An ingenious machine that has driven down the cost of manufacturing glass and steel enclosure doors incorporates a single stepper drive with pneumatics to produce a finished product every four minutes.

Phil Howlett, a director of Helbar Automation, says the project began several years ago, when a company making the doors invited adhesive manufacturer Loctite to come up with a better way of bonding glass to the steel sides.

At the time, the customer was bonding glass to powder coated steel using polyurethane. The adhesive was difficult to apply, the process was messy, and the bonded products needed to be stored for three days to cure. This had to be accomplished in such a way that there was no possibility of the components moving with respect to each other until the cure was complete. Unhappy with this state of affairs, the customer wanted a faster process and one that could be completed on the bonding machine.

Loctite quickly came up with a commercial product that could be cured by ultraviolet light in 30 seconds, and contacted Helbar Automation with regard to the supply a suitable machine. Meanwhile, the adhesive specialist began a test programme, making a few bonded doors by hand. The doors were temperature cycled and one was even sent on a trip by cargo ship to test its response to extremes of temperature and its resistance to handling damage.

Having perfected the bonding process, the requirement now was to exploit it in a production environment. The brief to Helbar highlighted the fact that the customer was making 24 different sizes of door, in varying lengths from 1.25 to 2.4m and widths of 600 or 800mm, and in batch sizes as small as one. So the customer wanted programs to make doors of any size, selectable from a touch screen, in such a way that no damage would occur should the wrong program be selected for the parts actually on the machine.

In order to save space, it was decided that 365nm wavelength metal halide curing lamps and adhesive applicators would be mounted on a gantry, which would move along the length of each door as opposed to having doors move through the machine. One of the design considerations was that the machine had to have a completely open bed, so that metal sections and glass could at some time in the future be robotically placed on it. In theory, curing could have been achieved using static lamps, but they would then have been very long and consequently expensive. The resulting machine with its moving lamps is said to be first to be so equipped in Europe.

Metal side pieces are placed on the machine bed, and held in position by clamps mounted on SMC pneumatic actuators which turn and pull down. Adhesive is then placed on the steel by traversing the gantry, which is driven along the length of the machine by a stepper motor and toothed belt. The stepper motor and driver were both supplied by Parker Digiplan. Electrical engineer Mike Hodnett says Helbar uses stepper or servo drives, according to whichever seems to be most cost effective for a particular application. Customers can specify suppliers or leave it to Helbar to select components from reputable suppliers. Control is via a Siemens S7 plc with a Siemens Simatic touch screen.

Glass is placed and pulled down on to the adhesive beads by Festo vacuum suction cups. The lamps are then switched on automatically and the gantry reverses over the length of the door. Damage to the machine from any attempt to make a different sized door than would be possible with the parts supplied is prevented by employing a large number of sensors to ensure that metal and glass is encountered where the program expects them to be and nowhere else. Festo pneumatic cylinders change the machine bed width for different door widths and also raise and lower the working gantry on Unimatic slides. Damage would certainly occur if they were to encounter an obstacle, so capacitative sensors are used to detect metal and infra red optical sensors to detect glass. Measurement is also made of suction vacuum at each cup to ensure there are no cups sucking down non-existent glass.

The remaining problem was how to ensure operator safety. Ultraviolet light is damaging to eyes and skin, yet access to the machine is needed to place steel sections and glass in position and to remove completed doors. The solution involves placing each machine in a manufacturing cell walled round with ultraviolet light opaque polycarbonate. Light beams inside the cell but outside the machine prevent it operating if a human worker is detected. Safety logic is enforced by three special Pilz PNOZ safety relays. A model X3 is associated with the emergency stop button; another X3 works with the machine guards; and a model X4 links with the light curtain.

www.helbar-automation.co.uk http://www.honleuv.co.uk

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