replacing aft upper skin on fuselage

[johneeb]

..... The best way to make the new skin is to remove the old skin completely while supporting the fuselage in an appropriate support or jig. Unbend the old skin on a flat temporary table. Match drill the new skin using the old skin as a pattern. Do not cut the new skin using tin snips, it will stretch the edges. Trim the edges using a draw knife using the old skin as the pattern while the old skin is clecoed to it. After cutting the edges of the skin with a skin knife as I refer to it you can clean up the edges with a Vixen file.......

Jim, please explain "skin knife". Do you have a picture?

[Ryan Smith]

..... The best way to make the new skin is to remove the old skin completely while supporting the fuselage in an appropriate support or jig. Unbend the old skin on a flat temporary table. Match drill the new skin using the old skin as a pattern. Do not cut the new skin using tin snips, it will stretch the edges. Trim the edges using a draw knife using the old skin as the pattern while the old skin is clecoed to it. After cutting the edges of the skin with a skin knife as I refer to it you can clean up the edges with a Vixen file.......

Jim, please explain "draw knife". Do you have a picture?

As well as a skin knife. I tried Googling both...no dice.

[johneeb]

Thanks Ryan, Skin knife is really the question I meant to ask.

[Metal Master]

Attached is a drawing of a Poor man's skin knife. (in PDF)I have ones I have made from Hacksaw blades. However I have found that a Plexigalss scribing tool which has a folding steel blade with Knife edge on it is a better option and I have made two of them. I remove the Knife edge and basically grind it flat the same as my drawing attached here. I used hacksaw blades for years the same as my drawing.
Using the tool after you have drilled all of the holes in a skin using the original skin as a tool template. You use the Skin knife to draw along the edge of the old skin. You can do long runs like the aft or forward edge of a wing skin. you can also follow complex shapes. It takes approximately one scribe along the full length the skin for each one thousandths of thickness. .025 aluminum equal 25 strokes of the skin knife, then you can bed the scrap piece up and it snaps right off. Dress edge with Vixen file. New part will be exactly the same dimensions as the original part. The skin has to be laid out flat. I have ran the skin through the roller backwards to remove curvature and it may stretch a little but it has never been a problem.

861657-ScanFile001.pdf

(261.89 KiB) Downloaded 624 times

Hope this helps
Jim

[johneeb]

Thanks Jim, very interesting and a good lesson for today!

[bagarre]

Can't wait to try the draw knife technique! That's a really good idea.
I wouldn't have thought of it in a million years. When cutting aluminum, I think about shearing, not scoring.
Thanks!
FYI, Could make a great half page article with a photo or two for the newsletter.

[Bruce Fenstermacher]

Jim, that is a neat tool. I also wondered what you called a draw knife. And what would a draw knife that could also be called a skinning knife be. When I think of skinning I think of skinning a animal which is a specialty knife in it's own.

I thought your draw knife might be what some around here call a carpet knife:

This example is marketed as a Linoleum knife. This knife might also work very well for your technique.

Jim, as David mentioned, we always need material for the 170 News. I'll probably use your short blurb and drawing for one. If you have any more to add or would like to write a slightly more detailed instruction and email it to me if you have anything to add, that would be great. Also a good high res picture of your knife examples would be great. A high res photo taken from a smart phone is fine so long as you send it at original or max res should you be asked when sending. brucefenster at gmail dot com.

[bagarre]

There's enough photos and info in this thread to make a feature length article on replicating and replacing skins.

[gfeher]

One more pic. A plexiglass scriber. I like that option. They are cheap enough, you get a nice molded handle, and much of the blade shaping has already been done. Thanks Jim for taking the time to put together the drawing and explaining it to us.

Plexiglass Scriber.png (33.78 KiB) Viewed 17489 times

[bagarre]

There is a similar knife for cutting cement backer board that might work as well.

fd21de7a-2bc1-40f3-8d70-a0774d71c5ac_1000.jpg

[blueldr]

The knife shown above by Bruce is also a valuable tool to own if you're into hand picking grapes when you're going to be making wine, or if you're just into a little extra farm labor income.

[Bruce Fenstermacher]

David, I've just acquired one of those backer board knifes. Thought I'd seen something similar to Jim's drawing recently. Now when I find that knife again, where did I put that, I hope I remember what it is I could use it for.

As I recall drawing it through the backer board, the one I had dug a wide groove. I'd think it might have to be a bit sharper point.

[bagarre]

I was going to try mine tomorrow on a piece of scrap.
It's not exactly the right edge. it's sharpened to a point so it would dig more of a Vee.
I think a sharp flat chisel would be better and dig more of a groove that actually removed metal vs moving it to the side.

A vixen file would clean it up either way but there'd be less clean up work with a flat edge.

I wonder if you could make a very shallow router bit with an .025 guide shoulder for a dremel tool and cut the pattern that way. Slipping off the pattern would trash the panel tho.

Really can't wait to try this out before I cut the belly skins.

[bagarre]

I'm getting close to taking this on.
There are a few rivets where the skin extends up the front door frame above the gear box.
How on Earth do you reach these to buck them?!?
I see how to reach all the others (with the floor removed) but those have me stumped. Is there a way to reach up into that box structure with floor off?