BIW Processing Methods Fixture Design Principles

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PROCESS OF PROJECT EXECUTION ACTIVITY

	PROCESS FLOW	WORK/CONTENT DESCRIPTION	OUTCOMES
1	Request for Quotaion	Sharing of Drawings – 2D with dimensions & geometric contraints, 3D CAD (IGES/STP), Volume requirement per day, Project Deadlines & Commercial Targets	Dimensions –critical & reference and geometric constraints, Material Specifications, Safety regulations & mandates, Functional requirements
2	Manufacturing Feasibility Discussions	Technical discussion on weldability of material specified, weld feasibility across intended joints & locations, feedback & recommendations on non feasible welding locations, feedback on dimensions & tolerances considering welding distortions	Joining process intended – MIG/TIG/ResistanceSpot/Seam/Projection Welding, Adhesive epoxies,/sealants, etc Manufacturing Feasibility covering Weld Length, Weld Sequencing, Volume requirement per day & Welding layout
3	Commercial Offer Submission	Working out commercials thru estimation considering part layouts & orientations – designing preliminary fixture layouts and by that defining resting block dimensions, types of clamps, number of locating pins and its material specification for reduced wear rate, trunions & base plate including frame structure estimation of material cost & conversion cost. Estimation of volume of blocks that are to be heat treated & surface area of blocks that are to be electroplated. Finalising on bought out elements – Motor, gear box, bearings, sensors, shafts, eletrical fittings, pneumatic cylinders, PLC elements,etc Estimation of Assembly, Rework, Installation & Commissioning cost. Preparation of project timelines for Design & detailing, Simulation, Manufacturing, Calibration & Assembly trials phase using Gantt charts aligning to the customer requirement. Commercial estimation is also done thru previous project experience.	Commercial offer aligning to customer target. Welding Layout aligning to volume requirement The Make, technical specifications & no off of the Robots, Welding Power Source, Robot Controller, Voltage Stabilisers, Motors, Gear Boxes, Rotary Indexers, Proximity /LVDT Sensors, Pneumatic Cylinders, PLC elements, etc Detailed week wise Gantt charts aligning to customer timeline

4	Order Confirmation & LOI/PO receipt	Order confirmation will be based on optimal techno-commercial offer submission among the list of suppliers. For the supplier selected, Letter of Intent (LOI) or Purchase Order (PO) will be released with customer terms and conditions. LOI is also used as a supporting document for securing loan from banks.	Business confirmation thru LOI/PO. Material Standards, Manufacturing Process Standards, Quality Testing & Inspection Standards will be shared by customer.
5	Detailed Activity Chart preparation	Detailed Activity Chart for 1)Design Phase - 3D Concept Design, Customer Verification & Feedback, Detailed Design, Simulation & Drawing Release, BOM preparation, Preventive Maintenance Plan 2)Manufacturing Phase - Raw Material Ordering & Procurement, Inhouse Machine allotment, External Processing for special process like Laser Cutting, Wire Cut EDM, Hard Chrome Plating, etc. Bought-out Material procurement & its BOM verification 3)Quality Inspection Phase – Inspection of Individual Blocks thru First Principle method, Rework & verification, Assembly CMM inspection 4)Trials Phase – Assembly trials, Sample Submission, Customer Testing & Validation, Customer End Installation & Commissioning, Trials & Proving to intended capacity per day	Detailed week wise or day wise gantt charts aligning to customer requirement will be prepared and submitted to customer for weekly review purpose.
6	Design Concept Preparation & Customer Approval	3D concept design for each individual process stage comprising of child parts involved, weld joint description & sequencing, welding length, robot accessibility, fixture design aspects like resting, locating, clamping &orienting, fixture poka yokes, part detection sensors, need for rotary indexers, design cycle time & its alignment to demand takt time. 3D concept design will be reviewed with customer for verification & approval. From the review, customer feedback points like additional clamping requirement, rotary indexing mechanism, tolerances of locating blocks,etc will be signed off.	Final 3D concept design, Customer DVP sign off

