BASICS OF BIW FIXTURES - Part 3

Question No.4 - What is resistance welding  & its application in automotive sector? 

Answer:

ELECTRIC RESISTANCE WELDING:

Resistance welding is a welding technique widely used in manufacturing industry for joining metal sheets & components. This type of welding is very much used in BIW assemblies most common. The weld is made by conducting strong current through the metal combination to heat up & finally melt the metals at localized points predetermined by design of the electrodes or the work piece to be welded. A force is always applied before, during and after the application of the current to confine the contact area at the weld interfaces and in some application to forge the work piecees. Resistance welding is used in mass production for welding sheet metals,wire & tubes. It is also used in welding bars, boxes, cans, rod pipes, & frame metals of medium & high resistance such as steel, stainless steel, monel metal & silicone bronze which are easy to weld.

Types of Resistance welding are,

1. Resistance Spot welding
2. Resistance Seam welding.
3. Resistance Projection welding.
4. Butt welding.
5. Precursion welding.  

                                     Figure 1. Classification of Resistance Welding

I. Resistance spot Welding (RSW):

• The process is used for joining sheet materials & uses shaped copper - chromium or zinc conium alloy electrodes to apply pressure and convey the electrical current through the work piece.
• Heat is developed mainly at the interface between two sheets, eventually causing the material being welded to melt, forming a moltel pool, the weld nugget.
• The molten pool is contained by the pressure applied by electrode tip & the surrounding solid metal.

Working Principle of Resistance Spot Welding:

The resistance spot welding process use two copper alloy electrode to concentrate weld current and forced between work to be welded. Produce coalescence in one spot for heat generated from resistance to the electrical current through work piece held together under pressure by electrode. It process usually the thickness range of sheet metal is 0.5 to 3.0 mm. the small spot is quickly heated until melting point, in this reason a nugget of welded metal after power supply removed. When the current is applied too long, which result in molten metal become expelled as welded splash (or) can be made a have right through the material become welded.  In this spot welding used for welding steel sheet metal.

                     Figure 3. Force/Current Vs Spot welding cycle graph

Advantages of Resistance  spot welding:

1. Short weld time and hence high weld speed.
2. Highly adaptable to automation & Robotics.
3. Typical BIW contains 5000 welds.
4. Strength & durability of aautomobile laregly depends on the quality of RSW.

Disadvantages of Resistance spot welding:

   Figure 4. Resistance Spot Welding

II. Resistance Seam Welding: 

• Seam welding is a resistance welding process for joining metal sheets in continuous,often leak tight,seam joints by directly applying opposing forces with electrodes consisting of rotary wheels.
• The current & the heat generation are localized by the peripheral shapes of electrode wheels.

Working Principle of Resistance Seam Welding:

• Two electrodes made from copper & in circular form are used. An impulse current is applied through one of the discs & pressure through the other on the materials that are being joined.
• When the material passes between electrodes they rotate. In this way, the electrode remains in contact with the material for a long time & forms a weld joint progressively unlike spot welding which forms an entire joint at once.
• Seam welding core principles are similar to that of the Resistance spot welding. Time, pressure & electricity are used to weld metals together. It is a type of electric resistance welding.
• If the current is quickly turned OFF & ON a continuous fusion zone of overlapping nuggets is formed then this process is called as STICH WELDING PROCESS.

                 Figure 5.  Resistance Seam welding

Advantages of Resistance Seam Welding:

          Figure 6. Resistance Seam Welding

III.  Resistance Projection Welding:

• In the projection welding, as per the name, different projections are formed for effective welding. Projection Welding is one of the types of resistance welding and its working principle is quite the same as the resistance welding. The only difference here is that projection or embossed joints are used for the welding purpose.
• Projection welding is a resistance welding process for joining metal components or sheets with embossments by directly applying opposing forces with electrodes specially designed to fit the shapes of the work pieces.
• The ccurrent & the heat generation are localized by the shape of the work piece either with their natural shapes or witth specially designed projection .
• Large deformation or collapse will occurs in the projection part of the work pieces implying high.  

Working Principle of Resistance Projection welding:

As per the definition, different projections are formed in this welding technique. Here, the metal pieces that are to be joined are kept in between the two electrodes. A larger pressure force is applied to the electrodes. As current is passed through the system, the heat formation takes place due to the internal resistance of the metal workpieces. One point that you must note down here, is that the heat generation takes place due to the internal resistance of the metal workpieces rather than an electric arc. Those projections concentrate the heat. As the pressure applied to the electrodes increases, this projection collapses and the formation of the fused weld nugget takes place. Thus, a quality weld is formed.

The exact working of the projection welding can be understood by referring the below image.

            Figure 7. Process of Resistance Projection Welding

You can see in the above image that the projection means the embossed joints are formed on one of the base metals and then, these base metals are kept in between the two electrodes and force is applied perpendicular to the electrodes.

