Building cabinet - need a support rail?

[Richfilth]

I'm in the process of making my first sizeable piece; a floor-standing cabinet with a bookshelf on top, and I've got a question about strengths.

The whole thing is made of pine boards, and the cabinet itself is 400x19mm pine, arranged in a cuboid (1000mm x 600mm). There's a support pillar at the back (70x20mm stock), the back (4mm ply) will be tacked to the back of all four boards (in a rabbet), and all boards have dowels in each corner. I can PM a pic to anyone interested, since I'm not allowed to post links yet.

Is this strong enough? Considering the span of the top board, I'm wondering whether I should add a horizontal beam/rail/joist(?) underneath the top board to prevent it sagging over time. I didn't really want to add another vertical post in the middle at the front, as it would interfere with the internal space.

I'm not an experienced cabinet maker, so any input would be appreciated

 

[Eric The Viking]

What are you going to use it for? Books and vinyl records (and cast iron cylinder blocks) are heavy, your collection of antique origami, possibly not so much.

Seriously, 600mm isn't so bad for most things. You could fix a batten at the back underneath each of the shelves (along the shelf). I've done that with some success with kitchen units, and they typically only have 12mm melamine faced chipboard (MFC) for the shelves, or even thinner, and wider spans. 1/2"w x 3/4"h gives a surprising amount of extra rigidity, and is usually hardly visible.

It also doesn't have to be rectangular section to work. You might use a fairly steep chamfer on the bottom front corner of the batten, to take the sharp shadow off it. This would greatly soften the appearance, but it would still be mechanically effective as a stiffening element.

I think for 19mm on a 600mm span, you're probably worrying too much, but I'm happy to be corrected. I'd be more concerned about only dowels for the shelf supports at the sides. Putting the shelf ends in a dado (shallow slot) would greatly increase the strength and could either be a design feature, or made to look invisible if you stop the dado before the front edge.

E.

Hint, if you spin out your reply over several messages, you'll get to the minimum to allow you to post images!

 

[Richfilth]

Actually the span is 1000mm, height is 600mm, but there's that support at the back to prevent major sagging. The top shelf sits on top of the sides, so the dowels are only for locating. The bookshelves above (200mm wide, 1750mm high) will then sit on top of the cabinet, and each shelf will be dadoed... but that's an issue for another day. For now I'm more worried that the cabinet top might cup over time, which will prevent the doors from fitting in smoothly underneath. But your comment on kitchen cabinets makes me think that maybe I am worrying a bit too much!

Cheers for the feedback.

 

[John15]

Try this, it may help. http://www.woodbin.com/calcs/sagulator/

John

 

[Richfilth]

Thanks John, that link was fantastic. Even with 22kg (40lbs) of stuff on that shelf, the sag would only be 0.6mm which is easily within my tolerances.