BIW Fixture Design Basics

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BIW DESCRIPTION

BIW stands for Body in White

BIW refers to the body shell design of an automotive product such as cars. It is just a sheet metal welded structure. BIW will not have doors, engines, or any other moving parts.

BIW is termed before painting & before moving parts (doors, hoods, and deck lids), the engine, chassis sub-assemblies, or trim (glass, seats, upholstery, electronics, etc.) have been assembled in the frame structure.

BIW components are joined together using welding (spot, MIG/MAG), adhesive bonding, riveting, brazing, etc.

In car design, the body in white phase refers to the phase in which the final contours of the car body are worked out, in preparation for ordering of the expensive production stamping die. Extensive computer simulations of crash-worthiness, manufacturability, and automotive aerodynamics are required before a clay model from the design studio can be converted into a body in white ready for production.

BIW PARTS

• BODY SIDES
• 1) A PILLAR
• 2) B PILLAR
• 3) C PILLAR
• 4) ROCKER BEAM

• UNDER BODY
• 1) FLOOR MEMBER
• 2) CROSS MEMBER
• 3) FRONT RAILS
• 4) REAR RAILS

• ROOF
• 1) ROOF BOW
• 2) ROOF RAIL

• FENDER
• 1) FRONT WHEEL ENCLOSURES
• 2) REAR WHEEL ENCLOSURES

• CLOSURES
• 1) TRUNK MEMBER
• 2) BUMPER BEAM
• 3) WINDSHIELD FLANGE
• 3) ROOF
• 4) DOORS
• 5) DECK LID

BIW NOMENCLATURE

BIW by its nomenclature is the appearance of a car body after it is dipped into a white bath of primer (undercoat paint)— despite the primer\'s actual gray color.

FIXTURE & ITS TYPES

1)Proto Fixtures

Fixtures which are manufactured for usage in the events for Part Proving, Testing & Tryouts.

Part development & proving phase makes use of fixtures which are not hardened by heat treatment (for withstanding rigorous wear rates during mass production) and they also will not be coated, electroplated or surface treated (for improving surface roughness/ for inducing hardness or toughness appropriately).

The fixtures are manufactured using conventional methods, but in most cases they are manufactured using advanced process like Laser Cutting, Rapid Prototyping techniques, etc using advanced materials like composites, carbon fibres, etc.

Essentially, Proto Fixtures in Fabrication are used to check the dimensional correctness, layout constraints in assembly and mass production manufacturing feasibility.

2) Pre Production Fixtures

Pre Production Fixtures are used when there is contingency arising out of design change in a particular part or sudden volume spike up in order to meet the market demand.

Pre production fixtures are manufactured thru conventional fixture manufacturing methods, but the blocks in fixtures which need to be heat treated or surface engineered will not be carried out due to time constraints.

Also, there will not be any automations like part detection proximity sensors or PLC actuated rotary indexers, or in some cases without pneumatic or hydraulic clamping.

Pre Production fixtures are essentially called as semi tooled fixtures or soft fixtures.

3)Mass Production Fixtures

Mass Production Fixtures are fixtures which aid for the demanded volumes of production at the required pace with robust preventive maintenance plan.

The fixtures are designed in a such a way to meet the demand takt time at that particular stage.

The fixtures will be in complete functionalities with blocks heat treated & hardened to withstand adhesive or abrasive wear and electroplated or coated to improve surface finish or surface engineered to induce hardness or toughness appropriately.

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

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

TYPES OF BIW STATIONS

BIW assembly line comprises of the following,

1)Tools – Geo Tool, Re-spot Tool, Marriage Tool & Framer Tool

2)Grippers – Material Handling Grippers, Process Grippers

3)Stands – Pedestal Stand, Gluing Stand, Docking Stand

4)Miscellaneous – Curing, Buffer Stand, Racks, Palettes, Trolleys, Conveyours

Geo Tool

Tool where part geometry is defined and it ensures that geometry is not distorted in and after welding.

Panel trim-lines which are defined together are welded accurately.

The pin tolerance for Geo pin is -0.15mm.

Eg: Panel Hole Size Φ20 mm, Geo Pin Dia Φ 19.85 mm

Respot Tool

Fixture in which the remaining weld spots which are not achieved in geo tool are welded from the pre-welded panel from the geo tool.

The pin tolerance for Geo pin is -0.28 mm.

