Nya Svetsaren 1, 1999. - Luleå University of Technology

undoubtedly a very costly solutionand one with reduced productivity.Another solution could be toadapt the process to water-plasmaoperation. This would call fora water table, an air muffleraround the torch and a largercutting power source to providethe increased energy that is neededfor under-water cutting. Alsoa solution which calls for high investmentsand has inherent featuresthat will reduce productivity.To the knowledge of the author,none of the solutions has sofar actually been put into operation.Cutting speeds N 2 /O 2Cutting Speed N 2Plate thickness (mm)Cutting Speed O 2Fig. 5. Cutting speeds versus plate thickness for MS for water-plasma cutting withO 2 and N 2 respectively.Speed (m/min)Ergonomics in generalThe development of controls forCNC cutting has brought about ageneral improvement in ergonomics.Just consider the followingexamples – marking equipment,automatic nesting, automaticcutting bridges, colour monitorfacilities, automatic height control,flame control and monitoringby sensors and advanced programmingfacilities.In the case of cutting installations,ergonomic improvementsworth mentioning include automaticplate alignment, tables withtransport facilities for the outloadingof parts and double tables,roller bed arrangements,special overhead cranes, specialarrangements for easy slag removal,all features that have helpedto reduce the heavy physicalwork and the man-hours used permetre cut.Development targetsProductivity in the broadest senseof the word is the driving forcewhen it comes to selecting the developmenttargets for the cuttingprocess as applied in cutting installations.This focuses attentionon the operations that are neededafter cutting, as a result of a moreor less perfect cutting performance.In plain terms, the answersto quality issues such as accuracy,freedom from dross,squareness and bevel quality.In the case of different cuttingprocesses, we find different combinationsof answers to thesequality issues.At the present time, dross-freeand accurately-cut parts are basicallyregarded as the major targetsto aim for, because the cost associatedwith dross removal by postprocessingcut parts is considerable,as is the lack of precision forwelding operations that, more oftenthan not, follows after cutting.Another productivity issuewhich has a direct bearing on thenature of the cutting process isthe minimising of process downtimedue to wear to torch tips,electrodes and nozzles. Systemsfor monitoring, surveillance andfast and precise exchange modesare in demand for all processes.It is also relevant to mentionthe constant design attention thatmust be paid to the interactionbetween the aforementioned developmentof controls and thecorresponding electrical and mechanicaldesign of cutting machines,as regards their dynamicresponse to rapid movementswith up to five degrees of freedom.A very high degree of dynamicresponse is an absoluteprerequisite for realising the potentialadvantages offered by anysophisticated cutting process.Finally, modern IT developments,utilised correctly with advancedsensor technology, havemade great advances in diagnosticsystems possible. This trendwill continue and lead to furtherimprovements relating to preventiveupkeep to minimise downtime,to secure fast troubleshootingand provide input for qualitysurveillance.Water plasmaIn order to reduce the post-processingof cut parts and also toobtain higher cutting speeds atreduced energy consumption, thereis a move towards cutting withoxygen, O 2 , instead of nitrogen,N 2 (Fig. 5).Investigations appear to indicatethat the running costs, interms of the consumption of gas,energy and consumables, end upat the same cost level, whereasthe use of O 2 improves the drossfreerange of plate thickness andpermits a 40% increase in cuttingspeed in 12 mm steel plate. forexample. As the energy that isneeded is lower as a result of theexothermic reactions with O 2 insteel, changing to O 2 increasesthe possible plate thickness for agiven size of power source.When it comes to the precisionand accuracy that can be obtained,it is likely that water-plasmacutting, no matter which gas isused, will lose its foothold tosome degree and laser cutting willbe preferred. In the case of newinstallations, it will be possible totake the special precautions thatare needed to operate a laser-cuttinginstallation, while paying dueconsideration to its environmentalimplications. This will be touchedupon at a later stage.Laser and dry plasmaThe dry processes have the inherentadvantage of being dry, whichmeans that they do not need awater table, a considerably moreexpensive arrangement than a36 Svetsaren No.1 1999