RESISTENCE SPOT WELDING
And Squeeze-Type Resistance Spot Welding Equipment (STRSW)

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Resistance spot welding is the most frequently employed welding method in the production process. It is therefore also the method of choice selected for the repair process. 

This method is based on the principle that a resistor heats up when an electrical current flows through it. With spot welding tongs, the panels to be welded are pressed together by the electrodes by means of a lever mechanism. The electrode tips have a rounded shape to keep the electrical resistance low. This assures that a high current can flow through the circuit closed by the electrode tips. The high resistance heats the material until it becomes plastic and melts together. The pressure of the electrodes prevents the material from flowing away.

1 Transformer 2 Electrodes 3 Mechanism for generating the pressure

When the weld spots are set, the electrodes heat up. Over time, this can cause the electrodes to become soft and can blunt the electrode tips (active area) and reduce the welding pressure with detrimental effect on the quality of the weld. In order to avoid this potential problem with the quality of the weld, welding tongs with appropriate cooling capabilities are to be used. This especially applies for the welding of thick or galvanized plates, for which higher currents and greater pressures as well as longer welding times are required. The Spot Welding Tongs required by DCAG have a secondary circuit which is water-cooled for maximum effectiveness (transformer, electrode arms and exchangeable spot electrodes). The welding capacity remains at a constant high level and the active area of the electrodes hardly changes. This assures consistent weld quality on all types of plate within the usual time span.

The cooling effect of welding tongs cooled by compressed air is less effective and is therefore not recommended. To minimize detrimental effects on the quality of the weld, cooling-off phases should be scheduled (e.g. after several consecutive weld spots) whenever the welding tongs are subjected to the above-average strain of welding thick or tempered plates.

(Kx602088)         1) without water cooling		(Kx601244)                2) with water cooling

The transformer is usually located in the welding tongs. Devices with separate transformers lose some of the advantage of weight due of the power supply cables. A higher connected load and fusing may also have to be taken into account.

DaimlerChrysler AG has begun using new high-strength boron steels in select vehicle models. These boron steels are extremely tough to cut and to weld properly. DCAG has therefore specified the newer, Inverter Technology Squeeze-Type Resistance Spot Welders (STRSW) when performing structural automotive collision repairs according to Mercedes-Benz repair procedures. MBUSA, LLC introduced two (DCAG approved) models in the 4 th Quarter 2003 and one additional (DCAG approved) model in the 2 nd Quarter 2004. These three are the only approved units which are qualified for the MBUSA Collision Center Certification Program.

To summarize the selection of the three DCAG approved models/brands 3:

No.	Manufacturer or Brand	Model	SSEP Order Number3+4
1.	Elektron USA1	Elektron: Mi 100	112-515875
2.	Tecna via Cebotech	Tecna: 3650	505-3650
3.	W+S2 via RAE	W+S: ATL-W	521-ATLW

1 see note below concerning specific acceptable model Elektron: M83 units 2(W+S = Wielaender+Schill; RAE = Reliable Automotive Equipment 3 Available via the MBUSA Standard Service Equipment Program call toll-free: 1-888-458-4040. 4 See attached Excel spreadsheet for specifications and a scope of delivery comparison.

Important: It is strongly recommended that an initial site inspection be performed by the equipment manufacturer to determine if your shop has the necessary electrical power supply (voltage and amperage) available to run these STRSW.

(Kx605483) Sample plate (unbuttoning test)		( Kx600468)

QUALITY ASSURANCE AND UTILITY VALUE RETENTION:

In addition to quality testing by means of the unbuttoning method described above, particular attention must be paid to the condition of the electrodes. The electrode surfaces must be checked for cleanliness and roundness before use (reworking the tips with an electrode cutter may be required). Satisfactory welds cannot be produced with flattened or burnt-out electrodes.

A sample panel should be spot-welded to check the quality of the weld spots and the settings of the welding tongs (pressure, amperage, welding time). The welding sample can be clamped in a vice and the plates separated using pliers (unbuttoning test). If the weld spot on one plate remains intact and a matching hole is produced in the other plate, then the weld is considered satisfactory (see figures above).

NOTE: Before purchasing any spot welder, it is strongly recommended that an initial site inspection be performed by the equipment manufacturer to determine if your shop has the necessary electrical power supply, e.g. voltage and current (amperage), available to run these STRSW. This ensures that the power supplied by the shop to the welder is sufficient and is designed to handle the high demand of the connected loads.