Preparing a joint through Spot Welding between two sheets

  • Objectives:
  1. To acquaint the students with the procedure of spot welding and to know the working procedure of welding tools and safety equipments.
  2. To develop the confidence in spot welding operation. 
  • Theory:

Spot welding is “the welding of overlapping pieces of metal at small points by application of pressure and electric current”. It is the join by welding at a number of separate points.

Spot Welding occurs in three stages:

  1. Electrodes seated in a weld head are brought to the surface of the parts to be joined and force (pressure) is applied.
  2. Current is applied through the electrodes to the workpiece to melt the material.
  3. Current is removed but electrodes remain in place at force to allow the material to cool and solidify.

Work-pieces are held together under pressure exerted by electrodes. Typically the sheets are in the 0.5 to 3 mm (0.020 to 0.118 in) thickness range.Weld timings range from 0.01 sec to 0.63 sec depending on the thickness of the metal, the electrode force and the diameter of the electrodes themselves. The term “spot welding” is also used in the laser welding process to describe the spot welds generated by single pulses of laser light.Spot welding is a resistance process, relying on the electrical resistance of the components to generate heat when a current is passed through them. The heat generated is proportional to

  • The square of the current (I²)
  • The resistance of the components. (R)
  • The time for which the current is passed. (T)

Weld current:

Weld currents typically vary from 500A on very thin materials (<0·2mm) up to 30kA for heavy sheet of projection weld applications. 

  • Weld time:

The thicker the material, the longer it will take to heat it up to the required temperature. In general, therefore, weld times increase in proportion to the thickness of the components.

  • Weld Force

The purpose of the force applied to the welding electrodes is to contain the expansion which occurs when the metal is heated. If this expansion is not contained, metal will be expelled from the weld area, leaving reduced nugget size and the possibility of an incomplete nugget composition, adversely affecting weld strength. Weld force depends upon several parameters like- metal thickness, type, current etc.

  • Spot diameter

The heat generation is represented in terms of ‘heat per unit area’. Spot diameter increases with material thickness (too small a spot on sheet of a given thickness which requires a given current to heat it up and a given force will indent too far into the sheet, causing splashing (metal expulsion) and adversely affecting weld strength.)

A standard 5mm diameter spot has a surface area of 19mm2. 

  • Drawing of Spot Welding set up and Job:
    Fig 2: Drawing of the job.

     

    Fig 3: Reference diagram for measurement of spot position.
  • Equipments used:
  1. Welding power supply
  2. Tool holders
  3. Electrodes
  • Raw Material: Mild Steel (MS) sheet.
  • Working Procedure: 

Preparatory:

  1. Collect and check the raw material for shape and dimensions.
  2. Collect spot welding equipments.
  3. Collect safety equipments.
  4. Check the specifications of the spot welding setup.
  5. Acquaint with the working knowledge of the spot welding process.
  6. Acquaint with the knowledge of the use of the safety equipments. 

Operation:

  1. Take the two prepared workpieces (120mm X 20mm X 1mm) and smooth the surfaces by emery paper.
  2. Remove the air gap in between two workpieces and hold the two pieces firmly in perfect way.
  3. Check and ensure that proper alignment on the workpieces and hold the two pieces in between two electrodes.
  4. Set the time and current and press the paddle by using leg.
  5. Hold the job for 5 seconds.
  6. Place the work piece in flat position of the table after welding.
  7. Inspect the work piece visually to ensure that the surfaces are free of defects and distortion.
  • Experimental Table:
Spot no. Voltage

(V)

Current(A) Size of Job (mm x mm x mm) Time taken Observation Remarks
1
2
3
4
5
6
7
8
9
10

 

Spot Distance

Spot position P1 P2 P3 P4 P5 P6
Required position length wise from one end (mm) 5 20 40 60 80 95
Observed position length wise from one side (mm)            
Observed position length wise from other end (mm)            

 

  • Discussion: 
  • Safety Precautions:
  1. Always wear proper protective clothing during spot welding.
  2. Loose clothing should always be avoided. Wear apron which will not burn during welding.
  3. Never handle the hot job by hand. Use gloves for this purpose.
  4. Tools which are not being used should always be kept in their respective places.
  5. Welding machine should not be operated without the working knowledge of the machine and in the absence of instructor.
  6. Welding shop should be well ventilated to reduce fumes and hazardous gases generated during welding.

 

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