How to Join MDF


Wood fibers, glue, moisture and hydraulic pressure yield a product known as medium-density fiberboard or MDF. Cabinetmakers, builders and woodworkers use medium-density fiberboard. The fact that MDF has no grain is an advantage: It can be cut in any direction. Woodworkers use spline joints to join two pieces of like material together. The spline joint is a piece of wood centered between two pieces of MDF to connect them. When glue and clamps are employed, an MDF joint is nearly invisible and almost indestructible, as long as the MDF is thick enough.

Things You'll Need

  • Table saw
  • 3/8-inch dado blade
  • Fir plywood, 1/4-inch-by-3/4-inch-by-48 inches
  • Wood glue
  • Rubber mallet
  • Bar clamps
  • Glue scraper
  • Place two pieces of MDF together. Check for gaps, open seams or defects where the two pieces join. If you see any such issues, rotate and fit the pieces until two edges fit tight. If none are perfect, use a table saw to trim edges and test again. Draw perpendicular lines across the joint with a pencil when you're satisfied.

  • Install a quarter-inch dado blade on a table saw. Raise the height of the blade three-quarters of an inch from the top of the table to the top of the tallest tooth on the blade. Set the fence a quarter-inch from the right side of the blade.

  • Turn on the saw. Stand one of the MDF pieces on the table saw. The perpendicular lines should be vertical along the bottom. Hold the MDF against the fence with your left hand.

  • Push the MDF over the blade with your right hand while applying even pressure with your left hand. Repeat this step with the second piece of MDF to cut two identical channels along the edge measuring 1/4 inch wide and three-quarters of an inch deep.

  • Measure the length of the joint. Cut a strip of 1/4-inch-by-3/4-by-inch-48-inch fir plywood to the measurement, using a miter saw.

  • Apply glue to the dado channel on both pieces. Insert the plywood strip into the channel on one of the pieces. Place it across two sawhorses. Place the other piece of MDF on the sawhorses. Insert the extended edge of the plywood strip on the first piece into the channel on the remaining piece. Tap the two pieces together with a rubber mallet.

  • Place two bar clamps, evenly spaced, across the top of the conjoined pieces. Place two bar clamps underneath the pieces, alternating the clamps from top to bottom. Apply even pressure to all four clamps to ensure that the MDF stays flat. Stop adding pressure when glue oozes out the seam. Wipe off excess glue with a damp cloth. Allow the clamps to remain on the MDF for at least one hour for the glue to dry.

  • Remove the clamps. Scrape off dried glue with a glue scraper.

Tips & Warnings

  • If you don't have a table saw available, rout the dado channel with a hand router.
  • Test-fit the plywood strip before adding glue. If it's too tight, sand it and test again until it fits snugly but not tight.
  • The majority of MDF is manufactured sheets that are three-quarters of an inch thick or thicker. Thinner pieces are available, but joining thinner pieces together is difficult -- if not impossible -- and not worth the effort.
  • MDF produces fine, powdery sawdust when cut or milled, and it also contains formaldehyde. Wear breathing, eye protection and provide adequate ventilation when working with MDF.

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