The Importance of Hole Quality and the Pneumatic Straight Drill in the Aerospace Industry

To many who are uninitiated to the intricacies of aircraft assembly, drilling is a simple process. However, in the aerospace industry, where hole quality could be the difference between efficiency and catastrophe, drilling is paramount and closely studied like rocket science. In this context, aerospace manufacturers must give utmost attention to the primary tool used in boring holes - the pneumatic straight drill.

Over a million holes are made to put together a commercial aircraft, and the quality of each one depends to a large extent on the drill used and the skill of the operator. 

This article will take a deeper look into the amount of work and thought that goes into each hole, the role that the pneumatic straight drill plays, and tooling options for the best hole quality.  

To gain a better appreciation for the complexity of drilling, many factors must be considered. 

For an aircraft to withstand the various stresses involved in flight and still be a viable business commodity, different materials are used in combination during manufacturing. The two most commonly used in the body of an aircraft are aluminum alloys and composites of carbon fiber. Each type of material behaves differently when drilled. 

There was a time when as much as 70% of an aircraft was made of aluminum alloy[1]. It was durable, lightweight, and inexpensive, which made it the ideal material for the aerospace industry. Over the years, developments in the manufacturing process of other materials have diminished the use of aluminum alloys in aerospace applications. An aircraft today is typically made of only 20% aluminum.

The aluminum alloy most widely used in aircraft manufacturing is the 7075[2], which contains around 6% zinc. It has great mechanical properties, including ductility. However, its ductile property causes three major problems in drilling: susceptibility to the formation of chips, evacuation of chips, and build-up at the cutting edge.

The use of carbon fiber in aircraft manufacturing is on the rise, especially in the latest generations of commercial aircraft. It has replaced aluminum as the primary material for the aerospace industry. Aluminum has the best weight-to-strength ratio among most metals, but the "specific strength"  of carbon fiber reinforced polymers[3] or CFRPs, is even better.

The impact of the reduced weight of an aircraft made mostly of carbon fiber is far-reaching, from reduced operational costs to increased payload capacity to expanded flight range. However, it is not without challenges, specifically in assembly. Tried-and-tested drilling techniques that have been used for years do not apply to composites, which are prone to delamination or separation of the layers at the drilled part.

The size of the holes and their position relative to each other[4] have a huge impact on the stress that the material will be able to withstand. Hole size is measured in terms of the "edge margin," which is the distance between the center of the hole to the edge of the material. It is calculated based on material thickness, the diameter of the fastener, and the critical load on the material.

Hole spacing is measured from the center of one hole to the center of the adjacent hole. Spacing that is too close will increase the stress concentrations on the material and reduce its fatigue life. Conversely, when drilled too far apart, each hole will be subjected to a higher load, which would also lead to material failure.  

Burrs are sharp deformations at the edge of the material as a consequence of drilling. It is one of the biggest problems drilling operations are facing. Exit burr, in particular, can significantly degrade hole quality and functionality, hindering the assembly process. If managed poorly, it can also affect the fatigue life of the material.

It is virtually unavoidable, such that additional actions are usually needed to deburr them. In the aerospace assembly, where as much as 1.3 million holes are drilled per aircraft, reaming and deburring operation costs could account for up to 30%[5] of the whole structure.

For CFRPs, delamination must be considered as well. This happens when the thrust force of the drill exceeds the critical thrust force for delamination[6]. In the final assembly of an aircraft, as high as 60% of the composite laminate parts are rejected due to drilling-induced delamination. Each hole takes about five to six hours to rework, even with the latest CFRP repair technology.   

Closer attention to drilling parameters can help minimize burr formation and may even negate the need for deburring in some instances. The cutting speed and the feed rate are the two most studied parameters that could greatly reduce burring. On the other hand, the delamination of CFRPs can be managed by adjusting the feed rate of the drill, which is directly correlated to thrust force. An optimized setting of both cutting speed and feed rate, coupled with the appropriate type of cutter material, is essential in achieving better hole quality.

The LBB16S high-speed pneumatic straight drill is one of the many tools used in drilling. It is primarily designed for vertical drilling and drilling in narrow spaces but has enough features that make it useful for a wide range of applications. It is equipped with an ergonomically designed rubber grip handle that can reduce vibration and temperature differences. With its low noise level, high power-to-weight ratio, and good throttling capability, hours of highly productive and risk-free drilling is easily achieved.

The LBB series of pneumatic straight drills have a maximum free speed ranging from 1200 r/min to 26000 r/min, which can be reduced to 50% using the trim valve. This flexibility is useful in achieving the optimum drilling parameters for the desired hole quality. It has a 0.47 hp, 350 W motor that is lubrication-free, packed in a body that weighs from 0.55 kg to 0.7 kg, depending on the model.   

Hole quality is the precursor of any aircraft's safety and functionality. It has been and continues to be the subject of many studies-- all of which point to drill quality and operator skill as major factors. 

Atlas Copco has the most advanced line of high-quality pneumatic straight drills that can make a hole as intended, each and every time you drill.

Contact us now for a free quote on our best ergonomic tools. 

References:

1. Standridge, Michael, Aerospace materials — past, present, and future, Aerospace Manufacturing and Design, August 13, 2014

2. Mraz, Stephen, Basics of Aerospace Materials: Aluminum and Composites, MachineDesign, June 19, 2014

3. Wiseman, Daniel, Advantages of Composite Materials in the Aerospace Industry, Medium, January 17, 2018

4. Durham. Bryce, Determining Appropriate Levels of Robotic Automation in Commercial Aircraft Nacelle Assembly, MIT, June 2014

5. Avila, Miguel, et. al., Strategies for Burr Minimization and Cleanability in Aerospace and Automotive Manufacturing, escholarship.org, July 1, 2006

6.Geng, Daxi, et . al., Delamination formation, evaluation and suppression during drilling of composite laminates: A review, ResearchGate, February 2019