Bulky shelves

[jfc]

How do others go about making thick shelving . I normally have two peices of MDF for the top and bottom and strips of MDF in the middle to give what ever thickness i need . The problem with this is i have end grain mdf top and bottom and two joints that may crack the finish over time . So what methods do others use to get over this ?

[jfc]

I'm not sure if i like the look of a shadow gap , mind you it's better than a crack in the paint i suppose . What other methods are there ? I can only think of mitring the face as Moretoolsplease did on his book cases .

[Scrit]

50mm MDF? You must be strong!

Scrit

[jfc]

No chance , i've got to build a whole tv unit and two book shelves out of it . I've worked with 50mm MDF before , never again !

[pj]

You could plant on a thin 3 or 6mm piece of mdf on the front and sides and then use a round-over to disguise the edge. Works well on solid wood but probably will not stop the cracks in painted mdf, but at least they will be at a natural edge and not in the middle of a section.

Philip

[engineerone]

jason why not make them as torsion box shelves instead??

they would then be lighter, but stronger, and can be any thickness that you want.

if you use say thin ply for the top and bottom surfaces. then mitre the edges and use an upright of mitred ply too.

the outer edges would be glued to the internal spars.

so if you had a shelf that was 500 deep by 700 long by 100 high,
you could make it the following way.

3mm top and bottom, meaning that the spars would be 94 tall, by about 12mm wide, and spaced 100 centres except outside one which would be at 685. the outer upright would be 100 by 3mm with mitred top bottom and ends.

torsion boxes do not move in anyway like the same way as mdf, and would be less likely to split the paint.

you could then spray them.

paul

[pj]

Paul

Don't forget about the mounting. Better drilling a couple of the internals for available bar and gluing in before nailing the top down.

Philip

[jfc]

Paul , the torsion box is what i was saying before but i dont think 3mm top and bottom is big enough . 15 mm would be the smallest i would go , not sure why really , just instinct .

[engineerone]

chek out the woodworkers journal american web site, recently there was an article by ian kirby about torsion boxes, and he used much thinner than 15mm.

i guess it depends on what you put on it, but remember that
the uprights really take the load, so the nearer they are together, the thinner the top surface can be.

paul

[mrgrimsdale]

Yes "torsion box" if that's the right word, or does it mean something more specific? MDF crap for shelves anyway.
Solid wood spars/boards for front and back, edge rebated to hold ply both sides glued and pinned. Door construction really, with as much inner core of extra spars as the load requires.

cheers
Jacob

[dom]

senior said:

could just buy 50mm MDF, got some today, bye heck that finishes off the hernias, I now have two lumps in my throat

So we won't talk about the 75mm stuff then

I'd use 6mm it's easier to mitre.

[Scrit]

jfc said:

Paul , the torsion box is what i was saying before but i dont think 3mm top and bottom is big enough . 15 mm would be the smallest i would go , not sure why really , just instinct .

Your instinct is wrong. A torsion box derives it's strength from being a stressed skin - rather like an aircraft wing. And they donn't make them from 15mm thick ally, do they?

Scrit

[mrgrimsdale]

Yes - light and strong like a ply door where even a light cardboard beehive in-fill makes a tough object.
So is "torsion box" a proper engineering term or just something thats come up in woodwork mag?

cheers
Jacob

[jfc]

But i'm not making an airplane i'm making a book shelf ;D

Thinner MDF , i can see all kinds of construstion problems using that as it tends to have a mind of it's own . I'd spend most of the time making sure it stayed flat . Solid timber would invite the cracking as it moves .
I'm also making the supprts of the unit thicker so the whole construction will be the same as the shelves .

[mrgrimsdale]

jfc said:

snip
Thinner MDF , i can see all kinds of construstion problems using that as it tends to have a mind of it's own . I'd spend most of the time making sure it stayed flat
snip

That's why you'd use thin ply. Just like a cheapo door.
Just been looking at "Torsion box" as a term - it seems to be a newish one in woodwork but is redundant as it doesn't say anything more than just "box" i.e. any box is a "torsion box".
So what we are talking about is just "box" construction. Although you could strengthen a box with diagonals to make it more resistant to twisting i.e. "torsion".

cheers
Jacob

[jfc]

Ply would also cause its own problems with finishing etc . It must do or people would be using ply instead of MDF wouldnt they ?

I was thinking of running all the face edges past the spindle moulder to make a 45* and just butting any parts that are not seen .

Im making lots of this sort of thing .

[engineerone]

jason, i think the ply vs mdf is a different thing
it is to do with costs.

whilst i accept that ply quality has reduced to some extent, if you get baltic it will still generally be better.

it all depends i think on the amount of internal ribs.

paul

[mrgrimsdale]

jfc said:

Ply would also cause its own problems with finishing etc . It must do or people would be using ply instead of MDF wouldnt they ?
snip

They do! Where necessary.

[engineerone]

the other point is obviously mdf is easier to use for those of us who are less skilled

but weight for weight it is less strong than ply, and also requires more support for the stength.


paul

[jfc]

Maybe , but a mitred plywood edge is a nightmare . Weight is not really an issue here as anthing i make is going to be lighter than 50mm MDF and there is no take off issues to deal with ;D

[pj]

Looking at the example shelves you could butt the sides, back and any internal ribs and use a mitre-lock bit for the front edge as it's the only edge where a split would be noticeable. Mitre-lock bits are quick and east once set up. I find 45's with no biscuits/tongue awkward to clamp.

Philip

[engineerone]

jason, i agree that mitring ply is a pita, however i was trying to suggest a method which involves the smallest joint line.

the other ting would be to peel back a couple of layers at each joint so that the upright is the thickness of a single ply, and thus the joint line will only be that wide.

paul

[mrgrimsdale]

Aha I can see the photo now. It wouldn't download earlier.
The perfect design for the "door" type box construction, couldn't be better. The dark edges all being dark colour solid wood (oak, mahogany, iroko etc) squarish section, rebated to leave a narrow lip say 6mm. Or planted on facings to softwood frames - more work, less timber, small saving if any.The panels all say 5mm ply with light colour timber face such as birch (the most common), the edges and struts out of sight all softwood. The carcase as one piece, the shelves separate with secret fixings.

cheers
Jacob

[JasonB]

I usually make the shelf from at least 12mm MDF top & botton with ribs of MDF about 50mm wide at regular intervals and one along the front. If its a painted finish then I stick a strip of 2mm MDF over the front, by the time you have knocked off the arrises the joint does not show. Did these curved ones that way

Jason

[dom]

You can't use the mitre lock on thin mdf.

Look Jason, use 6mm MDF.
You said yourself that mitred ply is crap.
3/4 mm MDF is too thin and the edge won't give a very wide glue joint.
In fact if you have a lot to make you should just lip them in solid wood, because if you're mitering dozens of shelves you're going to need a lot of clamps and tape and hands.
Just used loadsa glue make the lipping oversize and use an edge trimmer to get a perfect edge.
Now get on with it and stop whingeing

The End