Aerospace fabricators apply digital control to hydraulic stretch former
Aerospace fabricators apply digital control to hydraulic stretch former
Keywords: Siemens, BHW, CT Systems, Stretching
A highly specialised forming machine, designed to fabricate airframe components and fuselage skins, has recently undergone extensive development at Wigan based BHW (Bellhouse Hartwell Aerospace Group) (Plate 1).
The Loire stretch former is used to produce skins for engine cowlings,reverse thrusters, wing and fuselage components. This hydraulically operated machine is capable of movement in eight axes and uses large, winged carriages which stretch metal sheet over specially constructed tools, held on a central beam. With full order books from many of the world's leading civil and military aircraft manufacturers, BHW needed to improve the accuracy and repeatability of control of the Loire machine. This was a particularly important aspect for orders placed for duplicate parts, required some months after the originals had been manufactured. In order to reduce set-up time and provide accurate matches of the components produced originally, some form of intelligent control was vital.
Plate 1 Special front panel of the Loire eight axis system
BHW approached Huddersfield machine tool specialists, CT Systems (UK) Ltd for ideas. It was decided that the addition of digital CNC control would improve the stretch former's performance significantly. An advanced, multi-axis controller was needed, but it was important to retain both flexibility of operation and simplicity of set-up. CT Systems decided to add a Siemens Sinumerik 840D integrated with hydraulic HLA Simodrive modules. As CT Systems' director Chris Haigh explained:
The 840D is a multiple axis machine control system. We were able to provide six axis movement of the carriages plus the additional two axis positioning needed on the central beam, which tilts from end to end.
With so many axes to control, connecting sensors and servos would usually involve a large amount of wiring. However, by using a Profibus communications network and Siemens ET200 I/0 modules, the amount of hard wiring was said to be reduced significantly. The BHW system is novel in the UK because it is believed to be the first machine tool to integrate Sinumerik 840D control with hydraulic servo movement. While the trend for electric motors is gathering pace in the machine tool market, the Loire equipment was already fitted with hydraulics prior to the upgrade. In addition, CT Systems felt that electric motors would be unable to provide the 400 tonnes force required when stretch forming material.
Like many highly utilised machine tools, the Loire stretch former was in almost constant use at BHW before the upgrade took place. In order to minimise down time during the addition of the digital CNC control, development work and programming were completed off line, as much as possible. The incorporation of Profibus is thought to have played an important part by reducing development time and allowing the use of distributed analogue modules for all the machine sensors.
A number of the machine tools functions available with Sinumerik digital control have now been added to BHW's system, including diagnostics, part programming and the ability to store large amounts of production data. "Storage capacity is particularly important for batch manufacturing" said Mr Haigh.
Details available from: Siemens Automation & Drives. Tel: +44 (0) 161 446 6645; Fax: +44 (0) 161 446 5280; E-mail: white@plcman.siemens.co.uk;Web site: http://www.siemens-industry.co.uk
