a code of practice 'approved by the director of safety ... · final machine design the layout...

Machines for high explosive component

manufacture - how to specify

Malcolm Gull

A WE Aldermaston

Abstract

Components are manufactured from energetic materials such as high explosive - sohow do you ensure that the machines are Tit for purpose'. AWE have a long historyof machining components from these materials and to date no life threateningincidents haveoccurred, so howistliisachieved. Simply, AWEoperatesavery strictsafety culture that includes the buildings, equipment, operating procedures andmost importantly people.

This paperfocuses on the equipment aspects and more specifically describes

how AWE specify machinetoolsfortheprecisionmacliiningofsecondary explosivesIt gives the case history of a machine that started out in life as a standard computernumerically controlled (CNC) lathe before being modified for use in an explosiveprocess buildingprocess building.

Introduction

Explosive machining is a class of operations that involves the cutting of explosivematerial in specifically licenced process buildings, where it is necessary to providespecial safety features to preclude the possibility of accidental detonation of theexplosives.

All machining of explosives at AWE must comply with the requirements ofa Code of Practice '"approved by the Director of Safety, AWE. This dictates thatall machining of explosives is by remote control in specially licensed processbuildings with the operator housed in a separate, fully protected control room The

Transactions on Engineering Sciences vol 23, © 1999 WIT Press, www.witpress.com, ISSN 1743-3533

136 Laser Metrology and Machine Performance

machining is viewed by means of a remotely controlled closed circuit television(CCTV) link, to give both close up and long range views of the operation and theworkroom.

Machine tools are either specially adapted from 'off-the-shelf types orpurpose built to comply with both the requirements of the MOD (PE) ExplosiveRegulations, PI and AWE'sElectrical Safety Code.w AWE's Electrical Safety Codesupplements the Statutory Regulations currently in force and does not cancel ornullify any of them.

Only those explosive compositions that have gone through a rigorousschedule of Proof Machining Tests as defined in AWE's Code of Practice ̂ areallowed to be machined on these machine tools.

Definitions

Explosives - Any chemical compound or mechanical mixture which, when subjectedto heat, impact, friction, shock, or other suitable initiation stimulus, undergoes a veryrapid chemical change with the evolution of large volumes ofhighly heated gases thatexert pressure in the surrounding medium.

This definition is very broad and for the purposes of this paper must be furtherdefined. Explosives may be separated into two major categories: high and low,categories that are still rather broad. High explosives are chemicals that can detonateand can be classified into primary (those that can be made to detonate very easily),secondary (those that are more difficult to detonate). This paper focuses on themachining of secondary explosive components.

Process Building ^- A building authorised by a competent authority for theproduction, inspection, assembly, breakdown, testing, renovation or repair ofexplosives.

Hazardous Area Electrical Classification - Category B Zone Z which is defined as"An area in which a combustible dust is, or may be, present as a cloud during normalprocessing, handling or cleaning operation in sufficient quantity to be capable ofproducing an explosive concentration of combustible or ignitable dust in a mixturewi tli air."

Safety ProceduresThroughout AWE there is a requirement for facilities to be operated under a formalAuthority to Operate (ATO). AWE's Company Safety Procedures provide theframework of tasks to be undertaken and documents to be created to enable an ATOto be issued. One of the main documents involved in this process is the Safety Casewhich provides the formal demonstration that a safety assessment has been carriedout and that all hazards associated with all operations have been identified and therisks arising from them reduced to a level which is as low as reasonably practicable(ALARP).


Laser Metrology and Machine Performance 137

Explosive hazards and controls

Secondary explosives, although difficult to detonate, are still energetic materials thatcan react violently and as such should be protected from abnormal stimuli orenvironments, including friction, shock, elevated temperatures, pressure withconfinement, impact, deformation, electrical sparks, contamination and contact withincompatible materials.

