Metal Chips and Swarf
Chips come off hot, razor sharp and fast. How to handle them, how to clean them up, and the rules on compressed air.
Four reasons a chip is dangerous
Four reasons a chip is dangerous
A chip is not scrap until it has cooled and stopped moving. Straight off the tool it is every one of these at once.
- Hot
- Where an aluminum chip forms, the cut runs roughly 250–650 °C (500–1200 °F), depending on the alloy and the speed. Chips do not look it.
- Sharp
- The leading edge of a chip is always razor sharp. It is a freshly cut edge and it will open a hand.
- Strong
- A long continuous chip is a steel ribbon. If it wraps a sleeve or a limb it can pull you into the machine.
- Slippery
- Chips on the floor roll underfoot. This is one of the most common ways people get hurt without the machine touching them.
Never clear chips with your hands; Stop
Never clear chips with your hands
Do not reach into a machine to clear chips while it is running, and do not pick up a long stringy chip by hand at any time. Stop the spindle, then deal with it.
Gloves
- Gloves may only be used for cleanup after the machine is shut down.
- Never wear gloves at a running machine. A glove catches where bare skin would pull free, and takes your hand with it — see Machine Hazards and Housekeeping.
Brush
- A brush may be used while the machine is in operation — your hands stay outside the machine. If clearing the chip means reaching in, stop the spindle first.
- On a CNC machine, take particular care: the tool can move without warning, and the enclosure invites you to reach further in than you should.
Compressed air — there are limits on this; Watch out; Checklist
Compressed air — there are limits on this
Compressed air is convenient and it is the option people reach for first. It is also regulated, because a chip in a moving air stream is a projectile and air forced against skin can be driven into the body.
The rule
OSHA permits compressed air for cleaning only where it is
reduced to less than 30 psi, and only then with
effective chip guarding and personal protective equipment
(29 CFR 1910.242(b)). All three conditions, together — not the
pressure alone.
Chip guarding is anything that stops a chip reaching eyes or unbroken skin: a protective cone nozzle covers the operator, and a screen or barrier is what covers everyone else.
In practice
- The 30 psi is measured at the nozzle, dead-ended — blocked by the work or a hand — and is met by a limited air nozzle or a regulator, not by the supply pressure upstream of it.
- Eye protection, every time. Everyone nearby too — you do not get to decide the range.
- Never at skin, yours or anyone else’s.
- Never toward another person, and check who is behind the machine first.
- Never into an enclosed space.
- If air would just relocate the mess into the air or across the shop, use a brush or the vacuum instead.
Cleaning up; Stop; Checklist
Cleaning up
- Use cleaning equipment or gloves to handle chips for disposal — the sharp edge does not dull because the machine stopped.
- Clear the machine so only the residue in the bottom tray remains.
- Sweep the floor, and mop if the floor is wet with coolant.
- If cleaning equipment such as a vacuum gets wet, open it up and let it dry before it is put away.
Chips and fines do not go in the same place
- Chips and swarf go in the recycling container. That container is for chips only.
- Fines go in the trash. Dust from sanding, grinding, buffing and polishing, and what comes out of a dust collector — never the recycling container.
Why — two reasons, and the second one is the safety reason
- The recycler pays for clean aluminum. A clean load gets the aluminum price; mix another metal in and the whole load drops to a much cheaper one. Fines pick iron up — off a shared belt or wheel, off steel work at the next bench — so fines do not go in.
- Compare how often each container actually leaves the building. A chip recycling container fills slowly and can stand most of a semester between pickups; the trash goes out daily. Fines belong in the stream that leaves, not the one that stands in the corner for months quietly collecting more of them. Anything whose problem is accumulation does not go in the container designed to accumulate.
Two thresholds, same observation. “There is iron in the fines” is a real problem for the recycling price and not a fire problem — and the reason both are true at once is that the two thresholds are orders of magnitude apart. The price threshold is a trace: a few steel chips are enough to drop the whole load to the cheaper price. A thermite reaction needs iron oxide — rust or grinding scale, not bright steel — at something near three parts oxide to one part aluminum by weight, plus an ignition source far hotter than a spark. That is most of the pile, not a trace. Most rules in a shop have a number behind them; knowing which number you are being measured against, and how far you actually are from it, is the difference between following a rule and understanding it.
Fine dust burns; chips do not. Aluminum chips and swarf are hard to ignite. The fire hazard is fines — and then only when a cloud of them is thrown into the air and meets an ignition source, or when a great deal of rust or steel grinding scale is mixed into them. Do not let fine dust accumulate, and do not raise a cloud of it cleaning up.
If fine metal debris does ignite, do not fight it. The ABC extinguisher on the wall is the wrong tool and will scatter it. Alarm, evacuate, and tell the fire crew it is a metal fire. See Fire Safety.