Mdf

Also called: medium density fibreboard

Medium density fibreboard, usually known simply as MDF, is a manufactured board made by pressing very fine wood fibres into flat sheets rather than sawing them from a log. Because those fibres point in every direction instead of running along a single grain, the direction the fibres of solid timber all follow, MDF behaves differently on the bench from solid wood or plywood.

How MDF behaves

Without a grain to give it stiffness, a large sheet of MDF will sag under its own weight if it is left unsupported, so long panels need bearers or need to be fixed down soon after cutting. The upside of having no grain is flatness: full sheets of plywood are rarely as flat as MDF straight off the pallet, though once a sheet is cut down into the smaller panels most jigs and fixtures call for, that difference matters less. MDF has become common enough in furniture making that the USDA (2021) notes it is "frequently used in place of solid wood, plywood, and particleboard in many furniture applications."

Joining and reinforcing it

Because MDF has no long grain to plane or run a moulding into cleanly, its exposed edges are usually covered rather than left bare. Brett (2005) describes edging sheet material such as MDF and melamine-faced chipboard with either iron-on tape or a strip of solid timber lipping, held in place with masking tape while the glue sets. Where extra thickness is needed, two sheets of MDF can simply be glued face to face, a trick woodworkers also use with plywood, to build up a panel without buying thicker stock. For mitre joints, where two pieces meet at an angled cut rather than square across, made in sheet materials such as plywood and MDF, Rogowski (2002) recommends reinforcing the joint with a plywood spline, a thin strip of plywood glued into a matching slot cut across the mitre, since the angled cut alone leaves only a small surface for glue to hold.

Cutting it

MDF blunts ordinary high-speed steel (HSS) cutters quickly. Bird (2004) notes that, unlike carbide-tipped cutters, cutters with a very hard metal edge, HSS router knives cannot be used on manufactured boards such as MDF or plywood. Carbide-tipped blades and bits are the standard choice for cutting or routing it.