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Plywood joint for a cash-strapped newb

PostPosted: Tue Jun 23, 2015 10:07 pm
by plectrudis
I just built a workbench for myself, and my husband wired a couple of outlets in the garage for me, so I'm ready to rumble! Except I'm broke! (Dangit)

I can't afford to buy a 5x10 trailer at the moment, so instead I'm going to start on the cabin--specifically, on the walls. The Little Ferdie is going to be just shy of 11' long, and 4' tall, so I'll need to join some plywood together for the walls (and later the floor). I'm planing to use 1/2" exterior grade plywood attached to 1x2 poplar ribs.

QUESTION 1: What is the best join for me to use?

(1) My relevant equipment currently consists of a 2nd-hand router (no relevant bits that I'm aware of) and a kreg jig
(2) I'd like the join to be a smooth as possible, as I'm planning to paint the walls
(3) I'm totally inexperienced with the router and only lightly experienced with the kreg jig
(4) I'm willing to buy a part or tool that will serve me well, but I'm looking for a used Ford kind of a solution, not a BMW solution

QUESTION 2: Do I need apply anything other than wood glue to strengthen the join?

I'm pretty sure I've heard of people putting fiberglass on their seams--do I need to do this? Bearing in mind my lack of $$$ and ignorance of fiberglassing? (I'll learn it if I need to, but if it's not really that critical, then... not).

Thanks for the advice!

FWIW, these are the bits that came with the router, and below that is my mockup of the Little Ferdie



Re: Plywood joint for a cash-strapped newb

PostPosted: Wed Jun 24, 2015 4:53 am
by GerryS
You will probably find the lap joint is you friend here....butt joints (kern) aren't terribly strong, although they have their place... ... ap-joints/

I'd practice on scrap wood first before you try to do anything large. Be sure the glue you use is quality stuff. I recommend titebond III, as it is water proof and very strong. Learn about working time of glues...once they hit the air you've got a limited amount of time to complete the glue up and to get weight or clamps in place.

Putting fiberglass over the top will give you strength and better water protection. Both are important, as plywood and water don't mix...personally, if I were building (I cheated...I have a camp-inn) I would either varnish every single piece of wood or glass and resin'll get lots of opinions on this.

I'd think of it more like building a wooden canoe, which is actually a fiberglass boat using wood as a form...there are lots of resources. Look to kayak and canoe building forums, as well as home built airplane forums (airplane is going to be mostly about vacuum bags and styrofoam...that's probably overkill)

Good luck....take it slow to start. Focus is "good enough, " and not for perfection. Practice on scrap materials, and use google to your advantage. Look to sources other than teardrop specific forums, and have fun. There's a lot to learn....

Re: Plywood joint for a cash-strapped newb

PostPosted: Wed Jun 24, 2015 8:02 am
by noseoil
Have to agree with Gerry, a lap joint is what you need. Basically, you will be cutting a 1/4" deep X 1" wide slot into the edge of the sheets you will be joining. ALWAYS do a mock-up of the joint & setup prior to cutting the actual material. There are two ways to guide the router. If the router came with an edge guide (a "shoe" which slides in the frame of the router, the base plate, to give you an edge offset) you can set the depth at half the thickness of the material & make small passes to get the 1" wide slot.

If it didn't come with an edge guide which attaches to the router, you can use a straight board clamped to the plywood panel as a guide to give you a clean, straight edge. This is a bit more "fiddly" as you will be measuring the offset from the edge of the board to the inside edge of the cutter, which is your work line. In either case, do a mock-up to get the depth you need and the width of the slot you will be cutting for the joint.

It has to be pretty close to perfect to get panel joined properly & give you a nice finished look to the build. Always have a cushion in the moaning chair when you're doing this type of work. You can sit in the chair first to make sure it's correct (sketching, thinking, etc.) or afterwards once a sheet is destroyed. Make sure to start with the sheet oversized, just in case you need to run the joint more than once. You can use TB 2 or 3 for the glue, but the main thing is to have a nice, clean joint to start with before you do any glue.

