Example of the strength of glued mitre joints

[Stone Message]

Hi Everyone

Ages ago a really good friend of mine encouraged me to use the folding mitre technique to make really strong boxes and trays with MDF as the core material. Provided you use good quality PVA glue there is generally no need to use any dominoes, biscuits, screws or nails. I have made large drawers, many boxes and all sorts of trays using this method and not once has a joint failed.

It all depends on a well set up mitre saw (my Kapex is spot on) and some accurate 90 degree cuts on a tracksaw cutting station. The technique is perfect using MDF but is not really recommended for solid or plywood. In the latter cases I would enhance the joints with splines.

Here is the video:


Peter

 

[derekcohen]

Hi Peter

I always enjoy your videos. keep them coming

It is relevant to point out that a mitre joint using MDF is no different from a butt joint. MDF does not have a grain, and will present the same face whether upside down or inside out

What is more important is what to do when you join mitred joints of solid wood? And without using corner reinforcements. My recommendation is to first "size" the joint. This involves rubbing glue (e.g. Titebond ll) over the face of the mitre, allowing it to sink in and get close to touch dry. Before it is dry, add more glue to the faces and clamp the lot together.

This should create a strong joint. The main reason why mitre joints are not strong enough is that the "straws" soak up the glue and the joint is glue-deprived. Block the straws by sizing, and the problem goes away.

Putting my money where my mouth is, I recently glued up a new plinth for a turntable. The wood is Jarrah ...

The corners are braced internally, but this has more to do with creating a rigid structure than a glue joint ...

When it comes to any corners not closing up perfectly, the easy way to fix these is by burnishing with glue and a smooth, round shaft.

Regards from Perth

Derek

 

[Stone Message]

Hi [member=4358]derekcohen[/member]

Many thanks for this. Yes, with solid wood I use methods similar to yours or add splines using my spline sled:


Cheers.

Peter

 

[Packard]

Actually, 3/4" thick plywood works amazingly well using the fold up technique.  I've been using it for years.  With 3/4" thick plywood, 3/8" is actually face grain to face grain glue up area.  The balance is end grain to end grain.  So once assembled it has an equivalent to about 1/2" to 1/2" face grain glue up area.  Which is going to be pretty strong. 

When I am concerned about the strength of the joint I rip triangular sticks from a 3/4" thick board yielding a 3/4" x 3/4" x 3/4" equilateral triangular stick, all face grain.  I then glue those into the entire length of the corners.  The exterior of the plywood box looks really finished and the triangular inserts to not bother me.  I generally use a micro-pinner (23 gage) to hold the pieces in place.  But I have also used thin strips of wood long enough to become slightly bowed corner to corner to hold the pieces in place while the glue dries.