All explosive machining rooms have ageneral Category B Zone Z classificationwithin the process buildings in which exposed explosives are present. All electrical

equipmentpresentwithintheexplosiveroomsmust comply witlitlieMOD requirementsas defined in the MOD (PE) Explosive Regulations and the Explosives Storage andTransport Committee (ESTC) MOD Leaflet No.7. w

These documents define the level of earthing protection required to preventelectrical discharge whether electrostatic or from electrically supplied equipment. Italso dictates the level of dust protection that must be applied to prevent explosivesdust entering electrical equipment. For instance the Category B Zone Z classificationfor electrical equipment requires that, as a minimum, they must comply with BS EN60529 a for degree of protection 1P65 (In .lex of Protection). Those parts that do notcomply with IP65 must be air pressurised in accordance with BS 5501 ^. Thedocuments also define the dust protection measures that must be taken to preventexplosive dust from entering mechanical mechanisms such as gearboxes, etc.

The machine's exterior surfaces should be customised to prevent ingress ofdust into such areas as slides, screws, drive motors and bearings, the normal sourcesof heat and pinching action and as far as possible eliminate hot spots and pinch-points. ThesedusttmppingsandpincWngsealing or the use of non-cavity fixings.

The documents specify the maximum temperature that exposed surfaces areallowed to reach (e.g. for Category B Zone Z, the maximum permitted surfacetemperature is 55°C and more importantly the maximum allowed under any faultcondition is 100°C).

Any substance that is likely to be exposed to explosives (e.g. lubricants,paints, rubber seals) must be tested for chemical compatibility with all the explosiveformulations intended to be used.

Operation of the machines are from either of two control stations; Local andRemote, which are interlocked to ensure singularity of control. The Local controlpanel is usually adjacent to the machine and has all the controls, including a monitor,necessary for setting-up procedures only. The Remote control panel has full CNCcapability and all the necessary indications to advise the operator of all conditionsof the machine tool and ancillaries relating to machining conditions. This control isusually located in a hardened control room many metres away from the machine.



Additional safety systems are also utilised such as redundant toolpath monitoring systems to prevent any unplanned motion of the tool or workpieceand an integrated system of safety interlocks and fail-safe devices. The interlocksystem is designed to establish three main lines of defence; providing an integratedsafety system with the following features:

1. Prevention of simultaneous operation of both workstations.2. Prevention of personnel access to the machine during CNC controlled

explosives operations.3. Prevents unattended operation of the CNC machine tool during

explosives operations.These safety principles are implemented through the use of mechanical and

electrical interlock keys and through electromagnetic reed relays located on all controlpanels and personnel access routes.

Technical specification

A specification is defined as 'L4 statement of the attributes of a pro duct, process orservice " w. Most importantly the attributes should be measurable and the limits ofacceptance and compliance with the specification should be already defined. Thetype of specification usually employed by the writer is the 'Performance Specification'.This is a method of specifying that is concerned with requirements for performanceand appearance rather than indication standards for quality and workmanship. Thisis not to imply that we are not interested in these two features, merely that we transfermuch of the design responsibility onto the machine tool builder.

There would appear to be three main schools of thought with regards tospecifying the acceptance criteria for machine tools. Firstly there is 'good reputation'concept, this is where machines are purchased on the basis of the manufacturer'sgood reputation with no acceptance criteria specified. Then there is the concept thatonly workpiece test parts are specified. These can be classified as 'standard-type' asdefined by national or international standards organisations or 'non-standard-types'which usually take the form of customer workpieces. Thirdly there is the policy wherethe acceptance criteria include a combination of parametric checks, where the variousgeometric elements of error are measured individually followed by functional checkswhere the machine's overall accuracy and performance capability is measured. It isthis third approach that has been adopted by AWE for all its machine tool purchases.

Case Study - Hardinge HXL CNC lathe

This case study involved a requirement to manufacture precision high explosive (UNclassification 1. ID) parts withi.i an existing explosive processing facility. A CNCHardinge HXL lathe that had been in storage for several years was identified as thepossible solution. Following a study to identify the hazards, a Risk Assessment wasconducted to ensure that the technical specification minimised the risks. It should be



noted thatthefmalcostofthelathe,aftermodifications,wasdoublethatpurchase price - safety has a price.

