Thread: C5 gto
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Old 08-01-2017, 02:09 PM
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heinke heinke is offline
MetalShaper of the Month Jan 2018
 
Join Date: Nov 2009
Location: Prescott, AZ
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Post Making GTO trunk lid (cont.)

A progress update on making of GTO trunk lid. Now that the frame is fabricated, it needs some small adjustments prior to fitting the skin. First is it needs to be leveled such that the outside edge is 1/16” below the surrounding body surface. The skin is 1/16” thick. I do the leveling by hand and use a piece of .063 thick scrap as a guide.

Once the edge height is set, the edge gap needs to be set. I’ve decided to shoot for a 1/8” final gap around the trunk so a 3/16” gap is needed for the frame. I use the sharp points on a caliper to scribe a mark for 3/16” gap prior to cutting with aviation snips.



The last bit of preparation prior to starting on the skin is to polish the frame. The car’s interior is polished aluminum so this lead me to do the underside of the trunk lid as polished as well. Truth be told, I hate polishing metal. It’s a laborious, dirty task that takes a lot of time. But I do like the look when it’s done.



Now on to making the trunk skin. I started by making some surface templates from ľ” wide 20-gauge steel as guides for the crown and arrangement. This is a technique I had learned from Lazze.

I cut the Al alloy 5052 sheet and added a low crown using the Ewheel. When I bent the skin into arrangement and looked across the surface, I could immediately see issues in the form of 3 good sized high spots. Not good.



I first tried removing the high spots using a leather faced flat wood slapper over a sandbag. This didn’t work due to the large size of the high spots and the skin had already work hardened. Next I tried hammer and dolly using off dolly technique. It worked some but it was clear the metal needed to be annealed before it was going to move much using that technique. So, I decided to try a shrinking disk. This put a good amount of heat into the metal but galled the surface badly. It didn’t remove the high spots either and was doing more damage than good. I decided that a shrinking disk is not good for use on aluminum.

After thinking about it and recalling the physics at play as described by Lazze when I took his metal shaping class years ago, I came on another approach to remove the high spots. It’s not intuitive as it involves using the ewheel and metal stretch. The concept is that metal with a compound curve consists of metal surrounding the compound curve that forms a “frame”. The sheet metal inside the “frame” is raised up to form a curve because it is constrained by this “frame”. If the metal forming the “frame” is altered this will affect the shape of the metal inside the “frame”. For example, if the metal in the “frame” is stretched, the curve of the metal inside the frame will be reduced because there is less constraint holding in the shape.

Now to test the concepts on my warped skin. I carefully wheeled the metal around the high spots making sure to not also wheel on the high spots. The high spots disappeared. It worked! With careful, low pressure wheeling the skin was starting to look good again and I could clearly see that it could be rescued.

I however decided to start over with a fresh sheet given the galling/gouges left from the shrinking disk.



For my second attempt, I decided to use a different approach to this very low crown skin. I decided to bend it into arrangement first and then add the shape. This is reverse order from my first attempt. This seemed to work better as I could frequently check the shape with the templates and focus the wheeling to only where it was needed.







I got the skin shaped to match the templates on my second attempt and gave a sigh of relief. It was time for final trimming before attaching the skin. I had left an extra ˝” around the edge that needed to be trimmed off. Just to be sure, I re-checked the shape with the templates after trimming. The arrangement was still there but a good part of the crown was gone. Ah, the ˝” that I had cut off was acting as part of the “frame” and I had altered it. Oh well, more wheeling and I had the desired crown back in the skin.

Bottom side of skin with flange turned, ready to attach.



That’s it for now.
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