Everyone talks about wings, engines and assemblies, but the nuts and bolts of the operation are the poor relation. And that's a shame as it is a fascinating and integral part of production, and their accurate measurement can make production lines more efficient.
There are a wide variety of permanent and non-permanent fastener types used in the assembly of the body and wings of the aircraft, holding a complex structure of panels, cladding and ribbing together. But what is a fastener? A fastener is a device that mechanically joins or affixes two or more objects together.
When engineers design an aircraft, they need to consider the type of joint the fastener will be applied to, i.e. shear or tension. They will also need to consider the type of load that will be transferred through the joint. Aircraft loads may include those experienced during flying, towing, wind-gusts, landing, take-off – the list is endless, and fasteners need to take account of when and how these loads are transferred throughout the aircraft. Fastener numbers and diameter are calculated to transfer this load.
The dimensions of fasteners are key to the structural safety of the aircraft and to fuel efficiency by reducing drag or making surfaces aerodynamically efficient. And in the production of aircraft the size of rivets and fastener holes needs to be consistent and measurable. They typically range in diameter from 3mm (0.12”) to 30mm (1.18”) and if the fastener hole is countersunk, the countersink diameters typically vary from 7mm (.28”) to 50mm (1.97”).
A more measured approach
Aerospace manufacturers measure radius, diameter, angle and depth of countersink holes. In addition to this, with the advent of composite structures, they also check the edge condition on the pilot hole/countersink intersection and flushness of the fastener to the panel surface.
The reason engineers need to measure the angle of the countersink and depth relative to pilot hole is to calculate how tight a fit the fastener will be. If there is not a suitable edge on the pilot hole/countersink intersection, cracks can occur and the effective strength of the fastener is compromised. Likewise, checking the flushness of the countersink to the surface of the panel means if drilled at an angle the fastener too will sit at an angle in the countersink hole. Consequently, if the fastener or rivet protrudes from the surface this will prevent a smooth flow of air over the panel which will create drag and affect fuel consumption when the aircraft is in flight.
Third Dimension, a developer and manufacturer of non-contact, optical, precision profile measurement systems, has found a neat solution that can prevent this issue before the fastener is fitted. If a fastener is fitted incorrectly, it is then too difficult to drill out. Third Dimension's GapGun laser measuring tool enables preventative inspection of countersinks to be carried out quicker than ever, on the wing or on setting samples for drilling tools.
On any given aircraft, there are typically thousands of fastener holes to check and control per wing. The speed and portability of the GapGun system can be used to dramatically reduce time spent on the metrology inspection process and improve the classification of concessions.
Third Dimension has developed a cost effective solution that historically could not be achieved when moulds were cast to measure each and every hole. The GapGun laser measurement system can take the same measurement in a matter of seconds with the capability to measure all the required features within a countersink hole.
Working closely with key customers during development stages, the company has tested and proved measurement capabilities to implement the best working solutions into the customer's manufacturing and quality testing facilities.
The key advantages of the system are:
• Quick, accurate and traceable results - operators on the shop-floor can use the GapGun system to take fastener measurements, typically in just a few seconds, with accurate data instantly available and automatically stored to facilitate full aerospace traceability. Unlike casting moulds, GapGun takes a fraction of the time and digitally records, time, date, operator and feature ID against every measurement taken.
• No surface damage - taking non-contact measurement, manufacturers eliminate the risk of surface damage.
• Modular design – there are a unique range of interchangeable laser measurement heads, each with its own calibrated resolution, which have been designed for complete flexibility.
• Unprepared surfaces - GapGun's patented laser sensor head technology means the laser dynamically adjusts to the surface condition it is measuring so that operators have the flexibility to measure almost any surface finish, very accurately, without preparation. This ranges from the primed green iodised surface to the finished gloss white surface of a section of a wing or body part.
Recently Third Dimension launched its new GapGun Pro GapGun which uses optical triangulation to measure gaps, shapes and forms incredibly accurately. It has much improved speed, twice as fast as the previous model, greater ease of use, portability and ruggedness. The company is developing new solutions to fasteners.
A tool you can trust
GKN Aerospace is deploying the GapGun to measure radii on items which have been produced in aluminium and has found the GapGun to be an accurate and consistent tool in this task. It is seeking to create radii of known and repeatable sizes, cut to precise depths. They use the GapGun to calibrate and set the tools used to machine them. GKN Aerospace's use of GapGun also has a set of stand-offs, that centre and locate the measurement in respect of the hole.
These guarantee that GapGun's results are repeatable and consistent. It covers a wide range of hole diameters, from 4mm to 25mm holes. GKN Aerospace and Third Dimension are currently developing new solutions for the GapGun in relation to the setting of steps and gaps, countersink hole measurement and manufacture of carbon fibre components.
“The metrology of fasteners is an important area of aircraft manufacture. It's important to be able to measure holes and rivets accurately every time,” explains Third Dimension's chief business development officer, John Kane. “We continue to find new solutions which will make this aspect of the development and the production process more efficient, and more repeatable.”
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