7	Detailed Drawing & BOM release	3D concept design will be fragmented into individual part drawings with dimensions, tolerances & geometric constraints. Consolidated Bill of Materials will also be released.	2D Part Drawings with dimensions, tolerances & geometric constraints. BOM
8	Manufacturing & BOP ordering	RM procurement and subsequent processes like Wire Cut EDM, Vertical Milling Centre Operation, Turning, Co-ordinate Drilling, Jig Boring, Cylindrical & Surface Grinding, etc will be taken up inhouse. Processes like Laser Cutting, Hard Chrome/ Nickel Chrome Plating or any Electroplating processes, Heat Treatment like Tempering, Case Hardening etc, will be externally sourced & carried out. Motors, Gear Boxes, Rotary Indexers, Pneumatic Cylinders, Proximity Sensors, LVDT sensors, Kinematic Mechanisms for Clamps, PLC controllers, LED display,etc will be procured as bought outs externally.	Individual Components as per 2D drawings will be manufactured. Standard Bought out elements will be sourced and procured aligning to our technical requirements (Clamping Force for Pneumaic Cylinder Bore x Stroke, etc)
9	Quality Inspection	Each individual part will be inspected against 2D drawings and certified. Non conformances will be reworked. BOP parts will be verified as per its BOM.	Quality inspected parts for assembly
10	Assembly & Quality Inspection	Installation & Assembly of individual parts manufactured and bought out parts will be carried out. To counter tolerance stack up deviations, standard shims will be added/ surface will be grinded to reduce the error Quality Inspection will be carried out using CMM and reports will be released and accordingly drawings will be updated.	Assembly of manufactured fixture elements and bought out components. CMM inspection of the complete assembly
11	Installation of Mechanical & Electrical components, Calibration & Internal Trials	Mechanical components like Pneumatic Cylinders, Rotary Indexers (Servo Motor, Clutch & Gears), etc and Electrical components like PLC, Sensors, Robot Controller, Voltage Stabiliser and checking for Robot Accessibility of weld locations in design layout. Components welded in controlled configurations are checked for dimension inspection.	Configuring relative working of Robot, Indexer in the design fixture & layout using PLC and welding samples for inspection

12	Packaging & Transportation	Packaging of fixture elements, mechanical components, electrical components, fasteners & spares elements along with operations manual, preventive maintenance activities, problem troubleshooting guide, etc, in properly dampened housing enclosure and transported to customer in exclusive vehicle.	Solid Packaging and Transportation ensures defect free product
13	Installation, Commissioning, Calibration & Online Trials at Customer end	Installation, Commissioning of Fixture elements, Mechanical Components & Electrical components in intended welding layout along with Robot, Conroller, Welding Power Source, enclosed in a Welding Cell, WIP Jigs / conveyor setup. Calibration and subsequent online trials initiation followed by sample production and proving to product specification and followed by improvisation on process cycle time by reducing air cut time of robot, parallel processing of robot, weld speed optimisation, etc.	Consistent output conforming to product specifications with scope for process cycle time improvisation
14	Production Trials & Training of Customer Professionals	Production Trials involve steady ramp up to intended volumes as per customer requirement. Training to customer working professionals on loading & unloading pattern, clamping, declamping sequence, online problem troubleshooting, preventive maintenance procedures, etc	Consistent output conforming to product specifications at design takt time
15	Buy off meeting and final hand over	Proving the line to intended capacity, educating on standard operating procedure of machines, problem troubleshooting methods and preventive maintenance plan to be followed. It also includes settling out pending commercials and complete hand over of asset to customer.	Achiveing greater than 90% Line productivity. Asset transfer to customer

TYPES OF JOINING PROCESS

S No	TYPE	PROCESS
1	Mechanical Fastening	Riveting Bolt & Nut Fastening Shrink Fitting Metal Forming Process – Hemming
2	Metallurgical Joining	Metal Inert Gas Welding (MIG) Tungsten Inert Gas Welding (TIG) Laser Welding Plasma Arc Welding Resistance Spot Welding Resistance Seam Welding Resistance Projection Welding Brazing & Soldering
3	Chemical Joining	Adhesive Bonding

1) MECHANICAL FASTENING

1A) Riveting

Riveting is a permanent joining process where two components can be joined by means of long cylindrical rivets inserted through holes, which are drilled on the components prior to riveting. While welding requires edge preparation when plate thickness is more, riveting always requires pre-drilled holes on the component, which reduces load bearing capability of joined structure.

