Drills for High Tolerance Hole Making in Composite Materials

Each carbon fiber material has its own demands that increase the risk for delamination or splintering.  However, these drills and a countersink tool with a microstop for chamfering use reamer geometries that are ideal for rivet and bolt holes in carbon reinforced plastics and metallic stack materials because they reduce this risk, ensuring the stringent hole tolerances are met with exceptional finish and quality.

Hole making in composites such as carbon fibre reinforced plastic (CFRP) using hand-held drilling machines is an everyday procedure at aircraft factories around the world. The sheer size of structures, such as the main fuselage and wing spars, means it’s often impractical to use a fixed bed machine tool, or even a smaller power feed model.

But taking a portable handheld machine to the aircraft structure is not without its challenges, particularly when it comes to tooling. Tools for handheld machines must demonstrate unique qualities to overcome inherent process drawbacks, such as deflection and the presence of non-axial forces. With the latest aircraft programs containing around 50 per cent composite by weight, achieving competitive advantage in machining operations is vital, particularly as some structures contain thousands of rivet and bolt holes. With this in mind, selecting the right cutting tool via assured applications support has never been more important.

DRILLING, REAMING, COUNTERSINKING
A combination of Precorp’s specialist technology and Sandvik Coromant’s tooling and Application Center expertise are providing aerospace manufacturers with a number of solutions for the hand-held drilling of composite materials. For example, the new CoroDrill452 is based on H10F uncoated carbide substrate and offers optimum performance when drilling, reaming and countersinkingCFRP, both unidirectional and woven types, such as M21E andBMS8-276.

These drills, which are stocked as standard from 2.5 to 12.7 mm (other sizes made to order), are specially designed to offer low thrust force due to an innovative split point geometry design and smooth drill exit resulting from a left-hand helix on a right-hand drill. Here, sharp cutting edges generate high quality holes in airframe structures offering tolerances of ±0.025mm using a drill bush, although this can be improved further to ±0.01mm using the range of reamers (CoroDrill 452.R-C). The contersink tools (CoroDrill 452.C-C) offer users a choice of 100 deg or 130 deg chamfer angles.

OVERCOMING THE VARIABLES
When drilling holes in composite/metallic stack materials, a number of variables are introduced to the process due to the presence of aluminium or titanium layers. Although the metallic and composite layers are very different in terms of machining properties, yet again CoroDrill 452 can meet the challenge when using hand-held drilling machines.

How? Well, the secret lies in the ability of CoroDrill 452.1-CM to utilize a double margin flute design and self-centering drill point to help optimize accuracy. Both pilot and non-pilot versions are available depending on application and again, a low thrust force design helps users reduce burr upon exit. Reamers (CoroDrill 452.R-CM) are also available for machining composite/metallic stack materials.

When drilling and reaming CFRP/aluminium stacks, cutting speed is typically around 120 m/min, although this should be reduced to around 20 m/min when producing holes in composite/titanium stack materials due to the tougher nature of the material. It should be possible to maintain a feed rate of 0.05 mm/rev for both materials types.

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