16 years ago
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 ?