Spark Erosion Machining
INTRODUCTION
Spark erosion machining (also referred to as ‘die sinking’) is an electrical discharge machining (EDM) process employed for the production of high tolerance complex components and machine tools encountered, for example, in the aerospace and medical sectors and in the plastics industry.
The process involves the generation of sparks beneath the surface of a dielectric fluid which is flammable and is therefore inherently hazardous. The quantity of dielectric fluid present can range from a few litres on small machines up to several hundred litres in large machines. In some instances, several machines may be linked to a single dielectric system. Dielectric fluids were originally petroleum based, but more recently a range of synthetic dielectric fluids have become available with flashpoints in excess of 100⁰C.
Modern spark erosion machines are both complex and costly and need to be used to maximum effect. Allied with the fact that spark erosion machining is a slow process, it is common for machines to run unattended for long periods, including overnight.
RISK ASSESSMENT AND CONTROL
Spark erosion machining and allied EDM processes are covered under RISCAuthority RC29: Recommendations for spark erosion machining, which provides full details of the hazards involved and the fire precautions which should be taken, to which all Consultants should refer.
The key measures in RC29 are summarised as follows:
Operation of spark erosion machines included as important consideration of the fire risk assessment.
The provision of appropriate fire detection and suppression systems, particularly where machines are left running unattended for prolonged periods.
Where possible, the provision of fire compartmentation to the spark erosion area or the removal/protection of combustible construction and linings in the immediate vicinity of the machines.
Selection of dielectric fluid with a flashpoint as high as possible, ensuring that the working temperature of the fluid is at least 30⁰C lower than the auto-ignition temperature of the fluid.
Machines installed, operated and maintained in accordance with the manufacturer’s instructions for which staff should receive appropriate instructions and training. Particular attention should be given to ensuring that float switches, thermostats and other safety devices are functioning correctly.
Critical machine safety controls to be provided including, where appropriate, the following:
A low fluid level shut-off device, normally in the form of a float switch. (preferably duplicated).
Pulse monitoring and correction facilities (also referred to as anti-arc monitoring). This is essential for machines which are to operate automatically with minimum supervision.
A thermostat (preferably duplicated) to close down the machine should the temperature of the dielectric fluid rise above a predetermined level – normally set at least 10⁰C below the flashpoint of the fluid.
A back-off switch to ensure that the upper electrode does not rise above a pre-set distance above the workpiece, preventing it from reaching the surface of the dielectric fluid.
An anti-arc control device.
Further details of these safety controls are contained in RC29. In the case of modern machines, the first three items in the above list are normally incorporated as part of the standard safety facilities. As regards the remaining two items, whilst these may be encountered on older machines, on newer machines they have in some cases been superseded by the development of sophisticated anti-arc monitoring technology.
A typical modern spark ersosion machine
A typical spark erosion machine with fitted with a ‘Firetrace’ automatic fire suppression system. At the base of the machine head can be seen the red detection tubing, with the CO² discharge horns above.
Firetrace Ltd has a range of automatic fire suppression systems for the protection, for example, of all types of automated machinery and has worked closely with some of the leading manufacturers of spark erosion machines. The Firetrace system employs linear detection tubing which is installed throughout the risk areas of the machine, which activates automatic discharge of the chosen extinguishing agent - normally CO2. The system is pneumatically operated negating the need for complex electronic detectors or control panels and, as such, can be very cost effective – average installation expenditure <£2500.
SURVEY EXPECTATIONS
Consultants should remain alert to the presence of spark erosion machining, which will be commonly encountered in precision engineering risks. Where this is found, risk control considerations should be based on the provisions within RC29 and particular care taken where machines are run unattended for long periods, particularly overnight.
Details of such operations must feature in the survey report and the appropriate risk improvements raised, which in all but the smallest of cases should include the provision of an appropriate automatic fire suppression system integral to each machine. This is very important for machines which are left running unattended overnight, which in certain circumstances may also give rise to the need for an appropriate fire detection system or, in the case of multiple machines, possibly sprinklers to be installed.
When raising a risk improvement for automatic fire suppression, it is important that the standard RiskSTOP wording is employed which requires that the system be designed and installed by a company which is approved by a suitable third party certification/registration scheme, of which the Loss Prevention Certification Board (LPCB) LPS 1204: Requirements for firms engaged in the design installation, commissioning and servicing of gas extinguishing systems and the British Approvals for Fire Equipment (BAFE) SP 203-3 schemes are examples.
In cases where there are doubts as to the extent of in-built safety controls incorporated, particularly where dealing with older machines (circa pre-1990), suitable risk improvements should be made for which standard wordings are available.
Recognition must also be given to any client specific instructions and conditions relating to unattended machinery operations which should be paramount.
Where automatic suppression systems are encountered, Consultants are reminded of the need to ensure that these are correctly maintained, including annual servicing by a competent engineer.
FURTHER REFERENCE