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In the present study, a fully controlled experimental protocol is used to investigate the difference in milling Invar and HexTool® for composites curing tools. The main difference is the use of the different strategies: conventional milling is better than climb milling, high feed/low speed better than low feed/high speed, high depth of cut better than low depth of cut, air coolant better than water coolant, diamond coating better than solid carbide, sand finishing better than chip removal, torus tools better than ball nose tools. Therefore, using metal machining strategy on composite curing tools leads to cracking and delaminations in the composite surface which causes the resin flow to infiltrate these cracks during the molding process. Consequently, parts demolding will destroy the surface of the curing tool and make it unusable without repair. The new technology approach will lead to a higher integrity of the composite surface and enhance the life cycle of the curing tool.

On CNC Machines, drilling holes under perfect condition is possible. For drilling holes into titanium, composite and aluminum stacked materials the specific cutting condition can be selected. Furthermore surrounding conditions such as peck cycle, MQL and force and torque monitoring can be easily adapted. When drilling holes in the final assembly, CNC machine tools cannot be employed due to sizes and accessibility. Power Feed Units or Automated Drill Units ADUs are very handy, flexible and depending upon the jig extremely rigid. Whenever a machine tool does not fit, ADUs are highly recommended. In comparison to machine tools, conventional pneumatic ADUs can be used with one fixed set of feed, speed and micro peck only. Due to that a compromise in cutting condition has to be chosen in drilling stacked material with different layers.

The present paper characterizes the difference between metals and composite repairs in aerospace application. The main difference, focus on 5 axis scarfing instead of 2 axis sheet metal cutting, using a ball nose tool in comparison to a Torus type end mil. Even high feed / low speed strategy comes out to a better surface finish and longer tool life. Using the higher step over on special typed end mills, this increases the tool life and consequently, lowers the cost per part. The new strategy and process understanding leads to more economic in onsite and out site repair solutions.

This article describes the physical background and the experience in the drilling of carbon fiber and aluminum-carbon fiber stacks. Low temperatures and intelligent chip removal technologies are the most important requirements for dry drilling or to avoid the MQL (minimum quantity lubrication). The drilling in one shot and in IT8 quality is mandatory. In case of machining metal, like aluminum or titanium, a lot of heat is generated by the tools and the cutting process. Machining of composites, the material and the tool should remain as cold as possible even by drilling without external or internal coolant. A new drill design is now developed, qualified and patented by MAPAL that allows the dry drilling of metals at very low temperatures also. We are now able to drill, all batches (composite / aluminum) without MMS. The high drilling feed and due to that, the shorter contact length between the tool and the material stack also gives us approximately twice the tool life.

Ever since the advent of fiber reinforced polymer materials in the field of Aerospace, Metal-FRP stacks started to gain importance due to their superior fatigue performance, phenomenal low weight and good specific strength. However the machining, specially drilling these multi stack materials has always proved to be a challenge for the field of manufacturing and assembly. Drilling holes in only metal with a drill (metal drill), the material removal is through a process of clear shearing since the tool is much harder and sharper than the base material. The tools hence wears at a much slower and gradual rate, also the malleable properties of the machined metal compensate to the reduced cutting capability of the worn out drill. These properties of the machined metal act like a ‘FAIL SAFE’ mechanism during the machining process assuring a trouble free fail safe environment during the drilling process. However, drilling FRP composites is altogether a different story.

Drilling holes into metal with MQL (Minimal Quantity Lubrication) is a normal procedure, because the drill is designed for drilling metal and the malleable capability of the metal compensates for the insufficient cutting capability of a worn out drill. Drilling composite materials using the same drill (designed for drilling metal) is a different procedure, because composite fibers are not malleable like metal at all. Due to this fact the tools become very hot trying to forge composite fibers like metal. The elastic behavior of the composite and the delamination inside the hole makes the tool temporary smaller than the diameter of the drill. The hole in the metal part of the stack remains slightly larger due to the heat and the thermal expansion rate. This paper shows how to drill metal and composite with the same diameter, so that achieving H8 quality is no longer a dream.