Corner joints in Plywood

[John HaineParticipant @johnhaine32865]

Worth knowing that there is a Cuprinol product "Wet Rot Hardener" that's made for repairing things like window frames that, well, get wet and rot. It is great for MDF as it soaks into the surface very readily and hardens to a rock like consistency which (though I haven't tested it) is probably pretty waterproof especially is you apply a few coats and follow with varnish.

[peak4Participant @peak4]

Posted by Neil Wyatt on 26/01/2017 16:30:49:

12mm is the absolute thickest it can go to keep the weight down; mdf will be no good it will get wet by design although it won't be left out in the weather.

I was going to use WPBP exterior ply, as used on boats, rather than plain hardwood (birch I assume) ply.

Neil

Neil, I came across something called Medite Tricoya a while ago. It's effectively waterproof MDF.

It, or something similar, has been used to build a hide/viewing screen at North Cave Wetlands. It's untreated and exposed to the elements.

Not cheap, but it's all relative depending on how much you need, and whether it's more suitable then ply for your application.

I've never used it, and don't have any connection with this firm. **LINK**

and a youtube link **LINK**

[Martin KyteParticipant @martinkyte99762]

You need one of these.

**LINK**

Then just glue it all up.

regards Martin

[Ian S CParticipant @iansc]

I made the base for a ST S9 by gluing the mitered corners of a ply wood box, I formed a step inside and glued the (black marble), Malamine covered MDF top in. PVA glue

Ian S C

             

 

Edited By Ian S C on 27/01/2017 11:39:31

[Neil WyattModerator @neilwyatt]

Posted by JasonB on 26/01/2017 16:49:31:

Posted by Neil Wyatt on 26/01/2017 16:30:49:

Titebond and hide glue are a bit too much faff.

Neil, you just squeeze Titebond out of the bottle same sort of consistancy as PVA, small brush to apply to all surfaces. Out of interest Titebond also do a liquid hide glue that can be used straight from the bottle but teh Titebond II will be better.

Birch ply is generally WBP bonded and being a denser timber that the far eastern hardwoods gives a better board. Also (depending on grade) has less voids and surface blemishes so will also be stronger and better looking.

MR (Moisture Resistant) MDF is available and is what I use for most things inside as is the even more durable External MDF.

I'm only familiar with the liquid hide glue (used it to stick a violin neck back on that had previously been repaired with GOK what).

I went for 'exterior' ply with no blemishes on any side of three 4×8 sheets. The hardwood ply 'suitable for exterior use if sealed' looked cheap and nasty in comparison.

Got it cut to size (B&Q use centimetres! "I could do inches but the figures have worn off the scale&quot but nice and accurate for comb joints. Probably won't do loads of small combs, more like 100mm ones. Not sure whether to saw, saw & chisel , rout and saw or just rout. I will experiment on an offcut or two first.

[Neil WyattModerator @neilwyatt]

Posted by Martin Kyte on 27/01/2017 11:20:22:

You need one of these.

**LINK**

Then just glue it all up.

That would do nicely if I had a router table.

[Sam Longley 1Participant @samlongley1]

You do not state the box size but I have just made a couple of 2ft square boxes 6 inches deep.

I made the 6 inch sides from 20 mm planed softwood which i grooved 3.5mm 5 mm in from each edge.

I assembled the softwood as a frame pinned & glued at the corners with the base in 3mm ply set in the grooves & glued which not only reinforced the box held it nice & square. One of the 4 sides was reduced to just below the groove on the upper edge & the lid was made as a sliding piece of 3mm ply, The piece cut off the one side was fitted to the edge of the ply to act as a finger pull & maintain the look of the box.

Over the years we made quite a few smaller boxes like this as it is so easy & quick. You can make it deeper if you use ply for the sides & if the base is grooved into the sides it will hold it together Ok.You do not need thick ply for the lid & base if the box contents are not heavy. You can hinge the lid but if grooving for the base then the same set up means you can do it all in one go & you do not have the hassle of fitting hinges & making a catch etc

[mark smith 20Participant @marksmith20]

I use proper hide glue ,its fantastic stuff and can last for centuries ,though not outside .Though im biased because im a musical instrument maker. Liquid hide glue (ready made ) on a violin neck i wouldnt recommend ,its nasty stuff .

These joints on my personal violin that i play  were made in 1695  ,how many modern glues can last that long.

Edited By mark smith 20 on 27/01/2017 19:37:28

[Neil WyattModerator @neilwyatt]

Posted by mark smith 20 on 27/01/2017 19:22:35:

These joints on my personal violin that i play were made in 1695 ,how many modern glues can last that long.