Final machine design

The layout for the process building where the lathe is sited is shown in figure1. The lathe and Local control are in the machining room with the magnetics cabinetssituated in an adjacent plant room. The Remote control panel in the Control Room issituated forty metres away from the lathe.

lathe

magneticscabinet

Fig.l Facility Layout

The following photos of the lathe demonstrate how the lathe was designedto comply with all the regulations and with all the hazards controlled. Figure 2 showshow the original lathe was configured before all the modifications took place..

Fig.2 Original lathe



Figure 3 shows the final machine after it had been installed into the machineroom. The first things that are apparent are the 'relative' clean lines of the machine tominimise dust entrapment and the lack of interlocked guarding around the machine.

Fig.3 Modified lathe

The guards were removed for smoothness of design and to provide visualaccess to the work area by the CCTV. In order to comply with the PUWER Regulationsit was necessary to make the machine room itself act as the guards. This is achievedby an integrated system of door and control panel interlocks that prevent personnelaccess into the machine room during CNC operations.

The figure clearly shows the access covers, on the machine around the Localcontrol and on the base, secured by a large number of screws (the gaskets are notshown). This is one of the mechanical lines of defence to prevent dust from enteringinto enclosures containing non IP65 components. Ail these enclosures are pressurisedwith clean filtered air to 25 mbar. Figures 4 and 5 show this air pressurisation



arrangement. The large gauge on the front base of the lathe displays this airpressurisation pressure and to the right are the gauges for the main air pressure,collet closure pressure and tailstock air pressure (disabled).

Fig.4 AirPressurisation Fan

Fig.5 Pressurisation inlet / exhaust



Figure 6 shows the lathe inside the process facility. This shows the positionof the two CCTV cameras; one mounted above the lathe to show the work zone andthe other mounted on a mobile stand to monitor the complete lathe including all thecontrols. (It should be noted that all information displayed on the Local CNC displayis also displayed at the Remote control panel). Both cameras are adjustable for tilt,rotation and zoom from the Remote control panel.

Fig.6 Final machine layout

Just above the lathe's work zone is a vertical copper pipe for delivering the waterdeluge in the event of a fire. Behind the lathe there is the black flexible conduit carryingthe power and control cables to the magnetic cabinets in the next room. To maintainthe blast containment characteristics of the machine room this conduit is routed



through the wall via a special blast proof transit housing (see figure 7). Also clearlyseem are the earth bonding cables which connect all metallic items to the main facilitybonding system.

Fig.7 Special transit housing

Figure 8 shows the Remote operator panel housed in the control room. At thetop of the panel, are the two CCTV screens, whilst just below the left hand screen arethe two camera controls. Below these are the two video recorders which automatically

Fig.8 Remote control station



record all the CCTV images. On the right hand panel is the original CNC control panel,which, because this room is away from the machine requires no dust protection.

Summary

This report gives an overview of the AWE process for modifying machine tools forprocessing secondary explosives. It illustrates this process by the case study of aHardinge CNC lathe that was recently commissioned for use in an AWE explosivemachining environment

References

1 "Code of practice for explosive machining at AWE", 19912 MOD (PE) ExplosiveRegulations 19863 AWE'sElectrical Safety Code (AWE/CSP 3/VoI 2)4 MOD (PE) ESTC Leaflet No.7 "Safety conditions for explosive installations

and equipment for buildings and areas containing military explosives"5 BS EN 60529:1992 "Degrees of protection provided by enclosures (IP code)"6 BS 5501 Part 3:1977 "Electrical apparatus for potentially explosive atmospheres"7 BS7373:1991 "Guide to the preparation of specifications"

British Crown Copyright 1999/MOD

Published with the permission of the controller of Her Britannic Majesty'sStationery Office


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