Week 7:- Base Unit Design Challenges 1

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

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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.

A typical Base unit and its parts

The typical parts of the base unit are as follows:

1)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 diffrent units like rest units, clamp units, pin units etc.

Typical base plate

2)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. Standard pedestals are available in different heights. Pedestals are also available in different inclinations. Depending on the size of the base plate, the number of a pedestal for a fixture can vary for load distribution.

3)Trunking Channel: Trunking channel a duct or tube that carries all the electrical wire and pneumatic pipe connections for units of the different tool and units. 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.

Typical truncking challenge

4)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. 

Typical valve box

5)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.

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.

Typical eye bolts

6)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.

7)Weld bar: Weld bars are used after the levelling is done by levelling screw. Weldbars are steel plates along the sides of the base plate of a pedestal. Weld bars are used to position the positions permanently.

2. What are the points need to account while designing a Base unit?

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• Working height 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 specific country will apply.
• 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 and plate plate welded to tube.

     The base unit of fixture must be strong enough so that deflection of the fixture should be minimum. This deflection of 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.

     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 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?

     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 of the dump unit:

• To ensure proper resting of the panel being welded and to restrict its motion.
• To ensure there are is no rotation in the panel and to fix it either by pins or side blocks or magnets.
• To ensure there is enough clamping force in the cylinder so that it doesn't move while welding.
• Dump unit should have correct shimming for adjustment of the panel.

considerations for the dump unit:

  A dump unit can have a slide cylinder or a clamp cylinder attached to it. Cylinder validation is important in case of a dump unit as the weight can be too heavy for the cylinder to dump. In such case, calculations are done and force exerted by the cylinder is checked against the weight of the load being carried. If cylinder force is more, it is validated or we use a more powerful cylinder.

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

  The following are the points to be considered while designing the dump unit otherwise it results in errors and waste of time and money

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 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.

3) 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.

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. The centre og gravity of the unit amd the distances between the cylinder centre lines and end effectors.

5) 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 a 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 accessibilty. 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, guideblocks, 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:

1)hardened guide ways

2)LM guide way

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

1)Manual slides

2)Hydraulic/pneumatic slides

3)CNC slides

It may also be classified depending on application as follows:

1) Horizontal slide

2)Vertical slide

3)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:

1)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.

2)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. 

3)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.

4)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.

5)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.

6)Stroke: We need to select the length of stroke based on the work area we want to get cleared.

7)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.