I'll tell you in 2400

I was put off hide glue for life because when I was in the scouts we used to have to collect male holly and glue red cotton wool 'fruit' to it with heated up hide/hoof glue (which stank) before taking it to decorate the church.

My joints have to last three years. An experiment shows (1) my tenon saw is blunt, and as it's a hardpoint I need a new one, (2) plywood doesn't like being chiselled or cut with a blunt saw and (3) it will be much better if I match the width of the fingers to a chisel.

Still have a 340mm wide test piece glued up with gorilla exterior glue to dry overnight.

Perhaps I should use this stuff: http://www.pennelcomonline.com/en/Penn-Elcom-12-Double-Angle-E08762000/m-8920.aspx

[Steve PaveyParticipant @stevepavey65865]

All this talk of dovetails and finger joints – pah! Just get yourself a Festool Domino, a 4 mm cutter and some dominos and do it properly Neil. A man of your means etc etc.

Seriously, while a Domino is what I would use (cos I've got one for the day job), I have made most of my drawer boxes in the workshop with finger joints done on the saw table (router table also just as good), and glued with Titebond. Most of them carry quite a weight and none have failed after 8 years.

Another technique I have used for speaker boxes (in mdf) is a plain mitre – cut the mitres, lay the four pieces end to end along the bench with masking tape under the joints, apply the glue and fold it all up. Another couple of strips of tape all the way round, stretched a bit to apply a bit of tension, and leave to set. Titebond again, and it should also be ok with ply, but if you used epoxy resin I'm certain it would produce a very strong construction.

[pgk pgkParticipant @pgkpgk17461]

My experience with ply is use a router when possible otherwise it's a pfaff precutting all the lines with a stanley knife and using a fine tooth saw to avoid the frayed bits.

[Paul LousickParticipant @paullousick59116]

Strong corner joints can be made by glueing the ply together and covering with fibreglass tape and resin. Then sand smooth and paint/varnish. A super strong joint can be made by stitching the ply together with copper wire before covering with the fibreglass tape. This is a method for making lightweight canoes and boats.

Edited By Paul Lousick on 27/01/2017 21:12:50

[Neil WyattModerator @neilwyatt]

Well the router seems to work best on this stuff.

What about a rabbet joint?

[John ReeseParticipant @johnreese12848]

Posted by Neil Wyatt on 27/01/2017 18:08:08:

Posted by Martin Kyte on 27/01/2017 11:20:22:

You need one of these.

**LINK**

Then just glue it all up.

That would do nicely if I had a router table.

A perfect justification to buy one more tool.

[Steve PaveyParticipant @stevepavey65865]

Rebate joint – waste of time. No greater strength than a butt joint, and obviously takes longer to cut. Slightly better would be a tongue and groove type joint, such as would be used on an internal door lining, but that is likely to fail if there is any racking (parallelagram distortion).

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Steve Pavey on 27/01/2017 21:17:54:

Rebate joint – waste of time. No greater strength than a butt joint

.

Sorry, Steve … I have to disagree … That's why I suggested stripping three plies off [several posts ago] ideally as a housing, but a rebate would be better than nothing; it removes one 'degree of freedom' at each joint.

MichaelG.

[Mark CParticipant @markc]

For reference, the Gorilla glue is the same sort of stuff as the D4 adhesives I think? They are a thick brown liquid with a consistency about the same as not very runny honey if that makes sense. As Gordon W mentioned, don't open it and leave it sitting about for a couple of months 'cos it will no longer be runny!

It sets really quickly – the Everbuild stuff I get takes about 3-5 min's and once the foaming has finished (gone yellow) it is more or less set. It is fully set within an hour or so and I tried pulling two rough bits of building timber (C16) apart as a test – it simply ripped the face of the wood after a half hour setting time. As it is viscous it allows you some scope to manoeuvre the joint on assembly but don't let it get on anything you don't want stuck… It has the same affinity for fingers as super glue but with a little less setting speed (once it's on it is going to have to wear off)

Mark

[Mark CParticipant @markc]

Also, the ply was probably made using a relative of gorilla… perhaps assembled by one as well?

Mark

[Neil WyattModerator @neilwyatt]

I've ordered some Titebond 3 as well – on 'next day guaranteed' – I might as well get something from a Amazon Prime free trial membership before it runs out.

[Neil WyattModerator @neilwyatt]

Posted by Steve Pavey on 27/01/2017 21:17:54:

Rebate joint – waste of time. No greater strength than a butt joint, and obviously takes longer to cut. Slightly better would be a tongue and groove type joint, such as would be used on an internal door lining, but that is likely to fail if there is any racking (parallelagram distortion).

