Stretching your plywood...just 1 technique

[artwebb]

Thanks for fixing link. Have worked with wood before but not plywood and never needed this sort of joinery, just framing (houses) and building simple furniture. I remembered the terms (vaguely) from IA but could not remember the actual techniques

[afreegreek]

here is another alternative to spline cutting.. it is a finger joint cutter. it will work on wood from 7/16" to 1-3/8" thick. I have one of these little beauties. it's one of those things you get and then wonder how you did without it.

fantastically precise but a little on the fussy side to set up. you cut with one set up but you must fiddle with test pieces to get the bit "centred" so to speak. as you cut the first half with the router on the face of the piece and the second half with the router on the back. "centring" makes both halves of the joint line up flush on the face.

BTW, your test pieces have to be exactly the same thickness as your good stock and you must fuss every time you change material thickness. it works for other joinery too.

http://www.leevalley.com/wood/page.aspx ... 46174&ap=1

[Rlowell]

Here is one more method that I will be using on my build of a Weekender...Pocket Holes...From Kreg. Kreg states that if you space the holes about every 4 to 6 inches with the first one from the edge at 2 inches you will have a real tight bond (with or without glue). I think I will use glue just to make sure. Just make sure that the wood is square. I will be adding two feet onto the eight. Hopefully I can sent a picture next week. What do you think?

[rwelp]

I'm not sure how well the screws will hold in the edge of plywood. I've not had very good luck with that.
Rob Welp

[synaps3]

I'm not sure how well the screws will hold in the edge of plywood. I've not had very good luck with that.
Rob Welp

They don't! That's the whole point of the joints...

[afreegreek]

butt joining plywood with pocket holes.. the issue is the holding power of screws in the plywood edges. you can get around that by drilling and glueing in dowels through the plywood to screw into. drilling through the ply is the strongest way but you will see the dowels in one or both sides (one side if you stop drilling before you come all the way through). if this is objectionable then drilling and glueing them into the edge will work although it will not be as strong, and because the dowels will be oriented so you'll be screwing into end grain you'll have to choose your predrill size carefully lest you split the dowel driving in the screw. a 3/8" diameter dowel is the minimum, 1/2" would be better. however, you'll need to be using 3/4" ply to use them. brush a minimal amount of glue in the hole and on the dowel then tap it in nice and slow so as not to split the ply due to hydraulic lock. using a clamp across the hole will make it a little safer.... if you want to go full monty, you can drill dowels into the pocket side too for the screw head to land on. since this half will be marred by the pocket holes, going through (or almost) the ply won't matter. drill, glue in, and flush up the dowels before drilling the pocket holes.

[tinkerman]

Hey all, when "Streatching'' plywood another good router method is to use along mortice and tenon joint or a deep tounge and groove.
WIth a double fluted bit cut a rabet on both sides of one end of a sheet on the other a slot cutter bit is used doing t or more passes to make the tongue fit. PL the snot out of the joint and pipe clamp till the glue cures.
Want a smooth joint, wipe away all excess PL using a putty knife. if your joint is visible and you want it to look seamless use bondo as afantastic filler. It is sandable and paintable.

[afreegreek]

Hey all, when "Streatching'' plywood another good router method is to use along mortice and tenon joint or a deep tounge and groove.
WIth a double fluted bit cut a rabet on both sides of one end of a sheet on the other a slot cutter bit is used doing t or more passes to make the tongue fit. PL the snot out of the joint and pipe clamp till the glue cures.
Want a smooth joint, wipe away all excess PL using a putty knife. if your joint is visible and you want it to look seamless use bondo as afantastic filler. It is sandable and paintable.

this can also be done with just the one slot cutting bit if that's all you have. you have to cut the slot first on one piece then adjust the cutter to rebate the second piece from both faces leaving a tongue.

it's best to get the bit as close to centred on your stock as you can and run the slot twice, once from each face of the first piece. this will guarantee a centred slot.

then rebate the second piece from both faces starting with the bit set to cut the rebate a little shallower than you need. then with a couple of more adjustments and passes you can creep up to the perfect fit..

perfect fit does not mean tight fit... as with all interlocking joints you have to have enough slop to allow you to apply the adhesive and bring the joint together without splitting the slotted half. this fit will depend on the type of adhesive you use. remember it's the adhesive that gives the joint it's strength not the interlock. that is for reenforcement and alignment.

a good rule of thumb is to be able to fold a piece of writing paper or a dollar bill over the tongue and still be able to insert in in the slot.

[mwallace61]

Thanks for posting this one! It got bookmarked about 30 seconds into it...

[Rlowell]

OK...I give in...pocket holes are out for "Stretching plywood".
A lot of great ideas! Thank you again. I will consider all and let you know what works for me and my budget. I have built the sub frame using the pocket holes and it seems to be very strong.

[misterW]

Just curious what some of you would use to provide pressure lengthwise while gluing if you were building, say, a 12 x 4 piece from an 8 x4 and a 4 x 4. I don't have any pipe clamps that long.

[doug hodder]

You can always just butt them together and screw a piece of scrap to the joint to hold it, or make some 2x2 stretchers with a block to fit over the end to make up the difference for the short clamps. I didn't do that...just thumped them together with a mallet and weighted them and let the epoxy cure. Use wax paper on the bottom of the joint so that it doesn't glue itself to a bench/floor etc...same thing on the top side. Watch clamping as too much pressure will make them want to bow at the joint without weight. Just what I've found. Doug

[misterW]

Do you think the spline method is strong enough for making a 5x12 sheet that isn't reinforced on the inside? I mean, if you just took 3 8x4 3/4 inch plywood sheets, splined them together, then cut it down to 5 feet? I noticed the picture of the big teardrop on the first page seemed to have a lot of wood reinforcing it on the inside.

[doug hodder]

I'd cut them down to almost your correct height prior to gluing them up just for ease of moving them around. I glued up 6'4" x 11' and no backing. I'd think another 6 inches would be no problem. Doug

[misterW]

Thanks much for the great information. I'll let you know how it works out.