The empirical part of the sheet metal flat pattern process is in the selection of the postion of the neutral axis. The position of the neutral axis is part of the CAD part file and is a variable that can be adjusted. The neutral axis is the position in the thicness of the material where the material on the inside of the bend compresses and the material on the outside of the bend stretches. It is usually taken as about .45 of the material thickness from the inside of the bend, but naturally this changes with the material and more important with its' condition. For example the ribs are formed at -0 temper and then heat treated. Skins are formed in the -T3 temper so you have to use a slightly different neutral axis variable. But once you have established a table of values and verified it with some test bends, then you can adjust the value of the neutral axis with some confidence.
I do agree that there is some "art" to making all this happen, but I think the amount of manual intervention (fudging) is less than you might imagine.
Yes, you should try to get a tour of the plant, the machines are very impressive.
But, here's the bigger question....when they build the assembly of parts in the CAD system and locate the holes, THEN how much do they need to fudge the hole positions on the real parts so that the holes line up?
I think this is a real art. The manufactured pieces can not be zero-tolerance accurate, and more important, the parts can not be pulled or stretched to achieve the perfect mating assembly that would exist in theory in the CAD system. So, I think they need to go back and fudge the hole locations a little to account for that. I don't know if that is trial-and-error, or if they have rules of thumb, or what.
Or, I could be wrong -- maybe the holes as positioned in the CAD parts work perfectly and the real parts fit together well enough that holes line up. That would be really amazing.
I'd love to know, if Scott or anyone would like to tell us.
I am not involved in the computer portion of the process but from what I do know about it, Terry explained the basics of it pretty well. It can get complicated though. Especially with conical shapes and situations were a flat skin is being installed with a twist (like on the bottom of the RV-12 wing tip), or where shrinkage of material has to happen such as a flange being formed on a tight radius part (such as aileron or flap nose ribs). Over time, some processes proprietary to Van's have been developed. It is for this work that Ken Krueger (head of Van's engineering) received EAA’s August Raspet award this year at OSH.
The basic process of making parts to build an RV -
- Design the airplane in CAD with detailed models of all parts (proper bend radii on flanges, desired hole positions and diameters, etc.
- Flatten out the part to the shape it would be when punched from a flat piece of sheet stock.
- Use this computer file to make a program file for the punch machine. This requires choosing all of the proper punch tools, and choosing the order all of the different punch operations are executed, to most efficiently punch out the part. This process is done by the punch machine operators.
- Once the flat parts have been punched they may be ready to pack into a customers kit (flat skins), or they may require more manufacturing steps.
There is 3 primary processes that are used for part forming
Press Brake - This is used for the majority of parts that only have one or more straight bent flanges on them. A couple of the machines are CNC which allows for doing a variety of different bends (different angles on each flange, etc) on a single part so the part only has to be handles once (the program resets the machine for each bend and then starts over when that part is finished) A non CNC machine is also used for simple parts that have just one bend on them. The press brake is also used to form tight radius curves into skins (such as the nose portion of wing leading edge skins) by pressing the skin into a block of urethane with a round die (think long smooth pipe laying on the skin surface).
Roll Forming - As the name implys... a curve is roll formed into a part (think curved fuselage skins and wing leading edge skins).
Hydro-forming - This is the one that is the most interesting. The flat aluminum part is pinned (that is what the tooling holes in parts are for to a male mold (at Van's referred to as a Form Block) and then pressed into a thick layer of very hard rubber. Even though the rubber is very hard, under many tons of pressure the rubber extrudes around the form block. Any material that was overhanging the edges of the form block, bend around the corner until it is tightly against the side of it. This is how all parts with curved shapes get formed (and some with straight flanges if it is deemed the most efficient). Because of the high pressure of the rubber, if you machine low areas into the form block it will pres the material into them and make stiffening beads like are seen in bulkheads and wing ribs.
Obviously, the form block tool requires a specific level of precision when it is made to have the part form to the shape intended. The angles of the sides, radii of the corners, and the temper of the material all interplay to effect the end result. This is something that the entire engineering and prototype shop teams have developed to a high level of precision, which produces parts that to this day still even impress me with how well they fit.
The process of making the form block is another whole story in it self... I will just say that it involves modeling a tool that will fill the entire inside area of the finished part shape and then reproducing the tool shape in aluminum.
Some parts require 2 of these production processes (hydro and press brake, or roll form and press brake).
That's the bare bones basics of it.
As far as how much "Art" plays into the whole process? I would refer to it more as "experience".
There are a lot of different things that come into play when designing and then producing parts like what are in RV kits. It to a large degree relies on the experience that has developed from doing this for quite a few years, and on the team work between the designers, prototype builders and production floor people to make it all work.
It is very satisfying for all of them to hear people talk about how amazed they are that the parts fit so well.
