General Welding Precautions
General Welding Precautions
General
For ease of reference, the diagrams show only the type of weld used in repair where it varies from that used in production.
The replacement welds in the welding diagrams are denoted by the following symbols:
1. = Single/Multiple thickness plug welds
2. = MIG seam weld
When carrying out welding operations the following criteria must be observed:
- Where resistance spot welds have been used in production, these must be reproduced with new spot welds in replacement where possible. All such reproduction spot welds must be spaced 30 mm (1.181) apart;
- When spot welding, it is recommended that test coupons of the same metal gauges and materials are produced to carry out peel tests to ensure that welding equipment being used can produce a satisfactory joint. Plug welds must be used if a satisfactory spot weld cannot be produced;
- The electrode arms on hand-held spot welding guns must not exceed 300 mm (11.811) in length;
- Single-side spot welding is not acceptable;
- Brazing and gas welding are not acceptable EXCEPT where they have been specified in production;
- Where 3 metal thicknesses or more are to be welded together it is imperative to use MIG plug welds to ensure joint strength;
- MIG plug welds must be used in repair joints where there is no access for a resistance spot welder. To replace each production spot weld, an 8 mm (0.314) approx. hole must be drilled and/or punched, and a MIG plug weld then made in its place. The number of plug welds must match exactly the number of spot welds which have been removed;
- Where holes are left in an existing panel after removal of the spot welds, a single MIG plug weld will be made in each hole as appropriate.
Electronic Control Units
The electronic control units (ECU) fitted to vehicles make it advisable to follow suitable precautions prior to carrying out welding repair operations. Harsh conditions of heat and vibration may be generated during these operations which could cause damage to the units.
In particular, it is essential to follow the appropriate precautions when disconnecting or removing the airbag DCU.
Equipment
Prior to commencing any test procedure on the vehicle, ensure that the relevant test equipment is working correctly and any harness or connectors are in good condition. This particularly applies to electronic control units.
Seat belt anchorages
Seat belt anchorages are safety critical. When making repairs in these areas, it is essential to follow design specifications. Note that High Strength Low Alloy (HSLA) steel may be used for seat belt anchorages.
Where possible, the original production assembly should be used, complete with its seat belt anchorages, or the cut line should be so arranged that the original seat belt anchorage is not disturbed.
All welds within 250 mm (9.842) of seat belt anchorages must be carefully checked for weld quality, including spacing of spot welds.
WARNING: Body parts incorporating seat belt anchorages MUST be renewed completely if damaged beyond repair, as the welds in these areas are safety critical and cannot be disturbed.
High Strength Steels
Body panels are being increasingly manufactured in high strength steels to meet design requirements for safety and weight saving. As panels in high strength steels cannot be visually identified by the repairer, and as they can be more sensitive to excess heat than would be the case with low carbon steels, it is advisable that the following procedure be observed at all times.
While individual repairs will differ in detail, the following Panel Replacement Procedure has been devised placing emphasis on ease of repair and the elimination of unnecessary work. Where replacement of a particular panel involves departure from the Panel Replacement Procedure, a note to that effect is included in the relevant panel replacement operation.
Straightening
Whenever possible, structural members should be cold straightened under tension. Do not attempt to straighten with a single pull but rework the damaged area using a series of pulls, releasing tension between each stage and using the opportunity to check alignment
Body jig
Unless damage is limited to cosmetic panels, all repair work to body members must be carried out on a body jig, to ensure that impact damage has not spread into more remote parts of the structure. Mounting on a jig will also ensure that the straightening and panel replacement procedures do not cause further distortion. If original dimensions cannot be satisfactorily restored by these methods, damaged structural members should be replaced.
WARNING: Damaged areas should be cut away using a high speed saw, NOT an oxy-acetylene torch.
As a rule, body dimensions are symmetrical about the centre line. A good initial check for distortion is therefore to measure diagonally and to investigate apparent differences in dimensions.
Inspection
Every accident produces individual variations in damage. Each repair is influenced by the extent of the damage and the facilities and equipment available for its rectification. Most accident damage can be visually inspected and the approximate extent of damage assessed. Sometimes deformation will extend beyond the directly damaged area, and the severity of this must be accurately established so that steps can be taken to restore critical body components to their original dimensions. An initial check can be carried out by means of drop checks or, preferably, trammels. Gauges are available which will accurately check for body twist. Where repairs necessitate renewal of a critical body component it is recommended that a body jig is used.