But as the applied force increases those sharp projections collapses and the formation of the weld takes place at the weld surface. The above image illustrates the formation of the weld nuggets as well as the collapsing of the sharp projections.

Advantages of Resistance Projection Welding:

• As the above stated, this welding requires a very small supply of current and thus, it saves the electricity usage. So, less electricity requirement and a longer electrode life are the two most prominent benefits of this welding process.
• While doing spot welding there is a limitation on the thickness of the metal that has to be welded. But in this welding, almost metals of all thickness are welded.
• It can be used effectively for welding of the joints which are on the complicate locations.
• The heat balance is an important part of any welding process and this welding gives a good heat balance while welding.

Disadvantages of Resistance Projection Welding:

• This welding process is not applicable for some types of the coppers and brasses.
• Projection formation is a quite complicated process and it takes time to form the projections. It is very difficult to form the spherical projection and a skilled person is required to form such projections. While making those projections, a height of the projection has to be maintained properly.
• This process is not applicable to all types of workpieces. The composition of the metal workpieces has to be considered while this process and it has some limitations.

Application of Resistance Projection Welding:

As Projection welding is mostly used for the mass production. It has many applications such as:

• Automobile industry uses projection welding to a very large extent.
• Used for mechanical fixing of automobile body structures like weld nuts etc. 
• This welding process is also used for the fan covers and hollow metal doors.
• It is also used for producing the compressor parts and for the semi-conductors.
• Have you heard about the diamond segment welding? In the diamond segment welding, projection welding finds its applications.

            Figure 8. Resistance Projection Welding

IV. Resistance Butt Welding:

• Resistance butt welding is used to join components of similar cross section by making a weld across the entire section in a single operation. Heat is produced in the weld region by resistance to the passage of the welding current through the parts, which are held under a preset end force. As the material heats, the force forges the soft material to consolidate and complete the joint.

• Resistance butt welding is thus a solid state process. The force across the interface causes deformation which brings the surfaces into sufficiently close contact to make a weld, and there is some expulsion of material which carries oxide film and contaminants out of the joint.

• There are two types of Butt welding : a.Flash butt welding. b. Upset butt welding.

       Figure 9. Process of Resistance butt weld

Application of Resistance butt welding: Applications include,

1. wire and rod joints up to about 16mm diameter, including chain, and narrow strip joints such as automobile road wheel rims.
2. Butt welding used in automobile constuction of the body, axles, wheels frame etc.

                          Figure 10. Butt welding

V. Precursion Welding: 

• Percussion welding (PEW) is a type of resistance welding that blends dissimilar metals together. Percussion welding creates a high temperature arc that is formed from a short quick electrical discharge. Immediately following the electrical discharge, pressure is applied which forges the materials together. This type of joining brings the materials together in a percussive manner.
• Percussion welding is similar to flash welding and upset welding but is generally considered to be more complex because it uses an electric discharge at the joint, followed by pressure being applied to join the materials together. Percussion welding is used to join dissimilar metals together, or used when flash is not required at the joint.
• Percussion welding is used on materials that have small cross sectional areas.

Principle of Precursion welding:

Heat is generated by a high current arc between the two points to be joined. The high current melts a thin layer of the metal and the surfaces are brought together in percussion manner to get welded.

Advantages of Precursion welding:

1.  Parts with different thermal conductivities can be easily welded. 
2. Very quick process time, usually takes less than 0.1 second.
3. It causes very little damage to the material which is close to the weld.
4. Without any danger of annealing, hardened surfaces may be welded.
5. Heat balance is not a problem because the heat is concentrated at the ends of the work pieces.

Disadvantages of Precursion welding:

1. The process is limited to butt welded joint only
2. The joint used in limited to about 1.5 to 3 cm²because of control of paths of arc is difficult.
3. The process cannot be used for welding heavy sections larger than 600 mm².

Application of Precursion welding:

1. This process joins similar or dissimilar metals.
2. This process is used for welding fine wire leads to filaments in lamps and terminals.
3. This welding is useful to make large contact assemblies for relays etc.
4. It can join wires and rods by butt joints.                                    
5. This process is used to attach metal tips to valve stems.
6. This method is also used to weld pins, studs, bolts and so on.

     Figure 11. Resistance Precursion welding

APPLICATION OF ELECTRIC RESISTANCE WELDING IN AUTOMOTIVE SECTOR:

• The conventional steel body of a car, on an average, contains 4500 spot weld joints. Resistance spot welding is the principle joining method used in automotive industries and has been for many years.
• Welding is invariably used in the automotive industries for joining variety of structural components and engine parts. The constant demand for new improved material requirement for automotive applications necessitates the development of innovative joining techniques.
• RSW (Resistance spotwelding) is automated and used in the form of robotic spot welding in automotive industries to weld the sheet metals to form car body. Industrial robots spot welding the car body in production line is shown in the picture given below.

