Corner joints in Plywood

[Neil WyattModerator @neilwyatt]

I tested my rather crude finger joint to destruction -the glue wasn't fully set, but where it had stuck the problem was the face veneer peeling off.

I think the finger joint is the only type that will give me enough edge to edge contact, and I will have to squeeze in brackets with wooden runners to keep the case that slips in from contacting the brackets..

I will have to make some sort of jig for routing the fingers and use a facing board across where the router will emerge.

[Gordon TarlingParticipant @gordontarling37126]

FWIW, my experience of woodglues is this – Gorilla glue is great stuff, albeit with limited shelf life. However, it expands somewhat during curing, so is able to force joints apart if not firmly clamped. Secondly, I've used Titebond III and found it to produce a very brittle joint – could have been me, though I don't think so.

[Neil WyattModerator @neilwyatt]

Posted by Gordon Tarling 1 on 28/01/2017 10:26:58:

FWIW, my experience of woodglues is this – Gorilla glue is great stuff, albeit with limited shelf life. However, it expands somewhat during curing, so is able to force joints apart if not firmly clamped. Secondly, I've used Titebond III and found it to produce a very brittle joint – could have been me, though I don't think so.

That's interesting, most places on the web say the wood fails before the titebond.

An experiment witha simple jig and a facing piece of softwood and I can rout fingers OK.

[Russell EberhardtParticipant @russelleberhardt48058]

Some useful information on marine grade plywood and adhesives here: Robbins Timber

I have found them to be a very helpful company. They have cut hardwood to size for me and shipped it to France at half the local price.

Rather than fixing metal brackets to the joints I would laminate a couple of layers of fibreglass mat in the corners with epoxy to give a bond over a wider area. Just a thought.

Russell

[Neil WyattModerator @neilwyatt]

Looking back at some comments,. a bit more background.

I'm making a mobile display unit in perspex, this will sit on a table-height base in two parts, the bottom will have retractable wheels and the top, which the display sits in, also has to allow it to be inserted from beneath to allow it to all fit in a car.

All was simple originally as it was going to be three separate units, but when it was realised that the three would not fit in a car, it became complicated as the display part needs to be as large as possible while the overall length and width are restricted. It also needs to be light enough for easy handling with minimal protrusions and a good appearance. It also means the top part can will be boxed in at the top, giving strength, but the plan to box in the bottom as well has had to be abandoned. With a board top and bottom and no restriction on brackets, it would have been easy to make the top robust enough, even using butt joints.

The bottom part is not a problem as I have plenty of room fit braces and brackets to it.

What I've decided to do is use 50mm wide fingers, but also dowel through them with 6mm dowels to lock everything square AND use a low profile bracket at each of the bottom corners. belt, braces and a piece of string (thanks Compo).

[Mark CParticipant @markc]

Jason,

Out of interest (and I don't know how technical your knowledge of these glues is), are all PU "gorilla" type glues structural (D4)? I know that all PVA adhesives are not suitable for permanent construction joints. I think it may be the case that PU is always water proof as opposed to PVA which generally does not like to be wet for any length of time (it came as a bit of a revelation to discover that I should use acrylic sealer under tiles rather than PVA).

Mark

[JasonBModerator @jasonb]

As far as I know all the PU glues are durable to D4 spec. I tend to use Titebonds PU glue and occasionally their PU construction adhesive which is like a PU Gripfill but does not foam

Depending on the spec of the PVA it can be anything from D1 to D4, D3 is OK for occasional wetting or a humid enviroment eg a bathroom but the cross linked D4 PVA is the only exterior PVA.

One thing to remember with the PU adhesives is that moisture is that catalyst so on very dry wood or man made boards it can help to just dampen one part which helps kick it off. Also oily woods like teak and iroko benifit from a wipe down with acetone before bonding to remove any oil.

And while we are on about PU adhesives they may foam to fill an open joint but you should be aiming for a tight joint as the foam has little strength.

