Countersink? No problem!
The use of rivets is one of the most common standards for the permanent assembly of structural parts in aerospace and automotive sectors.
For reference only, the image on the right shows a typical distribution of different rivet types in a famous aircraft with a very recent design.
The use of rivets is one of the most common standards for the permanent assembly of structural parts in aerospace and automotive sectors.
For reference only, the image on the right shows a typical distribution of different rivet types in a famous aircraft with a very recent design.
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| Tratto da: http://www.france-metallurgie.com/2011/04/ |
Despite some minor limitations, this system offers useful advantages in the assembly process:
- Rivets are cheap
- The procedure is easy and fast
- They are available in many different types covering any need
- There is a long history and experience in their use leading to a good reliability guarantee
- Compared to other permanent fixations, they can be disassembled quite easily with specific tools
- They allow a calculated residual flexibility of the assembly
- They allow easy and fast repair even on the field
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When rivets are used to assemble the external layers on the supporting structures, they need to fulfill another very important role because the resulting assembly is directly exposed to the airflow: the final surface must be as smooth as possible in order not to affect the aerodynamic performances.
This specific need is very common in the aeronautic and aerospace field and all designers solve it by selecting rivets with countersunk head.
It becomes therefore evident the need of an accurate hole on the surface prepared with a countersink: the rivet head needs to be hidden below or in line with the external surface profile.
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| Tratto da: https://www.aircraftcompare.com/helicopter-airplane/piaggio-p180-avanti-ii/356 |
Just to reinforce the importance of the final surface quality the rivet head flush requirement is directly expressed on the assembly drawing through specific symbols (like a welded joint) and described in detail through dedicated quality procedures involving, very often, also a source qualification need.
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| Tratto da: http://www.aslgroup.eu/en/fleet/12/piaggio-p180-avanti-ii |
In order to better understand how important is to respect
the perfect surface continuity we can consider that, on
some aerodynamic very demanding design, the aircraft performances are so sensitive that just the surface cleaning has a perceivable impact on the overall performances.
The standard way of performing the rivet assembly is manual with specific tools that allow improving the reliability of the process.
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| Tratto da: http://www.northerntool.com/shop/tools/product_200451725_200451725 |
Generally, the structural parts and the sheet metal pieces are NC machined and prepared with smaller holes in order to guide the manual tools used to create the final hole and countersink prior to assemble the rivet.
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Sometimes only one of the two element to be assembled is pre-drilled in order to allow compensation of assembly misalignments; in this case, operator’s job become even more complex and sensitive, thus requiring higher manual skills.
There are a few reasons for the aircraft manufacturer to choose this way rather than finishing the hole and the countersink on the NC machine:
- The hole and the countersink must be perfectly perpendicular to the surface
- The countersink depth need to respect quite a tight tolerance in order to avoid rivet head to be above the surface. We must consider that the material thickness tolerance can be easily equal or even higher than the countersink one
- The position tolerance need to be very tight as well in order to guarantee that the two parts will match during the assembly
- Even if in a modern design all the parts are 3D modelled (it’s not the same in an older aircraft) the production processes of a structural part and a sheet metal one do not guarantee the same level of precision
- The sheet metal is a flexible piece and it’s very difficult to create a fixture that maintain the overall surface in the 3D model theoretical position
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| Tratto da: http://www.chinadaily.com.cn/bizchina/motoring/2014-09/17/content_18612888.htm |
Everything said above is valid even if the sheet metal is classical aluminum or composite based. In this second case, some other points must be considered:
- The dust produced is very dangerous for the operator's health
- The material cut is very critical and need better control of the cutting parameters
- The material surface control is even more complex
Until the business remains focused on small quantities and the market is ready to reward the “hand-made”, and the consequent high value added, as a “plus”, it’s possible to sustain the manual production process with respect to massive production competitors.
How does all this match with NC 5 axis machining?
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| Tratto da: http://www.youtube.com/watch?v=Ih89unYl0-g |
When the scenario changes to big numbers (typical of the civil market) with reduced prices and margins, the only solutionthe only
solution to stay competitive is the process standardization and automation, so the aerospace industry needs to find a partner who can help it achieving critical goals.
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| Tratto da: http://www.capmac-industry.it/en/aerospace/ |
On the market, it is possible to find solutions that substitute the operator job with a quite complex machine that request to introduce the full assembly jig into the machine in order to drill and rivet the components replicating the operator’s gestures.
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| Tratto da: https://www.youtube.com/watch?v=Ih89unYl0-g |
This strategy is very expensive, requesting a huge space allocation and presenting many times issues due to aircraft structures accessibility limitations.
Breton is following a different way, allowing to save the huge amount of money necessary for the previous type of investment and keeping the maximum flexibility to apply the solution to any type of component.
