Thread: Joining question

Nick

My first thought was that you should bevel the edges (on a tablesaw) and, this way, you will end up with cleaner corners when they are joined. I would then add biscuits for additional strength, although I am sure that this is unnecessary since a good epoxy glue would be stronger than the MDF itself.

Regards from Perth

Derek

I think it depends on how you want to finish them and what sort of gear you've got.

Getting a mitre joint right would be very tough on anything but a half decent tabelsaw - but it is probably the best joint. If you want a coloured finish (black, white etc), consider using black or white melamine - getting a painted surface right on MDF is not fun. You can also get veneered MDF of course - or veneer it afterwards. Or cover it in carpet/frontline etc.

I'd go for biscuits and rebated joints personally, using plain MDF as thick as practical in the circumstance, then veneer the whole lot.

There really isn't any need for anything but sound butt joints. These will give you heaps of glueing surface and be more than adequately strong enough. The only reason to rebate or use biscuits, etc. would be to help locate the bits as you assemble - particularly if your design calls for internal baffles, etc. Use any old PVA glue, align carefully, and skew-nail to hold them til the glue sets. (Remember to set the nails). After assembly, any proud edges can be sanded flush. If you doubt the integrity of this method, make yourself a small box and try dismantling it after a week!
If done well, a good paint job will cover the joins, and can look quite snappy, but veneering or similar may be preferred. Sets I made back in the early 70's (from chipboard - MDF's arrival was still some ways off) and veneered, are still sound - to my own amazement (speakers have been replaced in most cases!). I used contact cement for the veneer, too, which makes it even more amazing they've held up so long.
Building with pre-veneered board is best left for people with the appropriate gear - making long, accurate mitres isn't easy for anyone - doubly so if you are trying to preserve clean edges on chippy veneer that is a fraction of a mm. thick! Veneering a nice, clean, square box is simple.
Good luck,

If you have a router and a table you might want to consider a lock mitre joint. Lock Mitre Bit


Grunt

Duderduderini,

MLCS sell those lock mitre bits for US$38.50 plus freight, as opposed to AUD109 from Carbatec.

Rocker

This thread is rather old, but I found it whilst searching through the archives.

I'm a beginner. Does anyone have experience with these Lock Mitre Bits?

Are they also suitable for joining different thickness stock, say 3/4" pieces to 1"?

tktran

The bits are good. You will need a specific size depending on the thickness of the material you are using.

Trav

Tktran,

As far as I am aware, lock-mitre bits only work for cutting along the grain, and for joining pieces of the same thickness. If you wish to join pieces of different thickness, you could use a drawer-lock bit.

As a beginner, you might consider using a rabbet joint instead of a lock-mitre or drawer-lock joint. Those bits take a bit of getting used to.

Rocker

Trav and Rocker,

Thank you very much for the prompt reply,

I'm also looking to build a speaker box out of 18mm or 25mm pre-veneered MDF, but don't have access to a table saw. Even then, I hear it's not easy to get perfect 45 degree cuts to make perfect right angle joints. I thought this lock-mitre router bit might do the trick, hoping that it would be relatively easy to achieve good results.

My panels will be over a metre long...

My first speaker box was built around this time last year, constructed from 18 and 25mm MDF using butt joints, later veneered and finished. The finish turned out fantastic, but there's some lift/bubbling on the veneer thus my decision to choose pre-veneered MDF. Also, and to my surprise, pre-veneered MDF is more economical than the sum of MDF and raw veneer... at least from my local supplier.

tktran,

Actually, the job you intend to do with it, using veneered MDF, is well suited to the lock-mitre bit. They are trickier with solid timber. I would suggest using the smaller bit and 18 mm MDF in a router table with a high fence. The larger lock-mitre bits are rather intimidating.

Rocker

Originally Posted by Rocker

The larger lock-mitre bits are rather intimidating.
Rocker

Sorry, am not sure what you mean?

I've routed a hole in a piece of 1" thick MDF, and found that it required 2 passes? (ie. 15mm first go, another 10mm the next)

Originally Posted by tktran

Sorry, am not sure what you mean?

I've routed a hole in a piece of 1" thick MDF, and found that it required 2 passes? (ie. 15mm first go, another 10mm the next)

tktran,

Now you have me confused too. Lock-mitre bits are not intended for routing holes, and certainly not for making two passes. They are for routing the edges of stock so as to make matching profiles that lock together. Hence the bit must be set up in the router-table so that the centre of symmetry of the profile is equidistant from the fence and from the router table surface. See the diagram in the CMT catalog.

Rocker

Rocker,

Sorry I threw you off with the second sentence. Oh boy. Let me re-write that last point.

You suggested using the smaller lock-mitre router bit, with 18mm MDF. So I meant to ask, why is the larger lock-mitre bit "intimidating?" In what way?

It is because of the sheer size or in cutting thick stock? Or another reason?

In the past I have used a large and long straight-cut router bit, using it to cut (a hole in) thicker 1" MDF. The stock was was either too thick, or my bit too blunt/poor quality(?), that I had great difficulty getting a clean cut. Eventually I had to do it in 2 passes...

I think I understand the idea of how the lock-mitre bits work and their intended application. It looks very promising for make mitre joints out of pre-veneered MDF. Now I wonder if it works for joining the top and bottom panels to the other 4 sides of the box...

Tktran,
Ordinary 2-flute router bits are not suitable for boring holes in wood or MDF. For that, you should use a suitable drill bit in an electric drill, or, if you want to use a router (say, to ensure that the hole is vertical, and you haven't got a drill press), use a spiral upcut bit. These are expensive in Australia, but reasonably cheap if ordered from Lee Valley in Canada or MLCS in the US.

The lock-mitre bit won't work for attaching the top of a box to the sides. I don't have experience of making boxes out of veneered MDF, so am not sure of the best way of attaching the top. Probably your best bet would be with a simple butt joint, adding mitred wooden edging strips to the top.

Rocker