A rivet is a mechanical fastener composed of a head on one end and a cylindrical stem on another (called the tail) which has the appearance of a metal pin.

Typical applications include pressure vessel, gas cylinder, boiler,

Automotive body panels, etc.

Self Piercing Rivets in BIW automotive

Self-piercing riveting (SPR) is a joining technique for the automotive applications of aluminum sheets. SPR machines use electrical motors to drive a rivet into the sheets.

The combination of self-piercing rivets (SPR) and robotically-applied structural adhesives creates a robust joint with improved tensile-shear, cross-tension, fatigue, and impact performances. The rivets clamp the adjacent metal panels together to allow the adhesive-bonding agents sufficient time to cure while the assembly goes through the body and paint shops.

1B) Bolt & Nut Fastening

Bolt, Nut & Screw fastening are mechanical methods for securing parts. In this, Fastener Heads selection is important phenomenon linked to the mechanical effects. Conical heads produce undesirable tensile stress in the mating parts and should be avoided. Bolt or screw heads that have a flat underside, called pan or round heads, produce less harmful, compressive stress. Use of flat washers under both nut and fastener heads helps distribute the assembly force over large surface area.

The rule of thumb for the minimum distance between fastening drill hole to the parts edge should be at least the diameter of the hole or twice the part’s thickness, whichever is greater

1C) Shrink Fitting

Shrink-fitting is a technique in which an interference fit is achieved by a relative size change after assembly. This is usually achieved by heating or cooling one component before assembly and allowing it to return to the ambient temperature after assembly, employing the phenomenon of thermal expansion to make a joint.

Shrink fitting is accomplished by heating a part so that a hole in it expands, after which another piece may be fitted, usually under pressure, into that hole. The outer piece then contracts as it cools, creating a tight seal. Some sinter-like bonds may form, but shrink fitting works primarily by friction bonding. It requires thermal energy and less force is needed to achieve the final bond. If the material to be shrink-fitted is metallic, heating may be accomplished by induction.

A common method used in industry is the use of induction shrink fitting which refers to the use of induction heating technology to pre-heat metal components between 150˚C and 300˚C thereby causing them to expand and allow for the insertion or removal of another component. Other methods of shrink-fitting include compression shrink fitting, which uses a cryogen such as liquid nitrogen to cool the insert, and shape memory coupling, which is achieved by means of a phase transition.

1D) Metal Forming Process – Hemming

Hemming is a technology used in the automotive industry to join inner and outer closure panels together (hoods, doors, tailgates, etc.). It is the process of bending/folding the flange of the outer panel over the inner one.

Roller Hemming using Robot

It uses a standard industrial robot integrated with a roller hemming head to provide a flexible solution to closure manufacture. The flange of the outer panel is bent over the inner panel in progressive steps by means of the roller hemming head. The process allows the advantage of using the robot controlled hemming head to hem several different components in a single cell. Minor changes and modifications to panel hemming conditions can also be accommodated allowing a quick and cost-effective reaction.

The robot can also be used for other tasks, by, for example, equipping it with a tool changing system. This will allow it to also operate dispensing equipment for adhesives and sealants, or a gripper which will enable it to carry out panel manipulation within the assembly cell.

RESISTANCE WELDING & ITS APPLICATION IN AUTOMOTIVE SECTOR

2E) Resistance Spot Welding

The process involves applying pressure and heat to the weld area using shaped alloy copper electrodes which convey electrical current through the weld pieces. The material melts, fusing the parts together at which point the current is turned off, pressure from the electrodes is maintained and the molten “nugget” solidifies to form the joint. The welding heat is generated by the electric current, which is transferred to the workpiece through copper alloy electrodes. Copper is used for the electrodes as it has a high thermal conductivity and low electrical resistance compared to most other metals, ensuring that the heat is generated preferentially in the work pieces rather than the electrodes. The amount of heat depends on the thermal conductivity and electrical resistance of the metal as well as the amount of time the current is applied. This heat can be expressed by the equation:

Q = I2Rt “Q” is heat energy, “I” is current, “R” is electrical resistance and “t” is the time for which the current is applied.