Breton has developed specific machines and several solutions to support this type of challenge and has also a very strong and experienced process development team available to support customers before and after the integration of a fully comprehensive solution.
The only ground condition is the availability of a 3D part model, Breton takes care of anything else.
For each of the previous points supporting the benefits of a manual riveting process, Breton has a specific automatic solution leaving the final fastening to the operator but with the hole and countersink already prepared in the correct position, shape and depth:
- Position precision is not an issue for any Breton equipment that is designed to achieve the best 5-axis tolerance on the market.
- A specific automatic probing head is capable to calculate the real surface position in respect to that of the 3D model and calculate the corrections in order to recover depth of cut and surface perpendicularity
- The machine position is automatically modified without any operator assistance before drilling the hole and countersink
- With this solution the fixture only needs to keep the piece well fixed but it can leave to the machine probing capability the real shape calculation
- The composite dust issue is solved avoiding the operator to be exposed to it during the material cutting
- The composite integrity is guaranteed using a special developed solution that avoids any contamination and completely removes any dust
- All the cutting parameters can be controlled much better than in a manual mode
- The software is also capable to monitor the surface stability during the cutting operation and can be programmed to react in different ways according to the material responsiveness.
- The structural part can be machined on the same trimming machine in order to have the same level of precision. This way is the first step to guarantee a good assembly performance.
They have both been designed at Breton’s and patented due to their specific and unique capabilities on the market.
The special probing head is stored in a specific holding device on the machine, protected and located outside the working area; when necessary, the machine automatically picks it up, while keeping the tool change capability monitored through a specific application in order to avoid any collision with the device.
The system is composed of three mechanical transducers managed by a specifically developed NC control application in order to acquire the true position and orientation of the piece surface around the hole to be done.
The drilling program is a standard one where the machining cycle is substituted with the Breton routine to activate the countersinking head.
The typical drilling process follows these steps:
- The machine collects the special probing device
- The machine picks up and measures the cutting tool (using a specific Breton routine)
- The machine sets up the probing device on a reference gauge integrated in the machine (using a specific Breton routine)
- The machine probes the surface in the theoretical position and orientation
- The real position and orientation is recalculated by Breton software
- Depending on the surface type and scratch sensibility, the machine position and orientation are corrected by either keeping the probes in contact with the piece or retracting the machine from it (average time of the full probing and position adjustment is 6 seconds)
- The drilling and countersinking cutter proceeds along the real hole axis until the probes detect the correct depth of cut (this time depends on the type of material, layer depth, type of cutter, etc..). The system is capable to respect a depth tolerance of +- 0.03 mm on a stable surface (or +- 0.06 mm if the surface is not perfectly supported and fixed by the fixture)
The machine head is also equipped with a special dust extraction hood in order to collect all the dust generated by the drilling process.
There is no limitation in spindle performances as all the probing systems are static and connected to the head flange without any external wire.
Depending on the material type, the head can supply compressed air, spray oil mist or pressurized coolant up to 40 Bar as the probing devices are fully sealed.
The following pictures explain the capability of the special head comparing the results of countersinking with and without the Breton solution:
This sequence is useful to understand better how the full process is working:
The system can be installed on any Breton machine giving the customer huge advantages compared to the manual process, spending just a small portion of the investment requested by more sophisticated solutions.
(Breton is not yet assembling the rivets…).
Summarizing the points of strength of Breton solution:
- One machine to trim, drill and countersinking
- Precise process control
- Small investments in a machine accessory
Since one of Breton’s major points of strength is its capability to hear the customer’s requirements, we are working to further improve this system reviewing the design and testing a contactless solution.
Test Case: Composite panel
Machine type: Eagle 1500 2T K80
(5 axis overhead gantry machine)
Machine size:
X=8000 mm
Y=4000 mm
Z=1500 mm
Machine accuracy:
X=+-0.02 mm
Y= mm
Z= mm
Number of countersinks on each side of the part: 55
Countersink depth: 0,31 ± 0,08 mm
Process stability achieved: Cp 1,34
Cpk (lower) 1,14
The following graph shows how the system recovers the hole surface perpendicularity starting from the theoretical one, measuring the real surface orientation (red lines in the graph), correcting the head A and C axis and checking the final result (green lines in the graph).
After this first phase, the system starts monitoring the countersink depth comparing the probes data with the target values.
The final result, achieved on a composite part not perfectly stable on the fixture, is shown in the process control chart here below:
Consideration:
The process stability is very nice giving a big safety margin. The customer chose to further increase the safety margin moving the average countersink depth in the lower tolerance direction (as show by the Cpk value).
For more information please write to mail@breton.it.
Thank you for the attention and best regards.
Bye-bye
Sergio Prior
