Week 7:- Base Unit Design Challenges 1

Q1) What is the Base Unit? What are the typical parts of the Base Unit?

                                 Base unit is one of the main units in a BIW Fixture. A Base unit provides the platform to mount all the other units like rest units, clamp units and pin units etc. Base unit also mounts the components like Valve Bank, Junction Box, Eye Bolts, trunking cable path etc.

 

typical parts of the Base Unit

• Trucking Channel: Trunking Channel is a duct that carries all the electrical and pneumatic connections (wires and pipes) for units of the tool. It has a thickness of few millimetres. Valve Box: Valve Box is a standard part which contains all the valves for the pneumatic units.

• Base Plate: Base Plate is a machined plate made of steel or Aluminium of thickness 25mm to 40mm on which the holes are drilled to mount different units like rest units, clamp units, pin units etc.

• Valve box:Valve Box is a standard part which contains all the valves for the pneumatic units. It is a kind of control unit The valve box is the box which includes different solenoid valves which is used to control the vales. 

• Measuring hole cover:The measuring hole is drilled over the base plate which having exact coordinates and acts as reference for the other holes. Due to irregular shape of the car panel the holes coordinates are not nominal like 250.0, 310.0 But it like 250.62,310.84. These holes are covered by using some plate to protect against dust, damage etc.
• Eyebolt:Eye Bolts are the bolts that are used to lift the fixture for transportation by inserting hooks into them. For example, we usually use M20 bolts that are bolted on the base plate. Used for transportation purpose, sometimes forklift holes will be provided.

 

• Pedestal: Pedestal is part used to connect the base plate with a floor plate. The pedestal can be a standard part or a make part, depending on the size and height of the fixture. 
• Weld bar:Weld bars are used after the levelling is done by levelling screw. Weld bars are steel plates along the sides of the base plate of a pedestal. Weld bars are used to position the positions permanently.
• Floor Plate:They are also known as foundation plate. They are grounded by using a fixed bolt. minimum of 10 mm clearance should be given between pedestal base plate and floor plate.

Q2) What are the points need to account while designing a Base unit?

• Working height: It depends on the nature of the manual or robotic welding fixture. For manual fixture we need to consider the working height 850-1100 mm, depending upon the working height of the worker standard of a specific country will apply (in few countries human height is higher or lower e.g. (US, Japan).If base structure is made with base plate having tubular structure welded below it we can reach to height 100 mm to 250 mm. In this case to reach the working welding height we need to provide additional some leg structure which may have top & plate welded to the tube.
• As designer it is important to know how big base plate or bigger base welded structure will be handled during the fabrication & handling process. Special care needs to take care of flatness if the base size is very long.
• A base plate size should cover all the units with its outer periphery including opening.
• A base plate is preferred to be designed with respect to BCS coordinates.
• A base plate should be drilled with datum holes / measuring holes in line with a pitch of 1000 mm in general.
• Number of datum holes is depending on the size of the base plate.
• Datum holes should be drilled at nominal values of coordinates, & in terms of 50 x 100. (In general) ex. X=100, Y 600, Z-250.
• Datum holes should be drilled with dia 100 mm away from other units.
• Datum holes should be covered with cover plates.
• Depending upon the weight, eye bolt size is selected for lifting holes on base plate Generally, 4 eye bolt provision is made You also need to select the location of eye bolt considering the rope/ belts used while shifting the structure or complete fixture are not damaging any units (rest, clamp or location units) nearby.
• Depending upon the component size & weight of the total units you can decide whether you need a fabricated base or just plate. Mostly fabricated bases are used made of square or rectangular tube with the plate.
• Most of the OEM makes 4 dowel holes at 4 corners & put small aluminium plate marking X Y&Z coordinates of dowel holes. If the base length is too long few OEM' s follow the methodology of having holes at 500 mm distance.
• The best unit of the fixture must be strong enough so that deflection of the fixture is as minimum as possible. This deflection of a fixture is called because of forces of welding, clamping of the work piece.
• The base unit of the fixture should have the mass to prevent vibration & chatter.
• The base unit may be built from simple sections so that other units may be fastened with screw or welded necessary. Those parts of the fixture that remain permanently with the fixture may be welded. Those parts that need frequent changing may be held with screws.
• The base unit should be parallel to the floor to adjust this there should be provision of the height adjusting mechanism.

