Wall profile; outside plywood necessary?

[airdonut41]

Hi all,

I'm new to this forum and excited to be here! As I'm drawing up plans for my first teardrop, I keep running into a question: why do people put 1/8" (or thicker) ply on the outside of their structural wall only to cover in aluminum sheet? My plan is to use 3/4" ply for structure (with cutouts for insulation), 1/8" ply on the inside (sanded and painted to save $$$ vs. expensive plywood). But then on the outside, is there a reason not to put the aluminum sheet directly onto the 3/4" ply+insulation? Is it to keep the insulation in place for a floating skin?

Dan

[tony.latham]

why do people put 1/8" (or thicker) ply on the outside of their structural wall

Good question.

Sandwiched walls form a torsion box. The exterior/interior skins add stiffness to the wall that a single-sheet wall doesn't have.

I use 1/4" ply on the outside for two reasons. The first is that I run screws into the cross members and need the extra meat. The other reason is that the cross members fit into dados that are cut into the skeleton. The exterior skin forms the backside of this slot.

You can see skeleton dados here:



And you might be able to see how things fit here:



Good luck with your build.

Tony

[airdonut41]

Ah, yeah, that makes sense. I had been imagining that the 3/4" skeleton plus spars would be plenty of structure. I imagine it would be, but I realize now there are other benefits of the sandwich you describe.

After watching your video and a few others, I realized that I had neglected to properly consider how I would attach the inside of the roof. I had previously been thinking I would just nail/staple 1/8" ply to the spars (which would be directly nailed/glued to the 3/4" skeleton), but offsetting the outer 1/4" relative to the 3/4" skeleton by [inside ply thickness] + [spar thickness] means that you have a lip to which you can glue that 1/8" bit! Much easier, and should give a better continuous curve without showing any fasteners.

Another question for you, Tony: What's the purpose of the pieces you're gluing (that are currently clamped) in the third picture you posted here http://www.tnttt.com/viewtopic.php?f=21&t=73078? To add some more meat when attaching the roof?

[tony.latham]

I think the lip on the wall is a big deal. No trying to wrangle the headliner under the spars and it makes for a nice tight interior joint. Plus you can pre-finish (varnish) the interior walls on the bench without worrying about scraping them while installing the headliner.

Another question for you, Tony: What's the purpose of the pieces you're gluing (that are currently clamped)

That's roof blocking. The blocking is needed to attach the roof and also to lock the spars in place.

I cut the skeletons the same size as the exterior skin (using a 1/4" template) and then trim the shelf and floor areas off. The offcuts for the shelf are later used for the blocking.

This photo is for CNC milling of the skeleton if one wants to go down that tube but it shows the blocking --that is later cut to fit in between the spars.



Here's a quick video showing the headliner installation.






Tony

[airdonut41]

That's a great idea. I was thinking it would be painful to make the blocking, but the method you just described makes fitting trivial. Thanks for taking the time to respond! You may have just saved me many hours.

I wish I had access to a large CNC setup. Most of the fabrication I have done has been metal machining on a non-CNC mill, which, unfortunately, doesn't exactly match up to the task you're describing.

[noseoil]

The "poor man's milling machine" is just a decent router. If you get a 1 hp plunge base model & a few bits, that's really all you need for machining everything on a teardrop. Since you already have the skills to set up & run a mill, just set the clock back a few years & think analog, that should work well enough for the one-off stuff you'll need to do on a build.

[Steve Frederick]

The "poor man's milling machine" is just a decent router. If you get a 1 hp plunge base model & a few bits, that's really all you need for machining everything on a teardrop. Since you already have the skills to set up & run a mill, just set the clock back a few years & think analog, that should work well enough for the one-off stuff you'll need to do on a build.

I’ve built all of my projects with a router, bushings and a template.


Sent from my iPhone using Tapatalk

[airdonut41]

I'll definitely be using a router with flush trim bit, particularly for making nice interfaces for sealing. But man, the CNC would make such short work of all of the wood cutting for this project.

One last question on this topic: is there a benefit to using 1/8" ply over the roof insulation before skinning in aluminum? Given the spars and loading directions, it seems like you don't need the additional strength that you get from sandwiching the walls. And given that the aluminum will seal anything, it seems unnecessary to add the additional wood. Am I wrong about that?

Just got the steel to build my trailer and the aluminum skin; I'm excited to start digging into this project!

[Steve Frederick]

On the projects with an aluminum roof; I used a 1/8” ply base. This provides a backing for dent resistance and gluing. See my website or The ShopManual.

[airdonut41]

The dent resistance makes sense (as does surface area for gluing), but it seems from reading around on here that floating the roof skin (only fastening under the trim) should be a better way to attach the skin to avoid thermal expansion issues. Others seem to suggest flexible glue, but honestly, all of these seem like they'll hold the aluminum in place just fine.

[rjgimp]

...just set the clock back a few years & think analog...

Sometimes I think the world might turn a lot smoother if more of us thought that way a bit more often about a lot more things.

[twisted lines]

...just set the clock back a few years & think analog...

Sometimes I think the world might turn a lot smoother if more of us thought that way a bit more often about a lot more things.

But there is no way I could push a 1/4 “ cutter through 3/4” mdf without the router on

[tony.latham]

One last question on this topic: is there a benefit to using 1/8" ply over the roof insulation before skinning in aluminum? Given the spars and loading directions, it seems like you don't need the additional strength that you get from sandwiching the walls.

My roof systems start out with an 1/8" Baltic birch headliner, 2" spars and 2" of foam board, topped off with another layer of 1/8" Baltic. It's all glued together of course and the system becomes a torsion box just like the walls (but stronger since it's 2-1/4" thick).

It's stiff. I could probably stand on it. But without that top layer, it wouldn't be nearly as strong.





Tony

[tony.latham]

I wish I had access to a large CNC setup. Most of the fabrication I have done has been metal machining on a non-CNC mill, which, unfortunately, doesn't exactly match up to the task you're describing.

I wish I had access to a CNC for my builds too... but I didn't.

There's a guy that has taken my CNC files to a commercial shop and they are cutting the wall pieces for him. They are charging $400 and that includes the materials. I think that's cheap and it'll really jump-start his build. Full-sheet capable CNC machines are out there and are probably looking for work.



Tony

[twisted lines]

If it only fit on 10'