Week 7:- Base Unit Design Challenges 1

WEEK - 7

Base Unit Design Challenges 1

1. What is Base Unit. What are the typical parts of the Base Unit?

• A base unit is the foundation of a fixture. It is the unit to which all other units are mounted. The base unit must be strong and rigid to support the weight of the workpiece and the forces generated during the manufacturing process.
• Base units are typically made of steel or aluminum. They can be fabricated from scratch or purchased as pre-fabricated units. Pre-fabricated base units are available in a variety of sizes and configurations to meet the needs of different manufacturing applications. Base units should be designed to be as versatile as possible. This will make it easier to adapt the fixture for different workpieces and manufacturing processes.

• Typical Parts of Base Unit:
• Base Plate:A flat supporting plate or frame at the base of a column, designed to distribute the column's weight over a greater area and provide increased stability.The base plate has no of holes drilled on it which will act as mounting points for clamp unit, pin unit and so on. Base Plate is a machined plate made of steel or Aluminium of thickness 25mm to 40mm on which the holes are drilled to mount diffrent units like rest units, clamp units, pin units etc.
• Pedestal:Pedestal is a part or an assembly (welded structure) that joins the base plate to the oor plate. Pedestal can be a standard part or a make part, depending on size and height of fixture. Standard pedestals are available in different heights. Pedestals are also available in different inclinations.Depending on the size of base plate,the number of pedestal for a fixture can vary for load distribution.
• 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. The valve box is the box which includes different solenoid valves. The pneumatic fixtures work on air pressure provided to it. So the arrangement of air pressure inlet & outlet is done within this valve box. The trunking channel includes all the pressure inlet cables going to the pneumatic parts of the fixture & protects them from damaging while operation is in process.
• Eye Bolt:Eye Bolts are the bolts that are used to lift the fixture for transportation by inserting hooks into them.
• Measuring hole cover : The measuring hole is drilled over the base plate which having exact cordinates 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.
• Floor Plate:Floor Plates is grouted to the floor with xed bolt on the ground. Floor plates are also called foundation plates. There is a clearance of 10mm between pedestal base plate and oor plate. This gap is used to level the entire tool using leveling bolts. Weld bars are used to position the positions permanently. Weldbars are steel plates along the sides of base plate of pedestal.

2. 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 500-1100 mm , depending upon the working height of the worker standard of 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 you can reach to height 100 mm to ~250 mm.In this case to reach the working welding height you need to provide additional some leg structure which may have top and plate plate welded to tube.Study the gun ergonomic properly then design the unit height and then base top height.

• Eyebolt Size:It is important to know how big base plate or bigger base welded structure will be handeled during the fabrication and handling process.Depending upon the weight eye bolt size are selected for lifting holes on the base plate.Generally 4 eye bolt provision is made.It 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).
• Plate Thickness:Few OEM's recommend thick base plate 40mm or 50mm for the child part assemblies (e.g BMW). 40/50mm plate makes a very heavy structure but some OEM's follow methodology.
• Depending upon the component size and weight of the total units you can decide whether you need fabricated base or just plate.
• The base 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 caused because of forces of welding, clamping of the workpiece. The Base unit of the fixture should have the mass to prevent vibration and chatter.
• The base unit may be built from simple sections so that other units may be fastened with screws or welded whenever 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 the screws.
• The base unit should cover all the units with its outer periphery including the opening. The base unit should be parallel to the floor, To adjust this there should be the provision of the height adjusting mechanism.
• Most of the OEM makes 4 dowel holes at 4 corners and put small aluminum plate marking X,Y &Z coordinates of dowel holes.If the base length is too long few OEM's follow the methodology of having dowel holes at 500mm distance
• The base unit should cover all the units with its outer periphery including opening. The base unit should be parallel to the floor, To adjust this there should be the provision of height adjusting mechanism.

