Drilling Advanced Aircraft Structures with PCD (Poly- Crystalline Diamond) Drills

Transcription

1 ATC-18 Drilling Advanced Aircraft Structures with PCD (Poly- Crystalline Diamond) Drills Richard Garrick Precorp Inc., Utah U.S.A Copyright 27 SAE International ABSTRACT With the increased usage of Carbon Fiber Reinforced Plastics (CFRP) in the newest generation of commercial aircraft, the opportunity for using PCD drills has also increased. PCD has long been the preferred solution for the drilling of CFRP. However, given the manufacturing demands of commercial aircraft, a single drilling solution would be required to drill all possible material stack combinations which include CFRP stacked with aluminum (Al) and CFRP stacked with titanium (Ti). All of these possible combinations must be drilled with accuracy, excellent surface finish, minimal coolant usage and minimal exit burr, over the course of thousands of holes in the least amount of time possible. Precorp Inc. has developed a PCD drill that successfully drills all possible stack combinations while meeting the quality criteria. This drill is referred to as the 86 series PCD drill. The 86 series has been applied to an automated drill/fastening system as well as pneumatic power feed applications for the hole generation in CFRP, CFRP/Ti and CFRP/Al stacks. INTRODUCTION Increasing fuel prices combined with a competitive market have forced builders of commercial aircraft, such as Boeing and Airbus, to change the structures of commercial aircraft to be lighter and more efficient. This has called for a higher usage of CFRP. The first commercial aircraft to incorporate CFRP in bulk as the primary structure was the Boeing 777, first put into service in This plane incorporated 1 tons of CFRP in its structure. The next generation of aircraft, the Boeing 787 and the Airbus A38 will incorporate approximately 35 tons of CFRP in their structure. This fundamental design change in these aircraft will translate to a 2% more fuel efficient plane compared to other models of it same size. Structurally, 5% of the weight of the Boeing 787 will be CFRP. 2% will be aluminum and 15 % will be titanium, the remainder being steel and various materials. 1 The use of these materials combined with the manufacturing demands of the commercial industry presents new problems. Traditionally, aluminum has been the major component of the commercial aircraft structure. Easy to drill, the industry has instituted automated drill and fastener systems that allow a hole to be drilled with inexpensive carbide or high speed steel drill bits and a fastener to be inserted in 4 seconds. This industry now must apply these same automated systems to materials that do not allow for the use of a high speed steel and carbide due to the abrasive nature of the CFRP. In addition, PCD has long been the preferred method of drilling CFRP, 2,3 however tungsten carbide drill bits have been the preferred tool for drilling titanium. Given the current science, these automated systems would require a tool change per the material stack drilled. In Addition, traditional pneumatic, power feed drilling methods will continue to be used in this new generation aircraft. This process consists of a portable drill motor, mounted on a drill plate with mating holes in the required drill pattern. The drill bit in the drill motor is supported by a bushing. The drill feed is controlled by a threaded shaft which passes through a pneumatic driven motor that feeds the drill bit when the pneumatic motor is actuated. This process is a less rigid process than a computer controlled process and more susceptible to drill bit failure as a result. PCD has never been considered a solution for this application given its fragile nature. In the new generation aircraft structures, two materials are stacked, and then a hole is drilled and a fastener is inserted, thereby fastening the two materials together. In the majority of the cases, the two materials stacked are CFRP/CFRP. However, in areas of high load and stress, CFRP may be stacked with aluminum or titanium. Although the majority of the holes are in CFRP/CFRP or CFRP/Al which are easily drilled using PCD at high speed and feed rates, the limiting factor for the manufacturing will be the drilling of the titanium which is traditionally drilled at lower speed and feed rates using tungsten carbide drills..25" drill Stack Material CFRP/CFRP CFRP/Al CFRP/Ti Tool Material PCD PCD WC Layer 1 Speed (rpm) Feed (in/min) Layer 2 Speed (rpm) Feed (in/min) Life(holes drilled) Table 1. Speed and Feed matrix for.25 drill in different stack materials

