In brief
Most accidents on CNC machines happen when the door interlock is disabled, during setting and when the workpiece is not clamped correctly. Swarf is not cleared by hand; soluble cutting oil can cause skin disease through prolonged contact and respiratory problems when its mist is inhaled, so its concentration, cleanliness and extraction should be monitored regularly.
Where do accidents happen on a CNC machine?
At first glance, an enclosed CNC machine looks like one of the safest machines: machining happens behind a closed door, and the operator only loads the part and starts the program. Accidents in CNC operator safety happen exactly when this sense of security is broken: when the door interlock is disabled, during setting and the first part, when the part is not clamped tightly enough, and when someone tries to clear swarf by hand. Two long-term health risks are added to these: the effects of soluble cutting oil on the skin and on breathing.
In Bursa's automotive suppliers and machinery manufacturers, CNC lathes and machining centres are at the heart of production. Machining and welding work (25.53.01) is in the hazardous class on the 2025 list; if laser cutting of metals (25.53.03) is done in the same workshop, that work is in the very hazardous class and may change the workplace's class. The class determines specialist and physician time, training hours and examination intervals. CNC machine safety, however, rests on the same four headings on every machine regardless of class: the door, workholding, swarf and cutting fluid. You can see your minimum specialist and physician time with the OHS obligation calculator.
Why are CNC door interlocks disabled?
The door interlock is the safety device that prevents the door from opening while the spindle is turning or the axes are moving, or stops movement when the door is opened. On site it is bridged for reasons such as shortening cycle time, taking measurements during machining, clearing accumulated swarf between cycles or "temporarily" overriding a faulty sensor. The methods are well known too: a spare actuator key, a magnet, tape or a change to the control circuit.
The consequences are severe. A hand, glove or arm that touches a rotating chuck or tool through an open door is pulled in within a very short time. A broken insert or a loosened part is thrown out of the enclosure. The door window is also a guard: a cracked, scratched or oil-degraded window may not stop an ejected part. Many manufacturers specify a replacement interval for these windows; check your machine's user manual.
The correct way to set up and run the first part is to use the setting mode provided by the manufacturer: reduced speed, hold-to-run control and limited axis movement. The mode selector key is held only by the authorised setter. A machine with a faulty door interlock is not used in production until it is repaired. For the design side of machine safety, see our article on machinery safety and CE marking.
CNC workholding: ejected parts and broken tools
Workholding errors can cause ejections energetic enough to test the window even with the door closed. Common errors and measures:
| Error | Possible consequence | Measure |
|---|---|---|
| Chuck clamping pressure not set for the part and speed | The part coming free of the chuck at high speed | Check clamping pressure and the maximum permitted speed for the chuck in the manufacturer's table |
| Unsuitable or worn chuck jaws | The part slipping, vibration, tool breakage | Choose jaws suited to the part diameter; replace worn jaws |
| Machining a long part without a tailstock | The part bending and being ejected | Use a tailstock or steady rest for parts with a high length-to-diameter ratio |
| Chuck key or clamping tool left in the machine | The key being thrown at the start of the cycle | Choose a spring-loaded key; visually check the inside of the machine before the cycle |
| Vice or clamping elements left loose | The part shifting, the tool striking it | Define the clamping torque; run a dry cycle on the first part |
| Program or offset error | The tool striking the part or the chuck | Simulation, single block and reduced feed for the first run of a new program |
Running the first cycle of every new part and program under the setter's supervision, at reduced feed and in single block mode, prevents most collisions. For the periodic inspection status of machines and fixtures, see our article on periodic inspection intervals.
Swarf: cuts, burns and eye injuries
The most common injury on a CNC machine is a cut while clearing swarf by hand. Long, spiral swarf is razor-sharp and hot; if it is pulled while wrapped around the part or chuck, it can drag the hand with it. The rules are simple:
- Swarf is removed only after the machine has stopped, with a swarf hook, brush or suitable tool; it is never pulled with bare hands.
- Inserts with suitable chip breakers and cutting parameters are chosen to prevent long swarf.
- Cleaning swarf with compressed air blows swarf and oil droplets into the eyes and onto the skin; if unavoidable, it is done only with a low-pressure, shielded gun and while wearing safety glasses.
- Gloves create an entanglement risk when working near a rotating spindle. Cut-resistant gloves are for swarf and part handling done while the machine is stopped.
- Safety glasses are always worn at the machine.
Soluble cutting oil: skin and respiratory risks
The product called "boron oil" on Turkish shop floors is a metalworking fluid mixed with water. It provides cooling and lubrication, but comes into direct contact with the operator's skin and the air they breathe.