Resistance Spot Welding in BIW

A car BIW contains between 4000 - 5500 weld points with between 200 to 400 different material thickness combinations. Application of magnitude of current & its timing across the geometry of the BIW panels is programmed thru Robot and its controller. Clamping force for the panels should be in optimal range, as excessive clamping results in buckling of the component or damage to the skin surface, while insufficient clamping force results in distortion.

Spot Welding of Reinforcements with A, B, C Pillars, Door Lids, Floor Boards, in BIW

2F) Resistance Seam Welding

Seam welding is the process of joining metals at the seam by the use of electric current and pressure. In seam welding a series of overlapping nuggets is produced by electrode wheels that turn as the parts to be welded are fed between them. The process thus produces a continuous leaktight seam.

An electrical resistance between the two materials to be joined causes them to heat up at the seam. This is also known as contact resistance. The welding current is of primary importance in seam welding. The amount of heat generated at the seams will depend on the magnitude of the current flowing through it. However, cranking up the electricity too much can cause expulsions and electrode deterioration. Alternating current (AC) is still the most preferred type of electric current used in seam welding.

Automotive components like Fuel Tanks, Radiator housing, etc are seam welded.

2G) Resistance Projection Welding

Projection welding is a resistance weld where the design or shape of the part is used to make discreet individual point contacts to concentrate the current during the welding process. In most applications multiple small projections are formed on one surface of the parts to be welded. These projections can be round dimples, elongated ridges, circular, or the extended corners of weld nuts. 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. The 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.

BIW utilises Resistance Projection Welding in applications of threaded nuts joining onto sheet metal panels and thereby aiding in screws, nuts fastening.

FUSION WELDING, TYPES & ITS APPLICATION IN AUTOMOTIVE SECTOR

2A) Metal Inert Gas Welding (MIG)

Metal Inert Gas (MIG) welding is a gas metal arc welding (GMAW) process that uses a continuous solid wire electrode which is heated and fed into the weld pool from a welding gun. The two base materials are melted together which causes them to join. The welding gun also feeds an inert shielding gas alongside the wire electrode, which helps protect the process from airborne contaminants.

MIG/MAG welding is a versatile technique suitable for both thin sheet and thick section components. An arc is struck between the end of a wire electrode and the workpiece, melting both of them to form a weld pool. The wire serves as both heat source (via the arc at the wire tip) and filler metal for the joint. The wire is fed through a copper contact tube (contact tip) which conducts welding current into the wire. The weld pool is protected from the surrounding atmosphere by a shielding gas fed through a nozzle surrounding the wire.

The wire is fed from a reel by a motor drive, and the welder moves the welding torch along the joint line in the event manual welding, and in the event of robotic welding, control unit operates between robot, welding power source and wire feed unit. Manual MIG/MAG welding is often referred as a semi-automatic process, as the wire feed rate and arc length are controlled by the power source, but the travel speed and wire position are under manual control.

The process usually operates with the wire positively charged and connected to a power source delivering a constant voltage. Selection of wire diameter (usually between 0.6 and 1.6mm) and wire feed speed determine the welding current, as the burn-off rate of the wire will form an equilibrium with the feed speed.

Robotic MIG Welding of BIW child parts, Mufflers, Brackets, Shafts ,etc

In this process, wire feed rate is equal to electrode melting rate in order to maintain the arc.

Welding power source works as per the above equation and controls the current and voltage parameters to aid in spray transfer of metal droplets.

The control unit synergises the robot servo movements, wire feed electric drives, rotary indexing motor drives.

2A) Tungsten Inert Gas Welding (TIG)

Tungsten Inert Gas (TIG) welding, also known as Gas Tungsten Arc Welding (GTAW) is an arc welding process that produces the weld with a non-consumable tungsten electrode. In the TIG welding process the arc is formed between a pointed tungsten electrode and the workpiece in an inert atmosphere of argon or helium. The small intense arc provided by the pointed electrode is ideal for high quality and precision welding.