2 The challenge set forth is to develop a common drill that can be used for all material stacks, to maximize life and minimize cycle time in both CNC drilling environments as well as pneumatic, power feed environments. MACHINING TITANIUM The following characteristics of machining titanium are the reasons why PCD has not been considered a valid solution to drilling titanium. Titanium is a poor conductor of heat. Heat generated by the cutting action does not dissipate quickly. Therefore most of the heat generated during the cutting action is directed to the cutting edge causing the edge of the tool to be subject to high thermal loads due to the relatively low specific heat, thermal conductivity and density of titanium. 4,6 Titanium has a strong alloying tendency or chemical reactivity with materials in the cutting tools at tool operating temperatures. Titanium has a low modulus of elasticity. This requires rigid systems and the use of sharp, properly shaped cutting tools to prevent the material from moving away. Titanium fatigue properties are strongly influenced by surface damage. High surface finishes must be maintained to maximize fatigue properties. Titanium s work-hardening characteristics are such that titanium alloys demonstrate a complete absence of built up edge. Because of the lack of stationary mass of metal (built up edge) ahead of a cutting tool, a high shearing angle is formed. This causes a thin chip to contact a relatively small area on the cutting tool face and results in high bearing loads per unit area. The high bearing force, combined with the friction developed by the chip as it rushes over the bearing area, results in a great increase in heat on a very localized portion of the cutting tool. 4 Figure 1. broken PCD edges of drill after drilling Titanium Given the brittle nature of PCD, it is clear that it would be unable to work in the same operating range as carbide if it were to be a viable solution for drilling titanium. The use of low cutting speeds would be required. A change of 2 surface per minute to 16 surface feet per minute using carbide tools results in a temperature change from 8 to 17 F. 5 PCD would clearly need to be applied at the lower end of the operating range with regard to speed without applying excessive thrust to the tool so as to cause the point to fail. Given these characteristics of machining titanium, tungsten carbide is the preferred method for drilling titanium. Keeping these characteristics in mind, once applied correctly, the tungsten carbide tool simply wears. If these same principles of machining titanium were applied correctly to a PCD drill, under the proper drilling conditions, a PCD drill would be effective in drilling titanium since the ultimate condition is normal wear. PCD drill failure in titanium is a mechanical failure. Due to brittle nature of PCD, and the work hardening properties of titanium, classic failure is an edge failure. Figure 2. Showing point failure due to excessive thrust In order for PCD drills to be used successfully, the following would need to be achieved:

3 1. A robust drill design with the strongest possible geometry, while maintaining proper geometry for cutting other materials such as CFRP and aluminum. 2. Speeds and feeds would need to be optimized so as to find the narrow window in which the balance of forces is such that PCD may survive any mechanical failure. 3. The coolest possible drilling environment must be used. PCD VEINED DRILLS PCD veined drills were first patented in 1987 for use in the printed circuit board industry. Since that time, PCD veined tooling has found application in the medical, ceramic, green carbide, metal matrix composite, automotive and aircraft industries. PCD veined drills are manufactured by grinding a slot in a tungsten carbide blank. The slot is filled with diamond powder then cycled through a PCD sintering process using a high temperature/high pressure press. The end product is a tungsten carbide blank with a vein of PCD. This vein configuration may be ground at any geometry needed for the cutting tool. This blank is then ground into a cutting tool with cutting edges consisting of PCD. The advantage of the PCD veined drill is its ability to have positive cutting geometry. It also allows for smaller sized drills to be manufactured. Non-veined PCD drill technology incorporates a flat, silver solder brazed wafer. These tools do not have positive geometry and are limited by the braze joint surface area for smaller sizes. 3 PCD DRILLS USED IN DRILLING TITANIUM In the development of a PCD drill for titanium, the first step was to apply technology already proven effective in CFRP and CFRP/AL. Understanding what would be required, a stronger drill, run at slower speeds was developed. This drill is referred to as the 86 series. Figure 4. Carbide drill used for drill CFRP/Ti stacks Once operating parameters were optimized, a.25 PCD veined 86 series was used in drilling CFRP/Ti and compared to a standard carbide drill commonly used in the industry today for drilling CFRP/Ti stacks. A Makino A55 horizontal machining center was used with flood coolant. A peck cycle of.4 peck depth was used for the titanium portion of the stack. Peck cycles are commonly used for CFRP/Ti stacks to prevent damage caused to the CFRP by the titanium chip exiting the hole..34" thickcfrp/.242" thick Material titanium CFRP RPM 6 Feed 6"/MIN Ti RPM 7 Feed 1.16"/min Peck Depth.4" Table 3. Operating parameters of.25 diameter 86 series, PCD veined drill in CFRP/Ti stack Figure Series Veined PCD Drill