Skin effects of soluble oil: Working with wet hands for long periods and repeated contact with the fluid can lead to dryness, cracking and irritant contact dermatitis on the hands. Preservatives and additives in the fluid, or certain metals dissolved from the workpiece, can cause allergic contact dermatitis. Once an allergy develops, sensitisation is usually permanent; when the worker is exposed to the same product again, the complaint recurs. Oil-soaked work clothes and an oily rag carried in a pocket keep the contact going all day.
Respiratory effects: On a machine running at high speed, the fluid forms a fine mist. This mist can irritate the airways; long-term exposure has been associated with respiratory diseases such as asthma. Poorly maintained fluid degraded by bacteria increases this risk.
The measures are:
- Control at source: Use an oil mist extraction system on enclosed machines; open the door only after the cycle has finished and the mist has been extracted.
- Fluid maintenance: Keep the concentration within the manufacturer's recommended range and measure it regularly; a bad smell, foam and colour change are signs of degradation. Clean the tank and lines regularly and change the fluid at the set intervals.
- Skin protection: Use nitrile gloves for contact with the fluid while the machine is stopped; wash hands with soap and water, not solvent or thinner, and moisturise at the end of the job. Change oily work clothes regularly.
- Chemical information: Read the product's safety data sheet; the sensitising substances in the content and the recommended PPE are written there. For how to read the sheet, see our article on the safety data sheet.
- Measurement: Where oil mist and chemical exposure measurements are needed, we plan them with authorised laboratories and enter the results in the risk assessment.
Start-of-shift checklist for CNC operator safety
- Does the door interlock work? Does the cycle not start with the door open?
- Is the door window cracked, scratched or cloudy?
- Does the emergency stop button work?
- Are the chuck jaws and clamping elements sound, and is the chuck key in its place?
- Are the level, smell and appearance of the soluble oil normal?
- Is oil mist extraction working?
- Is the floor clear of oil and swarf?
- Are safety glasses, suitable gloves and a swarf hook at the machine?
Posting this list at the machine and having the shift supervisor check it weekly allows habits such as bridging the door interlock to be noticed early.
Before a new operator is left alone at the machine, induction training should have shown them practically at the machine: the location of the emergency stop, the function of the door interlock, the sequence for clamping and unclamping parts, safe removal of swarf and the rules for contact with soluble oil. These topics are repeated in the job-specific risks section of basic training; in the hazardous class this section is delivered face to face. Who holds the authority to set up and change programs should also be in writing; this authority is given to people whose training and experience are documented. For the general arrangement of machine guards, see our article on machine guards.
Bursa Sistemer OSGB provides OHS specialist services to metalworking workshops in Bursa and neighbouring provinces; on the workplace physician side, skin and respiratory complaints are monitored under health surveillance. For other risks in the sector, see our metal and machinery page.
Frequently Asked Questions
Can the setting key be used to work without the door interlock?
Setting mode is for setting up with the reduced speed and hold-to-run control provided by the manufacturer; it is not a way of running series production with the door open. The mode selector key should be held only by the authorised setter, and the rules for its use should be in writing.
Should a CNC operator wear gloves?
Gloves create an entanglement risk when working near a rotating spindle or chuck. Cut-resistant gloves are used for part and swarf handling while the machine is stopped, and nitrile gloves for contact with soluble oil.
Does a soluble oil allergy go away?
Dermatitis caused by irritation often recedes when contact is reduced. In allergic contact dermatitis, however, sensitisation is usually permanent; the workplace physician assesses how contact with the substance causing the allergy can be prevented.
Are CNC machines subject to periodic inspection?
Yes. Lathes and milling machines appear in the Annex III tables of the Work Equipment Regulation; unless the standard or the manufacturer's instructions set a shorter interval, periodic inspection is carried out at least once a year. The inspection is carried out by authorised persons registered on EKİPNET (the Ministry's register of authorised inspectors).
Does a CNC operator need a vocational training certificate?
A vocational training certificate is mandatory for the jobs in the hazardous and very hazardous classes listed in the annex to the regulation. Check whether your operator's job is on the list in our article on the list of occupations requiring vocational training.
A certified specialist matched to your hazard class: site inspections, approved logbook, training plan, annual plan and report follow-up.
Bursa SİSTEMER OSGB Technical Team — written from the field experience of our OHS specialists and workplace physicians. Legal information is valid as of the publication/update date; contact us for an assessment specific to your business.