Week 7:- Base Unit Design Challenges 1

1) What is the base unit? What are the typical parts of the base unit?

Ans: A base unit is the unit designed to mount all other units (clamp unit, pin unit, rest unit etc) of the tool also other parts necessary for a tool like pneumatic valave box,trunking cable path will also be                     mounted on to a base unit.

                                         A BASE UNIT                                                                                                                                        WELDED BASE STRUCTURE

DIFFRENT TYPE OF BASE UNIT

                  FIG. 01                                                                                                                      FIG.02

Below are the some parts of a base unit in general 

• Trunking channel
• Base plate
• Valve box
• Measuring hole cover
• Eye bolt
• Pedestal
• Weld bars
• Floor plate 

Typical parts of a base unit

a) BASE PLATE

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

                                                                                                              A BASE PLATE

FUNDAMENTALS OF BASE PLATE DESIGN

• 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 BCS 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 boxplate.
• Datum hole should be drilled at nominal value of coordinates and in terms of 50 & 100 in general Ex. X= 100, Y=-600, Z=250.
• Datum holes should be drilled with diameter 100mm away from the other unit.
• Datum holes should be covered with cover plates.

b) PEDESTAL

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

                              A PEDESTAL UNIT

FUNDAMENTALS OF PEDESTAL DESIGN

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

c) TRUNKING CHANNEL

A trunking channel is a duct which accomodated all the pneumatic and electrical hoses and wires for the units of the tool. The trunking channel includes all the pressure inlet cables going to the pneumatic parts of the fixture and protects them from damaging while operation is in process.

            A TRUNKING CHANNEL

d) VALVE BOX

Valve box is standard part which will accomodates the pneumatic valaves for the units. It is kind of control unit. The unit valave box is the box which includes different solenoid valaves which is used to control the valaves.

                                                                             A VALAVE BOX

e) EYE BOLT

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

                                                                                                         A EYE BOLT

f) MEASURING HOLE COVER

The base plate has some drills to mount various units like clam,rest,pin etc. and every drill have some coordinates to define the exact location of the hole.Due to irregular shape of the car panel the drilled holes coordinates are not like 110.0,150.0 but like 110.62,150.84. The measuring hole is drilled over the base plate which having exact coordinates so that all other holes can take reference from it.The measuring hole number depends on the size of base plate.These holes are covered by using some plate to protect against dust,damage etc.

g) FLOOR PLATE

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

                                   A FLOOR PLATE

FUNDAMENTALS OF FLOOR PLATE DESIGN

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

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

Ans: Following points need to be taken into consideration while designing the base unit.

       a) Working height: It depends on the nature of the manual or robotic welding fixture.

                                   For manual fixture we need to consider the working height 850-1100mm depending upon the height of the worker stanadard of a specific country will apply( in few countries human                                             height is higher or lower eg.(US, Japan)

                                   If base unit structure is made with base plate having tubular structure welded below it, we can reach to height 100mm to 250mm. In this case to reach the working working height we                                           need to provide additional some leg structure which may have to and bottom plate welded to the tube.

       b) As designer it is important to know how big base plate or bigger base welded structure will be handled during the fabrication and handling process. Special care need to take care of flatness if the base                 size is very long.

Fundamental of base plate design

• 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 BCS 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 boxplate.
• Datum hole should be drilled at nominal value of coordinates and in terms of 50 & 100 in general Ex. X= 100, Y=-600, Z=250.
• Datum holes should be drilled with diameter 100mm away from the other unit.
• Datum holes should be covered with cover plates.

c) Depending upon the weight eyebolt size is selected for lifting holes on the base plate. Generally four eye bolt provision is made. You also need to select the location of eye bolt considering the rope/belt used          while shifting the structure or complete fixture are not damaging any units ( rest,clamp or location units) nearby.

d) Only base plate or welded structure.

    Depending upon the componenet size and weight of the total units you can decide whether you need a fabricated base or just plate. Moatly fabricated bases are used made of square or rectangualr tube with        the plate.

e) Most of the OEM makes 4 dowel holes at 4 corner and put small aluminium plate marking x,y and z coordinates of dowel holes. If the base length is too long few OEM's follow the methodology of having holes        at 500mm distance.

f) The best unit of the fixyure must be strong enough so that deflection of the fixture is as minimum as possible. The deflection of a fixture is caused because of forces of welding,clamping of the work piece.

g) The base unit of the fixture should have the mass to prevent vibration and chatter.

h) The base unit may be built from simple section so that other units may be fastened with screw or welded 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 screws.

i) The base unit should be paralled to the floor, to adjust this there should be provision of the height adjusting mechanism.

3) What is dump unit?

Ans: Dump unit is the complex unit to design due to space constrains there is less space available and have to adjust all the pats in available space.It consists of dump cylinder to perform the operation of clamping and unclamping car panel.

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 carrying clamp mylars,pin and 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.

                                    A DUMP UNIT

Function of 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.
• 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 ched against the weight of the load being carried. If the cylinder force is more,it is validated or we use more powerful cylinder.

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

Ans: 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.

a) 3-2-1 Principle

The resting location of the panel being welded is checked in accordance with the designed mylars,pins and clamps. This will be done by the assembly being dumped onto the panel to operate. It is important because it is the essential reason for creating a dump unit.

b) Mass consideration

The assembly being dumped should have as little weight as possible without compromising the strength. Less weight means less force is recquired to push and pull the assembly.

c) Time of opening and closing

The time of opening and closing of the clamp should be less. This basically depends on the capacity of the cylinder and the weight of the assembly being dumped. More time will result in an increase in the cycle time leading to more idle time.

d) 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 the operation and then pull it back after the operation is complete.The centre of gravity of the unit and the distance between the cylinder centerline and end effectors.

         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 the 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 forces is more than the required force, the cylinder gets validated or else we change the cylinder.

e) Open condition

We also need to check the open position after the cylinder validation because the panel should not fowl with the unit while removed.

5) What is a slide unit?

6) What are the points you need to consider while designing a slide unit?

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

a) 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.

b) 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.

c) Locking mechanism

The locking mechanism ensure 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 position accuracy will be hampered.

d) Stopper blocks

Stopper block 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 the proper resting and location without a stopper,it may dent the panel.

e) Sensor unit

Sensor unit helps to check the position of the slide. When the position 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.

f) Stroke

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

g) Open position

The only moving parts are that one the slides,so we need to check the following with a gun or other unit nearby. Only after this has been checked that we can approve the design.

      Slide unit in working position for LWB variant                   Slide unit in working position for SWB Varient