# How to build a box frame - the right way?



## JayArr (Sep 18, 2018)

Hi All

I need to build a box to go over the generator on my trailer. It will serve two purposes. First it will keep the generator out of the rain/weather and when the generator is running it will muffle the noise so I'm not bothering the neighboring campers so much.

I've got the base already, it's 1" poplar that sits inside a metal frame now I need a removable box to go over it all.

I need to be able to remove the box to service the generator, do oil changes, replace filters etc but this will be infrequent. For simplicity I'd like to build five sides of the box as one piece that just sits down over the generator and gets bolted to the sixth side which is the bottom the gen is on.

I don't want the edges to be plywood to plywood. In my experience they aren't too strong, the plywood splinters and it doesn't look too good. Keep in mind this box is on the tongue of my travel trailer so it's subjected to 65MPH winds as I drive down the highway. I thought I would build a box frame out of maple or poplar pieces with inside dados that the five plywood panels would get glued and nailed into. This way all the edges are solid wood and should wear well so I'm not building it again in three years.

The first conundrum is, when building a box frame, do I build the top and bottom square frames and put the four stiles in between them or do I make four stiles longer and use the rails to connect them? Is one way better than the other or is there a third way to join three pieces in a corner?

Maybe that's the core of the question, what's the best way to join three pieces of wood in a corner?


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## shoot summ (Feb 21, 2014)

I wouldn't build a cover for a generator out of wood.

I would mimic trailer construction, aluminum channel and some sort of panel material from metal, likely aluminum sheet.

Insulate on the inside with a heat resistant product to help mitigate sound.


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## TimPa (Jan 27, 2010)

if you go the stile and rail route, i would rabbet the edges of the joining edges, and glue/screw them together. but, i would agree with above, aluminum angle with marine grade ply panels may be a longer lasting box. 

also, you may want to address heat dissipation while it is running...


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## woodnthings (Jan 24, 2009)

What about heat and exhaust when the generator is in use OR during cool down. The wood won't allow it to air cool if and when it's covered.
Aside from that being obvious, a plywood box, from 3/8" thick will still be a bit heavy to lift on and off, thicker would be even worse. A lightweight metal frame from 1/8" X 1 1/2" aluminum angle having sheet aluminum panels pop rivited on would be my choice, weather proof and fireproof.


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## JayArr (Sep 18, 2018)

I don't have a NASA budget here, I just want to muffle the genny from 76db down below 60db at 21 feet.

Air/exhaust - there will be baffle openings with fans to move air through.

Weight - once it's mounted on the tongue it stays there, weight isn't a huge factor. I may have to remove the cover a couple of times a year at most.

Heat - it's not really a big deal, a muffler gets hot enough to burn skin and maybe hot enough to ignite a board that touches it - if it was there long enough - but not hot enough to ignite a board located several inches away from it.

A frame of aluminum angle would be sweet! Someday I'll own a tig welder and work with aluminum but right now I don't. Trying to connect three pieces of aluminum angle iron with screws would be a nightmare. Sometimes you just need to KISS.

Noise shielding is a big function here. Metal doesn't dampen noise very well but 1/2" plywood does.

Building the walls out of aluminum then covering them in a sound proofing is a possibility but there just isn't any great upside. It's not cheaper, it's not easier to build, it won't end up any lighter and it won't work any better.

I've got a woodshop, let's use it to solve this problem - rabetting the stiles seems like a good idea, certainly less work than tenons, time to get a sketch pad out and figure out how to do that.


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## _Ogre (Feb 1, 2013)

i wouldn't put a box over a generator, it just creates too much heat
get a washer/dry box and throw it over your generator when it's running
i think your genny will overheat and seize within the hour
punch what ever holes you think you need for cooling, it'll still overheat

most generator manufacturers design low noise mufflers for most gennys
i'd choose that path before i cover a running genny


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## JayArr (Sep 18, 2018)

I agree with the low noise muffler, I've already purchased that and it will help. I've also bought rubber isolation feet/mounts designed for air conditioners and mounted the genny on those.