Re: Plywood joint for a cash-strapped newb

PostPosted: Wed Jun 24, 2015 8:19 am
by KCStudly
Realize that on a short budget there will either be many compromises or your project will take a very long time to complete. It can be done, tho, so don't get discouraged. :thumbsup: I do recommend that you plan out your build thoroughly, so that you can get a realistic idea of the scope of work and the true long haul cost involved. The quickest way to waste money is to make avoidable mistakes (there will be enough with a good plan... a lot more with no plan), or to get in too deep and realize that the costs are ballooning and the time is taking way more than planned due to budget restrictions.

First thing's first: Safety. Before even thinking about using the router you will need safety glasses with side shields, or at least a set of cheap goggles. The first way to blow the month's budget (or worse) is to get junk in your eyes and have to go to the doctor (or worse). I also like to use those cheap foam ear plugs.

Check your used router for any play in the shaft; there shouldn't be any end (in/out) or radial (side to side) play and the bearings should feel smooth. The depth adjustment should operate smoothly and lock up solidly, if not you can usually take this apart to clean out sawdust, and make a tension adjustment. Some older routers don't index the shoe (or base plate) concentrically with the bit, so you may get inaccuracies if you rotate the base while traveling relative to the work. On my older router I use a simple piece of bent wire clamped onto the collet like a curb feeler and rotate it around the shoe to center it before tightening the base screws.

The bits you show are high speed steel (not carbide) and the guided ones only have guide posts, not bearings. You can get by, but you will find yourself having to clean the bits more frequently, they are much more susceptible to overheating, and the guide posts will leave more of a mark and/or can scorch the wood. Carbide bits with guide rollers are a real asset. Most builders will have a bottom bearing straight trim bit for trimming overlaps flush (make sure it is long enough to make the cuts needed), a top bearing straight cutter for following templates, and at least a 1/4 inch round over.

IMO, in order to make a TD that is not overly heavy, you will almost certainly need to rip material at some point in the game (buying milled white board can really add up the cost); if you don't have access to a decent table saw, beg borrow or trade labor with a friend when the time comes. When the table saw that was in my work space moved out of the picture. at first I bought a cheap entry level table saw. Big mistake; it was crap. For about the same money and some foot work, I was able to buy an old clunker cast iron saw and get it to work really well.

Lots more thoughts, but I see I have rambled on...

With only a router, a lap joint is strongest. You could also buy a slotting bit and do a spline joint or biscuits.

Re: Plywood joint for a cash-strapped newb

PostPosted: Wed Jun 24, 2015 8:21 am
by KCStudly
... depending on where your seams land you could also do simple butt joints with backers.

Re: Plywood joint for a cash-strapped newb

PostPosted: Wed Jun 24, 2015 3:26 pm
by tony.latham
QUESTION 1: What is the best join for me to use?

A lap joint would be fine. A scarf joint would be a bit better. Scarfing may sound a bit over your pay grade but look at this video. The boat guy's much perfer a scarf over a lap joint.

You may have to codger someone into whipping the saw jig out, but I think that portion of the task would take about fifteen minutes. You'd have to do a bit more research on the glueup but it's simple.

QUESTION 2: Do I need apply anything other than wood glue to strengthen the join?

Not if you've done a decent job of "joinery." If you go with a lap joint, clean your cuts up with a bit of sandpaper on a block of wood.

Either way, I'd probably design your build to put the middle of the joint at the bulkhead.

Here's a couple of pieces of 1/4" plywood that have been cut with a router and ready to glue. Note the router guide is still clamped to the left piece:


And here's the two pieces clamped and gluing. (I've got the pieces blocked up underneath.) There's a 2x6" under the joint and covered with wax paper. The bottom plywood is screwed to this 2-by, outside the joint, to keep it from moving around. After the glue was applied to both joints, I screwed the top piece to the 2-by, covered the joint with more wax paper, laid a piece of 1" pine on top and screwed it tightly to the joint.

That glue-up was for an aluminum sheathed 'drop. If it had been for a woodie, I'd tack the pieces of ply with small finish nails, and "clamp" with several five gallons of water on top of the pine board. You can fill the holes later.


But a good scarf joint would look better.

Tony :beer:

Re: Plywood joint for a cash-strapped newb

PostPosted: Wed Jun 24, 2015 7:36 pm
by plectrudis
Thanks, all! I had been considering a rabbet joint, but I see no one seems to recommend it. Is it not suitable for 1/2" plywood? Or is the overlap insufficiently strong?