TIG welding must be operated with a drooping, constant current power source - either DC or AC. A constant current power source is essential to avoid excessively high currents being drawn when the electrode is short-circuited on to the workpiece surface.

TIG welding is used in Mufflers – Body Muffler, Baffle,CAT, SS Tube Welding, Shafts, etc

2C) LASER Welding

Laser welding is a process used to join together metals or thermoplastics using a laser beam to form a weld. Being such a concentrated heat source, in thin materials laser welding can be carried out at high welding speeds of metres per minute, and in thicker materials can produce narrow, deep welds between square-edged parts.

Laser welding is more usually accomplished using higher power densities, by a keyhole mechanism. When the laser beam is focused to a small enough spot to produce a power density typically > 106-107 W/cm2, the material in the path of the beam not only melts but also vaporises, before significant quantities of heat can be removed by conduction. The focused laser beam then penetrates in to the workpiece forming a cavity called a \'keyhole\', filled with metal vapour. This expanding vapour or plasma contributes to the prevention of the collapse of the molten walls of the keyhole in to this cavity.

Furthermore, the coupling of the laser beam in to the workpiece is improved dramatically by the formation of this keyhole. Deep penetration welding is then achieved by traversing the keyhole along the joint to be welded or moving the joint with respect to the laser beam. This results in welds with a high depth to width ratio.

Under the action of surface tension, some of the molten material at the leading edge of the keyhole flows around the keyhole cavity to the back, then cooling and solidifying to form the weld. This leaves the weld cap with a chevron pattern, pointing backwards towards the start point of the weld.

LASER Welding is used to establish high precision weld joints and finds its application in welding of lower thickness HSLA steels, Dual Phase Steels, Complex Phase Steels & TRIP steels (Transformation Induced Plasticity).

Higher accuracy & lower tolerance transmission elements like ratchets coupled onto gears, cltich discs,etc are laser welded.

2D) Plasma Arc Welding

Plasma welding is very similar to TIG as the arc is formed between a pointed tungsten electrode and the workpiece. However, by positioning the electrode within the body of the torch, the plasma arc can be separated from the shielding gas envelope. Plasma is then forced through a fine-bore copper nozzle which constricts the arc. Three operating modes can be produced by varying bore diameter and plasma gas flow rate:

Microplasma: 0.1 to 15A.
The microplasma arc can be operated at very low welding currents. The columnar arc is stable even when arc length is varied up to 20mm.

Medium current: 15 to 200A.
At higher currents, from 15 to 200A, the process characteristics of the plasma arc are similar to the TIG arc, but because the plasma is constricted, the arc is stiffer. Although the plasma gas flow rate can be increased to improve weld pool penetration, there is a risk of air and shielding gas entrainment through excessive turbulence in the gas shield.

Keyhole plasma: over 100A.
By increasing welding current and plasma gas flow, a very powerful plasma beam is created which can achieve full penetration in a material, as in laser or electron beam welding. During welding, the hole progressively cuts through the metal with the molten weld pool flowing behind to form the weld bead under surface tension forces. This process can be used to weld thicker material (up to 10mm of stainless steel) in a single pass.

Plasma Arc Welding Applications

Microplasma Welding was traditionally used for welding thin sheets (down to 0.1 mm thickness), and wire and mesh sections. The needle-like stiff arc minimises arc wander and distortion.

Medium Current Plasma shows greater tolerance to surface contamination including coatings.

Keyhole Welding exhibits deep penetration and high welding speeds. Compared with the TIG arc, it can penetrate plate thicknesses up to l0mm, but when welding using a single pass technique, it is more usual to limit the thickness to 6mm. As the welding parameters, plasma gas flow rate and filler wire addition (into the keyhole) must be carefully balanced to maintain the keyhole and weld pool stability, this technique is only suitable for mechanised welding.

2H) Brazing

Brazing joins parts by heating them to more than 840°F and applying a filler metal that has a melting temperature below that of the base metal. Filler metal flows into the joint by capillary attraction.

