Maintenance of mechanised arc welding equipment and process fault finding

Bulletin Vol.27, Issue 4, April 1986

J Weston

John Weston, CEng, BE, AIM, MWeldI, is head of the Automation Section in the Production Systems Department.

To ensure continuous production, mechanised arc welding equipment must be maintained in first class condition. This article provides a suggested 'Preventive maintenance schedule' for users of mechanised and robotic arc welding equipment. MIG/MAG process fault finding guidance is also provided which leads the operator from the weld defect to the likely cause.

The advent of robotic welding systems has brought an increased, and indeed essential, need to consider maintenance. Planned preventive maintenance becomes a must. Furthermore, knowledge of likely causes or sources of poor welding conditions is also required so that emergency maintenance can be applied to rectify unforeseen problems quickly.

Comments on the subject, made in 1971 ¹ , are today still relevant and worth repeating: 'Automatic welding ... should have as much emphasis placed on it as, say, on machining to fairly close tolerances'; Strict control on the shop floor is imperative involving programmed changes for contact tips and wire drive wheels, and preventive maintenance...'; 'Changing (contact) tips compares with throwaway tungsten carbide tool tips - throwaway following a present amount of production'.

The capital cost of robot welding systems dictates that they should work as continuously as possible to ensure a satisfactory return on investment. Use of correct maintenance procedures helps ensure that this requirement is met.

Preventive maintenance

In preventive maintenance scheme the system as a whole must be considered, the units within that system and the components of each unit. In a typical installation, the units involved include the controller, the robot, the workpiece manipulators, the jig and fixtures and the welding equipment. The components comprising a wire feed 'unit' are, for example, the wire feed mechanism, the wire feed control, the wire feed rolls, the conduit liners and the umbilical for services.

A suggested preventive maintenance schedule, (Table 1) considers these items in terms of their shift , daily, weekly or longer interval attention requirements. This listing is offered as a guide which must be modified according to the specific recommendations of the equipment suppliers and in the light of practical user experience. As examples, the supplier may require that the motor unit on the robot be checked biannually and the user's experience may indicate that the contact tips be changed twice per shift.

Table 1 Preventive maintenance schedule

Emergency Maintenance

For example, a displayed error 'E937 X AXIS POSITION ERROR' might, in the manual, be diagnosed as: 'increase in the motor load, or a wiring failure, or an obstruction to the robot motion'. The operator is then referred to the maintenance manual for the detailed checking and fault elimination procedures. With some machines the error codes may also relate to process faults. 'E136', for example, might be 'low shielding gas' and the operator is directed to 'check the gas regulator for setting and for the gas volume remaining'.

At present, most mechanised and robotic welding systems use the MIG/MAG welding process and the 'Emergency maintenance guide' presented in Table 2 is biased towards this process. These lists together with additional comments, see Appendix, have been developed from earlier studies, ^2-4 general Welding Institute experience and specific knowledge gained from the Institute's work with robotic welding systems.

Table 2 Emergency maintenance checks

Specific problems

The high and nearly continuous arc duty cycles of robot work cells have highlighted the need for regular attention to welding plant. In this section several specific problems are discussed.

Copper particles, flaked, or scraped from the welding wire, have been known to build up to such and extent that arcing has occurred in the vicinity of the feed rolls causing serious damage to the equipment. These particles can also be drawn into the wire feed conduit/ liner, leading to erratic wire feed, defective welds, and ultimately stopping the process.

Changing to a copper free wire may not completely avoid this problem as reports have been received (e.g. ref. 5 ) of accumulations of residual drawing compounds sufficient to block the conduit. Regular cleaning and blowing out of liner and wire feeder will help avoid these problems.

Experience has shown the importance of ensuring accurate alignment of the wire feed system components and this has been highlighted in ref 5 and 6. When combined with correct feed roll pressure and continuous support of the wire, the feed should be positive and particle accumulation problems minimised.

Setting correct roll pressure still poses problems and is largely a matter of experience. One recommendation ⁶ suggests increasing feed roll pressure until the welding wire cannot be stalled by squeezing the wire between the fingers. (This should only be done in a 'no weld' condition.)

If roll pressures are too low the wire may slip at the rolls leading, for example, to burning back. On the other hand excessive pressure may lead to flattening of the wire, again giving feeding problems. Should a wire jam and a stoppage occur, then all the wire between the feeder and the contact tip should be replaced.

Particular care is required when setting feed roll pressure for flux cored wires: these consumables are particularly prone to deformation when pressure is excessive.

Wire feed problems often arise as a result of poor arc starting but the use of high voltage arc initiation systems ⁷ should lead to improvements.

The high operating duty cycles and rapid motions involved in the operation of robotic systems can cause nuts and screws to loosen leading to many faults, e.g. loss of positional repeatability, and cyclic flexing of conduits and cables often leads to more rapid wear than that experienced in equivalent manual operations.

Summary

The advent of high capital cost mechanised and automated arc welding systems has made use of preventive maintenance schedules an economic necessity. Work plans which minimise the down time resulting from unscheduled faults must also be followed and it is hoped that the guide tables presented here will help in both these areas.