Q3) What is a Dump unit?

                Dump unit is where you assemble component if you have certain constraint. Here you will have clamp unit, pin unit attached panel will be assembled. So if you have certain constraint you cannot assemble in normal way, Dump unit uses it in different way. It consists of dump cylinder to perform the operation of clamping & unclamping car panel. A dump unit can have more than one cylinder to perform such operation. The first cylinder dumps the assembly carrying clamp Mylar, pins and then this assembly performs the required operation in this unit, we should make sure that in the open position of panel, we should be able to remove the panel.

Q4) What are the points you need to consider while designing a Dump unit?

• 3-2-1 principle: Once we have designed the rest mylars, pins and clamps and added a cylinder for clamping. We will check the resting location and clamping of the panel being welded. This will be done by the assembly being dumped onto the panel to operate. It is important because it is the reason why we are creating a dump unit. The positional accuracy of the panel is of utmost importance.
• Mass consideration: The assembly being dumped should have as less weight as possible without reduction in the strength of the assembly. This is because the lesser the weight, lesser will be the force required to push and pull the assembly. We have to consider factor of safety as well.
• Time of opening and closing: The time of opening and closing of the clamp should be less. This basically depends on the power and capacity of the cylinder and the weight of assembly being dumped. We need to keep it less because if it takes say 2 seconds to open then for that time we will not be able to unload the panel which will increase the cycle time resulting in more idle time.
• Cylinder Validation: Cylinder validation is an important aspect of the dump unit or any clamp unit design because the cylinder should be able to push the load for operation and then pull it back after the operation is complete. The centre og gravity of the unit and the distances between the cylinder centre lines and end effectors.
• Open Condition: We need to also check the open condition after the cylinder validation because the panel should not fowl with the unit while removal.

Q5) What is a Slide unit?

                       Slide unit are positioning units which are used to position the fixtures, weld gun, units, robots into position for further operations.

here are different types of slide units:

• hardened guide ways
• LM guide way

Slide units can be classified into three types on the base of the prime mover used:

• Manual slides
• Hydraulic/pneumatic slides
• CNC slides

It may also be classified depending on application as follows:

• Horizontal slide
• Vertical slide
• Angular slide

Q 6) What are the points you need to consider while designing a Slide unit?

• Weight carrying Capacity:There are different kinds of guide rails available in the market, each having a different load capacity. Hence, we calculate the weight of our assembly and calculate the weight capacity of the rail. Hence, we choose rail based on its load-carrying capacity.
  
• Push and Pull Forces:We need to check the push and pull forces and check if it is enough to push and pull the slide especially when the slide is at an inclination. 
  
• Locking Mechanism:The locking mechanism ensures that the slide does not move while it is at open or retract position. If the slide moves while welding, it can damage the panel as the positional accuracy will be hampered.
  
• Stopper Blocks:Stopper blocks are used to stop the slide at the required position. It is important as if the slide unit has rest mylars and pins, they need to be stopped at their respective position for proper resting and location. Without a stopper, it may dent the panel.
  
• Sensor Unit:Sensor unit helps to check the position of the slide. When the operation is going to start, the operator pushes the button and the slide moves forward, the sensor gives this signal of the presence of the unit to the PLC which then performs the next operation.
  
• Stroke:We need to select the length of stroke based on the work area we want to get cleared.
  
• Open position:The only moving parts are that one the slides, so we need to check the fowling with a gun or other unit nearby. Only after this has been checked that we can approve the design.