Eg: Panel Hole Size Φ20 mm, Respot Pin Dia Φ19.72 mm

Marriage Tool

Marriage of Side Body, Under Body, Front & Rear Body.

Marriage Tool or Hemming Tool is a fixture in which the inner panel assembly and outer panel assembly are welded together.

Framer Tool

Roof members will be attached to the output of the marriage tool.

Gripper

A fixture which holds the car panel and is fixed to the manipulating end of a robot or an end effector.

Gripper is called as End of Arm Tooling (EOAT)

Grippers are frame based structures with light in weight and a rigid base to aid in flexibility in handling.

Material Handling Gripper

Grippers for handling the car panel from one station to another station

Process Gripper

Grippers to aid in processes like pedestal welding, pedestal riveting, gluing, sealing, etc., and also for handling the car panel from one station to another.

Combo Gripper

It is a combination of Gripper and weld gun

Dual Gripper

It is for handling the car panel from one station to another station, with two sided gripper.

Stands

Gluing & Sealer Stand

Gluing gun or sealing gun fixed to a standard welded stand grouted to the floor.

Docking Stand

When robots are selected for multitasking like material handling & welding, which cannot carry out both due to limitations like payload capacity of the robot, space constraints, etc., the robot will drop and pick the gripper or gun after completing the respective operation with the help of the docking stand.

Date Stamp Stand

Date embossing onto car panels from date stamping device mounted onto the stand.

Pedestal Gun Stand

Weld Gun mounted onto Stand. Process Gripper carries the car panel to weld the spot with this fixed weld gun.

TERMINOLOGIES IN BIW FIXTURE

1)NC/ Mylars / Fingers – Elements actuated by pneumatic/hydraulic clamps for ensuring proper clamping force. Since the proximity of these elements are nearer to welding, they may be coated with Tungsten/Titanium Nitrates which are of high refractoriness.

Also, Copper including its alloys like brass, bronze do not take part in welding and essentially these elements are made of it as well.

2) PLP, ALP – Primary Locating Points & Auxillary Locating Points

PLP determines the primary location and it is aided with auxillary location for arresting all 6 DOF’s. This is a reference for maintaining all other dimensions in a part.

3)Dowels, Rough Locators, Fasteners, Shims

Dowels maintains the relative position of two elements in a fixture.

Addition of Shims are used to adjust positions along x,y & z co-ordinates to rule out the error due to each individual tolerance stackups.

4)Number Plates, Co-ordinate Plates, Name Plates

Co-ordinate plates contain information on reference co-ordinates for calibration purpose. Name & Number plates are for identification purpose.

5) Clamping Unit

Clamping units are mostly actuated by pneumatic or hydraulic cylinders. Manual toggle clamps are also preferred in some cases. Sufficient clamping force is mandatory to prevent any lift due to welding heat. Excessive clamping force also leads to buckling/damage to the part being welded.

6) Reed Switches, Proximity Sensors, LVDT

Reed switches & Proximity sensors are used for Part detection and location purpose.

LVDT is a transducer used in revising machines to correct the part profiles tampered by processing and material handling.

7) Chemical Grouting

Groutings are mandatory to fix the machines rigidly onto the ground. Further, chemicals added to ground (sand particles) acts as a reinforcement and dampens the vibration during machine operation.

8) Rotary Indexers

Rotary indexing is achieved thru servo motors for ease of material loading & unloading as well as to improve the reach of the robot during operation.

9) PLC

Programmable Logic Controllers are used for automation of a welding cell, so that the robotic controller, welding machine, rotary indexer and fixture clamping & declamping units are working in unison as per the programming.

10) Poka-Yoke Andon Signals, LCD/LED Lights

Andon signals (LED) are visual signals indicating the health of the electrical equipments in operation. They are also used for Cycle Start & Stop indication.

LCD displays the sequence of operations in the cycle.

LED lights are also visual for clamping, declamping & cycle start – stop purpose.

11) Skin Surface Fixturing

Skin Surface Fixturing involves elements like pads of steel elements reinforced PU rubbers/ Thermoplastic elastomers which are both refractory and of higher hardness.

Also, the elements should be of improved surface roughness to avoid any dents/damage to the parts.

12) Conveyors

Conveyors are for transferring parts from one to another stage by the use of magnetic rails, and where possible usage of gravity is also intended to reduce power consumption.