The idea that your genny will overheat is just not true. RV's enclose their generators in little cubicals and they run just fine and there are millions of them on the road. Generator boxes are not a new idea, you can buy them, look up Zombie Box. I've watched dozens of you tube videos of home builds and the generator is fine, some of the guys don't even use fans and there are no overheating problems. One guy addressed the idea that the gen would overheat and said he ran his 6 hours a day for two years so far without problem. Another guy told how the army digs a hold drops the genny in and covers it with plywood and dirt to keep it quiet.

Go to youtube and search DIY generator box, you'll see dozens of people who have done this without overheating the generator. I believe they are more resilient than you think.

Anyway... I started this thread at WoodworkingTalk to find the best way to connect three pieces of wood in a corner. I have one suggestion of rabetting the boards and using glue and screws. 

I've also thought of half lapping the two bottom rails and then pre-drilling up through the half lap to screw into the stile. 

I'm open to suggestions


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## BigCountry79 (Jun 2, 2021)

A simple box might collect too much water on top. I would suggest you build in an overhang and an angle so the water drains off the lid and drips to the side.

Joinery with decking screws and butt joints will be just fine. Just get exterior sheathing and screw it together. Make the top oversized, and screw it together from underneath with either pocket screws, or attach a 1x2 under the roof along the edge of the box, then screw it from below. 

Consider adding a sheet of drywall inside to better reduce sound. 

Dont make it removable, just leave one side so you can easily unscrew it.

I think cooling will be an issue in hot places like everyone says.

How are you planning to fill it with gas?

How do the you tube videos make the box?


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## J_L (Apr 22, 2014)

I've done what your considering, and the generator overheated. I ended up with enough ventilation holes to negate the noise blocking benefits. RV generators are different than stand alone generators and are meant to dissipate the heat differently. If I were in your shoes, I'd make a mock up box out of some inexpensive material and run the genny with your normal load and see how it reacts.


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## J_L (Apr 22, 2014)

For whatever method of joinery you end up using, I'd recommend some large epoxy fillets on the inside corners using some epoxy thickened with cab-o-sil. This will give it a lot more rigidity which is necessary on the road.


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## shoot summ (Feb 21, 2014)

JayArr said:


> I've got a woodshop, let's use it to solve this problem - rabetting the stiles seems like a good idea, certainly less work than tenons, time to get a sketch pad out and figure out how to do that.


Then I would build the box out of plywood, and wrap the edges in aluminum angle, a hybrid approach.

You get the sound deadening of the ply, and durability, and you protect the edges with the angle.


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## JayArr (Sep 18, 2018)

The youtube videos vary but it's either plywood/MDF/particle board or this rigid lightweight tile backing board. There are some where they start with a metal box and then insulate the daylights out of it with foam boards and aluminum tape. Some are double walled with pink insulation in them, some use a professional deadener. The ones without a fan have larger intake/exit holes, it appears if you use a decent fan to blow into it you only need a 6" hole for the fan and an exit port.

I'm not trying to make it silent, I know that's not possible, I just want to reduce it from 76dB to 60db at 21feet which seems to be the standard for not being too annoying to the neighbors and for government run campgrounds. The newer inverter generators run around 68db under load so if I can be quieter then those then no one should complain.

No need to refill the gas, the engine has been converted to propane and runs directly off the propane system of the trailer. It's also electric start so my wife can turn on the gas valve and push a start button and it'll run. There was no way she could pull start an 8HP Briggs and Strattton so electric start was important.

I had to go google Cab-o-sil, it looks like a great idea for strengthening the seams - thanks J_L.

Slanted roof makes total sense, thanks Big Country!

I thought of outside edges Shoot Summ but my angles aren't all perfect 90s. It would work around the rails but the corners where the stiles attach are not 90 degree angles because this box sits on the tongue of the trailer so it's a trapezoid. see pics. The generator sits on top of the metal fame you see and plumbs and wires into the trailer in a permanent way.


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## JayArr (Sep 18, 2018)

More pics.

I fabricated the metal frame myself with my mig welder. I also moved the propane tanks forward on the tongue to make room and hard lined all the propane, added a port with a valve for the genny and installed a new, higher volume, regulator and switching valve mounted between the tanks so it's covered when they are.


