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CNC cutting, tolerances and sheet metal assembly questions

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Arfang

Well-Known Member
Supporting Member
Joined
Aug 10, 2014
Messages
157
Location
Switzerland
Good day all,

I'm trying to get a better understanding on sheet metal construction. As I understand, from looking at build logs and videos, rivet holes on the wing skin are drilled after being temporarily fitted on top of the ribs to make sure they're aligned with the holes already drilled in the rib flanges (or the other way around?) Same for fuselage skins, I saw people using some thin cardboard to get the accurate dimensions before cutting the skin panels. A considerable part of the assembly seems to follow the 'fit on assembly' principle. Is my interpretation correct? Could someone share ressources, be it personal experience, books, videos or websites to better understand sheet metal riveted construction? I already have Standard aircraft handbook and Sheet metal building basics, but I still struggle to fully understand the proper building and assembly sequence.

While I still have a lot to learn about sheet metal, I am familiar with milling, turning and designing metal parts with CAD which makes me choose metal over wood and composite. Plus I recently realized how left-handed I am working with wood. But I always worked with geometric dimensioning and tolerancing and trying as much as possible to avoid things like 'fit on assembly'. So, I had this idea of having all the parts cut and rivet holes ''drilled'' using a CNC cutter, be it laser, water jet or other, then flanging and riveting everything together.

The workflow would look like this:
Design in CAD, transfer the files to the cutter, cut and drill extrusion parts while the sheets are being cut, get the parts delivered and bend where necessery , assemble the entire thing. 90 percent of the aircraft is done, now you only have the remaining 90 percent to go.

Is CNC cutting all the parts, rivet holes included, a viable option, or standard practice? What level of accuracy would be needed to make it work? Assuming I have access to a CNC cutting machine and cost is not a concern.

Now my next concern is bending, like flanges on the ribs for instance. Let's say I got my parts all cut and drilled delivered, now I need to create flanges. What level of accuracy can one achieve forming flanges on ribs for instance? I remember seeing a video showing a way to create ribs using a hydraulic press and some MDF dye. Seems that this method would be able to provide a high level of repeatability. Are there other comparable methods and what would the largest part size you could practically use?

Are there any bending and assembly aspects that would prevent using pre-cut and pre-drilled parts?

As always, any help, suggestions and thoughts are appreciated.
 
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