Week 7:- Base Unit Design Challenges 1

Base Unit Design Challenges 1

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

Answer. A base unit is the unit designed to mount all the other units (Clamp unit, Pin unit, rest unit, etc.) of the tool,also other parts necessary for a tool like pneumatic valve box, trunking cable path will also be mounted on to a base unit.

• trunking channel
• base plate
• vlave box
• measuring hole cover
• eye bolt
• pedestal
• weld bars
• floor plate

Base Plate: Base plate is machined plate of 25 to 40mm thickness made of steel and aluminium,on to which the holes are drilled to mount be individual units like clamp unit,pin unit etc. of a tool.

Pedestal: A pedestal is a part which connects the base plate to the floor plate. A pedestal is a standard part in general,and in some case it can be a welded frame integrated with the base plate.

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

Trunking Channel: A trunking channel is a duct which accommodates all the pneumatic and electrical hoses and wires for the units of the tool.

Valve Box: Valve box is a standard part which will accommodates the pneumatic valves for the units.

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 transportinig purpose.

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 10mm maintained between the pedestal base plate and the floor plate.
• this clearance will be used by the levelling screw, for levelling the entire tool.
• weld bars are be used to weld the positions permanently,after levelling is done.
• A weld bar is a plate along the sides of the base plate of the pedestal.

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

Answer.

• 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 BSC coordinates.
• A base plate should be drilled with datum holes/measuring holes in line with a pitch of 1000mm in general.
• Number of datum holes is depend on the size of the base plate.
• Datum holes should be drilled at nominal values of coordinates, and in terms of 50 and 100. (in general)
• Ex: X=100,Y=-600,Z=250
• Datum holes should be drilled with Dia 100mm away from other units.
• Datum holes should be covered with cover plates

• 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 the base structure is made with a 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 welded to the tube.

2 As a designer its is important to know how big base plate or bigger base welded structure will be handled during the fabrication and handling process. Special care needs to be taken care of flatness if the base size is very long.

• Depending upon the component size and 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.

6 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.

7 The Base unit of the fixture should have the mass to prevent vibration and chatter.

8 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.

9 The base unit should cover all the units with its outer periphery including the opening.

10 The base unit should be parallel to the floor, To adjust this there should be the provision of the height adjusting mechanism.

Question: 3 What is a Dump unit?

Answer.

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.

Question: What are the points you need to consider while designing a Dump unit?

Answer.

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.

Question: What is a Slide unit?

Answer.

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

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.

Question: What are the points you need to consider while designing a Slide unit?

Answer.

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.