“T” joint with MDF—No power tools

Question

Disclaimer: I'm a complete newbie when it comes to woodworking so I apologize if I don't use good woodworking vocabulary.

I'm hoping to build a small hutch/monitor stand for my desk out of MDF. I'm choosing to go with 3/4" MDF because I'm a poor college student and want to go as cheap as possible. So far I've gathered that MDF and screws don't really get along and that I can use box joints for joining ends. I'm wondering if there is a good way to join two pieces of MDF in a "T" shape, like this:

I'm trying to avoid using any kind of hardware or joinery that takes up space below or above the "shelf" in the image: I need to slide an object on my desk onto the shelf and it will fit exactly.

I also don't have any power tools besides a circular saw and a power drill, so I can't cut holes for biscuits. I want each joint to be sturdy, but also aesthetically pleasing (I want my wife's support in future projects so I don't want it to look like some scraps were just slapped together).

I saw a joinery method called a "dado", and was thinking that this might work:

If this is a viable option, how deep should I cut the groove? The MDF is only 3/4" thick. How would I cut a blind dado (as shown in the image above) with hand tools?

If this is not a good option, why not? Are there any other methods I can use for a discrete but strong joint that can be implemented with hand tools?

Answer 1

So far I've gathered that MDF and screws don't really get along and that I can use box joints for joining ends.

There are two main problems with using screws with MDF: splitting and strength. If you drive a screw into the edge of a piece of MDF without drilling a pilot hole, it's likely that the screw will force the MDF to split. And even with a pilot hole, screwed connections in MDF are often weaker than in wood because the short, randomly-oriented fibers in MDF don't have a lot of strength. But there are a number of ways to overcome or at least compensate for those problems:

• pilot holes: If you're going to drive a screw into the edge of a piece of MDF, you really want to drill a pilot hole. A pilot hole makes space for the shaft of the screw, so that it doesn't force the MDF apart.

• use the right screws: Screws designed for use with MDF have wide threads so that they'll bite into more material, which helps compensate for the lower holding strength of MDF. Spax makes some are supposed to be self-drilling and self-tapping, so you may not need a pilot hole. I've seen those in home centers for about $7/box, where a box has 200 screws, so they're not too expensive. If you've ever assembled any flat-pack furniture from places like IKEA, you've probably seen Confirmat screws. These run about $10 per hundred, but you'll also want to pick up a drill bit that creates the stepped pilot hole needed for these screws.

• use more screws: Screws have less strength in MDF, but you can compensate for that somewhat by using more of them and being careful not to over tighten them.

• add glue: Like screws, glue isn't as strong when used with MDF as it is with wood because the material itself doesn't have much strength. But adding glue to a screwed joint can give it more strength than either screws or glue alone.

• use a wooden insert: If you don't have screws designed for MDF on hand, you can still make a strong joint by adding a wood insert to the MDF so that the screw has some wood to bite into. For your the T-joint that you're trying to make, you'd drill a fairly large hole though the thickness of the horizontal piece — maybe 3/4" or 1", positioned an inch or two from the joint. Then you'd fill the hole with a piece of wooden dowel sized to fit. That gives the screw some wood to bite into, and you can screw through the vertical piece into the horizontal piece and the dowel and get a fairly strong connection.

• use a metal insert: Same idea as above, but use a commercially-available metal cross nut instead of wood. You'd use a bolt instead of a screw, and your holes need to be drilled pretty accurately, so a jig is a big help.

I also don't have any power tools besides a circular saw and a power drill

That's going to make it hard to cut any type of dado. It's not impossible to do it with a circular saw, but the right tool for the job is either a router or a table saw with a dado cutter. I'd go with a plain old butt joint held with glue and screws meant for MDF, if you want to make a permanent connection, or bolts and barrel nuts if you want something you can take apart and reassemble.

Above all, don't be afraid of making mistakes. You're not building an heirloom here, and MDF is cheap. If you build something and it fails in some way (breaks, doesn't look good, etc.) then you'll learn from it and build a better one the next time.

Answer 2

Given your situation I recommend you consider dowels and glue.

Cutting an accurate dado with a circular saw is very tricky and mistakes there are hard to undo or hide. Generally that sort of thing is done with some sort of guided tool like a table saw or router against a guide.

MDF accepts glue very well and makes strong joint. The dowels (maybe three on each end/joint) will provide some mechanical strength and reduce the likelihood of the glue breaking from impact shock. You could run a couple screws in from each end to keep the joint tight while the glue sets. They would provide some additional mechanical strength as well. This also avoids the need for bar clamps which is how a better equipped shop might do the glue-up.

If you did go with a dado I'd go with 1/4" deep if you can keep the edge sharp, 3/8 (half-way through) if you weren't so sure. Once glued, the side boards will be just as strong as long as the fit is pretty tight. I might also use the screws trick for this approach just to keep things tight while the glue sets and provide some insurance thereafter.

Answer 3

You could use your circular saw and a clamped straight edge to iteratively cut out a dado. It won't have a flat bottom (because circular saw teeth typically aren't flat tipped) and you'll need to be insanely careful to get things lined up, but it'll work, and add strength.

Whatever your final plans for insetting the shelf, you could add a solid wood lip to the front of the shelf. This doesn't necessarily help with how it stays up, but it will strengthen the MDF sheet, which can sag in time and imperfect conditions.