A major street lamp manufacturer was experiencing a problem with an aluminium die casted product that requires eight screws to be driven into the casting in a set and controlled sequence. The difficulty it faced was that, at each screw position, a slightly different torque settings was needed.
The solution that AFTSL – Aluminium Frame and Tool Solutions Ltd – came up with when approached was to supply the customer with a programmable DSM Posi Arm system, linked to an electric screwdriver that is programmable and can be used with torque and angle parameters. The product would travel along on a pallet to the screw-driving station. Here, the pallet would be located in an exact position, so that the same datum position is achieved every time.
The screwdriver is an automatic start unit, this being achieved when the screwdriver is pushed down on the screw head, located on the workpiece.
The Posi Arm counter balances the screwdriver and ensures that the screws always go into the product straight, to prevent cross threads. The screws themselves are self-tapping. AFTSL can programme the screwdriver to drive into a set torque and then rotate at a set angle in reverse, before the final tightening is carried out.
The operator moves the screwdriver to position 1 and drives in the screw. The screwdriver controller controls the tightening sequence and signals back to the arm controller that the fastening was okay/ not okay, according to the pre-programmed parameters. If the fastening is sound, the person using the tool would move to position 2 and repeat the operation.
“We have the ability to reset the parameters, which would happen automatically once the screw point is within 10mm of the centre of the screw hole,” says AFTSL’s managing director Barry Bowen. “Then number 2 screw is driven in. If for any reason, such as the wrong screw length, torque was reached too soon – loose fixing – torque reached outside of parameters and clamped, or torque was not reached at all, for example, then the operator would simply remove the offending screw and try again; or start again on a new assembly.
Were they to try to carry on and move the screwdriver to the next position, the screwdriver would not start, as it would recognise that the previous fastening was not to specification.”
Once all eight screws are located correctly, the count will reset to zero, ready for the next assembly to begin. “We can control clamps from the system, so a component can be clamped in a fixture and released only when the correct amount of screws has been fixed in the controlled sequence and all the right torques,” adds Bowen. “Our customer has bought into this system, as it ensures that the lamps are sealed correctly every time. It enables them to track the tightening, as the castings change due to pin wear during the casting process.”
