Introduction: My Favourite Joinery Method for Plywood, MDF & OSB

About: Average Can Be Extraordinary

Sheet material such as Plywood, MDF and OSB are fantastic to work with. They are really versatile and easily available at your local DIY hardware store. The awkwardness comes when it's time to decide what joinery method to use. So in this post I'm going to show you my favourite joinery method for sheet material.

I've always liked the simple solution to problems and I think this joinery method couldn't get any simpler. It uses a router. a slot cutting router bit and some pieces of 6mm MDF.


Tools & Materials Used:

Step 1: The Star of the Show

The star of the show is the slot cutting router bit itself. The bit that I use has a cutting width of 6.3mm and a cutting depth of 9.5mm. I find this works great for joinery on 18mm material. The 6.3mm cutting width is a perfect fit for use with 6mm MDF. I cut my own MDF strips for use with this joinery method.

Step 2: How to Make the MDF Pieces

To make the MDF strips I simple cut scrap pieces of MDF to 18mm wide strips. I then sanded them on the belt sander, slightly easing the edges to add a chamfer. This helps the MDF strips slip into the slots easier. Especially when wood glue is added. They can be cut to exact lengths for each joint or cut into smaller pieces and stacked next to each other to fill the slot.

Step 3: Set Up the Router

The beauty of this method is that you don't have to be super precise with set up. I install the slot cutting router bit into the router and set the depth by eye to roughly centre on the sheet material I'm working with. As long as you reference the same side with the base of the router then the pieces will slot together perfectly flush, regardless of how far off centre you are.

Step 4: Cut the Slots

Cutting the slot in the workpiece is simple, thanks to the bearing guided bit. There is no need for a fence or any other alignment jig. I simply start the groove away from the edge and ease the bit out before reaching the other edge. The slot can be cut in the same way on the mating piece. Easing in and out at either edge so the slot isn't visible from the sides.

Step 5: Add the MDF Pieces

The MDF pieces can then be added to the slot. For this example I didn't use any wood glue but typically, you would add wood glue to the slots and the MDF pieces before assembly. When wood glue is added the MDF swells slightly and makes for an even tighter joint. It's then just a case of slotting the pieces together. Add clamps while the glue dries and you're left with a perfectly flush joint.

Step 6: 90 Degree Joinery

This joinery method even works for 90 degree joints. The first part of the joint is cut exactly the same way as before. The workpiece lay flat and the slot cut along the edge.

The mating piece needs the slot cut in the face of the material rather than the edge. This is a little awkward but it's still simple with the right set up. I clamp the workpiece in the jaws of my workbench in an upright position. A vise works great too. Behind the workpiece I add some scrap material. This is to help stabilise the router base as 18mm material is quite thin and the router base can rock slightly. With this secure it's just a case of cutting the slot. Again, easing in and out before reaching each end.

Chip out isn't a huge issue because it's inside the joint and will never be seen but you could use some painters tape to help prevent this from happening. If you're working with MDF you don't have to worry about this at all.

With the slots cut it's just a case of adding wood glue and the MDF strips into place. If you need to remove a strip for any reason and you're finding it difficult. Then the claw side of a claw hammer works great to lift it out. Then the pieces can be slotted together forming a 90 degree joint that is perfectly flush on the outside.

Step 7: Other Methods: Screws & Dowels

There are of course lots of different joinery methods you could use with sheet material. Another one that I like is ideal for when it doesn't matter what the joint looks like. I use glue and screws at the corners of drawers, leave the glue to dry and then replace the screws with wooden dowels. It has all the strength of dowel joinery without the hassle of alignment. Dowel joinery is great but it's a paint to get perfectly centred so using the screws first makes it quick and easy.

Step 8: Other Methods: Pocket Hole Screws

Another quick joinery method is pocket hole screws. They're really quick to drill and the resulting joint is a lot stronger than regular screws on their own. This method does leave large oval holes that will either need filling or at least hidden in some way.

Step 9: Other Methods: Biscuit Joiner

A similar method to the one I have just shown is a using a biscuit joiner. It cut an oval shaped slot into the work piece and then using ready made biscuits to fit in the slots and complete the joint. While you can cut your own biscuits, you would typically buy them which adds more expense. You also need to be quite precise with where you cut the slots. There is some wiggle room, but not a lot. If you want to join a large area then it requires multiple slots which increases the need for accuracy too.

Step 10: Give It a Go

I hope you have found this post useful. This joinery method is really fast and simple to achieve. I've had great results from it. Give it a try and see what you think.

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