Using edge mitre joint in plywood carcass

[matthew]

Phew, after what seems like forever, finally managed to get back in the workshop.

I'm taking a break from solid timber and making some storage units from 15mm birch ply, something like this:

Pretty simple stuff. They need to be quite strong - size is 800mm x avg 300mm (a few different sizes) and will be stacked 5 or 6 high (up to about 2m) and filled with books.

I'm not sure how to join the corners. The picture probably doesn't show this but I want the bevel the front edges inwards - this would be relatively straightforward if the corners were mitred, but I'm not sure if mitre & biscuits would be strong enough for the load?

Alternative is to butt them with biscuits - easier and stronger, and the sides won't really be showing anyway. But the front would look much nicer mitred and I can see the bevel being a bit awkward to achieve this way.

Any ideas much appreciated...

MTIA, Matthew

 

[engineer one]

don't use ply for this kind of thing myself, but were i doing it with mdf, which i will be later, biscuits would work for me.

normal practice is to set the angle so that the biscuits are nearer the inside than the outside to ensure more strength and less chance of break out.

are you putting a back on, cause this will ensure that the item will not move, thus possibly compromising the joint.

others might suggest a full width spline made also of ply, that is often mentioned in the older books.

paul :wink:

 

[spadge]

If you are going to paint the boxes I would use MDF with biscuits and mitred joints as Paul says because a mitered plywood front edge on the box will be difficult to finish.

If you are using veneered plywood I would still use mitres and biscuits at the corners but I would lip the front with some solid timber.

Good Luck

Grahame

 

[Nick W]

Another option with ply is to use a mitre lock cutter. They're a bit fiddly to get set up just so, but once you're there running off a batch of boxes is very fast.

 

[matthew]

Hi, as ever, thanks for all the helpful replies!

are you putting a back on, cause this will ensure that the item will not move, thus possibly compromising the joint.

Aha - I won't be putting a full back on, but probably a shorter one maybe 1/3 the height of the box to stop the books falling out! Hopefully this should help strengthen it.

If you are going to paint the boxes I would use MDF with biscuits and mitred joints as Paul says because a mitered plywood front edge on the box will be difficult to finish.

Actually, I want to keep the plywood edge showing! I'm using nice fine ply and that's the look that's required. Which is why I'm keen to mitre them - if butted the front won't look so good (not worried about the sides, they won't be visible).

But just worried a bit about the strength of the mitre joints when these are stacked maybe 6 high and filled with books. With some biscuits it should be strong enough?

Another option with ply is to use a mitre lock cutter. They're a bit fiddly to get set up just so, but once you're there running off a batch of boxes is very fast.

Oh that's a good idea! I'm not sure I can do it this time round cos I only have a 1/4"/8mm router at the moment But I'll certainly try and use one of these sometime...

 

[engineer one]

frankly as long as you fix the back to the centre upright, and join that properly with biscuits. then that will stop the whole thing racking, and
moving the end joints.

if you can arrange for all the boxes to line up with the centre support
when you make them 6 high,that should also help.

recently in one of the american mags they made a simple barristers bookcase, and to align them one above the other, they made the tops in two parts and the bottoms had a spare piece underneath, and this then allowed the two units to strengthen each other.

thought it was a good idea, so i adopted it for the boxes underneath my new workbench.

paul :wink: