace materials, the key is to obtain a complete machining solution, not just a product. A complete cutting tool solution includes not only the necessary geometry and design, but application engineering support as well. The knowledge and experience of the human resource combined with the advanced product to form a complete solution and achieve ideal results.

Part quality and process security require the best possible tool designed for the particular application at hand, whether it be composites, titanium or Inconel. But that tooling must be acquired from a supplier able and willing to provide guidance as to the proper way to run it for optimum performance. Education and training is key to getting the most benefit out of today’s advanced tooling designed for tough aerospace materials.

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