As you say PVA under tile is a no no, out with the ADP or similar which won't come back to life!

[Neil WyattModerator @neilwyatt]

>phew<

Titebond III has arrived!

Even more >phew< I've just done the second set of 50mm ringers for the upper box and, after easing in just one place they are a 'hold together' fit with no nasty gaps and only two cosmetic break outs which I can glue back in place..

Irony is I'm using my tiny 1/4" router rather than struggling with the 1/2" beast! I put it down while still running and it fell over and nearly got my shoe – oops!

Now work up the courage to fit all four pieces together around the 'shelf' and leave overnight to set, then I will drill for 30mm dowels, which should keep it solid and square.

Let's hope I can repeat the feat on the lower part.

Neil

[Peter TuckerParticipant @petertucker86088]

Posted by Neil Wyatt on 28/01/2017 16:32:21:

Irony is I'm using my tiny 1/4" router rather than struggling with the 1/2" beast! I put it down while still running and it fell over and nearly got my shoe – oops!

Neil

Fill out an incident report form and file it.

[BazyleParticipant @bazyle]

So it's a display table. If it ends up looking crap just throw a cloth over it

[Neil WyattModerator @neilwyatt]

Well…

If you get boards cut by B&Q in store, you can expect great repeatability but don't expect accuracy. Every cut I had made horizontally is short by about 7mm, and it means my horizontal shelf is too narrow. Fortunately I got offcuts the same length and a fair bit wider for cutting other bits out of (e.g. the tongues to locate top and bottom). one of these can be a new shelf. Losing 14mm in height is no problem as I had upped the height by 50mm as 750mm seemed a bit too low. Losing 14mm in height is no problem as 786mm is comfortably in the ballpark.

The finger joints on the first box seem to have dried nicely and securely, I've dowelled one side of four, but then my drill went flat. Once all four sides are dowelled I'll fit the shelf in, using dowels & glue only. This should make it resistant to collapsing. I then have to take out a few cutouts for handles, butterfly latches and a relatively large hole in one long side.

Neil

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 29/01/2017 10:03:25:

Well…

If you get boards cut by B&Q in store, you can expect great repeatability but don't expect accuracy. Every cut I had made horizontally is short by about 7mm …

.

Precision Machine with B&Q 'Setter'

Q.E.D.

MichaelG.

[EmgeeParticipant @emgee]

Was the saw blade about 7mm wide ? measure twice cut once still a sound idea.!!!!!!

Emgee

[John OlsenParticipant @johnolsen79199]

Well, I would go for the West System epoxy myself, but I guess having built a 30 foot steam launch with it has given me quite a lot of experience with it. I must have used well over a 100 litres of the stuff, for glueing, coating, and for the glass fibre sheathing over the strip plank hull.

I would have gone for finger joints myself, partly because I have a little jig for my Unimat three saw attachment that does very nice ones, and boxes I have made like that have been very strong and useful, with no need for brackets in the corners.

One thing about Gorrilla glue which I have also used…as has been mentioned if you get it on your fingers it is very hard to get off. Worse, it also stains them black. However there is a way to get it off. Get the rubber gloves that you forgot to put on when you did the glueing, and wear them for a while. The heat will make your hands sweat and after a while the glue will come off.

John

[Brian OldfordParticipant @brianoldford70365]

Posted by John Olsen on 29/01/2017 10:45:52:. . . . . . . . .

One thing about Gorrilla glue which I have also used…as has been mentioned if you get it on your fingers it is very hard to get off. Worse, it also stains them black. However there is a way to get it off. Get the rubber gloves that you forgot to put on when you did the glueing, and wear them for a while. The heat will make your hands sweat and after a while the glue will come off.

John

And whatever you do don't get PU adhesive on clothing either.

