MMC Magazine

Since the launch of the Boeing 787 aircraft in 2011, the application of CFRP has gradually increased as a new material for airframes, main wings and other aerospace components. Different from metals, CFRP is made of carbon fibre and resin; and the machining of this new material requires new techniques. We interviewed Gifu Aero Group staff who were engaged in the development of machining techniques for this important material.

What are the peculiar phenomena encountered when drilling CFRP?

– What is the background of the development?

Yanagida: Mitsubishi Materials has delivered drills to customers for machining CFRP for more than 10 years. Utilising the know-how we accumulated over the years we have improved drill functionality to provide a broader range of solutions that are applicable over a wide variety of CFRP materials.
CFRP has layers of carbon fibre and resin that are heat treated. Compared with steel, CFRP is around a quarter of the weight but 10 times as strong. It also features corrosion-resistance, heat-resistance and high rigidity. While the carbon fibre layer is hard but brittle, the resin layer is soft but more plyable.

Yamamoto: This is why CFRP machining produces phenomena that are funda-mentally different from machining metals. The major defects encountered in CFRP drilling are the generation of uncut fibres, delamination due to the layered structure and back countering of stacked CFRP and metal (back countering is caused by the metal chips wearing the sides of the hole in the carbon section of the stack as they travel up the flutes of the drill). In this project, we started by checking these phenomena to thoroughly explore the technical mechanisms that generate the defects.

Yanagida: There are two types of CFRP material used for aircraft parts. One contains resins with thermosetting properties and the other is stack material made by layering aluminium and titanium into CFRP. There are also two major machining methods, one is automated machining utilising for example a machining centre, and the other is to drill using hand tools. Due to these different materials and machining methods, it is extremely difficult to create one type of drill suitable for all processes. Therefore in this project, we developed the MC series of seven different drills for a wide range of CFRP materials and they were introduced to the market in April 2017.

MCA Groove Structure for Reducing Stack Materials Back Countering

– Would you please introduce some of the seven products?

Yanagida: I will show you two types, the MCA and MCC. The MCA is a drill for CFRP and aluminum stack materials. We sought to significantly improve the performance of the CFRP drills that have been available as special products for the past 10 years. We usually drill stack materials composed of carbon fibre and aluminium; whose machinability is completely different, with the same drill. The main problem is a phenomenon called back countering. As the drill penetrates the CFRP and machines the aluminium layer, the expelled aluminium chips can cut the CFRP surface. As a result, the hole diameters in the CFRP and aluminium layers differ. To prevent this, we changed the flute design of the MCA drill.

Yamamoto: We focused on the width of the flutes. The flute width is usually the same along the whole length of the drill; however, the flutes of the MCA gradually widen from the cutting edge to the top. First, we designed narrow flutes to generate compact chips, and then to widened them to help the chips flow along the flutes without interfering with the surface of the hole.

Yanagida: We applied Mitsubishi Materials MWS deep hole drill technology to provide the smooth discharge of chips. This was required to provide an increase in the surface quality of the hole, which was a common problem in the machining of both stack materials and deep holes. For the MCA drill development, we also utilized the technology of MHE drills that are used to machine automobile hubs. MHE drills are used to create holes for bolts on the hubs that connect automobile axles and wheels. The size of the hole diameter in each hub must be very precise, and the surface quality of the holes needs to be extremely high. Preventing the chips from damaging the surface of the hub requires the MHE to have narrower flutes than regular drills.

Yamamoto: As a result, MCA used the features and know-how of the MWS and MHE drills. This means that overall, the drill initially generates small chips that flow through the narrow part of the flute. The chips are then channeled through the broadened upper part of the flute and discharged without damaging the wall of the hole.

Positive Edge Prioritizes Cutting Quality

– Please tell us about the background of the developmant of the MCC drill?

Yamamoto: While MCC is designed specifically for the machining of CFRP, aerospace components also use stack materials. The automobile and wind generation industries also use CFRP materials. The customers that machine CFRP materials often need to drill holes in thin plates.

Yanagida: Reducing delamination at the end of the hole was the biggest issue when drilling CFRP. CFRP doesn’t have a layer of metal at the exit of the hole that stack materials have and doesn’t suffer from back countering. However, the exit of the hole can fragment, which prevents support for the cutting resistance generated when the drill penetrates into the CFRP layer and causes burring at the exit of the hole.