  Figure 12. Robotic welding cell in automotive industry

• The most common application of Resistance spot welding is in the automobile manufacturing industry, where it is used almost universally to weld the sheet metal to form a car.
• Resistance Seam welding is used in the assembly of fuel tanks to make leak proof in the automotive industry. The below picture represent the fuel tankof Royal Enfield bike 

     Figure 13. Fuel Tank of Royal enfield bike

• Automobile industry uses Resistance projection welding to a very large extent.
• Resistance Projection welding Used for mechanical fixing of automobile body structures like weld nuts etc.
• Resistance buttweding is used for wire and rod joints up to about 16mm diameter, including chain, and narrow strip joints such as automobile road wheel rims.
• Resistance Butt welding used in automobile constuction of the body, axles, wheels frame etc.

Hence, The Electric Resistance welding & its types are discussed in deatail with their working principles, advantages, disadvantages & Applications. Also the application of Resistance welding in automotive sectors are explained in detail.

Question No.5 - What is the 3-2-1 Principle ?

Answer:

FIXTURE:

• A fixture is a device which holds the work piece to be processed (welded) by using 3-2-1 principle.

FIXTURE DESIGN:

• Fixture must be correctly locate a work piece in a given orientation with respect to a cutting tool or measuring device or with respect to another component.
• Locating & clamping are the critical functions of any workholding fixtures.
• A work piece in a space can move in an infinite No. of directions for analysis, this motion can be broken down into 12 directional movements or degrees of freedom.
• All the 12 degrees of freedom must be restricted to ensure proper referencing of the work piece.

3-2-1 PRINCIPLE:

• The best & most cost effective method of part location is referred to as the 3-2-1 method.

(3) Minimum : Three locator blocks to establish part plane. Three locators or supports are placed under the work piece. Three locators are usually positioned at the primary locating surface. This restricts two axial movement downward & upward and four radial movements. Together, the three locators restricts six degree of freedom.

                   Figure 1. showing 3 minimum supports

(2) : Round locating pin a round hole that defines location in four directions (4-way) perpendicular to the plane previously established. This restricts four axial  movements.

            Figure 2. Showing round pin 1 locator 

(1) : Round locating pin  in a slot that defines two of the direction of other pin (2 - way). This restricts two radial movements.

               Figure 3. Showing Pin 2 locator

Hence, the 3-2-1 principle is defined.

Question No.6 - Define Body coordinate system ?

Answer:

BODY CO-ORDINATE SYSTEM IN BIW:

• The body co-ordinate system has been widely used in the automotive industries for drawing body parts, products & process design.
• A co-ordinate system is a reference system consisting of a set of points, lines & surfaces used to define the position of points in space in either two or three dimensions.
• In general Body Co-ordinate System is also called as car line & body line.

                       Figure 1. Body Co-ordinate System

• X = Longitudinat direction [Fore & After- F/A] or [Front 'O' Line - FOL].
• Y = Transverse direction [Cross Car - C/C] or [Centre 'O' Line].
• Z = Vertical direction [Up & Down - U/D] or [Bottom 'O' Line].
• The origin of the body co-ordinate system (OX) is defined at the front centre of the vehicle. It indicates the length of the car.
• The co - ordinate system (OZ) below its underbody indicates height of the car.
• The co-ordinate system (OY) its starting point is the centre of car body indicates the width of the car.
• The origin will be differ from car makers to car makers.

Hence, the Body Co-ordinate System is defined in detail.

Question No.7 - Elaborate Body plane system & its essentials.

Answer:

 BODY PLANES:

• Body planes are the reference plane in an automotive car. These planes are used to define the GD&T of all car parts in automotive domain.
• Body planes are three mutually perpendicular imaginery planes. These are generally considered to be present at the driver position or sometimes at the engine location or at the front end of the car depending on the customers.
• The measurements from the body planes to the parts are called Body line dimensions.
• These body planes are called as,

1. FOL - Front 'O' Line.
2. COL - Centre 'O' Line.
3. BOL - Body 'O' Line.
4. The measurement along FOL or X axis is called as F/A or Front Aft.
5. The measurement along COL or Y axis is called as C/C or Cross car.
6. The measurement along BOL or z axis is called as U/D or Up Down.

                     Figure 1. Body Planes

ESSENTIALS OF BODY PLANES:

Body lines dimensions are essential for the reason stated below,

1. It defines the exact position of each product in the vehicle with respect to a fixed origin.
2. It ensures the flawless design for assembly.
3. While designing lines, all the designes are carried out relative to the body lines, Viz. fixtures, turn table etc.
4. It assists in easy location of different parts in a vehicle.
5. It aims in creating a standardized system.
6. It assists in Quality checks.

Hence, the body planes and its essentials are defined in detail.