[Neil WyattModerator @neilwyatt]

Posted by Michael Gilligan on 29/01/2017 10:30:10:

Posted by Neil Wyatt on 29/01/2017 10:03:25:

Well…

If you get boards cut by B&Q in store, you can expect great repeatability but don't expect accuracy. Every cut I had made horizontally is short by about 7mm …

.

Precision Machine with B&Q 'Setter'

I think the chap (if you mean him by 'setter&#39 was doing it right, just somehow the scale was not well aligned with the blade. Still, only took 2 minutes with a hardpioint saw to get another bit to get a good fit. I'm going to dowel the shelf every 5 cm, hopefully enough if some idjut stand or sits on it.

[Neil WyattModerator @neilwyatt]

Posted by Emgee on 29/01/2017 10:39:30:

measure twice cut once still a sound idea.!!!!!!

He did; obviously the machine was set up wrong. It was consistent as I had some 450 wide and some 350 wide and they were both out by the same amount. I think the cut was a bit narrower than 7mm.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 29/01/2017 12:52:10:

Posted by Emgee on 29/01/2017 10:39:30:

measure twice cut once still a sound idea.!!!!!!

He did; obviously the machine was set up wrong. It was consistent as I had some 450 wide and some 350 wide and they were both out by the same amount. I think the cut was a bit narrower than 7mm.

.

[as per my earlier comment]

There's a possibility that the blade is 3.5 mm thick, and they just don't understand which bit is the piece you want; and have double-accounted.

MichaelG.

.

I presume it's one of these machines that they used:

https://qunud.files.wordpress.com/2010/06/bandq.jpg

Edited By Michael Gilligan on 29/01/2017 13:43:43

Edited By Michael Gilligan on 29/01/2017 13:52:17

[JasonBModerator @jasonb]

More likely operator error. You are supposed to move the board against the stop from which the scale is calibrated each time.

Lazy operator will put board against stop and cut Neils 250mm wide board then move saw to 600mm (250+350) and cut the 350mm board but infact that boaed will be 350-saw kerf

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 29/01/2017 13:53:56:

Lazy operator will put board against stop and cut Neils 250mm wide board then move saw to 600mm (250+350) and cut the 350mm board but infact that boaed will be 350-saw kerf

.

But Neil said they were all short [narrow] by the same ammount

MichaelG.

[JasonBModerator @jasonb]

Without knowing the rest of the cut list it is hard to say, could have cut a large piece and then the odd strips from the ends of the boards, Neil also said some in the other sizes were right, that was just an example of what can be done wrong.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 29/01/2017 10:03:25:

Well…

If you get boards cut by B&Q in store, you can expect great repeatability but don't expect accuracy. Every cut I had made horizontally is short by about 7mm, and it means my horizontal shelf is too narrow.

.

We evidently have a different understanding of Neil's statement, Jason

… I will can no more until he clarifies what exactly/precisely he meant.

MichaelG.

[Sam Longley 1Participant @samlongley1]

If they cut it the wrong size , why exactly does one accept it ?

Why not hand it back & say "  not what I asked for "– End of !!

Edited By Sam Longley 1 on 29/01/2017 15:26:22

[JasonBModerator @jasonb]

Just noticed Neil said it was the horizontal cuts in which case it is more than likely the operator left the board on teh rollers rather than dropping it onto the stops along the bottom.

There is a lever that lifts the roller slightly to make it easy to place the board on teh machine, you should then push the lever the other way which lowers the rollera and board so it sits on the bottom ledge which is where the scale is zeroed.

Guy in Michael's photo still has it up on teh rollers

 

Edited By JasonB on 29/01/2017 15:34:14

[Neil WyattModerator @neilwyatt]

The cuts were the horizontal ones, measured down from a ball race that runs along the top of the board, not from the bottom. So set to 45 cm and I got two boards 44.3 cm wide, then set to 35 cm and get two 34.3cm wide, then again 57.8cm and it came out about 57cm.

I don't think it was operator error as his vertical cuts were all spot on, which did involve moving up to the stop for each cut.

[Author]

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