Compound Curved Skin

Hi All,
I have a question re the forming of a sheet metal skin for an airplane.
I need to manufacture a skin in 6061-T6 0.032" thick. Basically the
shape is one quarter of a cone (with a taper ratio of approx 2/3 based
on the diameter and with the shape at both ends not exactly circular).
The complication is that the sheet is not exactly conic (which of
course would be a flat wrap). Instead there is approx 1" of 'bulge' in
each of the two edges. The one inch is measured from the cone shape
i.e. is is fatter than the cone by approx 1" mid way between the ends.
So the sheet is compound curved. Dimensions are approx 24" long and 24"
diameter at the large end.
The skin should be formed in the T6 condition without annealing and
heat treating if possible. It is difficult to find a furnace or salt
bath big enough to heat treat it where I live (I have tried all
options). I need to make a number of these and so a method that gives
some repeatability is desired.
The options as I see it are:
1. English Wheel ... I don't have one and dunno how to use.
2. Powered planishing hammer .... ditto.
3. Stretch Forming ... this is way out of my league !!
4. Crown rolling ... I keep finding references to skins like this
having been formed using 'crown' rolling in the aerospace industry. Can
any one enlighten me on what this process entails. From scant
information that I have it would appear that a mould of the skin is
made and the sheet roughly rolled and hammered to shape. The skin is
then sat in the mould and a 'crown' roll (I think like a ball shape)
used to roll and slowly stretch the skin against the mould. I have also
seen references to 'free crowning' in which the skin is stretched using
a polished dome shaped piece of metal without a mould. More information
on crown rolling would be greatly appreciated.
5. Could the skin be 'slapped' with a leather bag filled with shot and
slowly stretched against a mould ? Of course it would need to be
roughly shaped first.
Which is the most appropriate method ?
Are there other methods that I have missed ?
Skins like this are very common in light aircraft. Does anyone know how
any of the aircraft manufacturers make them ?
Thanks,
Stephen
Reply to
tdfsks
Loading thread data ...
Stephen, I don't know about crown rolling but I bet it's a variation of wheeling. 6061-T6 is pretty hard and won't take much deformation without cracking. I have no idea if what you want to do is OK with the T6 temper. But I do know where you can find out. Follow this link:
formatting link
. Get an idea of what you want to do. Also, books by Ron Fournier will be of use to you. Small investments with huge returns possible. Obviously, if you are building a plane then you already know that everything must be done right to avoid death and that in a stressed skin constructed object the skin's reliability is paramount. ERS
Reply to
Eric R Snow
6061T6 is rather malleable. It's often used for structural members such as angles, bar and tube. It's the 2024T3 sort of stuff that work hardens much faster and wil crack. Hammering a sheet to stretch it will often crack it sooner than wheeling it; something to do with the sudden impact, guess. "Bumping" is what it's called, and is done over a bag of sand or shot. Soft aluminums are formed this way, and don't know that 6061 would be suitable. Even the soft (3000 or 5000 series) metals need periodic annealing during the forming to prevent cracking during bumping.
Dan
Reply to
Dan_Thomas_nospam
Just out of curiosity, what plane are you building? You might try asking at rec.aviation.homebuilt. Tom
Reply to
Tom Wait
I'd preform it in a slip roll (with the rollers set tighter on one end to get the cone shape) then use an English wheel to get the bulge in the cone.
Rolls:
formatting link
'd expect any HVAC sheet metal shop to be able to do this part. Pretty simple.
English wheel:
formatting link
Hi All,
Reply to
RoyJ
I wouldn't have suggested this, but you did mention that you have a number of them to make. Hand-forming in production, well.... If you have no experience, it would be evident in the results.
We have been playing with hydroforming using urethane.
Your part is large, but the thickness aluminum you are using wouldn't require an insane amount of pressure. If you couldn't scrounge a press to do the job, you could build one easily enough. The tooling could probably even be made from wood, with a little steel reinforcement. I think it would be worth researching. Dad had told me years ago that homebuilders would laminate sawed shapes in masonite, bolting and guing them together, to make a lower die for forming aluminum teardrops ( read as compund curved) used to clear sparkplugs in cowlings, just as an example. This is basically the same thing on a larger scale. Seeing the results I got last week pressing an extreme amount of detail into a 3 x 3" aluminum sheet about the same thickness, I see no reason why it wouldn't be practical to do the same with one very large (and simple) feature into .032". With near perfect repeatablility (after the learning curve, of course ) You could expect a certain amount of springback. The worst demon would be the metal's tendency to bunch as a result of the compound curve, but I don't think you'd see much of that in the shape you are describing. I think you could also combat this by using a mating upper and lower die. In any event, do a little research, and experiment on a smaller scale.
Reply to
Jon Grimm
see
formatting link

Reply to
F. George McDuffee
Since you have a number to do, how about explosive forming? Find someone who understands explosives, & has the relevant licenses, naturally :) Basically, make a female mould in, say, concrete. Clamp your piece to it, & pump air out of the space inside. Sink the whole issue in a body of water, & fire a small charge above the metal. The explosive shockwave does the forming.
tdfsks wrote:
Reply to
David Brooks

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.