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  1. #1
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    Default Designing With MDF

    I just built a stack of shelves with MDF. Like a bookcase but with 400mm between shelves, meant for the kitchen.

    2m high. 1.2m long. 0.4m deep.

    Five shelves in all. First one 200mm off the floor and then up to the top with 400mm between each.

    16mm MDF.

    I wasn't game to trust the MDF to support the weight just by screws driven into the ends so I put cleats of MDF 40mm deep along the sides and rested the shelves on them and screwed them onto the side and screwed the shelves down onto the cleats.

    Making a much uglier job but much stronger, I thought.

    While assembling it I went to pick it up using the top shelf to lever it up from the floor and the screws just tore up and out of the MDF cleat! metric 8 gauge 30mm.

    Got me thinking about the whole thing.

    What's the professionals opinion of construction design when using MDF?

    Factors to take into consideration? Particular design rules to follow?

    Like I'm thinking when using 'cleats' like that I should use wood. Even softwood better than the MDF. But a 1" hardwood cleat made from dressed tomato stakes would be cheap and much stronger. Screw half a dozen 10 gauge 30mm into that and I could hand the whole thing from it I reckon.

    And the same thinking got me to suspect maybe you don't allow MDF to stand on the floor but instead put some solid wood feet on it? Mopping the floor etc is going to play hell with it isn't it? Just chips and knocks from a broom.

    Thought I'd come here and ask the knowledgeable for their comments, if anyone is interested enough to say anything....


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  3. #2
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    Mdf can be great for furniture but has considerable disadvantages too.
    It is very heavy, sags when used for unsupported shelves,splits when screwing into the edges unless you drill pilot holes, swells like cardboard if wetted.Also horrible dust when cutting.

    The trick is learning to deal with its shortcomings, shelves are best lipped with generous solid timber or at the very least fixed from the back so they aren't free floating.

    Mdf is my favourite carcase material for veneering work.

  4. #3
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    Yep. I've discovered all or most of those things already. These shelves are supported halfway so there's only 0.6m span.

    Should be 'lipped' you say. You mean across the front of the shelf have a strip of solid timber?

    Here's what they look like. https://plus.google.com/photos/11077...CImLzrOGj9Wbbw

  5. #4
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    Did you use glue when assembling the piece? I am surprised it just ripped the screws straight out.

    I wouldnt have used cleats. As you said, there ugly. I would have used 3 pairs of 8mm dowels to join the sides to the shelves. If I wanted to make it even better, I would have used a 3mm housing joint +dowels. Then I would screw the back on to the sides as well a few screws along each shelf to prevent sagging with glue (carefully place the back on, dont mess the glue). This just leaves the front edge of the shelves unsupported, and a 1200mm span will sag. Adding a faceframe kinda thing to the front of each shelf will help, something like a 50-65x19 hardwood. Attached using dado type join.

    Edit: dont need the faceframe lip for a 600mm span.

  6. #5
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    Couldnt get the attched link to work but yes a lipping of solid timber at the front/back or even both will sustantially stiffen any shelving.





    Quote Originally Posted by abrogard View Post
    Yep. I've discovered all or most of those things already. These shelves are supported halfway so there's only 0.6m span.

    Should be 'lipped' you say. You mean across the front of the shelf have a strip of solid timber?

    Here's what they look like. https://plus.google.com/photos/11077...CImLzrOGj9Wbbw

  7. #6
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    30mm screws just aren't long enough; you only end up with 15mm of bite after you've gone through the 16mm MDF and 4mm of that bite tapers to nothing at the tip, so you have, at best, about 12mm of useful thread.
    40mm screws are the bare minimum to be useful in that application, I'd be using 50mm myself.

  8. #7
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    Quote Originally Posted by Kuffy View Post
    Did you use glue when assembling the piece? I am surprised it just ripped the screws straight out.

    I wouldnt have used cleats. As you said, there ugly. I would have used 3 pairs of 8mm dowels to join the sides to the shelves. If I wanted to make it even better, I would have used a 3mm housing joint +dowels. Then I would screw the back on to the sides as well a few screws along each shelf to prevent sagging with glue (carefully place the back on, dont mess the glue). This just leaves the front edge of the shelves unsupported, and a 1200mm span will sag. Adding a faceframe kinda thing to the front of each shelf will help, something like a 50-65x19 hardwood. Attached using dado type join.

    Edit: dont need the faceframe lip for a 600mm span.
    No, no glue. Everything I build has to be built with disassembly in mind. Everything always screwed or bolted together.

