The Hatch: Round 2

[DudKC]

Needless to say, the hatch won round one.

Which is why it is currently on the burn pile.

We tried several things to salvage it, none of which were successful, so we decided to scrap it and try again.

I believe our main issue was spring back. We used 3/4 inch ply wood for the outer ribs, doubled up. with ripped 2 x 4's for horizontal ribs. We skinned it with 1/8 inch luan, and it sprung back on us, we couldn't get it right so we're starting over.

So now we're thinking about different materials, we thought about redoing the frame out of wood again, and using a more flexible skin that won't spring back on us. But, I'm building a woodie, so if at all possible I'd like to maintain that look on the hatch, and I don't feel comfortable just rebuilding the same hatch and hoping it doesn't spring back.

So then we thought of using 1/8 x 1 inch steel angle iron for the outer ribs, and possibly pre welding a strap of flat steel which would span the curve of the hatch. This design would hopefully be impervious to springback. Again thinking of ripped 2 x 4's for the horizontal ribs.

I guess I'm wondering if anyone else has used angle iron for their hatch frame, did it work? Was it difficult to attach a plywood skin to it? Any stories or suggestions would be much appreciated. Once we get this hatch figured out I can just tell it's going to be smooth sailing from here on out (*crosses fingers*).

Thanks for reading.

[George Taylor]

I had the same issue with my hatch. I used 3 layers of 3/4 ply for the sides that were glued and screwed. Then I tried to use hardi board/peg board. All that did was pull the ply wood arch out about 2 inches from the original shape. All I had to do was eliminate the wood skin and use the aluminium for the skin.

[DMcCam]

If memory serves, this was a brain child of Grant's (man is he good). One way to keep things really straight is to use 1/8" aluminum sheet flush cut 2 extra ribs out of it to match your outer rib contour. You can either glue it to the rib so it shows or sandwich it in between the two 3/4" outer ribs. The aluminum won't bend or spring back therefore neither will you hatch. Use the same thickness of stringers as you do the ribs so you have something to glue the plywood to. I would use Baltic birch instead of luan just because it won't de-laminate on you. Just an idea.

All the Best,

Dave

[DudKC]

If we did go the aluminum sheet skin route, I wonder what thickness to use. A supplier in town has 0.025, 0.032, 0.040, 0.050, 0.063, 0.080, and 0.090 I'm guessing those are all inches of thickness.

[DMcCam]

Hi Dustin,

Most use .040 aluminum either anodized or mill finish. We used anodized .040 for the sides and .063 for my roof because I wanted it seamless. If I was to do it again perhaps I'd go with a seam or two of .040. The .063 is 1/8" thick and is a bear to bend especially when it's anodized. Some have used pleated aluminum to great effect Like Woodbutcher http://www.tnttt.com/viewtopic.php?f=50&t=39036&start=15 You might ask him about the thickness, it sure is pretty stuff.

All the Best,

Dave

[markhusbands]

I used 1/8 red oak veneer ply across 3/4 inch ribs and nothing bad seemed to happen. I did rip it so that the veneer grain was bent in the flexy direction. I'm also planning on using draw latches in case it springs a little.

[CarlLaFong]

.063 is 1/16 inch, not 1/8

[DMcCam]

Hi Carl, Yep, you sir are absolutely correct...it just felt like 1/8" Even at 1/16th it would be worth having it sent through a roller.

Thanks for catching that Carl.

Warm Regards,

Dave

[DudKC]

So we'll probably try for .040 or .032. Probably as thin as I think I can get away with on the hatch skin. If we use the steel angle iron for outer ribs, would you attach to the angle with rivets?

[markhusbands]

When people are getting the springback, are they laying the 1/8 longways, or cutting the sheets in half and laying it so it bends with the grain? I tired to do the former to reduce seams, but there was so much more resistance i decided to do the latter and put up with more trim. What are other people doing?

[DudKC]

We laid it long ways too, our seam was running horizontal, not vertical. That probably made the springback worse.

[KCStudly]

Even aluminum or steel can spring back if it is not strong enough. The easy way to make the ribs much more resistant to springing open, regardless of the material used, is to make them deeper.

Instead of 1x2 nominal section ribs, I am using 1x true 3 inch deep ribs. Just need to make sure and leave clearance for the counter, cabinets and struts. Haven't gotten to the assembly stage yet, but I have confidence that when skinned it won't spring nearly as bad as some that have been cursed and tossed in the fire pit (... I hope).

[Fyddler]

I followed the directions for the Generic Benroy trailer. Lots of vertical ribs from 3/4" ply (6 for a 6.5' wide hatch) and lots of cross members. I also ran the 5mm plywood width wise, so it was easier to bend, but I am not building a woodie, so I didn't care about the seams. I also screwed the frame in place, and used my first sheet down the center of the hatch, let it dry, and then removed the screws. I had no noticeable spring back.

[danlott]

I am curious what construction method people having the spring back issues are using. I have not made my hatch yet, but here are my plans for building mine.

1. Build the hatch in place on the back of the trailer.

2. After framing is complete use some 1/8" scraps of plywood on the outside of the hatch to prevent the hatch from racking when moved.

3. Remove the hatch from the trailer, glue and attach the inside skin to the hatch.

4. Place the hatch back on the trailer. If any spring back has occurred I will then use some cheap cargo straps to pull the hatch back to the proper shape. I will attach the straps on the inside of the hatch, both top and bottom. I envision just using some drywall screws ran through the straps into the wood. I would then adjust the straps to ensure a good fit on the trailer.

5. Leave the hatch on the trailer. Remove the plywood scraps off the outside of the hatch, glue and attach the skin to the outside of the hatch.

6. Once the glue has had time to properly set remove the hatch and then remove the cargo straps.

Again I have not made my hatch yet, but this is my plan. My thoughts are you are just creating a torsion box. A torsion box will retain the shape that it was constructed in. If you do not remove the initial springback caused by the plywood on the first side you skin, the springback will be locked in with the second skin.
Dan