Make MDF Joints That Last
Cut a dado in plywood, and you're on your way to a strong joint. Cut a dado in medium-density fiberboard (MDF), though, and you'll weaken it enough to cause cracking and joint failure.
What makes MDF different is how it's made. Manufacturers compress glue and loose wood particles into a sheet that's hard and flat on the outside, but weak and crumbly on the inside. Because of this, MDF requires different joinery techniques than plywood or solid wood. But you can still assemble a strong joint using butt joints reinforced with dowels or specially engineered metal fasteners.
Glued dowels bond with the MDF fibers to strengthen joints. And unlike screws, dowels can hide below the surface. Of course, there's no margin for error when positioning dowel holes in mating parts.
Oversize dowel holes can weaken MDF, so use 1⁄4 " dowels for 1⁄2 "-thick stock, and 3⁄8 " dowels for 3⁄4 " stock. We prefer fluted dowels that allow excess glue to escape as the dowel is inserted. Also follow these tips:
■ Keep the first and last dowel locations about 11⁄4 " from the ends. Space the dowels 8–12" apart.
■ For the tightest joints, slightly bevel the mating edges of the dowel holes with a 1⁄16 "-deep countersink.
■ If a dowel can't be inserted and removed by hand, it may split the MDF upon assembly. Lightly sand the dowel with 180-grit abrasive until it fits. Then glue both sets of holes, and assemble the joint.
Screws made for fastening MDF can be installed faster than dowels, but with some precautions. Unlike conventional woodworking screws, MDF screws have deep threads that better penetrate and grip MDF fibers. Still, to avoid splitting, drill no closer than 2" from an MDF edge. Drilling a countersink or counterbore prevents fibers from mushrooming around the screwhead. Screws seldom provide enough strength alone, so reinforce the joint with glue. See the four great screw options below to find the right fastener for your needs
Use these fasteners made for MDF
Long sold as Confirmat screws, these require a stepped bit that drills a pilot hole, shank hole, and countersink. When fastening white or brown melamine MDF, hide screwheads beneath color-matching snap-on plastic caps. Available from Woodcraft (800-225-1153).
A lip around the head sits flush with the surface without the need for a special bit. Alternating deep and shallow threads provide extra grip to prevent the screw from pulling loose. Drill a 3⁄16 "-diameter counterbore for the head and center a 9⁄64 " pilot hole within.
#8×13⁄4 " screws, no. 30202, Rockler (800-279-4441).
A smooth shank at the top allows the threads to pull the workpieces together. The wide screwhead stops pull-through and can be recessed using a counterbore/countersink bit that also starts the pilot hole. Although this screw has a self-piloting auger tip, play it safe and drill a 1⁄8 " pilot hole into both pieces.
3" screws, no. 834206; Counterbore/countersink bit, no. 147950, Woodcraft.
Spax screws made for MDF have extended smooth shanks (like the Powerhead) that allows the joint to pull together. Serrated threads and an oversize point cut through the MDF, reducing splits. However, we still recommend drilling 1⁄8 " pilot holes.
#8×13⁄4 " screws, no. 939986, Home Depot stores, homedepot.com.