    Yep, ripped them up and out. Three at each side. 8 gauge 30mm.

    You would use 8mm dowels? In 16mm board? That's only 4mm each side to take the weight. The would be alright, you think? I used 6mm dowels. And didn't trust them. Well, not didn't trust the dowels but didn't trust the board - 5mm each side - not to give way.

    First thing my 9 year old did when I put them up was crawl into one - that's the kind of surprise loading they can be subjected to.

    Yep, the back and the sides are screwed onto the shelves.

    I have never done a housing joint. I've got a router for just that purpose but never done one yet. I think you're right. The way to go instead of those ugly cleats. 3mm out of 16mm in this context isn't too much. Could even go 4mm?

  9. #8
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    Quote Originally Posted by elanjacobs View Post
    30mm screws just aren't long enough; you only end up with 15mm of bite after you've gone through the 16mm MDF and 4mm of that bite tapers to nothing at the tip, so you have, at best, about 12mm of useful thread.
    40mm screws are the bare minimum to be useful in that application, I'd be using 50mm myself.
    Thanks for that opinion. I'll take it to heart. I was being lazy I guess. I had the 30mm for joining two 16mm, of course, and I just went with them, thinking there'll be no upward force on the shelves.

    Then I went ahead and applied massive upward force......

  10. #9
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    Quote Originally Posted by abrogard View Post
    No, no glue. Everything I build has to be built with disassembly in mind. Everything always screwed or bolted together.

    Yep, ripped them up and out. Three at each side. 8 gauge 30mm.

    You would use 8mm dowels? In 16mm board? That's only 4mm each side to take the weight. The would be alright, you think? I used 6mm dowels. And didn't trust them. Well, not didn't trust the dowels but didn't trust the board - 5mm each side - not to give way.

    First thing my 9 year old did when I put them up was crawl into one - that's the kind of surprise loading they can be subjected to.

    Yep, the back and the sides are screwed onto the shelves.

    I have never done a housing joint. I've got a router for just that purpose but never done one yet. I think you're right. The way to go instead of those ugly cleats. 3mm out of 16mm in this context isn't too much. Could even go 4mm?
    All of my joinery that I suggested is heavily reliant on glue. You could use a 10mm housing joint if you wanted too, but it must be glued. I said a 3mm housing +dowels. The housing will bear the weight of the shelf if the dowels wont support it, and the dowels will hold the sides in hard against the shelf. Using a 10mm housing doesnt leave enough material for a dowel into the side panel because you really need atleast 12mm of dowel in the side panels, but then you wouldnt even need the dowels.

    Really for MDF construction alot of traditional joinery just doesnt work. Sliding dovetails will probably break off. So it only leaves butt joins with knockdown hardware, housing joins with glue. or a loose tenon type such as dowels or dominoes with glue.

  11. #10
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    Quote Originally Posted by Kuffy View Post
    All of my joinery that I suggested is heavily reliant on glue. You could use a 10mm housing joint if you wanted too, but it must be glued. I said a 3mm housing +dowels. The housing will bear the weight of the shelf if the dowels wont support it, and the dowels will hold the sides in hard against the shelf. Using a 10mm housing doesnt leave enough material for a dowel into the side panel because you really need atleast 12mm of dowel in the side panels, but then you wouldnt even need the dowels.

    Really for MDF construction alot of traditional joinery just doesnt work. Sliding dovetails will probably break off. So it only leaves butt joins with knockdown hardware, housing joins with glue or a floating tenon type such as dowels or dominoes with glue.

    So, that's the point. 'Traditional...just doesn't work..' so what to do?

    And 'heavily reliant on glue..' not a general question but particular to people like myself who must build with disassembly in mind.. so what to do then?

    I'm thinking one needs a mix of materials.

    I'm looking for cheap, mainly, I must confess. That's why I use Mdf. And the main expense I'm trying to avoid is wide planks. Wood gets very expensive, doesn't it, once you want something wider than about 250mm or something.

    I did try tongue and groove floor boards once. They were pretty good. Maybe I should resurrect that idea.

    I built some shelves with construction pine 70 x 35mm framing clad in 3mm Mdf and shelves of Mdf 400mm deep. They're lovely and solid and don't look too bad. I lap jointed the pine.

    Only thing I didn't like was the 3mm Mdf. It's like cardboard.

    So I was thinking for the next one I do like that to make it of Mdf shelves, pine frame and thin ply backing.