4 Hole Size Exit Burr (IN) Size Size(CFRP)(in) Size(Ti)(in) Size (in) Exit Burr (IN) Hole Number Hole Number Standard Drill Hole Size Standard Drill Exit Burr Size Size (in) Size(CFRP) Size(Ti) Size (in) Series Hole # Hole # Graph 1. Graphs comparing hole size using 86 series veined PCD drill (upper graph) in CFRP/Ti stack compared to using a standard carbide drill (lower graph) The 86 series demonstrates a consistent hole size difference between the CFRP and the Titanium of approximately.4 over the course of two hundred holes. The carbide drill showed hole variance over.1 over the course of 34 holes. Graph 2. Graphs comparing exit burr size using 86 series veined PCD drill (upper graph) in CFRP/Ti stack compared to using a standard carbide drill (lower graph). Exit burr specifications for titanium most drill and fastener systems is for the exit burr to be less than.8. The PCD drill produced an exit burr less than.8 up to 14 holes. The carbide drill only produced 4 holes with an acceptable exit burr over 34 holes. Torque (Ncm) Torque(Ncm) Hole Number Graph 3. Torque measured on drill while drilling CFRP/Ti stacks

5 Thrust(N) (N) 2 15 Thrust(N) Hole Number Graph 4. Thrust measured on 86 series veined drill while drilling CFRP/Ti stack Torque and thrust followed consistent increases over the course of the 2 hundred holes. Accepted manufacturing practice is to re-sharpen a PCD drill at 335 N of thrust. This insures maximum life of the drill while not running a risk of breaking the drill. As shown in the pictures below, a wear land has formed on the cutting edge of the 86 series PCD veined drill after the 2 holes in the titanium. This is classic wear that is easily re-sharpened. The drill may be put back into service after re-sharpening and expected to achieve the same results as the new drill. Figure 4. Wear on 86 Series PCD drill after 2 holes PCD drills Applied To Automated Drilling Systems Having proved that the 86 Series can successfully drill CFRP/Ti stacks, the next step was to apply it to the automated drill fastener system in which a hole must be drilled to finish tolerances, and a fastener is automatically inserted in one clamping. This would require that the drill produce a hole within the.3 tolerance, with an exit burr that cannot exceed.8, an interlaminate burr that cannot exceed.3, and maintain a surface finish less than 64 micro inch. Exit burr size ultimately became the most difficult quality requirement to achieve therefore, exit burr size became the indicator of tool life and hole quality. The process of automatic drill and fasten systems is the part is clamped, a hole is drilled through the material, a fastener is inserted into that hole, and a collar nut is swaged onto the fastener. Traditionally, when drilling and fastening CFRP/CFRP, CFRP/Al and CFRP/Ti stacks, the hole is drilled, then the two panels are pulled apart, cleaned, de-burred, then put back together for fastener insertion. The challenge set forth is to produce a hole that exceeds quality requirements without the

6 need to break down the panel to clean and de-burr the hole..12 Burr Data One of the difficulties of the application was coolant was not an option due to requirement of not being able to clean the part. Instead, a minimal quantity lubrication (MQL) system would be applied. MQL is a cutting lubricant that has been atomized into an air stream that flows through the tool. MQL serves as a cutting lubricant and lacks the cooling properties of liquid coolant. Although it does reduce heat caused by friction, this poses problems as to the life of the tools due to the critical role coolant plays in the machining of titanium. In order to qualify the process of the automatic drill fastener system, sample coupons would need to be drilled using the qualified tooling. Those sample coupons would then be fatigue tested to determine the strength of the hole. Preliminary testing of various tools qualified two tools to be used in the fatigue coupon drilling test. They were the Precorp 86 series PCD veined drill and the Precorp 4DH series tungsten carbide drill. Deviation (Inches) Test Hole Number Graph 7 PCD 86 Series PCD Drill Burr Data.12 Burr Limits EXIT BURR inches Hole Diameter Delta Diameter (Inches) CFRP H DIA 1 inch max Ti H DIA 2 inch max Upper Limit Lower Limit Deviation (Inches).8.6 Burr Limits ILENBURR inches EXIT BURR inches Test Hole Number Graph 5 86 Series PCD Drill Test Hole Number Diameter (Inches) Hole Diameter Delta Test Hole Number CFRP H DIA 1 inch max Ti H DIA 2 inch max Upper Limit Lower Limit Graph 8 4 DH Tungsten Carbide Drill Graphs 7 and 8 show exit burr size produced by the 86 Series PCD drill compared to the 4DH series tungsten carbide drill. Once the 86 series and the 4 DH series were qualified to be used in the fatigue panel drilling, open hole fatigue panels were drilled. These panels consisted of two holes drilled into a titanium coupon. Seven coupons of each possible size (.1875,.25,.315,.375 ) were produced using both the 86 series and 4 DH series. Graph 6 4 DH Tungsten Carbide Drill Graphs 5 and 6 show hole size produced by the 86 Series PCD drill compared to the 4DH series tungsten carbide drill. PCD Drills Applied To Power Feed Applications Due to the nature of power feed drilling, PCD has not been considered a viable solution for use in power feed motors. The major failure in this application, is the PCD chipping while passing through the bushing.

