Structural use of back of cabinet

[nosuchhounds]

Im looking to make some drawer units for my shop and wondered if the back of a unit adds any structural integrity, especially if rebated or encased?

The units will.be made of 18mm ply and was looking at an overall height of around 700mm, width of around 500mm

 

[Spectric]

The back panel if done correctly will prevent the cabinet from lozenging, it adds a form of triangulation.

 

[recipio]

You don't want the carcass to rack which will jam the drawers so yes, a 6 mm back will prevent that. You could also use 12 mm ply unless you are trying to use up the 18 mm stock. Finding the optimum solution is the fun bit in furniture design.

 

[Jacob]

Im looking to make some drawer units for my shop and wondered if the back of a unit adds any structural integrity, especially if rebated or encased?
.....

Yes it will, but not in a good way.
Basically the back needs to be loose. If structural, any movement will be greater at the front instead, and drawers and doors could stick. Nearly all trad furniture has very loosely attached backs, thin ply, panels, T&G boards etc just nailed and not structural.

 

[nosuchhounds]

Yes it will, but not in a good way.
Basically the back needs to be loose. If structural, any movement will be greater at the front instead, and drawers and doors could stick. Nearly all trad furniture has very loosely attached backs, thin ply, panels, T&G boards etc just nailed and not structural.

So captured back with a bit of play in the groove? Deeper groove than the size of the panel?

 

[eribaMotters]

I would not worry about having the back loose as you are dealing with manufactured board and not solid.
At present I'm making a small bookcase unit out of 18mm MR MDF. It will be painted and the 9 mm back will be glued and pinned on before hand.

Colin

 

[Jacob]

So captured back with a bit of play in the groove? Deeper groove than the size of the panel?

Sounds good!

 

[rogxwhit]

Nearly all trad furniture has very loosely attached backs, thin ply, panels, T&G boards etc just nailed and not structural.

He's not doing trad, Jacob. Poke your head out of the trench for a minute. He's doing plywood. A trad carcase would quite likely have a construction (dovetailed corners, tenoned rails) that provided anti-racking at the front and back. Here, there's presumably little of that, so anything helps! So yes, fit the back panel tightly, and fix it in. Wood movement isn't a significant issue here. Structural movement is.

 

[Jacob]

He's not doing trad, Jacob. Poke your head out of the trench for a minute. He's doing plywood. A trad carcase would quite likely have a construction (dovetailed corners, tenoned rails) that provided anti-racking at the front and back. Here, there's presumably little of that, so anything helps! So yes, fit the back panel tightly, and fix it in. Wood movement isn't a significant issue here. Structural movement is.

Still applies. Any loads causing movement could show at the unbraced front more so than at the heavily braced back.
It's like a box with one side missing (the cupboard or chest front). The open side will be bendy (e.g. corner to corner) but the opposite side (the back) much less so.

 

[rogxwhit]

You're only right as far as you're right. That's no excuse for casting the whole caboodle to the winds. Give the job half a chance - it's got little chance at all otherwise. You want the whole thing to slop about? Come on, man, pull yourself together!

 

[Jacob]

You're only right as far as you're right. That's no excuse for casting the whole caboodle to the winds. Give the job half a chance - it's got little chance at all otherwise. You want the whole thing thing to slop about? Come on, man, pull yourself together!

OK he might get away with it but it just happens that I've recently seen a cabinet very carefully made with expensive timber and a solid back etc but the doors no longer fit well as they did, for the reasons I've given!
Good idea to look at a few cabinets and see how they are made and how they are getting on.

 

[Jacob]

OK he might get away with it but it just happens that I've recently seen a cabinet very carefully made with expensive timber and a solid back etc but the doors no longer fit well as they did, for the reasons I've given!
Good idea to look at a few cabinets and see how they are made and how they are getting on.

In fact it's simple logic:
In any structure prone to movement from whatever cause, if you stiffen part of it then there will be more movement in another part. If you strengthen the back then the movement will be at the front........

 

[rogxwhit]

In any structure prone to movement from whatever cause, if you stiffen part of it then there will be more movement in another part.

That's bunkum, Jacob. Your simple logic has yawning gaps in it ... ;-)

 

[Jacob]

That's bunkum, Jacob. Your simple logic has yawning gaps in it ... ;-)

Why?

 

[peter-harrison]

i

Why?

If I understand your argument right, it's like saying that if one of your legs is broken, you should break the other one to make it easier to walk!

 

[Jacob]

i

If I understand your argument right, it's like saying that if one of your legs is broken, you should break the other one to make it easier to walk!

er, not really, you haven't understand my argument right.
It's not an argument anyway it's a simple observation; it's just one of those things which most furniture makers all seem to know and follow. You only have to look behind a few chests of drawers to see.
Have a look at "Techniques of Furniture Making" Joyce. I haven't checked but I guess, if anything, he will say something similar.

 

[Hornbeam]

If the back is a plywood or mdf based panel and the grain of the solid timber is running along the edges of the panel ( which would be normal for a set of shelves or a cupboard)then it is better to rebate or groove and glue all round. And yes it will significantly add to the strength of the cabinet. If it is a solid timber panel or a slatted/T&G back yoh need to allow for expansion/contraction and should fit without glue and slightly loose

 

[Jacob]

If the back is a plywood or mdf based panel and the grain of the solid timber is running along the edges of the panel ( which would be normal for a set of shelves or a cupboard)then it is better to rebate or groove and glue all round. And yes it will significantly add to the strength of the cabinet.

Will add "strength" yes, but to the back, and shifts any movement to the front. Wouldn't matter for open shelves but with doors or drawers could cause them to stick.
Imagine an open ended tea chest on its side - if you stood on top of it the open end (where doors or drawers might go) would bend and distort, the closed end would stay square.

 

[Hornbeam]

Your tea chest analogy is slightly flawed but stay with it. If the base wasnt there or didnt offer any rigidity then the whole thing would collapse. by folding along the corners. The second point is that the chest is quite deep. The hallower the chest, the more the rigidity from the bottom is transferred through to the open end. If the chest was only 50mm deep it would be totally rigid. If it was 5metres deep then you would be correct that the base would offer minimal rigidity to the open end

 

[peter-harrison]

er, not really, you haven't understand my argument right.
It's not an argument anyway it's a simple observation; it's just one of those things which most furniture makers all seem to know and follow. You only have to look behind a few chests of drawers to see.
Have a look at "Techniques of Furniture Making" Joyce. I haven't checked but I guess, if anything, he will say something similar.

I don't think it is a simple observation. I've made lots of furniture out of both solid and manufactured boards. For the solid, I allowed for expansion/ contraction; the ply etc I didn't. I never had a problem with either, ecxept a couple of times when I ignored some basic rules like not sticking a long bit of end grain to a piece of long grain. I have had a look in Joyce. He says, "In all carcass work it should be remembered that while a well-fitted carcass back, grooved, screwed or glued in position will help to achieve overall stiffness and squareness, it will not automatically ensure such rigidity"