Week 7:- Base Unit Design Challenges 1

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

Base Unit:

Base unit is the unit of the fixture which is designed as such so that other units of the fixture can be mounted on it. Apart from these units, other necessary tools like pneumatic valve box, trunking cable path will also be mounted on the base unit.

Base unit is the fundamental componet of the fixture because all other components are mounted over it.so every fixture have base unit. The base unit design is different according to the panel, Design of clamp system, location system etc.

Parts of the Base Unit:

• Base plate Pedestal
• Trunking channel
• Valve box
• Eye bolt
• Floor plate

Welded Base Unit:

1. BASE PLATE:

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

2. 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 cases it can be a welded frame integrated with the base plate.

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

3. Trunking Channel:

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.

4. Valve Box:

Valve Box is a standard part which contains all the valves for the pneumatic 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 the pneumatic parts of the fixture & protects them from damaging while operation is in process.

5. Eye Bolts:

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.

When installing lifting eye bolts it is critical to ensure face is in firm contact with the mating surface. These lifting rings are intended for linear pulling loads in the direction of the thread.

They are not intended for use where the load is perpendicular to the thread. This is because in these instances the bolt could unscrew and move or it could be twisted and break

6. 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.

Q.1: What are the points need to account while designing a Base Unit?

1. Working Height:

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

2. Base Plate or Welded Structure:

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

3. Selection of Eye Bolt:

• 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.
• 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 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.

4. Number of Measuring Holes Required:

• We would need 3 holes to define the plane.
• Most of the OEM makes 4 dowel holes at 4 corners and put small aluminum plate marking X, Y &Z coordinates of dowel holes.
• If the base length is too long few OEM's follow the methodology of having dowel holes at 500mm distance.

5. Base Plate:

• There are few OEM’s, Line builders use only thick base plate. Few OEM's recommend thick base plate 40mm or 50mm for the child part assemblies (e.g BMW). 40/50mm plate makes a very heavy structure but some OEM's follow methodology.
• If we go for higher thickness it will not bend after machining and will have good flatness.
• General observation is if we go for 10-12 mm thick plate for large size for e.g. 2mx2m, after machining when we unclamp plate from milling machine the plate will not show good flatness.

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 100mm in general.
• Number of datum holes depends on the size of the base plate.
• Datum holes should be drilled at nominal values of coordinates and in terms of 50 and 100.
• Datum holes should be drilled with diameter 100mm away from other units.
• Datum holes should be covered with cover plates.

Q.3: What is Dump Unit?

Dump Unit:

The dump unit is used when we have any constraints and we cannot use our normal units like clamp unit and pin unit etc. 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, 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.

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.

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.

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

As 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:

1. 3-2-1 Principle: Like every other fixture unit, this unit too has the function to restrict the motion of the panel which is being worked upon that is being welded. Other units like clamp unit will be mounted on this unit to hold to panel at place. Thus it is logical and important for dump unit to follow this principle.

2. The dump unit must be placed on the fixture such that it does not block or collide with other units of the fixture and it must not collide with the weld gun.

3. Same goes with the open position of the dump unit. It takes more space than rest of the units as in other units like clamp units, only the clamp arm open and closes but for dump unit, open position is the opening of clamp cylinder with its other parts. Hence we must take care that wherever we place dump unit, we must have a lot of space.

4. Cylinder Validation: It is the process of analyzing the cylinder used by doing some measurements and calculations to know if the cylinder used is proper. In cylinder validation process we determine the force required to open and close the moving part of the unit. If the force obtained is within the limits of the force specified on the cylinder then we can use this cylinder else we have change the cylinder.

Q.5: What is a Slide unit?

Similar to the dump unit, sliding unit is also used where we have space constraints. It consist guide block, guide rails, sensor unit, stopper block, position locking unit and slide cylinder.

1. Slide cylinder: This helps in actuation of the platform from closed position to open position and vice versa.

2. Sensor unit: This unit senses if the panel has is at its position or not.

3. Guide rails: This helps in movement of platform between open and close position.

4. Position locking unit: It locks the open or closed position.

5. Stopper: It restricts the further motion when in open position.

Classification of Slide Unit:

Based on type of guide ways:

• LM guide way
• Hardened guide way

Based on prime mover used

• Manual slides
• Hydraulic/Pneumatic slides
• CNC slides

Based on application

• Horizontal slide
• Vertical slide
• Angular slide

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

Following points must be kept in mind while designing a slide unit:

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.Reserve stroke of 10mm to beconsidered in both forward and retracted condition.Cylinder and LM guides must be validated for load and stroke. LM guiderail length must be standard and more than actual stroke

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.