3. What is a dump Unit?

• A dump unit is a positioning unit used in BIW welding Fixtures to position car body parts when geometrical constraints prevent the use of standard positioning units, like clamp and pins. Dump units are complex to design due to space constraints and consist of a dump cylinder that clamps and unclamps a car panel. When designing a dump unit, it's important to consider that the designed parts should be small but still function effectively, and that the dump unit will contain a heavier dump cylinder.
• When we have some geometrical constraints then it is difficult to place units like clamp unit, pin unit etc. In this case the dump unit is used. Dump unit is the complex unit to design due to space constains there is less space avilable & have to adjust all the parts in available space. It consist of dump cylinder to perform the operation of clamping & unclamping car panel. The first cylinder dumps the assembly carrying clamp mylars, pins and then this assembly performs the required operation. In this unit, we should make sure that in the open position of the panel, we should be able to remove the panel.
• Functions: -
• Resting and restricting motion: Ensuring the panel being welded is properly rested and its motion is restricted
• Clamping force: Ensuring the cylinder has enough clamping force to withstand welding
• Shimming: Providing proper shimming for panel adjustment
• Panel rotation: Restricting panel rotation using side blocks or magnets
• Opening and closing: Performing opening and closing actions with a dump cylinder

4. What are the points you need to consider while designing a Dump unit?

• Dump unit is a complex unit which has a lot of parts. Hence, we need to be extra cautious while designing a dump unit because any mistake in design can result in irreversible errors that can create wastage of time and money. Hence, we should check all the points that should be addressed so that we do not face problems at a later stage. Points to be considered are mentioned below:

1) 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.

2) Mass - We need to make sure that the assembly being dumped should have as less weight as possible without compromising on the strength of the assembly. This is because the lesser the weight, lesser will be the force required to push and pull the assembly. Here, we apply material as aluminium on cylinder and steel on the rest of the parts to calculate the mass of moving assembly. We take mass a bit more than calculated for the factor of safety.

3) Time of opening and closing - We also need to keep in mind that the time of opening and closing of the clamp should be least possible and this will depend on the power 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.

4) 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. For this following calculations are necessary.

5. Calculating the Centre of gravity of moving assembly.

• Calculating "the longest perpendicular distance from a vertical line that passes through the C.G and pivot centre." The longest perpendicular distance could be in the work or in rest position or anywhere in between.

• Calculating "the shortest distance between pivot centre and cylinder line.
• "We add all these values in the table along with the cylinder bore and the results are shown whether the cylinder is validated or not according to the formula used in the table for push and pull forces. If the calculated force is more than the required force, the cylinder gets validated or else we change the cylinder.

6) 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.

5. What is Slide Unit?

• A slide unit can have a clamp or a pin or a rest unit attached to it whch slides forward and performs the required operation. On completion of the operation, slide moves back. It has a cylinder which pushes the assembly forward. Assembly is mounted on a plate, which is attached to guide rails. There are sensors on the extreme ends of the slide to sense the position of slide. It has stop docks on either sides of the slide to stop the cylinder at required position. It has a locking mechanism which ensures that the slide does not move in a particular position as it may affect the welding quality and may damage the panel. Slide units are commomly used at places wher there are space constraints.

• Slide units are used at places where there are space constraints and places where there is less accessibility. Sometimes, there are too many small panels near each other and there is not enough space as each panel requires resting, location using 2 pins, clamping and sensing. This becomes even more difficult when panel is at angles. To solve this problem, a slide unit mounts rest, clamp or pin unit or all of them on a slide using guide rails, guide blocks, a cylinder, locking mechanism, stop blocks and sensors. When a particular panel is to be welded, this slide moves forward, fixes the panel and on completion of operation, slide moves back and gets locked at its position. Similarly, the same is done for other panels.
• There 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

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

• A sliding unit comes under the category of complex units. Few points that need to be considered while designing a slide unit are:
• 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.

CONCLUSION

• The above given questions are answered with relevant content and image as mentioned.