Brazing has several advantages. Dissimilar metals can be joined. Assemblies can be brazed in a stress-free condition, and complex assemblies can be brazed in several steps by using filler metals with progressively lower melting temperatures. Materials of different thicknesses can be joined, as cast and wrought metals. Nonmetals can be joined to metals when the nonmetal is coated. Metallurgical properties of base materials are not seriously disturbed, and brazed joints require little or no finishing.

Brazing process employed in automotive applications is as follows;

1) Oxy-acetylene Flame Brazing and Induction Brazing are commonly used. In Flame Brazing, neutral blue flame is used , whereas in Induction Brazing, heat input is controlled thru potentiometer feedback control

2) The surfaces to be joined must be cleaned to remove all contaminants.

3) The surface of the base materials should be coated with a material called flux. Flux is employed to get rid of residual contaminants on the surfaces to be joined and for the prevention of oxidation.

4)The joint area must be heated to melt the flux. The molten flux’s chemical reactivity cleans the substrate and prevents further oxidation by remaining as a layer of liquid flux.

5) The filler metal is melted between the surfaces of the substrate, displacing the flux, wetting the surfaces and forming the braze joint.

Soldering is a process in which two or more items are joined together by melting and putting a filler metal (solder) into the joint, the filler metal having a lower melting point than the adjoining metal.

In soldering process, heat is applied to the parts to be joined, causing the solder to melt and to bond to the workpieces in a surface alloying process called wetting. Capillary action takes place when the workpieces are very close together or touching. The joint\'s tensile strength is dependent on the filler metal used. For electrical applications, a good solder joint produces an electrically-conductive, water- and gas-tight joint.

The distinction between soldering and brazing is based on the melting temperature of the filler alloy. A temperature of 450 °C is usually used as a practical demarcation between soldering and brazing.

CHEMICAL JOINING METHOD

3A) Adhesive Bonding

Adhesive Bonding is the process of joining two surfaces together, usually with the creation of a smooth bond. This may involve the use of glue, epoxy, or one of a wide range of plastic agents which bond either through the evaporation of a solvent or through curing via heat, time, or pressure. Many modern glues are carbon-based petrochemical derivatives, which can be used for a variety of joining applications including bonding metal and dissimilar materials.

One of the main advantages of adhesive bonding is that it allows for the even distribution of stress loads, thereby reducing the stress on the joint. Since they are applied inside the join, adhesives are also invisible within the assembly.

The properties of glues mean that they can form both an adhesive and sealant at the same time, while also resisting vibration and flexing stresses. The ability to act as a seal allows adhesives to help protect a join from corrosion.

While adhesive bonding can be applied to metal-to-metal, plastic, glass, rubber, ceramic and many other material combinations, the process can be used to join irregularly shaped surfaces while offering virtually no change in the dimensions or geometry of the parts.

Adhesives also add very little by way of weight to the materials being joined and can be used to quickly bond dissimilar substrates and heat-sensitive materials.

One of the greatest advantages of adhesive bonding is the versatility it affords, allowing for joining of a wide range of items as well as being easy to automate.

Adhesive Bonding application in Automotive BIW

In OEM weld shops, bonds are typically metal-to metal, and adhesives are used to supplement or replace spot welding or mechanical fasteners. The adhesive is cured using the heat of the e-coat oven, which is roughly 180 C for 25 – 30 minutes. Epoxies are the primary chemistry used in this environment, as they are best suited to survive these temperatures without degrading. In some cases, sub-assemblies are provided to the OEM weld shop from another OEM plant or a supplier. Because these parts are shipped between facilities before they are painted, two-component, reactive adhesives are sometimes employed, because these adhesives can develop handling strength before being cured with heat, which can improve dimensional stability of the final assembly. The adhesive selected for these applications are capable of surviving the paint process, so epoxy chemistries are most commonly selected.

On the OEM assembly floor, the primary structural bonding application is for installation of front and rear glass.

PRINCIPLE OF FIXTURE DESIGN

Arresting the Degrees of Freedom (DOF) for the part to be processed is the basis on which the fixture design is carried out.

3-2-1 Principle corresponds onto the above statement.

