Paul Hilton is Technology Manager - Lasers at TWI where he has responsibility for the strategic development of laser materials processing. As such he has been instrumental in the setting up and management of several European collaborative research projects. Dr Hilton has previously worked in the laser systems industry in the UK, and before that, was a researcher at an international scientific institute in France.
In common with all other manufacturing processes, the laser must be able to produce a technically viable process at an economically acceptable price. Paul Hilton set out to develop spreadsheets software capable of performing economic evaluations of laser welding systems for user defined applications.
The laser is a very precise and controllable energy source, which offers great flexibility in its usage. The capital cost of laser systems is often very high and the relative inefficiency of converting electrical power to optical power means that lasers are not often used as bulk heat sources. The applications in which (high power) lasers are used, mostly rely on the advantages a focused laser beam offers such as, localised low heat input, high process speeds, narrow treatment area, no tool wear and ease of automation.
The advantages and disadvantages mentioned above mean that the laser is best suited to high energy density processes such as cutting and welding. Efficient time usage of the equipment is also necessary to offset the capital investment. In order to construct an economic profile of a potential application for laser processing, it is necessary to consider the factors involved in the purchase and operation of the chosen laser system and their interactions. Such an analysis should have the capability to assess the capital, fixed and variable costs of owning and operating the system and to provide information on the cost of the parts produced.
Objective
To develop spreadsheets capable of performing economic evaluations of the use of laser welding systems for user defined applications.
Scope
In this project two spreadsheets have been developed in Microsoft Excel, which provide information to persons wishing to assess the costs involved in the purchase and operation of CO 2 or Nd:YAG laser based welding equipment. For sets of given input parameters the spreadsheet calculates the cost per hour of operation of the system, the cost per part produced and the cost per metre of weld produced. The costs are assembled from estimates of the fixed and variable costs associated with the chosen system, its operation and the production rate required.
The spreadsheets
One useful tool for this type of analysis is a spreadsheet which provides the user with a mechanism for assessing the economic implications of using lasers, and which can be tailored to the applications concerned. A benefit of this approach is the ability of the spreadsheet to provide 'what if' solutions to changes in input variables. In this way the relative importance of factors affecting cost per part can be established.
This report summarises the development of two such spreadsheets, produced specifically to evaluate the economic aspects of welding with high power laser sources. The first is suitable for systems based on carbon dioxide lasers, with powers in the range 1-45 kW and the second can be used to assess the economics of the newer generation of high power lamp pumped Nd:YAG solid state lasers, in the range from 1-5kW.
These spreadsheets can be used as stand alone items or their output can be used as input into the more general 'Process cost estimator' spreadsheet developed under CRP project 9803-11, Process cost modelling. In this way it becomes possible to compare the economics of the laser process, with other processes, such as arc welding, for a particular application.
The spreadsheet requires a series of inputs from the user and performs a series of calculations based on these inputs. For clarity, those cells requiring an input by the user have a white background and those cells containing the results of a calculation have a yellow background (or grey as in this report). The spreadsheet depicted contains default information for data on both CO 2 and Nd:YAG lasers and their use, but this data can be changed by the user to input, for example, running costs supplied by a particular laser manufacturer.
The spreadsheet is arranged assuming that a particular application or series of applications will be examined. As a result, the first input to the spreadsheet asks for general information on the application for which costs are being calculated. This information is not used for calculations but reminds the user/reader of the application to which the spreadsheet refers. The next series of inputs defines the production requirements for the application, the working practices of the company eg available hours per day etc, processing length per part and load unload time per part. The spreadsheet then calculates the required processing time per part and the minimum required processing speed that will meet the production requirement of parts per year or parts per day ( Fig.1).
A knowledge of the type of laser which will adequately provide the combination of required process speed and weld penetration is now required, along with a knowledge of its operation and use of consumables. If the user has no knowledge of the performance of high power lasers, then the input of TWI staff or a laser manufacturer will be required. It could also be the case that the production requirement and thickness combination precludes the use of a single laser source. In this case the user can change details of the working practice ( eg increase from one to two shifts per day), or increase the number of laser workstations.
