DGUV Information 209-027 - Machine Tool Fire and Explosion Prevention and Protec...

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Abschnitt 3.2 , 3.2 Measures against hot surfaces and other ...
Abschnitt 3.2
Machine Tool Fire and Explosion Prevention and Protection (bisher: BGI/GUV-I 719 E)
Titel: Machine Tool Fire and Explosion Prevention and Protection (bisher: BGI/GUV-I 719 E)
Normgeber: Bund
Amtliche Abkürzung: DGUV Information 209-027
Gliederungs-Nr.: [keine Angabe]
Normtyp: Satzung

Abschnitt 3.2 – 3.2 Measures against hot surfaces and other sources of ignition

In most cases, machine fires during operation are started by an incandescent chip, a grinding spark or an overheated tool. Therefore, reliable and adequate cooling of the machining zone should be provided by the MWF.

Figure 19 Worn indexable cutting insert

Figure 20 Hot run drill as a source of ignition

Sufficient flushing quantities

The MWF circuit should be dimensioned (pipe cross-sections, storage tank, pumps etc.) so that a sufficient quantity of MWF is available to flood the cutting zone at all times and for every tool. The necessary quantity of MWF depends on the type of machine type and the machining process. For information on the design of the MWF circuit see VDI 3035, VDI 3397 Sheet 1. Measures for best possible flooding are e. g.:

  • Cutting fluid flooding at low pressure (2 to 4 bar),

  • Flushing (abrasive tools etc.) with 30 l/min at high pressure (up to 100 bar),

  • Extinguishing of sparks with an additional MWF supply at the points of generation (e. g. lower nozzle during grinding).

The shape of the flushing nozzle (pressure, nozzle geometry and correct setting) is also relevant for the cooling effect and the degree of atomization. Flooding with large quantities and low pressures in the close vicinity of the tool is advantageous. By the installation of additional nozzles and their arrangement as "MWF rinsing curtains", mist volumes can be further minimized. It is necessary to correctly adjust and direct the nozzles towards the workpiece/cutting zone area.

Monitoring of the MWF supply

The precondition for an optimum and safe process is ensuring sufficient MWF supply from the very start and during machining. The hazard of tool breakage is thus significantly reduced. For monitoring the MWF supply, switches for high and low pressure, flow control devices or monitoring of the pump motor currents are mainly used.

Generally, a monitoring and a functional test of the sensors (e. g. signal verification) should be carried out by the machine control system. In case of disruption or failure of the MWF supply, the control system induces a separation of tool and workpiece (emergency draw-back) and a shut-down of the machine drive.

Process and tool monitoring

A "hot running tool" or even a tool breakage are considered to be the most frequent causes of a machine fire. A "dull" worn-out drill often causes a sharp increase in temperature at the cutting point and thus may act as a source of ignition. Therefore, tools should be checked for their condition and be exchanged when their service life specified by the manufacturer has elapsed.

By means of process monitoring, the above situations affecting safety due to tool wear can be recognized and the machine stopped in time.

The following possibilities exist:

  • Performance monitoring: Measurement of motor torque requirement at, for example the tool or workpiece spindle. Internal or separate device, easy handling, little susceptibility to malfunctions,

  • Monitoring of the structure-borne noise: Measurement of the sound generated during machining. If a tool breaks, for example, the breakage signal clearly differs from the machining signal,

  • Monitoring of the cutting force by sensors, e. g. piezo-quartz, elongation measuring tapes.