Rabbet not rebate, more or less a tongue and groove.

Neil

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 27/01/2017 21:46:23:

Rabbet not rebate

.

Both Wikipedia and my 'O level' woodwork teacher would dispute that.

**LINK**

https://en.m.wikipedia.org/wiki/Rabbet

MichaelG.

[Ian PParticipant @ianp]

Reading this thread and seeing all the different joint suggestions got me thinking….

Which joint to use depends a lot on the purpose of the box you are constructing, things like whether the external and or internal finish is visible or important, or whether the constructions has to as strong as possible without regard for extra bracing (internal fillets, angle plates etc). Probably the strongest method with the best appearance would be by using some sort of biscuit or domino inserts but these would be very time consuming for a one-off especially if one did not have the right equipment.

I would imagine that box (finger) type joints are a bit compromised if the box is made from plywood as half the fingers have grain in the wrong direction, but in practice would still be very strong.

The plain mitre method that Steve Pavey described has a lot going for it especially if there is a back or bottom bonded on so the four sides cannot lozenge. I occurs to me that a mitred the joint face is larger in area than a plain butt joint which must be a good thing. An even larger surface area of joint could be achieved by having a series of rebates (like a staircase in appearance) and these could even be sized to match the thickness of the individual plies. An OTT method especially if the outer most ply layer had to be a mitre for appearance.

I don't know if it could be applied to a right angle joint but I see joint in solid wood worktops, chopping boards etc, where long pointed wedges are cut into the ends of timber and two of these ends are mated into each other with such force that the joint is not always glued. Even if the idea would work on right angle joints I doubt it would be possible in a home workshop.

Out of interest it would be nice to know what Neil actually ends up using and what the boxes are actually for.

Ian P

[Steve PaveyParticipant @stevepavey65865]

Rabbet is the American spelling of rebate. It refers to a step on the edge of a piece of timber ie one side and one base, whereas a groove is set further in and therefore has two sides and a base to it. The main point of both is to provide location, prior to fixing with glue and/or pins or other mechanical fasteners. It doesn't provide very much strength to the joint in itself.

Ideally, a decent joint provides location, a good glued area and some sort of interlock and/ or mechanical fastening method.

Dowels, dominos, biscuits and splines all provide extra glue area, along with resistance to twisting. Finger or comb joints provide extra glue area, and resistance to relative movement in one plane. Dovetails provide extra glue area and resistance to relative movement in two planes.

Glueing end grain to end grain, or end grain to face grain, is usually weak. With man-made boards such as mdf this is not such a problem, especially if the joint is mitred to give extra glue area. Plywood is better then solid wood on end to end joints as about half the face being glued is face grain. With a decent modern glue such as epoxy resin or aliphatic resin, joints which rely only on having a decent area of contact are often good enough for many applications.

Someone earlier on in the thread mentioned a milling machine (Michael G?) – with a bit of careful setting up, something like a slitting saw or side and face cutter in a vertical mill could be used to cut a finger joint in all the pieces sandwiched in the vice in one setup (assuming that a router table wasn't available).

[BazyleParticipant @bazyle]

How about 45 degree plain mitre but a thin deep groove down the middle of each face cut with a slitting saw into which a piece of aluminium or steel angle iron is set to provide the strength. Thus incorporating some metalworking into the job and no worrying about weak end grain.

[JasonBModerator @jasonb]

MarkC

"For reference, the Gorilla glue is the same sort of stuff as the D4 adhesives I think"

D4 refers to the durability of the adhesive not its type, you can have D4 PU adhesive like Gorilla and you can also have a D4 cross linking PVA

IanP

"I would imagine that box (finger) type joints are a bit compromised if the box is made from plywood as half the fingers have grain in the wrong direction"

Ply has its grain running in alternate layers so you would have some long grain and some short grain in the joint

SteveP

"Someone earlier on in the thread mentioned a milling machine"

That was me, but it sounds like Neil is making quite lareg boxes so the boards are unlikely to fit vertically to allow a standard slot drill to be used to cut finget joints.

Neil if you do have a router you can buy a cutter that allows it to cut biscuit joints which woul dbe another option, basically just a bearing guided grooving cutter, size ) biscuits would do

Definately "Rebate" unless your name is Norm As Michael says better than a but joint, I have made plenty of drawboxes that way. Lock joint is another simple option for ply that can be done with a router and straight bit or tablesaw (with suitable guarding)

Edited By JasonB on 28/01/2017 07:46:51

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