Laser specific data are then entered which allow the spreadsheet to calculate the on-time of the laser in the processing cycle, and the processing cycle time. The spreadsheet then calculates the utilisation of the system as a percentage of the yearly available process time. This enables the user to establish early the efficiency (and potential unused time) for the system. At this point the user inputs the percentage of the fixed costs to be apportioned to this application, in future cost calculations. This enables the user to study a system producing several different parts within a working year and spread the fixed costs however required.
Costs are calculated under two categories; variable and fixed. In these spreadsheets variable costs include the running costs associated with the application, such as gases, electricity and the fixed costs include labour, maintenance, floor space costs and depreciation, the latter being calculated based on an assessment of the capital cost of the equipment required/used ( Fig.2).
Default data on the variable costs of operating a 6kW CO 2 laser and a 4kW Nd:YAG laser are included in the spreadsheets. The user can change any of these data based on information provided by laser manufacturers, gas suppliers, utilities companies etc. Variable costs relating to the use of the laser, gas utilisation in beam lines and at the process, and operating the workstation, can all be catered for. It is also possible to attribute pre-process costs and post-process costs per part if required, in this section. This part of the spreadsheet concludes by listing the total variable costs, calculated per hour of operation, per part produced, per metre of weld made and for the particular application being considered.
Fixed costs are input under the headings of labour, maintenance, depreciation and floor space. Provision is made for the cost of a system operator and any auxiliary labour required in terms of hourly rate and overhead factor (if required). The user can now calculate the capital cost of the equipment being used either as a single sum or as a sum built up from an itemised list of system components. A depreciation figure calculated linearly over a user-defined number of years provides an annual contribution to the fixed costs. Maintenance costs can be input in two ways. The first assumes the costs are proportional to the capital investment and a percentage rate can be input. Alternatively, an annual fixed maintenance sum can be included in the calculations. Floor space costs are simply entered as cost per square metre and total area required. This section of the spreadsheet concludes in a similar way to the variable cost section, with a listing of the total fixed cost per year, which is then apportioned to the particular application via the figure entered earlier in the spreadsheet, to provide the total fixed cost for the application and the fixed cost per part and per metre of weld ( Fig.3 and 4).
A cost summary is then provided which lists all the variable and fixed costs in terms of cost per hour, cost per part, cost per metre and cost per application. This section ends by adding variable and fixed costs and presenting these per hour of operation, per part and per metre of weld.
Two additional sections then provide a listing of the total costs related to the particular application and the ratio of these cost elements to the total cost for the application.
The spreadsheet concludes with an application synopsis, which can be printed as hard copy on a single sheet, and summarises the application providing the cost per hour of operation, the cost per part and the cost per metre of weld.
Default data and validation
The default data entered into each spreadsheet have been established via discussions with laser manufacturers, gas suppliers, system builders and utility supply companies. The values are typical of costs incurred in the UK and were validated in 2001.
To validate the spreadsheets several examples were run by several project leaders and the results compared. A single example for each spreadsheet was calculated manually, based on the software formulæ. In addition, for a specialist user, it is possible to configure another spreadsheet, specifically designed for CO 2 laser cutting, to simulate welding. When the same system and component were analysed this way, the resulting costs per part differed by less than 2% ( Fig.5).
Conclusions
Two spreadsheets have been developed to enable users to evaluate economic aspects of high power laser welding. One is tailored to the use of high power CO 2 lasers and the second to the use of high power Nd:YAG lasers. They can be used to assess the contributions to the operating costs per part and per metre of weld produced, from the various fixed and variable costs associated with a particular application. The spreadsheets are also useful in providing rapid estimates for a change in cost per part when varying selected input parameters.
The spreadsheets can be used independently or to provide input to the Process cost estimator software developed in project 9803-11.
Acknowledgements
The concept for the type of spreadsheet presented here was originally developed in the EUREKA project EU194 on 'Applications of high power CO 2 laser processing'. Sylvia Nasla, Gerard Parsons and Steve Riches all contributed to the development of the spreadsheets.
The majority of work undertaken to produce the spreadsheets featured in this report was funded by the Industrial Members of TWI, through the Core Research Programme.
