NOTE:
This correspondence is from Kent to Tom, the owner of a Spartan Executive which needs a cabin topskin, a heavily-formed compound shape, 8ft. X 3ft, of .032 2024T3 CLAD, and which must be POLISHED.
Tom,
Three days. Getting close, but time to go yet. My machine is good, but needed new bearings and careful twiddling for this degree of perfection. .032" is what we ended up with, after trying two sheets of .025. The stresses from heat treat and rolling mill all come out to show themselves during this process--urk. AND we find that the sheet is .001" or so thinner at the edges--ugh. Considering that we shim the machine .003" at a shot, and that the shafts are .0002" accurate, it all adds up. Kent
Kent,
Although I can't see a lot of the details, I think this looks absolutely superb!! I imagine with polishing, some of the marks may become less noticable -- you're the expert there. But, compared with the alternatives of either leaving the Spartan the way it is or going to some much more elaborate forming operation-- I think it's looking fine. Are you saying the imperfections from the rolling mill are from the original production process for the sheet or from the english wheel you're using?
Can you take some more shots of the imperfections you're worried about?> When you're satisfied you have it knocked, let me know and I'll start making the wooden forms for the sides!!
Thanks for the update.
Tom
The Tinman Respondeth:
Tom,
Photos of polished surface are impossible to reveal this stuff. Any photos look GREAT.
After attending Golden West Flyin and talking to 50-yr metalman from LA this past weekend: Sheets can vary quite a bit.
Some vary in thickness across the sheet and down the length. Ours is a few thousandths thinner down the length, at the edge, and in about 4" from the edge, tapering in to full thickness.
Heat treat can lock in stresses which come out during forming. This we have seen. Waves appear and tiny deviations in the polish in the surface.
Since my machine upgrades the finish of the sheets by its contact pressure forming them, the increased polish shows up stuff not ordinarily visible. Manufacturers might grain the sheets enough to conceal this and thus pass appearance tests.
Though I have formed much material like this, none have been so large. With large, percentages increase. Sigh. Overall, sheet is not nearly so bad as some I have heard about from the LA aircraft metalmen who have done this for 60 years.
Now, from the Detroit pressforming men who make thousands of large parts by pressing--(like the Spartan was made originally): Sheets from some mill runs are poor for pressing, some are good, ---and sometimes one run will be fantastic--so they run immediately to the phone to find out how to specify that particular process. Then, if they are lucky, the sheets may made to order from the rolling mill just like that, IF the pressing run is large enough to warrant the volume of sheet from the mill. Sigh. In the meantime, we constantly sort out the flaws, and determine exactly what is causing them. We simply cannot afford to let them accumulate. Kent
See also:
Aircraft Alloys
