Week 7- Challenge 1

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



Base unit is one of the key units in a BIW Fixture. A Base unit is used 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.

Base unit has a base plate which can be of different shapes. The shape of the base plate should be such that all the units can be mounted without any space constraints. Sometimes, if the fixture weights too heavy, base plate can be given pockets to reduce some weight.

Typical parts of base unit are given as follows:

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.

Pedestal is a part or an assembly (welded structure) that joins the base plate to the floor 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.

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.


Trunking Channel image is shown above

Valve Bank is a standard part which contains all the valves for the pneumatic units.



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.



Eye bolts M20 is shown above

Floor Plates is grouted to the floor with fixed bolt on the ground. Floor plates are also called foundation plates. There is a clearance of 10mm between pedestal base plate and floor 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?

Following points need to be kept in mind while designin base unit:

1) Base unit should cover all the units along with its outer periphery containing the open conditions.

2) Base unit is preffered to be designed in carline coordinates.

3) No. of datum holes to be drilled depends on the size of base plate.

4) Datum holes should be drilled outside a diameter of 100mm of unit.

5) Datum holes should be nominal and should be in multiples of 50 and 100.

For example; X=250;Y=-150;Z=100

6) Datum Holes should be covered with hole cover plates.

7) Datum holes should be drilled in line and at a distance of 1000mm in general.




Q. What is a Dump Unit?


Sometimes, it happens that there are some hard to reach panels and which cannot be fixed by simple units, in such cases, to fix them, we use a dump unit. A dump unit can have more than one cylinder to perform such operation. The first cylinder dumps the assembly carring clamp mylars, pins and then 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.

Functons of dump unit:

To ensure proper resting of 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 doesnot move while welding.

Dump unit should have correct shimming for adjustment of panel.


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 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.



Q. 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 perform the operation. It is important because it is the reason why we are creating a dump unit. Positional accuracy of 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 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 mass of moving assembly. We take mass a bit more than calculated for factor of safety.


3) Time of opening and closing

We also need to keep in mind that the time of opening and closing of clamp should be least possible and this will depend on the power of 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 important aspect of dump unit or any clamp unit design because 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.


Calculating the Centre of gravity of moving assembly.


Calculating \"the longest perpendicular distance from 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.


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.

Q. 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.

*The opening and closing of the slide should not fowl

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.

Guide rails should be lubricated well.
The lubricant has the following roles:
(1) Minimizes friction in the traveling unit to prevent seizure and reduce wear.
(2) Forms an oil on the raceway to decrease stress acting on the surface and extend rolling life.
(3) Covers the metal surface to prevent rust formation.

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



A slide 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 diifferent load capacity. Hence, we calculate the weight of our assembly and calculate the weight capacity of the rail. Hence, we choose the 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

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 required postition. It is important as if the slide unit has rest mylars and pins, they need to be stopped at their respective postion for prper resting and location. Without a stopper, it may dentthe 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, sensor gives this signal of presence of 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 gun or other unit nearby. Only after this has been checked that we can approve the design.