Joining/Fixing MDF

12 Apr 2010
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United Kingdom

Got a few DIY projects coming up; AV Unit, shoe rack and narrow bookshelf (for cookery books). Material is 18mm MDF.

Last time (AV unit), I did mitred corners with glue and screws for the outer carcase, and normal butt joints with glue and screws (edge to face, no joinery at all) for the internal shelves. Seemed to hold together really well.

This time, fancy trying something different. The shoe rack and bookshelf I'm not really worried about as the bookshelf shelves will probably only be 300mm wide.

For the AV unit, the outer dimension is 900mm wide by 650mm high. The plan is to have 1 long shelf near the top, with a vertical separator below it, about 364mm in from the left edge (so supporting this long shelf). Then they'll be 2 shelves from this vertical piece to the right, meaning about 500mm long shelves.

As it will slot into an unused fireplace, I wasn't planning on having a back - I can't anyway as power, satellite, speaker and network points are all chased in here. Yes at the back of the chimney breast! Racking shouldn't be an issue as it won't be able to move left/right, up/down etc.

The unit will mostly be fixed, with exception of the 2 short shelves on the right, which I planned on using kitchen cupboard peg things with positioned vertical holes.

My idea was to use dowels (or biscuits) and glue to assemble. How would that work with MDF?

PS: If I used dowels, rather than drilling a bit into the material to create a pocket, I would go all the way through. You won't see the dowels from the outside. Good or bad idea?
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Easy just to pin and glue [brad gun] or screw and glue , keeping 80mm from outer edge to prevent splitting [also pilot hole].
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Do you have to use MDF? Bloomin' awful stuff.

I see a lot of this, but if it was really that bad, would it still be made?

I actually had a good look at both MDF and Ply and a lot of the ply sheets were quite badly twisted.

PS: Yes. Already got it rough cut at B&Q.
Certainly MDF has its uses, but it sure has its limitations too.
Its an end of line is made from compressed dust and adhesive squashed together, and therefore it has very little edge strength.
It can be difficult to penetrate with nails, especially the pin variety and holds screws poorly, especially on the edge. It doesn't like to expand and therefore screws, if a fraction too big cause splitting - as do hammering in dowels. As there is no grain, glue has little effect as it doesn't absorb readily.
People should be aware that MDF dust is toxic, but so are presumably other man made boards.
For some flat pack furniture then sure, MDF can work, but more often than not its actually veneered chipboard thats used here.
I honestly think that creating projects with this stuff is a mistake - especially if the makers skills are limited. Far better (and pricier) to use birch faced ply or similar and add a timber lipping for an edge.
In my early days there was lots of blockboard and battenboard used - sadly that seems to be rare now.
John :)
If I ever made anything like what the OP is making I would use veneered plywood. I used it on a project on a narrow boat and the finish is great.
I would use veneered plywood.

The trouble is, I was umming and ahing for an hour between MDF, softwood ply, hardwood ply, birch ply, spruce ply. Basically all sorts of ply.

I went down to a few sheds and timber yards and pulled what they had out - all but the MDF was twisted or bowed.

Unless you have access to shop tools like planers, jointers etc. something that is perfectly flat (MDF) is the only option.

PS: Is it just me or is the weight thing with MDF a myth. I couldn't tell the difference between a full sheet of 18x2440x1220 MDF vs. various Ply sheets.
If you find sheets of ply bowed, they have been stored incorrectly and allowed to become damp. Best go somewhere else where the stuff has a fast turnover and is stored flat. The DIY sheds also have 1/2 sheet sizes plus a cutting service which helps with the transportation!
John :)

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