Week 7:- Base Unit Design Challenges 1

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

Base unit is a unit which is used to mount all the units such as clamp unit, pin unit, rest unit etc., of a tool and also other necessary table parts like pneumatic valve box, trunkin cable path.

Typical parts of base units are given as in the below figure,

Base plate:

The base plate is machined plate of 25 to 40 mm thickness made of steel & aluminum, on to which the holes are drilled to mount the individual units like clamp units, pin etc., of a tool.

Pedestal:

A pedestal is a part which connects the base plate to floor plate. A pedestal is a standard part in general & in some cases it can be a welded frame integrated with the base plate.

• A standard pedestal can be of different height & different sizes.
• Depends on the size of the base plate, the number of pedestal varies for load distribution.
• A pedestal is of different types with different inclinations.

Trunking Channel:

A trunking channel is duct which accommodates all the pneumatic & electrical hoses and wires for the units of the tool. It has thickness of few millimeters.

Valve box:

Valve box is standard part which will accommodates all the pneumatic valves for the 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 pneumatic parts of the fixture & protects them from damaging while operation is in process.

Eye bolt:

Eye bolts are used for the transporting of the tool, using lift hooks, fork lift holes will also be provided in some cases for transporting purpose. For example, we usually use M20 bolts that are bolted on the base plate.

Floor plate: Floor plate is grouted to the floor with fixed bolt on the ground. Floor plates are also called as foundation plates

• There will be a clearance of 10 mm maintained between the pedestal base plate & floor plate.
• This clearance will be used by the leveling screws, for leveling the entire tool.
• Weld bars are be used to weld the positions permanently after leveling is done.
• A weld bar is a plate along the sides of the base plate of the pedestal.

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

The points need to be taken into account while designing the base unit are:

Working Height:

• It depends on the nature of the manual or robotic welding fixture.
• For manual fixtures working height of 850-1100 mm is needed to be considered, depending upon the working height of the worker standard of a specific country will apply (In a few countries human height is higher or lower e.g. US, Japan).
• If the base structure is made with a base plate having a tubular structure welded below it then we can reach to height of 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 the top and plate welded to the tube.
• Try to study the gun ergonomics properly then design the unit height and then base top height.

Base Plate or Welded Structure:

• Depending upon the component size and weight of the total units we can decide whether we need a fabricated base or just a plate.
• Mostly fabricated bases are used made of square or rectangular tubes with plates.

Selection of Eye Bolt:

• As a designer, it is important to know how a big base plate or bigger base welded structure will be handled during the fabrication and handling process.
• Depending upon the weight eye bolt size is selected for lifting holes on the base plate.
• Generally, 4 eye bolt provision is made. We also need to select the location of the 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.

Number of Measuring Holes Required:

• 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 due to the forces of welding, clamping of the workpiece.
• The base unit of the fixture should have the mass to prevent vibration & chatter.
• The base unit may be built from a simple section so that other unis are fastened with screws or welded necessarily.
• 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 it there should be the provision of the height adjusting mechanism.

Fundamentals of Base unit:

• A base plate size should cover all the units with its outer periphery including the opening.
• A base plate is always 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 m in general.
• The number of datum holes depend on the size of the base plate.
• Datum holes should be drilled at nominal values of co-ordinates and in terms of 50 and 100. Example, X=100, Y=-600, Z=250.
• Datum holes should be covered with cover plates.

3. What is a Dump unit?

Dump unit is mostly used on different panels where the welds are not achieved easily. sometimes it happens that there are some hard-to-reach panels, and which cannot be fixed by simple units .in such case fix them we will use dump unit

The first cylinder dumps the assembly carrying clamp mylar, pins, and then this assembly performed the required operations. 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 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.
• The dump unit should have correct shimming for adjustment of the panel.

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

A dump unit is complex to design because of lot of parts, we have to be considerate while designing it as any errors during designing stage creates a lot of problem at the later stage which would result in loss of time and money and eventually more time and money to rectify the problems. Following are the points we 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 amd 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.

5. What is a Slide unit?

A slinding unit is majorly used for difficult panels like tunnel assembly to be welded. Slide unit can have a clamp or a pin or a rest unit attached to it which slides forward and performs the required operations. On completion of the operation, the slide moves back.

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Slide units can be calssified into two types on the base of the type of guide-ways used as

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

1)Horizontal slide

2)Vertical slide

3)Angular slide

- Slide units are used when we have some space constraints. So we dont have space to design different special units & to assemble it. In those case we use sliding units.

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

Slide unit comes under the category of complex units, few points that need to be considered while designing a slide unit are,

The following points must be kept in mind 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 open or retracted 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

The 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. Reserve stroke of 10mm to be considered in both forward and retracted conditions. Cylinder and LM guides must be validated for load and stroke. LM guide rail length must be standard and more than the actual stroke

• 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.