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## JayArr (Sep 18, 2018)

Here's the modified genny. By switching to propane I was able to shed the gas tank, I also removed the metal shrouding that it had to protect users from burns, this will help with heat dissipation. The original air filter was huge so I found a smaller aftermarket one and I bought the large low tone muffler and some flex pipe. You can see it in the right rear corner, I'll make a mount for that over the generator section. You can see the propane regulator in the front right, it connects to a modified carburetor and supplies the fuel. No choke needed with propane and you remove the float assembly. There are no moving parts in that carb! I've re-wired the field windings to change it from 240VAC at 15A to 120VAC at 30A since that's what my trailer is looking for and it'll connect to that large receptacle called a TT30R that the trailer will plug into. Oh, and I plumbed in a valve for easy oil changes.


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## JayArr (Sep 18, 2018)

So I finally got all of the mechanical aspects completed and could move onto the woodworking. 

I spent the afternoon in the wood-shop and built a decent wooden frame with rabbet cutouts for each of the plywood panels. It's all poplar and when finished it will get lined on the inside with sound dampening vinyl and foam.

Construction was fairly simple, I made the top and bottom trapezoidal frame rails from 1x1 with 3/8 rabbets cut into them full length. Then I made the stiles from 2x2 cut down to match the trapezoidal corner shape and I rabbeted them as well. Then I cut down the corners of rails to the rabbet edge in the shape of the stiles so when the stiles are placed the rabbet turns the corner.

The top trapezoid pieces added a further wrinkle since they needed a rabbet for both the side panels and a top panel. 

The hard part was getting it all together, I used some good outdoor glue and some pin nails and it eventually stiffened up and got stable. Once the glue completely dries I'll put the panels in and paint.


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## JayArr (Sep 18, 2018)

Well I got this almost completed. I cut panels to fit into the rabbets and used exterior glue and staples to fasten them. Then I cut holes for the fan intakes and exhaust and I built a pair of vent covers to go over the intake and exhaust to keep the rain out. Finally I lined the inside with a heavy vinyl material that dampens sound and will keep the neighbors from complaining about my genny noise.


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## JayArr (Sep 18, 2018)

Here it is all painted and with the vinyl inside.


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## JayArr (Sep 18, 2018)

Mounted on the trailer.


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## JayArr (Sep 18, 2018)

It still just looks like a plywood box but I'm pretty sure it will last years longer. The frame will keep the corners from splitting and none of the plywood has exposed edge. It's got two coats of primer and two coats of oil based white on it to protect it from the rain and the road.


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## J_L (Apr 22, 2014)

Looks good 👍 Have you ran the genny in the box yet?


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## JayArr (Sep 18, 2018)

I had it running briefly in the box before the vinyl was installed. It ran OK but was still a bit loud. Since then the governor gear broke so now I'm waiting for a part that I hope will arrive this week.


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## John Smith_inFL (Jul 4, 2018)

Jay - looks great.
(how are your fiberglassing skills ??).


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## JayArr (Sep 18, 2018)

Almost non-existant, but great idea.

How much extra do you figure it would weigh if I covered it in fibreglass?


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## John Smith_inFL (Jul 4, 2018)

questions abound about the interior temperature after running for a few hours.
I'm sure you could find some 1/4" thick heat resistant sound deadening panels for the inside.
glass would add maybe 2-3 pounds more: glass and resin don't amount to much weight compared to the massive strength and weather resistance it offers. EXTREMELY easy to learn. all the supplies you need are at your local Auto Parts Store and instructions are on the ole YouTube. (you can call this one your "prototype" and after you use it awhile, you can incorporate the changes into the final project.


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## JayArr (Sep 18, 2018)

I have another layer of sound deadening foam coming that has a thermal aluminum layer on one side and self adhesive on the other so the inside of the box will be all fireproof when done. I'll test inside the box with a thermocouple but I know the engine only gets to about 375F when running and that's the iron, not the air. With the fans constantly blowing I don't expect the box to get hotter than about 250.

I'll watch some you-tubes on fibreglass.


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