Materials · Sheet goodsbeginner~2 min read

Cutting MDF and HDF on a CNC

Short answer

MDF machines predictably in every direction with no grain and no tearout, which makes it ideal for spoilboards, jigs and painted work. Its dust is very fine and worth taking seriously, it dulls bits, and it swells permanently if it gets wet.

Why CNC people like it

Grain

None. It cuts the same in every direction.

Flatness

Very flat and consistent thickness

Tearout

None to speak of

Cost

Low

Detail

Holds fine detail well, though edges are slightly soft

Dust

Very fine, and the reason to run real extraction

No grain means no tearout, no grain direction to think about, and no surprises from a knot. For testing a toolpath, building a jig or making a spoilboard, MDF is the obvious choice.

The dust

Treat MDF dust as the main hazard of the material

MDF dust is extremely fine, so it stays airborne long after you finish and gets deep into your lungs. The binder is urea formaldehyde resin, and formaldehyde is the reason MDF dust is treated more seriously than plain wood dust. Boards labelled NAF or ULEF use other binders and are worth seeking out if you cut a lot of it. Run dust collection at the tool, wear a fitted respirator rather than a paper mask, let the air clear before you unmask, and do not blow it around with compressed air. See CNC safety.

The weaknesses

Water destroys it. Not warps, destroys. MDF swells irreversibly and goes soft. It is an indoor material with a finish on it, or it is temporary.
Edges soak finish. A cut edge is end grain in every direction and drinks paint. Seal edges before finishing or they look furry and dull.
Screws split it. The face holds screws poorly and the edge holds them very poorly. Pre-drill, and do not rely on edge screws for anything structural.
It dulls bits. Not as badly as some reputations suggest, but the resin content is abrasive.
It is heavy. A full sheet of 3/4 inch MDF is genuinely awkward to handle alone.

HDF and other relatives

HDF is denser MDF. It holds an edge better, is stiffer, costs more, and produces the same fine dust. Hardboard is, roughly, thin HDF, useful for templates and patterns. Ultralight MDF is lighter and softer, handy for large jigs and pattern work where weight matters.

Settings direction

Starting point MDF is uniform and forgiving, so it tolerates a wide range. It cuts more like a soft plastic than like wood: it does not burn easily but it does dull tools, and chip clearing is easy because the chips are essentially dust. Start with settings you would use for a medium hardwood and expect to be able to push depth of cut further than you could in maple.

Using it well

Spoilboards. The standard choice: flat, cheap, surfaces well, and sacrificing it is the point.
Jigs and fixtures. Stable, machines precisely, and you can make a new one when it wears out.
Painted signs and props. Smooth faces take paint beautifully once the edges are sealed.
Test cuts. Its predictability means a toolpath that works in MDF has been tested for shape, though not for how a grained material will behave.
Not for anything outdoors, anything that gets wet, or anything that needs to hold a screw under load.