    Mdf makes excellent wide flat boards. That's the big thing about it, eh? But it's pretty crook for almost anything else.

    If I get a thicknesser I could dress the pine and turn out some reasonable stuff that'd last for twenty years I think. And that pine's only $2 something a metre.

    Don't forget this is all DIY stuff for use at home, that's all.

  12. #11
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    MDF is good stuff. its stable after it has been sealed, and is fairly consistant in thickness +/- 0.5mm usually. To assemble without glue, just copy Ikea and use the knockdown hardware. It can get expensive when comparing raw materials vs flat pack from ikea/bunnies.

    To do it on the cheap and avoiding ugly cleats. 5mm housing, screw through side into the shelf. I would use a 50mm 10g hinge screw, these are pretty much the same as any wood screw, but the hinge screws have a smooth underside of the countersink part...dunno how to describe it. This allows the screw to pull hard up to the MDF without chewing out more mdf as u drive the screw further. Pre drill the shelf and sides with a drill the same as the screw shank size ~3mm, and countersink the side so the screw when fully seated sits exactly flush with side panel. I find driving the screws by hand for the last few turns is best to avoid stripping the mdf. This will leave visible screws on the outside of the side panels, can use cover buttons to dress it up a bit if u like. If the shelf wont be supporting much weight, you can do away with the housing.

    Leftover floorboards r awesome, I have about 200lm in the garage between 800mm-1500mm 130x19 spotted gum standard n better (mostly standard....very standard). Picked em up off ebay for less than 1.50p/m

  13. #12
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    Another option is special hardware such as these Hettich cam and dowel fittings. They are very easy to use and work well and give a neat and tidy appearance. Also perfect for disassembly.

    You do have to be meticulous with your drilling and hole positions, but when I used them I did all the holes free hand with a cordless drill and they all worked fine.

    Peter
    The time we enjoy wasting is not wasted time.

  14. #13
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  15. #14
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    Thanks for all that. Tons of stuff there. I looked at the knockdown fittings thing, downloaded this:

    https://timbertech.wikispaces.com/fi...n+Fittings.pdf

    Didn't know about them before. Very interesting. But seems to me that I get most if not all of what they do just by using screws. Knockdown all my stuff by unscrewing it.

    Downloaded the data sheets. They were good, too.

    But confusing, a bit.

    Seems to me one of them is saying that on 'two parallel supports' Mdf a 600mm span of 16mm can take 6.9kPa.

    Which is like 704 kg on a square metre. Is that right?

    My shelves being 600mm x 400mm they're 0.24 square metres and therefore could take a weight of 168Kg ? Sounds a bit much to me. Nice if it's true.

    But then I don't understand their 'parallel supports' in the first place.

    They're supporting at the edges I suppose? Along the edges. But extending onto the board by how much? Or doesn't it matter - the tested portion being that which lies between the supports, between the edges of the supports?

    And how wide is the piece? I'd assume it'd be as wide as the span - they're testing a square test piece.

    Thanks for the clues about the hinge screws. I saw them in Bunnings and turned away from them for no real reason, just because I didn't know any better.

    I understand why you'd say use a 50mm screw but I'm not sure I could reliable drill a straight pilot hole 50mm down into a 16mm piece. Maybe I could. I should try.

    I did find something out about pilot holes while doing that job. I drilled pilots for 10 gauge screws and then ran out of them. So I stuck 8 gauge in just to finish what I was doing.

    When the thing was assembled it wobbled all around. Took the 8's out and put in the 10's and it all tightened up. Now those 8's went in and drew up tight, felt fine. But obviously there was enough slack in those holes to allow movement. Who'd have thought it...

    I don't understand the 'housing' bit. What's 'housing' ?

  16. #15
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    "housing" joint. its basically just a trench the same width as the thickness of the shelf, and the shelf seats into the trench. a 5mm housing joint will have the trench being 5mm deep.


    to drill long holes into the edges of boards can be a pain. if you angle it too far one way or the other, you end up drilling out of the face of the shelf. If im worried that i might do this, ill use a piece of timber square to the shelf and use that to eyeball the drillbit vs the timber to keep me drilling pretty close to square.

    your right that the knockdown fittings are pretty much just fancy/expensive screws. for knockdown furniture, one of its advantages is that you can assemble it and disassemble it many times without ruining the connections (if your careful). screws tend to strip out the timber/mdf very easily when overtightened, eventually you would need a new fresh screw hole to get a decent grab.

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