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I have been working on a design for some cupboard doors. The requirements are: - Simple to build so that an low-skilled wood-worker (myself) can put it together. - Be strong enough that the door does not "sag" over time.

My frame is basically a rectangle of 80mm x 20mm oak. In-side the frame I will have an MDF board (with oak laminate) that is either 4mm or 6mm (I have not yet decided) that will be attached within a 4/6mm groved channel inside the frame.

I have made an attempt at a diagram below for the MDF board grove that I will cut into the oak frame, it shows how the MDF will slot inside.

   | |     <-- 4/6mm MDF board sitting within the groved channel
   | |
+--+ +---+
|  |_|   |
|        | <-- Oak 20mm frame cross-section view
|        |

The frame is constructed with dowelled and glued joints (three large dowels per join to stop twisting). And the MDF board will slot snug into the groves and will be glued.

The idea is that the MDF will keep the doors in shape.

Do people have suggestions / or confirm this is a good/poor design? / alternatives? (please not hammer and mortice joins... too difficult for me!)

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Your plan sounds fine to me, except I'd go with two dowels per corner. Also, since you're new to woodworking, doweling can be devilish if you don't have special doweling tools. So, if you don't, and you end up with some angular holes, or misaligned holes, I suggest you just drill the holes a little oversize, fill the holes with wood filler, insert the dowels, clamp, and scrape off the excess filler. Good luck. –  getterdun Jan 28 at 1:06
    
@getterdun Thanks for that, I have obtained a pillar drill for the dowels, so I think I can get the holes quite straight, but I don't suppose I need 3 dowels, maybe two is enough! Thanks for helping to confirm the design :) –  code_fodder Jan 28 at 12:17
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Although MDF is more stable than solid wood, I won't claim that it will not swell enough with enough humidity to break the joints at the corners as solid wood will if there is no room for expansion, did that my self with a single panel in a 24" cabinet door 20 years ago never again... The strength of the door needs to be in the joinery, not the panel. There really needs to be room for movement, maybe not as much as a wood panel, but something. Dowels will suffice for the joinery, keep it to 2 per corner. If you are going to cut the groove (dado) with a router or table saw, I presume you will stop the dado from the ends so it is not visible. It need not be exact where, as long as it is behind the joint and long enough to not foul the panel.

Take this a step farther and run the dados all the way through and a little deeper to 1/2 inch, and you have the basis for a simple joint that will be strong enough to hold without dowels. The trick is to use a smaller bit or blade than the panel get it roughly centered on the set up and run the stock (wood) the turn it the other way and pass it through again. With a little patience the dado will finish exactly in the center, and the proper width for the panel.

With that done, you can use the same bit to cut the ends of the stock for the top and bottom pieces (rails) they need to be 1 inch longer than the dimension between the horizontal pieces (stiles) so when the ends are reduced to fit in the dados it will give your finish size you need.

If the notion I pass by intrigues you, the next step is actually pretty simple too, but it depends in what tool you are using to machine your wood.

Edit 1/28/13

If you wish to use dowels, and don't mind seeing the holes at the outside corners of your doors, I would suggest this. Lay your corner out on a flat surface just as you would like it after it is assembled. Clamp it in place so it is square and near the edge of your support table, and drill in from the edge into the wood through one piece and into the other, gauging your depth to allow the dowel to be centered in both pieces. Use fluted dowels, the fluting allows excess glue to escape from the drilled hole. then plug the holes on the outside corners with an oak plug to blend in. You will need bar clamps for this, though a jig can be made to do the same with blocks screwed to a flat surface and a wedge of wood to draw it all tight. lay plastic under your joints or better yet, tape at the joint locations to keep the glue that will ooze out from gluing your frame to the workbench. Bar clamps will allow you to maneuver the frame after assembly to clean the glue on all sides, just be sure when you are done tidying up the frame is absolutely flat (no twist) and square, while the glue is drying. If it is a little out of square, a bar clamp diagonally across corner to corner will pull it back the little it may need. Have all this ready before you start, even make a dry run without dowels if possible, you will not want to try to find stuff while the glue is making its first "grab", it all needs to be at hand.

I say without dowels because if they get inserted without glue, they will still be more than very contrary to remove. Since the holes are all the way through, use metal drill bits that are smaller than your holes to line everything up temporarily.

I have used the turret style doweling jigs before, definitely not accurate. You can spend a lot of money on other doweling jigs, but the same money can be used on a matched router bit set too, the trick is, you need a router table and the router should have a 1/2" collet to hold up under the strain of cutting these joints. Back at the doweling. The clamping of the material will insure you can control how the drill is going, a piece of tape, making a "flag" on the drill bit can act as a depth gauge. All this will make certain the faces will match on assembly and the holes are in plane with each other on each part. This alone is most critical. The slightest misalignment in the way the holes line up to each other will create a twist in the door frame and the door will not meet the cabinet box (carcass) on all corners, one corner on the door will set away from the carcass. In a matched set of doors, this can really look bad....

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thanks for your answer. I guess you are suggesting that mdf can expand?, but I was sort of under the impression MDF was the most stable marterial (of the "woody" materials). Anyway, your other idea sounds interesting, but I can't quite understand what you mean. Are you saying cut a larger groove in one side of a join (with a router) and then cut two grooves (either side of the middle) on the other side and slot the two together? –  code_fodder Jan 28 at 12:14
    
Yes MDF is the most stable, but with humidity changes, any material that can take on moisture will expand, it is paper, more or less bonded together with glue and heat under extreme pressure. Do not let it get wet, it will swell back to its uncompressed size, usually double of what it was –  Jack Jan 28 at 13:12
    
The groove will be larger in depth, not wider than 1/4", but a 1/2" deep. Now I know you will use a router, I will post more in a detailed part regarding the groove making. Another question, do you have access to a table saw at all? The suggestion I can make will tremendously reduce the strain on your router if you do. There is a technique with the table saw that does this simply too. There may already be a post that does so on this site, but if you don't the router will work too. You will need to feed the material slowly to not overheat the cutting bit, when you start your milling. –  Jack Jan 28 at 13:25
    
I just realized a few more questions are in order. Do you have a router table? maybe better to ask what is in your tool selection? Table saw, router with table, router with 2 or 3 wing cutter, straight cutter, etc. Sorry to ask so much, when you are asking about doweling it all together. I will add more to the answer on that too. –  Jack Jan 28 at 14:31
    
I have a "chop-saw" I think its called (circular saw that pulls down from above), I have a hand-circular saw (you push it along by hand), not sure if either of those is a table saw? - I don't have a router table, but I have work benches and clamps etc... –  code_fodder Jan 28 at 14:38
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