Basic Knowledge For Spot Welding

Picture 1. Spot welding machine

(source : http://www.dmsf.ust.hk/spot_welding/spot%20welding.htm)

Basic Knowledge For Spot Welding by Welding Engineer DIPL.-ING. H. KRUEGER

Compiled and rewritten by Welding Inspector Muhammad Irsyad Junaedi, A.md

Content :

1. Welding Theory

2. Materials

3. Electrodes

4. Welding Machines

5. Machine control settings

6. Testing of spot welds

7. Machine qualification

8. Process certification

9. In-production tests

10. Safety requirements

11. welding mistakes

12. maintenance

1. welding theory

Welding is joining parts by heat and/or pressure. Resistant welding machine (simplifie)

Picture 2. Spot welding machine (block diagram)

(Source : http://www.milltech.in/spot_wielding.htm)

The resistant welding process has 3 steps :

A. Pressing

The parts to be joined are pressed together to acheive good surface contact and eliminate surface roughness.

Picture 3. Pressing

(Source : http://www.robot-welding.com/spot_welding.htm)

B. welding

The parts are still pressed together. Caused by the welding current, heat develops and the welding nugget forms.

Picture 4. Welding

(Source : http://www.robot-welding.com/spot_welding.htm)

C. Forging

When the material cools down after welding, the pressure force is still applied to avoid imperfection (crack, incomplete fusion or cavities)during contraction of weld area.

Picture 5. Forging

(Source : http://www.robot-welding.com/spot_welding.htm)

            For more detail about the explanation, see video below:

                          

 

or link below

http://www.youtube.com/watch?v=9LEDdXqOeWE

                                                     

Formula for the heat required for certain spot :

 [0,24 ò R.I² d.t ]

Remarks :

Q = Heat (kalorien)

R = Resistance ( ohm)

I = Current  (ampere)

T = Time ( seconds)

This formula cannot be used in practice, because the parameters R and I change during the welding process. But the formula shows us that the parameters resistance, current and time are most important for spot welding.

-          Resistance

Picture 6. Spot welding

(Source : http://www.robot-welding.com/spot_welding.htm)

R = R1+R2+R3+R4+R5+R6+R7

A.      R1 = R7 = Electrical condactivity of electrode material

B.      R2 = R6 = Contact resistance between electrodes and material to be welded

C.      R3 = R5 = Electrical conductivity of material to be welded

D.      R4 = Contact resistance between parts to be welded

Generally, the following formula valid :

R4>R2=R6>R3=R5>R1 =R7

R4 is the most important resistance of all because it is the highest resistance of this group. R1 and R7 are very low, because the electrodes are made from special copper alloys that have excellent conductivity.

-          Current

The welding current is determined by the following formula :

[ V = I.R ]

[ I = V/R ]

Remarks :

I = welding current ( ampere )

V= Voltage (volt)

R= Resistance (ohm)

The voltage of the welding machine can be controlled by the setting of the welding control unit. Therefore the current depends on the machine setting and resistance.

-          Time

The time is controlled by the “welding control unit”. The setting is made in “periodes”. The length of one periode depends on the AC power supply frequency.

With a 50 HZ (hertz = 1/second)  frequency, 1 period = 1/50 second = 0.02 second.

to be continous..