Just to make sure I've understood the process for a lap joint: I'd use a straight bit (preferably carbide), mark off a 1" area for the joint, and with the help of a clamped-on guide, run the router back and forth over the area repeatedly until all the material is removed to a depth of half the plywood and a width of 1". Repeat with the 2nd piece. Clamp or temporarily screw the bottom piece to the work are, use good quality wood glue (like titebond III), and clamp everything together or apply weights.

Noseoil, I've never heard of a moaning chair before, but I expect I'll be making its acquaintance soon. Thanks to you and Gerry for reminding me to make a habit of doing practice runs for all major operations.

KC, thanks a ton for the router advice. Frankly, I'm intimidated by the darn thing, so your tips for testing the machine and your list of key bits are very helpful.

Tony, the video dude does make it look easy, but I'm probably not up for the scarf joint this go round--perhaps on the Ferdie II ;-) The pic of the gluing stage is very helpful, though.

Re: Plywood joint for a cash-strapped newb

PostPosted: Wed Jun 24, 2015 7:57 pm
by tony.latham
mark off a 1" area for the joint

I don't know if there is a rule of thumb for joining large pieces of plywood with a lap joint. But for scarfing plywood, there is a rule; you should use a 2" scarf for 1/4" ply and a 4" scarf for 1/2" ply. Based on that old boatbuilder's rule, I'd suggest you consider making your lap joint 4" wide. I'd be gun shy of a 1" lap joint.

Perhaps other's could weigh in.


Re: Plywood joint for a cash-strapped newb

PostPosted: Wed Jun 24, 2015 9:25 pm
by pchast
A 4" joint is preferable. Take the time to set up a clamped guide and another piece of 1/2 ply on the outside of the lap. This will make sure your router has support on both sides. I go so far as to set a second clamped guide on the opposite side of the cut too. This will enclose the router in the cut. I'm not too comfortable working with a hand held router myself.


Re: Plywood joint for a cash-strapped newb

PostPosted: Wed Jun 24, 2015 11:01 pm
by noseoil
The reason I suggested using a 1" wide pass instead of something larger was because you said you were going to use poplar backing & edges for the build. If there's a poplar rib behind the joint, you will just need to be careful while handling it without the poplar in place. Once the panel is glued to the poplar, there is no way it can move if the glue does its job & you get a good bond & it sets up over night. My 3/4" side frames were just glued with butt-joints (with a 3/8" wood dowel) and only 1 1/2" wide. When the 1/8" birch skin went on, it was fine & still is. If you are using 1/2" exterior ply, try to find some 5 ply stuff as the 3 ply is pretty much junk.

Re: Plywood joint for a cash-strapped newb

PostPosted: Thu Jun 25, 2015 10:42 am
by Pmullen503
Some random advice:

On plywood splicing, a butt or lap joint is fine for flat surfaces. Scarfing is worth the effort on bent panels where strength is needed and where a "hard" area wouldn't allow a smooth bend.

On woodworking, I recommend that you look for a class at a high school or tech school. Not only will you get training in the basics but you can get access to equipment (or people who have the equipment) that you aren't able to afford. You may get a line on used tools (or wood) which can be a bargain. Ripping and planing your own rough stock is cheaper than buying finished stock. You may meet someone who will help you with tools and know how too.

Practice building the internal boxes or cabinets first to sharpen your skills. It will cost less and if you do waste materials it wouldn't be too costly.

On your trailer, don't start until you have a good plan. You are less likely to come up against costly remaking of parts. Consider thinner plywood. Good luan is cheaper, lighter and has a better surface finish than exterior plywood. In fact, I recommend you buy the best plywood you can. You won't miss the extra money years from now but will appreciate the quality. Using quality tools and materials saves time and often doesn't cost that much more.

Consider sandwich construction with insulation. It's more work and money but you'll enjoy your trailer much more if it's insulated. Visit a teardrop round-up in your area to see various examples, finishes and features you may want. Most people are happy to offer advice and talk about their trailers.

Get your trailer before you start on the exterior. You'll need a big flat work surface and the trailer is good for that. You'll know exactly how long and wide it so your shell will fit better.

Finally, have fun!