CNC safety

The realistic risk list

CNC routers are less immediately dangerous than table saws, and more insidiously dangerous than people assume. Nothing here is meant to frighten you off the machine. It is meant to make you spend your caution where it actually matters.

1. Fine dust, over years

The dust that hurts you is the dust you cannot see hanging in the air an hour after you finish. Wood dust is a recognised respiratory hazard and some species, including many exotics and western red cedar, are also sensitisers: repeated exposure can turn into an allergic reaction that gets worse each time. MDF dust is particularly fine and particularly worth avoiding.

• Run dust collection at the tool, not just a shop vac in the corner.
• Wear a proper fitted respirator for long sessions and for MDF, not a paper nuisance mask.
• Let the air clear before you take the respirator off. An air filter running for twenty minutes after a job does more than you would think.
• Sweep and vacuum rather than blowing dust around with compressed air.

2. Something leaving the machine at speed

A workpiece that comes loose under a spinning cutter can travel. So can half a broken bit. This is the one that can injure you in an instant.

• Eye protection, every time, including for the quick one.
• Do not stand in line with the direction the tool is pushing the work.
• Take workholding seriously, especially for small parts and for the last pass when tabs are all that is left.
• Watch the first minute of every job. That is when workholding failures and wrong-zero crashes happen.

3. Fire

Two fire paths matter. A dull bit rubbing in a deep slot gets genuinely hot and can scorch or ignite dust packed in the cut. And a dust collector full of fine dust with a spark source is a real hazard, which is why you do not vacuum up hot chips or smouldering material.

• Never leave the machine running unattended. Not for a coffee, not for a long 3D job, not overnight.
• Keep a fire extinguisher within reach of the machine, not across the shop.
• If something smells like it is burning, stop and find out why. See burning.
• Empty the dust collector rather than letting it pack solid.

4. Hearing

A trim router at 20,000 rpm cutting hardwood is loud enough to damage hearing over time, and it is a frequency that is genuinely unpleasant. Ear protection is cheap and you are standing next to the machine for the entire job.

5. The machine itself

• Nothing loose near the gantry. Sleeves, gloves, long hair, a hanging shop towel, a dust hose that is not secured. Gloves in particular: do not wear them near a spinning tool.
• Know where the emergency stop is without looking for it. If your machine does not have a physical one, add one. Reaching for a laptop keyboard during a crash is too slow.
• Unplug before you change a bit if the machine can be commanded to move from a computer that someone else might touch.
• Bits are sharp when stationary too. Most CNC cuts on hands happen during a tool change.

Material hazards

• Exotic hardwoods. Several are irritants or sensitisers. Cocobolo and other rosewoods have a reputation for it. Take dust seriously with unfamiliar species.
• Treated lumber. Contains preservatives you do not want as airborne dust.
• Reclaimed wood. Hidden nails and screws destroy bits and throw fragments. Check it with a metal detector before it goes on the machine.
• Composites and solid surface. Very fine dust, often with a mineral filler that is abrasive on both your lungs and your tooling.
• Aluminium. Chips are sharp and hot, and fine aluminium dust is flammable. Do not put it through a dust collector full of wood dust.

The habit that prevents most of it

Before every job, out loud: material secure, bit tight, zero correct, path previewed, nothing in the way, eyes and ears on, hand near the stop. It takes fifteen seconds and it is the difference between the machine being a tool and the machine being an event.