Joining 18mm ply to 22mm Mahogany

[sawdust maker]

Hi All
I am making a cabinet. I plan to make the base out of 18mm birch ply and the sides are 22mm South african Mahogany. Can I use a dovetail joint the two together? This cabinet will sit on a base unit.

Paul

 

[mr]

Hi Paul

I don't mean to be facetious but why are you considering a dovetail joint? It sounds as though it's a hidden internal joint so no need for the showiness and if its for strength it might be worth considering whether there's something else that might do the job equally well while being faster to execute.

Cheers Mike

 

[sawdust maker]

Hi Mike

My problem is that the side panel, made from Mahogany, is a face panel and I do not want to fix it to the base board in any way that will show from the outside.

Paul

 

[mr]

Stopped housing?
Biscuits?
Laid in on top of stretchers?
Will the ply base be load bearing ? I imagine that might make a difference.
I seem to recall Scrit posting recently about the merits or not of Dovetails in ply thoguh I may be completely wrong of course.
Cheers Mike

 

[sawdust maker]

Hi Mike

Biscuits would not work. The grain of the Mahogany runs vertically and biscuits would just push out of the end grain. Not sure what you mean by "Laid in on top of stretchers"

Let me try and describe the cabinet a bit more. It is the top part of a breakfront bookcase. Standing on a base unit. It is about 3' tall and 6' long. Four vertical boards of Mahogany make the sides of the three cupboards. The top and bottom is made from veneered 22mm ply. Only the outer side boards are causing me a problem. The two centre boards will be fastened into stopped housings. Hope this makes sense.

Paul

 

[Nick Gibbs]

If the outer boards are solid mahogany, and you are intending to fix them to veneered ply top and bottom you need to be quite careful because the mahogany will probably move with time and the ply won't. If the top and bottom are 'inside' the sides I'd be tempted to use a sliding dovetail. Effectively this is a housing joint, but with a dovetailed section on the 'tongue' that corresponds with a dovetailed groove in the mahogany sides. The dovetail can be worked on either or both faces of the tongue and the housing.

These are often tapered so that the joint tightens as you close it up because they can be pretty hard to assemble otherwise. With this joint you shouldn't need to use glue, so the ply and mahogany can move at different rates, but the mahogany sides won't be able to cup.

I hope that makes sense, and that I've interpreted your design correctly.

Nick

 

[sawdust maker]

Yes Nick, You make a lot of sense. I had overlooked wood movement. Puts dovetails out of the picture. Your idea of a dovetail groove would work on the two middle boards, but the end boards meet at their ends. I am now wondering if I should consider a mechanical method of joining the two boards. The knock down joints used on kitchen furniture but used in such a way that some lateral movement of the mahogany can occur.
Paul

 

[Nick Gibbs]

I'm not sure I understand the construction completely, but I too was considering a fixing too, but wasn't sure if you'd be able to hide it.

 

[sawdust maker]

Think of a MFI kitchen wall cupboard the joint between the side and the bottom is what I want to do. But in my case the side is solid Mahogany and the bottom veneered ply. As this unit sits on the bottom unit I could use a fastening that is exposed to the underside of the ply.

Paul

 

[jasonB]

What about pocket hole screws from below the ply panel, leave the front one as a standard hole and slightly elongate the ones in the middle & back to allow for movement. You could also add a couple of dry biscuits just for alighnment. If the sides and end of the ply sit on the base there will be no risk of the biscuits pushing out the end grain.

Jason

 

[sawdust maker]

Thanks Jason, but the problem is the side does not sit on the ply but butts up to the end. I am beginning to think I have built in a design problem that is not easily fixed. I think the easiest option is to make the base board out of solid mahogany, so they move together.

 

[jasonB]

The method I said was for the side to come all the vay down with the base butting against its side, I often use this method on cabinets made from flat panels, just leave the biscuits loose to allow movement.

What you need to ensure is that both the end of the base and bottom of the side panel are supported, like you said the leg on a kitchen carcase supports both.

 

[sawdust maker]

I know my problem now I keep forgetting KISS. If there is a complicated of doing a thing it the first way that springs to mind. Jason's idea of using pocket screws would work, but i don't have a jig. The method I am now considering is this, and its so simple.

I will glue the font 2" or so and dry screw the rest. I will use furniture screws as they are thinner normal screws and their wide threads should hold well in the end grain of the Mahogany.
Thanks for your help. I am sure I will be soon back with another problem :wink: