This Open House will be the best ever! There are a LOT of exciting projects going on right now. Hope to see you Jamo! I'll save a space in our new gym for you
Is the aluminum sheet the same thickness and weight as would be on an original coupe, but just a different alloy?
The alloy gives a considerable amount of strength to the part. 5083 is the highest strength alloy in commercial use (in the non-heat treatable alloys). The sheet in the video is very close to the original thickness.
Cobra Make, Engine: Classic Roadster 351W, T5, Red & White
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David @ KMU
David, what are the differences in the aluminum and processes being used by KM as compared to the mid 1960s ? In the UK magazine MotorSport, an advertisement mentions old world metal forming by long time craftsmen for restoration projects.
......thanks
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2014 Porsche Cayman S, 2014 M-B CLA 45 AMG,
Unkown:"Their sweet lines all but take my breath away, and I desire them as much for their beauty as for their use "
David, what are the differences in the aluminum and processes being used by KM as compared to the mid 1960s ? In the UK magazine MotorSport, an advertisement mentions old world metal forming by long time craftsmen for restoration projects.
......thanks
Original Cobras, and most aluminum cars of that era, were made from 1100 alloy aluminum. 1100 is pure aluminum. Original Cobras are extremely fragile cars. You can dent 1100 aluminum by simply pushing your finger into it It is fun to work with...but, it is so weak many people use alloy 3003 here in the US today. In Poland we use an old Soviet alloy (which has no equivalent in the US) to make our bodies. It is alloyed with iron. It is tricky to weld, but it really polishes up quite nicely--much better than 1100 or even 3003. Original Cobras were made from 0.050" thick aluminum sheet. In Poland we use 1.5 mm thick (0.059") aluminum. Most coach builders in the US (and England) use 0.063" (1/16") aluminum to build bodies today as it is far more durable.
Originally, Cobras were made by a combination of techniques. Deep shapes (say the area around the headlight) were pounded out on a sand bag with a rawhide or wooden mallet. Those rough shapes were then rolled smooth with a "wheeling machine" (AKA "English wheel" here in the US). It takes extremely skilled craftsmen to use a wheeling machine. The shapes were constantly fit back and forth against a buck (a model of the car body--typically a plywood "egg crate" design of some sort). All fitment was done by eye. As you can imagine, this left for quite a bit of "interpretation" by the craftsman. His interpretation may, or may not, have varied depending upon how close he was to quitting time. On CSX3104 the left fender is 1 inch narrower than the right fender and the fender wasn't wrecked. The left fender also had 1/2" of bondo in it on top under the original paint. (The fender really was that far out--but, that was "original.") Thinner aluminum was originally used because it was much easier (cheaper) to form by hand and it, of course, cost less than thicker aluminum.
We do not use a wheeling machine very much. We really only use it for prototype parts. It is exceptionally difficult to make two parts match up perfectly that are wheeled. I am sure many of you have seen beautifully wheeled parts. They are all nice and shiney. But, I am sure you have also noticed the welds are invariably filed smooth and left in a brushed state so the car looks like a mirror with stripes running all over it. Those transition areas are rarely smooth. In fact, I have never seen one smooth. Our panels don't look as nice as a wheeled panel, but our overall car is smoother. That is why we are able to polish the entire car to a mirror finish.
In Italy, they rarely used a wheeling machine. Parts were just beaten out and then welded together. After a bit of planishing (straightening with a hammer and dolly) the car was slathered in body filler and sanded smooth. If you have ever seen an old Ferrari you would be amazed at the amount of filler they used. There is no doubt the body shapes were stunningly beautiful nonetheless.
Originally the aluminum panels were gas welded together. Gas welding is very fast. Probably twice as fast as TIG welding. We also gas weld our body panels together in Poland. Repair work is generally done with a TIG. Incidentally, silicon bronze welds very easily with a TIG.
In terms of material science...
Young's modulus, or stiffness, is about 10,000,000 psi for aluminum. For comparison, it is 30,000,000 psi for steel. In other words, steel is 3 times stiffer than aluminum. Bronze is about 1/2 as stiff as steel in case you wanted to know.
That said...be careful!!! Hardness and stiffness does NOT always equate to strength.
Aluminum (the soft body forming alloys we use, like 3003-0) has a yield strength of 6,000 psi and an ultimate tensile of 16,000 psi.
Mild steel has a yield strength of 50,000 psi and an ultimate tensile of about 60,000 psi.
655 Bronze has a yield strength of 55,000 psi and an ultimate tensile of 85,000 psi.
As you can see, the question is more complex than it first may appear. Material science is fascinating.
For body parts we make here in the US, we use 3003 aluminum in the H0 and H14 state. 3003 means the aluminum was alloyed with manganese to make it a bit stronger than pure aluminum. H0 means its hardness is 0, or fully annealed. H14 is a little different. Each number, the 1 and the 4 mean something different. The 1 means "strain hardened" (by the rolling process) and the 4 means "1/2 hard." In other words, H14 was rolled until it was 1/2 hard. H0 is really only good for a few parts, or parts with a LOT of shape as it is so soft there is not much strength left in the part when you are finished. You have to move H0 a long way to get the hardness back up to something durable. Sometimes, we start with 3003 H0 and form the part 1/2 way. Then we anneal and form it the rest of the way. 1100 also comes in H0 and H14.
We also use 5052 H32, 6061 T-6, and 7075 T-6 to make parts. The "T" designates the aluminum was solution heat treated and aged to make it harder and stronger.
For superplastic forming we use (generally, but not always) alloy 5083 to form the part. 5083 works really well because it doesn't have to be heat treated and it has a very high strength when it comes out of the press.
For comparison, bronze is much, much harder than aluminum. Remember, bronze was used to make weapons for thousands of years after it replaced copper. Swords were invented in the bronze age (and not in the copper age) because bronze was finally strong enough to make a long blade. Copper wasn't strong enough to make swords (they did make copper axes, however, as evidenced by the "Ice Man.") We advanced from the copper age to the bronze age when we figured out how to alloy copper with tin and make bronze. (Now, there are many different kinds of bronze and I am just giving a very brief over view here of the history involved.)
The bottom line is, "old world metal forming" is a dying art. It is very hard to get a perfect part by hand. There really isn't anything that can compare to a super plastic formed part. SPF parts are simply stunning. We certainly still use the old methods in Poland, but there are modern ways to get much stronger and repeatable parts--like superplastic forming. We will make our new Coupe with super plastic forming.
Cobra Make, Engine: Classic Roadster 351W, T5, Red & White
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David.......WOW , thanks for the chapter on Aluminum 101
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2014 Porsche Cayman S, 2014 M-B CLA 45 AMG,
Unkown:"Their sweet lines all but take my breath away, and I desire them as much for their beauty as for their use "
Cobra Make, Engine: Classic Roadster 351W, T5, Red & White
Posts: 3,478
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David, another KMU question, progressing from from 101 to 201......
Basically, the reasons behind a forged aluminum product vs. machined, and the choice of aluminum, heat treat etc. Such as:
" Profil wheels are forged from 6061-T6 aluminum alloy; this aircraft-grade aluminum is heat treated for a higher yield strength than your typical aluminum wheel. This allows us to create stronger, more stable wheels, while maintaining a low weight. "
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2014 Porsche Cayman S, 2014 M-B CLA 45 AMG,
Unkown:"Their sweet lines all but take my breath away, and I desire them as much for their beauty as for their use "
David, another KMU question, progressing from from 101 to 201......
Basically, the reasons behind a forged aluminum product vs. machined, and the choice of aluminum, heat treat etc. Such as:
" Profil wheels are forged from 6061-T6 aluminum alloy; this aircraft-grade aluminum is heat treated for a higher yield strength than your typical aluminum wheel. This allows us to create stronger, more stable wheels, while maintaining a low weight. "
Forgings are used when grain structure is important. Aluminum is forged at around 800 degrees F (if I remember right) then squeezed in a hydraulic press. You need some 25 tons/square inch to make a good forging. As you can see, that is a tremendous amount of force. The press squeezes the hot, plastic aluminum into the shape of the die making it flow into the die. Curiously, if the temperature is too hot, the aluminum will literally crumble. It is quite interesting to actually see it happen in real life. As the aluminum flows throught the die, the grains tend to line up in the direction of the flow. Usually, (but not always!) the stresses the part sees in life are also in the direction of the flow and so the direction of the grain strengthens the part. Alloys that are typically forged are 6061, 6082, 7075. We have made parts in all these alloys.
Another advantage of a forged part is you don't need to remove so much material to make the part. A forging basically makes the part in the general shape you need. Many times all you have to do is drill some holes and machine a few surfaces and you have a usable part. There is a HUGE drawback, however. Dies are expensive to make. Dies are also expensive to set up. So, it only pays to make a die if you are sure you are not going to be changing anything. But, what do you do if you are into constant improvment--like Shingo? That is why over the years we have gotten away from forgings and gone to machining our parts straight from billet. If we have a better idea today, we don't want to wait until tomorrow (or next year like many companies) when we run out of forgings to implement a new, upgraded part into our proeduction. If we have a better idea today, we want to implement it TODAY.
I imagine, the quote above that 6061-T6 has a "higher yield strength than your typical aluminum wheel" is comparing 6061-T6 (forging alloy) to 356-T6 which is a casting alloy. There is no doubt 6061 T-6 is far stronger than 356-T6. Castings do have their advantages, like you can cast intricate shapes that are quite impossible to make with machining (think about the hollow water jackets in an engine block). However, the strength of 356-T6 is quite low and we really don't like using it for much of anything. As the years have gone by, we have eliminated almost all castings from our car. The only casting left is the differential housing now. Who knows, one day we might billet that too.
Happy to answer any questions, or go deeper if you want.
Kirkham Motorsports University Superplastic Forming Alloy Strength Demonstration
Linear Mode
