Getting CNC machine safety right  

By: Simon Fridlyand
Automation abates some dangers of working with machining centres, yet many safety standards must be applied to prevent serious injuries to workers and maintainers.

CNC technology has been a boon for the machining industry and has made working with machines safer for operators, but there are still many dangers in using these modern machines, for both operators and maintainers. This article guides you to the standards that must be followed to ensure a safe working environment.

CNC is Computer Numerical Control, and refers to a computer controller that reads G-code programming language instructions and drives a machine tool. CNC technology was developed in the late 1940s and early 1950s by the Massachusetts Institute of Technology Servomechanisms Laboratory.

The introduction of CNC machines dramatically changed the manufacturing industry. With it, curves became as easy to cut as straight lines, complex 3D structures were relatively easy to produce, and the number of machining steps that require human action were dramatically reduced.

Because automation of manufacturing processes increased with the advent of CNC machining, considerable improvements in consistency and quality were achieved. CNC automation reduced the frequency of machining errors and provided CNC operators with time to perform additional tasks. CNC automation also allowed for more flexibility in the way parts are held in the manufacturing process and in the time required to change the machine to produce different components.

Movements: The most basic motion for a controller is to move the machine tool along a linear path from one point to another. Some machine tools can only do this in the XY axes, and have to accept changes in Z separately. Some have two further axes of rotation to control the orientation of the cutter, and can move them simultaneously with an XYZ motion.

Tool changes: Originally there would be a G-code instruction telling the machine tool to stop so that a human operator could remove the cutting tool from the chuck and insert a new one. Modern machine tools have a magazine of different tools that can be changed by the machine pneumatically, hydraulically or electromechanically.

Dangers of operating a CNC machine

Each machine has its own characteristics and configuration. This should be considered when identifying the hazards and assessing the risks, but all of the following hazards will need to be evaluated:

1. Ejection of the workpiece or cutter

2. Contact with rotating cutters

3. Trapping and crushing caused by moving tables or machining heads

4. Unexpected movement or start-up caused by faults in the control system

5. Excessive noise emission, and

6. The production of dust and chippings.

Safeguarding

As control systems have developed, the degree of operator intervention required during the machining process has decreased. The addition of automatic loading and unloading facilities and automatic tool changing further reduce the need for a close approach to the cutting area. This enables the machine manufacturer to adopt safeguarding methods that differ from those used for manually operated machines.

Access to the cutting area during the machining process should be prevented by an enclosure or other effective means. Where access into an enclosure is necessary for loading/unloading the workpiece, for cleaning, setting or adjustment, or for tool changing, then access should be via a door which is interlocked so that it is not possible to gain entry while the cutters and other dangerous parts are moving.

The interlocking system should be designed based on a hazard assessment.

When the interlocked door needs to be opened to facilitate set-up or adjustment needs, the ANSI B11.23-2002 Standard -- Safety Requirements for Machining Centers and Automatic, Numerically Controlled Milling, Drilling, and Boring Machines -- stipulates the following: "When any interlocked movable guard is open or a protective device is suspended, powered machine movements shall only be permitted under the following conditions.

a) Axis movements are to be at a maximum rate of 2 m/min (6.5 ft/min) or a maximum increment of 6 mm (1/4 in.) at full feed rate. These movements shall be selected one axis at a time and may be initiated and maintained by one of the following means -- a hold-to-run control device; a rotary control device (e.g., an electronic hand-wheel); or a manual data input (MDI) followed by cycle start together with an enabling device.

b) The spindle speed shall be limited by its stopping performance that shall not exceed two revolutions. Spindle rotation shall only be initiated and maintained by one of the following means -- a hold-to-run control device; or a spindle start control device together with an enabling device.

The limits of speed or incremental distance defined in a) and b) above shall be monitored and if exceeded, the power to the drives shall be removed by a controlled stop (Category 1 -- see ANSI/NFPA 79).

The CSA standard

The CSA Z432-04 Standard for Safeguarding of Machinery, Clause 6.2.1.9.12.1, allows certain operations (i.e. setting, process changeover, fault-finding) to be performed, if required, with the guard/door open and the protective device neutralized.

This requires the use of a manual control mode that simultaneously disables the automatic control mode; permits operation of the hazardous element only by triggering an enabling device a hold-to-run control device, or a two-hand control device; and permits operation of the hazardous elements only in enhanced safety conditions (e.g. reduced speed, reduced energy/force, step-by-step).

Dust, chippings and noise issues

Dust and chippings should be effectively delivered to an extraction outlet that is an integral part of the machine.

On machines capable of producing noise levels greater than 85 dB(A) during machining, the enclosure should be designed to reduce noise emission.

Where effective close guarding of the cutters is used, other hazards such as those created by noise, dust, ejection from the machine or crushing or trapping points must also be addressed.

Tool changing mechanism

In some cases the enclosure for the cutting area will prevent contact with the tool changer. In other cases there may be a separate access door or access from the tool magazine. Such access doors should be interlocked with the tool changer and if there is access to the cutters, they also should be interlocked with the cutters.

During manual tool changing, there should be no risk of rotation of the tool holder while tools are being inserted or removed. 'Hold to run' or single-step pendant controls may be used to index the toolholder.

Material loading

Material loading may present a hazard when the operator's fingers, holding the workpiece, are in the area close to the moving jaws.

The ANSI B11.22-2002 Standard -- Safety Requirements for Turning Centres and Automatic, Numerically Controlled Turning Machines, Clause 6.17.2 -- stipulates the following:

For manual loading/unloading, means shall be provided to prevent the operator's fingers from being trapped. These means may include incremental movement not exceeding 4 mm (3/16 in.); closing speed not exceeding 4 mm/sec (3/16 in./sec); or the operation of the workholding device shall be controlled with hands outside the working zone (e.g., by two-hand control) together with a retractable workpiece support.

Maintenance activities

Standards require that maintenance activities shall follow strict lockout procedures when working with CNC machining centres. All sources of energy (electrical, mechanical, hydraulic, and any other source of energy) shall be locked out or dissipated and the system shall be brought to a zero-energy state.

Lockout/tagout provisions and procedures as per the CSA Z460-05 Standard, Control of Hazardous Energy - Lockout and Other Methods, are required to be in place and communicated to staff.

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