Yamamoto: We prioritised the sharpness of the MCC drills for smoother CFRP machining and reduced cutting resistance to prevent delamination. The most important aspect of the MCC drill is the sharp edge. Drills traditionally feature a negative rake angle to prioritise deflection resistance and prolong tool life. However, a negative rake angle is not capable of cutting hard carbon fiber layers smoothly and this meant that the MCC drill benefitted from a sharper geometry. While cutting CFRP smoothly, it also restricts delamination and the generation of uncut fibres at the exit of the hole. In addition, the 90-degree edge angle reduces the thrust at the beginning of the drilling process, which also helps to reduce delamination.

– What are the characteristics of the coating?

Yamamoto: CFRP has mechanical characteristics that cause abrasion immediately after the start of drilling with uncoated cemented carbide drills. To address this we applied CVD diamond coating to the MCA and MCC drills to increase wear resistance.

Yanagida: To maximise the sharpness of the drill edge, we needed to consider both the form of the edge and the size of the diamond coating particles. Mitsubishi Materials new CVD diamond coating particles are extremely fine, which significantly increases adhesion and we were able to increase tool life by approximately 10 times compared with conventional coatings.

– What have you done to improve sharpness?

Yamamoto: In order to increase sharpness, which was our priority, we sought the best method of processing the edge and maximising the helix, rake and clearance angles, which are all basic elements of all drills. We examined individual combinations of angles to identify the best match for preventing damage to the drill. In general, the greater these angles are, the better the sharpness becomes. However, cemented carbide is a brittle material and has limited deflection resistance.
In addition, the combination of the drill elements and materials determine the final performance, which meant that we had to test the drills repeatedly to gauge effectiveness. To increase the sharpness, the edge processing I mentioned earlier is also important. The conventional drills produced by Mitsubishi Materials have tiny imperfections on their edges due to pre-coating process. However, for the MCC drill the edge processing is completely different from conventional drills, which made it possible to create a really smooth, even edge. Utilising this new edge processing method enabled us to realize both sharpness and strength, which led to the prolongation of tool life and improvement of hole quality.

Yanagida: To help development of the MC series, we conducted joint research with the Vienna University of Technology (TU Wien) in Austria. When we needed to conduct machining tests for the prototypes with different thicknesses of coatings on the drills, edge forms and rake angles, we asked TU Wien for cooperation and acquired a lot of data from them, which we believe contributed significantly to the success of this major innovation.

Difficulties and Achieve-ments in the Development of New Materials

– What were the difficulties you encountered during the MCC drill development?

Yamamoto: Along with the challenges we faced, we also enjoyed learning about the challenges of CFRP machining. I was transferred to the Gifu Aero Group, Aerospace Dept. when it was established in October 2016 to begin working on the MCC drill development. I had experience in developing drills for metals, but this was the first time for me to work on drills for CFRP.

Yanagida: The developers in this department, including Yamamoto, created prototypes. We operate the tool grinding machines ourselves, considered the best conditions, including cutting angles, speed and types of grindstone and continue creating prototypes as we cultivate our sense as engineers.

Yamamoto: We repeatedly reviewed grinding conditions with a priority on ensuring sharpness. During those processes, we selected promising prototypes for futher testing and asked customers to check their quality and performance. When we heard the words, “This is much better than the drill we are using now,” we were naturally very pleased.

Yanagida: Since we design, manufacture and test prototypes by ourselves, we immediately detect even the slightest difference in performance. Yamamoto created the MMC prototypes and because of this he had a few ideas that he applied to product development. This enhanced our ability to deliver an outstanding product.

– Would you please tell us your plans for future drill development for CFRP materials?

Yanagida: The critical requirement in the manufacture of aircraft parts is safety. Prolonging tool life is also an important goal, but the quality of the hole is the priority and we strive to realize both. We predict that CFRP strength will improve and that the appearance of new stack materials combined with stainless steel will mean that overall, materials will become increasingly harder to machine. Mitsubishi Materials continues to conduct joint research with carbon fibre manufacturers and work with universities engaged in cutting-edge research to deepen our understanding of CFRP machining and out ability to respond to ever-changing market needs.

– As we conclude our interview, do you have anything to say to your customers?

Yanagida: JIS and ISO have not yet classified CFRP structures. There are many different types of carbon fibre resins, thicknesses and weaving methods. Therefore it is necessary to tailor drills to the material being used to ensure the highest quality of holes. We are ready to satisfy customer needs, so please feel free to contact us.

Yamamoto: The MC series is listed in the catalogue as standard products. However, I believe that the MC series should be tailored to fit individual customers. We strive to quickly and effectively satisfy the needs of customers, so please feel free to consult with us.