In free space, an object/a part is said to have 6DOFs’ (or 12DOF’s considering direction vectors). DOF’s are classified as below,

Translational Axis – Xt, Yt & Zt (X+, X-, Y+, Y-, Z+, Z-)

Rotational Axis – Xr, Yr & Zr (Xcw, Xccw, Ycw, Yccw, Zcw, Zccw)

12 DOF’s, going by the words is defined as below,

X+, X- ; Strifing, Swaying

Y+, Y- ; Elevating, Heaving

Z+, Z- ; Walking, Surging

Swivels Left or Right ; Yawing

Tilts Forward or Backward ; Pitching

Pivots Side to Side ; Rolling

Essentially, 3-2-1 Principle is summarized as below,

Datum Reference	Description	Constrained DOF’s
Primary Datum	Resting – by gravitation	Yt, Yr, Zr
Secondary Datum	Locating & Orienting – by locator pins, profile stoppers	Xt, Xr
Tertiary Datum	Clamping – by Toggle Clamps	Zt

Textbook Rule of Thumb for 3-2-1 Fixture Design Principle

Holding fixtures used in measuring systems and assembly operations often follow a 3-2-1 locating scheme to position parts. Under this scheme, three locators position a part in a primary plane or direction (e.g., high/low). Two locators then position the part in a secondary direction(e.g., in/out), leaving one locator for the tertiary direction (e.g., fore/aft). This approach fixes the part in 3-dimensional space and satisfies the six degrees of freedom constraint. For some product designs, manufacturers replace the three locators for the secondary and tertiary directions using two round pins, one fitting a circular hole and the other a slot. The pin locates the part in two directions (e.g., in/out and fore/aft). The slot then becomes the other locator for the secondary

direction (e.g., in/out).

BODY CO-ORDINATE SYSTEM

Every point in a BIW component is defined with respect to x,y & z co-ordinates in a three dimensional space. This forms the Body Coordinate System (BCS). This is represented by grid lines in mesh in three axis of reference about origin. For a given BIW part, for which the welding fixtures to be designed, reference is taken from the BIW part which is predefined with coordinates as per BCS.Therefore, this relates the each element in the fixture with respect to body coordinates and aids in design tracebility.

Body Coordinate System aids in measurement system applications of BIW. The defined coordinates are verified in manufactured BIW component by different inspection methodologies. The most widely used systems to measure automotive bodies and their stamped components are checking fixtures (hard gages / acceptance gages) and coordinate measuring machines (CMMs), which either are mechanical or optical (OCMMs). The use of a particular measurement system largely depends on application and measurement objectives. For example, manufacturers typically use coordinate measuring machines for larger, complex parts requiring numerous dimensional checks. The coordinate measurement systems are considered the most accurate and repeatable. The benefits are their flexibility in terms of adding dimensional checks and that they may be run using automated programs, reducing the need for measurement personnel.

Although coordinate measurement systems (both CMMs and OCMMs) offer tremendous flexibility and data collection efficiency, they often are not used for process control in press shops. To measure stamped parts for process control, most manufacturers rely on hard checking fixtures. The principal advantage of checking fixtures is that manufacturers can locate them near a press or a subassembly line. This helps provide quick feedback on process performance.

The design of checking fixtures (or acceptance gages) should be in line with production fixtures, for the fact that, any non conformance in the part observed by non answering of a pin in checking fixture can be traced back to the particular pin in the production fixture and can be corrected.

BODY PLANE SYSTEMS & ITS ESSENTIALS

Body Plane Systems depicts three mutually perpendicular reference planes that define geometric dimensions & tolerances of the BIW components. Body Plane System in design phase gives the layout of the product based on which BIW skeleton is finalised considering vehicle aerodynamics.

Body line dimensions from the reference planes are essential for the following reasons,

1)It defines the exact position of each component in the vehicle with respect to fixed origin.

2)It ensures flawless design for assembly

3)All the designs are carried out relative to the bodyline dimension viz welding fixtures, acceptance gages, revising fixtures, etc

Body Plane Systems and its corresponding Body Coordinate Systems, gives the relative metrics that aids in establishing standard relation across fixtures and gauges.

Component Geometric Dimensioning & Tolerancing defined by design in the basis of BCS should accommodate manufacturing process constraints like welding distortion, component elastic recovery (springback), sharp contours and allied thinning & weakening effect in the component, etc