Acknowledgements

The assistance of colleagues at The Welding Institute in compiling this article is acknowledged. The work was financed by Research Members of The Welding Institute and the Materials Chemicals and Vehicles Requirements Board.

References

Appendix

Causes of common faults in MIG/MAG welding

Porosity

1. Incorrect gas flow for the nozzle size. Nominally 10-30 litre/min. Small nozzles require lower flows. If too low, there is insufficient gas to give adequate shielding. If too high, gas flow aspirates air and causes porosity.
2. Mechanical disruptions to the gas supply, e.g. hoses kinked by robot motion, solenoids fluttering, detective gauges or pipes of too small a diameter, may all lead to low gas flows and porosity.
3. Gas leaks in the system aspirate air and cause porosity. To check: cover and/or block the shroud and look for continued gas flow from cylinder, this indicates leaks.
   These leaks may be in any part of the gas system.
   The gap where the wire enters the gun liner may be excessive and air may be entrained here as the wire feeds.
4. Blocking of the gas shroud with spatter during operation can cause nil, turbulent or asymmetric gas flows giving rise to air entrainment and porosity.
5. Asymmetry in the torch by design, or resulting from damage or poor construction may give uneven gas flows, air entrainment and porosity.
6. Cutting or shaping of gas shroud is not a good principle and may lead to uneven gas flows and porosity.
7. Asymmetry in the preparation, or of the taught path within the preparation, may contribute to porosity problems by giving poor access and gun attitudes, causing poor gas flow patterns.
8. Long gas shroud to weld pool distances are not good. (For solid wires 12-15mm is average and 20mm is long.) At long distances gas flow is easily disrupted.
9. Random draughts can disrupt gas flow and cause intermittent porosity. For example, opening of doors or windows, grinding on adjacent plates, fan heaters cutting in. Excessively powerful fume extraction may suck away shielding gas.
10. Erratic arc behaviour or surging arcs can pump gas flows causing entrainment and porosity. Check wire feed, earth connections, contact tips.
11. Large long weld pools may become unshielded in the freezing region and the use of trailing shielding systems can help.
12. Small quick freezing weld pools can trap gas which would normally be diffused from larger weld pools.
13. Plate cleanness is important in avoiding porosity, keep grease and moisture free. Paints may also cause porosity. Surface grinding in the preparation and for 25-50mm on the plate adjacent to the preparation before welding is recommended.
14. Wire cleanness is important, keep free of oil, moisture and rust to avoid porosity.
15. Welding gun angles have a noticeable effect; they are usually normal to the welding plane or up to 10° leading angle. When porosity occurs, use of the leading angle to push the gas flow in front of the weld head can give benefits.
16. Gas blow (porosity) may come from backing bars, backed joints or fillets. Even on clean plates the rate of gas build up can be sufficient to cause porosity in weld pools. Cure of this problem is difficult but best achieved by ensuring that the mating surfaces are freshly ground.
17. Gas entrainment in the root runs may be checked by grinding out or radiography. Subsurface root porosity can rise through the whole of a multi-run weldment.
18. Gas flow may be made more uniform by using a gas diffuser within the nozzle (as used in TIG equipment). Stainless steel wire wool may be used to give this action although care must be taken not to earth the contact tip to the gas shroud.
19. Gas flow insufficient before the arc strikes or after the arc is extinguished.
20. Water leaks in water cooled guns.
21. Excessive use of anti-spatter compounds on the work or on the welding gun.

Erratic wire feed

1. Dirty/damaged contact tips: these increase friction on the wire especially as the wire heats, leading to erratic feed.
2. Damaged/blocked and/or sharply bent liners: these also increase friction, again leading to erratic wire feed.
3. Kinks in the welding wire: these give problems in feeding through the rolls and give frictional problems in liners and contact tips.
4. Wire reel brake too tight: again the increased friction may lead to erratic wire feed.
5. Wire crossed on wire reel: this may result from the wire reel brake being too loose allowing unspooling of the wire which crosses: in severe instances unreeling is impossible.
6. Use of old/rusty wire: again a frictional problem: rusty wire may also cut and increase wear on liners.
7. Wire feed motor faults: any erratic behaviour in the wire feed motor is directly translated to the wire feed itself.
8. Wire feed system misalignment: this too increases friction in the system. Misalignment may also lead to scraping of the wire surface which removes copper or wire drawing coatings, blocking the system and causing erratic feed.Shaving of the wire, particularly in soft aluminium wires, may lead to problems in current pick-up.
9. Damaged or worn feed rolls: these may result either in increased friction or in lack of contact with the wire, resulting in intermittent wire feeding.
10. Roll pressure incorrect: if pressure is too low, slip occurs and feed becomes erratic. If pressure is too high, the wire may be flattened, the flats in turn jam against the liner and contact tip causing erratic feed.
11. Worn contact tips: when the hole in the contact tip becomes large current pick-up becomes erratic, possibly leading to arcing and welding of the wire and tip within the tip bore.
12. Undersized contact tips increase friction leading to erratic feed.