7 Figure 5. Typical PCD drill chipping when used in a power feed application. In order for the PCD drills to work successfully in a power feed application, it would need to be supported by tungsten carbide and the edges would need to be strengthened to survive the varying forces inherent in the power feed drilling process. In turning applications, k- lands are typically used to increase edge strength when turning in interrupted cuts. Those same principles have been applied to the 86 Series PT tool. Figure series PT tool showing minimal amount of PCD exposed to prevent chipping in bushing. Arrow shows tungsten carbide supporting PCD This tool was applied to the drilling of1 holes in a stack of.5 CFRP and.375 Ti, at 5rpm and.2 / rev, using a single Precorp 86 PT drill bit combined with a Quackenbush 15QDA drill. The drill diameter was Micro-cut coolant was used and delivered through the tool via a coolant pump. The 86 PT drill produced good quality holes with regard to hole size (see graph 9) in the CFRP/Ti. The testing was stopped at 1 holes to conduct a thrust measurment. Figure Series PT tool showing k-lands applied to cutting edges. When generating the flute of the 86 PT tool, less than.1 of PCD is exposed down the O.D. of the drill bit. Typical PCD wear is less than.2. The minimal PCD exposed provides the necessary protection for the PCD so that it may pass safely through the bushing of the power feed tool. Figure 8A

8 the aircraft industry. To maximize the usage of PCD and to reap the benefits of its usage, PCD may be applied to drilling CFRP/Ti stacks. Because temperature is a large factor in machining titanium, best results are achieved when using liquid coolant. The operating range of PCD veined drills in titanium is narrower than that of tungsten carbide however once optimized, it can be applied successfully. PCD tools may be modified with k-lands to strengthen the cutting edge to make PCD drills viable for power feed application. The application of PCD veined drills to CFRP/Ti stacks increases tool life and improves hole quality thereby reducing tool changes and increasing production and tool value. Figure 8B Figure 8. A) Cutting edge of new tool B) Cutting edge after drill 1 holes in CFRP/TI in power feed application, In an attempt to qualify this process for one up viability, exit burrs were measured. The 86 PT tool does not produce exit burrs less than.8 for a sustained amount of time. However, they averaged consistently below.2. CONCLUSION ACKNOWLEDGEMTS The Author would like to thank the following people for their support and input to this project. Dan Thurnau, Spirit Aerosystems Gary Calvert, Spirit Aerosystems Kim Nixon, Precorp Inc. Richard Hopkins, Vought Aircraft Tanni Sisco, Boeing Alan Merkley, Boeing Ed Feikert, Boeing Broetje Automation Bill Jones, Production Tool and Supply With the increased usage of CFRP in commercial aircraft, PCD will be used more abundantly throughout Pow er Feed 86PT 1 Holes Hole # Diam. CFRP Diam. Ti Graph 9. Holes size of 1 holes drilled in CFRP/Ti using 86 PT PCD drill. Portable Tool Burr Height Burr Height (in) Hole # Exit Burr Height Graph 1. Exit burrs measured on 1 holes drilled by 86 PT tool.

9 REFERENCES 1) Kamiura, Masayoshi, Toray Carbon Fiber Composite Materials Businesses, Toray IR seminar- No.7, June 6, 25 2) Abrasive Technology, Techview, AT Everlast TM Veined PCD Drill-A Case Study 3) Bunting, John, Precorp Inc., Drilling Advanced Composites with Diamond Veined Drills. 4) Machining Titanium and its Alloys. 5) Trucks, Dr. H.E., Machining Titanium Alloys, Titanium Industries Inc. 6) Deutsche Titan GmbH, Machining Titanium and Titanium Alloys Nov. 2