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By siggy_7
#1322057
I've not seen torsion boxes built with the internal ribs in only one axis, but my intuition tells me you're right in saying that the lengthwise ribs are the most important for your application. For the dimensions you're looking at, I would suggest in addition to a rib along the front and the back of the shelf you put one down the middle of the shelf - that's what I would build anyway, but it may be unnecessary. If you're going to include front to back ribs (again, I would if only for belt and braces comfort), I would space them at something like 200mm intervals - over a 900mm span, one each end plus three in between at 225mm centres would be fine I should think.

Regarding how to install the shelves, my first choice would usually be the little metal shelf supports that fit into 5mm holes in the carcass sides. If you want fixed shelves, then dowels (or dominos if you have the machine) would be the way I would do it - bump up the rib thickness on the end of the shelves to 18mm so the fixings have plenty of purchase. Housing joints would also work well, but fit and finish may be a bit harder to get a really tidy look.

I've not used MDF like this in a structural application, but I imagine it will be fine. I've had bad experiences with solid MDF shelves sagging where I've under-specified it, but for a torsion box of this thickness I can't see you running into issues. Someone more familiar with MDF may be of more use here.
By Jeffo
#1348851
(Belated) Thanks siggy!

TLDR: Is attempting a seamless join between tulip and MDF a disaster waiting to happen?

One of the few upsides of this lockdown is that I've had time to return to this personal project, which to my regret hasn't progressed much since I last posted...

By way of an update, I decided to make the shelves themselves out of 25mm MDF, lipped front and back with tulip wood. The rear tulip is flush with the shelf (25mm x 25mm), but the front is 25mm x 50mm. This provides a dropped lip for my hide the LED's behind.

For the verticals, I've taken a bit of inspiration from Peter Millard's approach and am sandwiching some 15mm MDF noggins between two sheets of 18mm MDF (giving a total thickness to the verticals of 51mm). The verticals will be faced by an additional piece of 18mm MDF.

As per the picture in my original post, I've cut all of the wood so that when it is constructed, the front lip of the shelves will be flush with the face of the verticals. I've only done a partial dry fit so far, but as I'm building this in-situ, inevitably (at least with my skill level) there will be some gaps between the (tulip) shelf lips and the (MDF) verticals/filler panels. I'll be using a combination of pocket holes, dominos and regular screws (each with wood glue) to assemble.

I've been investigating options for filler that I can use. Someone has pointed out that the tulip and MDF will move differently and that cracks might be inevitable...

Seems obvious now it has been pointed out to me. Has anyone any experience in creating a seamless joint between MDF and tulip (or similar)? Will a decent amount of wood glue and careful assembly mitigate the differential in movement? Any other tricks?

Otherwise I'm tempted to trim 10mm off the rear of each shelf so that the front lips sit back 10mm from the verticals. I won't be able to do this for the bottom shelf (that forms the top of the cupboard, with inset door); so there is a chance for some cracks there.

I sort of wish I'd followed suggested practice in designing in lines where lines naturally exist from the outset, rather than going for a flush finish. I do prefer the flush finish look and was prepared for the effort of sanding, filling, sanding, filling - but this potential for cracking is making me think again.

Open to opinions - do I take the risk, keep it flush and hope that cracks don't appear? Or redesign?

Thanks again for the advice.