Week 7:- Base Unit Design Challenges 1

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 channel, base plate eye bolt will also be mounted on to a base unit.

Fig shows: Parts of the base unit

• It is the fundamental component of the fixture because all the other components are mounted over it. So every fixture has base unit. The base unit is different according to the panel, design of clamp system, location system, etc.,

Typical parts of the base unit:

Base Plate:

Fig shows: Base plate of a fixture

• The base plate is a plate of 25mm to 40mm thickness.
• It has a no of holes drilled on it which acts as mounting points for clamp unit, pin unit,etc.,
• It holds the trunking channel, eye bolts, valve box and some other parts.

Pedestal:

Fig shows: Fxture having no of pedestals

• It is used to connect the base plate with the floor plate.
• It withholds the entire tool onto the floor plate. Hence it must be strong and rigid enough to withstand the forces and weight of the tool.
• It can be a standard part or a make part, depending on the size and height of the fixture.
• Standard pedestal are available in different heights.
• Pedestals are also available in different inclinations.
• Depending on the size of the base plate, the number of the pedestal for a fixture can vary for load distribution.

Trunking channel:

Fig shows: Trunking channels

• Trunking channel is a duct that carries all the electrical and pneumatic connections like wires and hoses for units of the tool.
• It has some thickness of few millimetres.

Valve box:

Fig shows: 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 and 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 being damaged while operation is in process.

Eyebolt:

Fig shows : Eye bolts

• Eye bolts are the bolts that are used to lift the fixture for transportation by inserting hooks into them.
• Generally M20 bolts are used as eye bolt which are bolted on the base plate.
• The main purpose is for transportation, sometimes forklift holes will be provided.

Floor plate:

• They are also known as foundation plate. They are grounded by using a fixed bolt.
• 10 mm clearance should be given between the pedestal base plate and floor plate.
• Weld bars are welded after the levelling is done by using levelling screw.
• Weld bars are steel plates along the sides of the base plate of a pedestal.
• They are welded to permanently maintain the proper position of the fixture.

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

Answer:

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

1. Working Height:

• It depends on the nature of the manual or robotic welding fixture.
• For manual fixture we need to consider the working height of the worker, standard of a specific country will apply. ( In few countries human height is heigher or lower e.g. The USA, Japan)
• If the base structure is mad with a base plate having tubular structure welded below it, we can reach to a height of 100 mm to 250 mm.
• In this case to reach the working welding height, we need to provide an additional some leg structure which may have top and plate welded to the tube.
• As a designer its important to know how big base plate or bigger base welded structure will be handled during the fabrication & handling process. The special case needs to take care of flatness if the base size is very long.
• Depending upon the weight eye bolt size are 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.
• 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.
• 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?

Answer:

• Dump unit is mostly used on difficult panels where the welds are not achieved easily. Sometimed, it happens that there are some hard-to-reach panles and which cannot be fixed by simple units, in such case to fix them we use a dump unit.
• The first cylinder dumps the assembly carrying clamp mylars, pins and then this assembly performs the recquired operation. In this unit, we should make sure that in the open position of the panel, we should be able to remove the panel.

Fig shows : Dump unit

Functions of the Dump unit:

• To ensure proper resting of the panel being welded and to restrict its motion.
• A dump unit should have 2 cylinders one is to clamp the mylars at the localized point and the other cylinder used to dump thewhole unit.
• In the dump unit,one assembly is fixed and whole fixed assembly will mount on the dump unit cylinder.
• Programming is the main part to function dump unit.Because when the 2 cylinders move at the same time programming will play the role.
• Cylinder validation is more important in the case of a dump unit as the weight can be too heavy for the cylinder to dump.
• In such a case, calculations are done and force exerted by the cylinder is checked against the weight if the load being carriend.
• If the cylinder force is more, it is validated or we use a morepowerful cylinder.

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

Answer:

Fig shows:Dump & slide unit

• A dump unit is a complex unit which has a lot of parts. Hence, we need to be extra cautious while designing a dump unit. Any mistake that made in design can result a lots of error that will create wastages of time and money. Hence we should check all the points that they are clearing all areas for all operations.
• Most probably we should check it on with the simulation software like ROBCAD, DELMIA V5 and process simulate if there any clash are hiding on catia these software will show the particular area of clashing.

3-2-1 Principle:

• This will be done by the assembly being dumped onto the panel to operate. It is very important to create a dump unit. The position of the panel orientation, pivot point and PLP points are important to check while designing.

Mass of the unit:

• We need to ensure that the whole assembly being dumped should have less weight as possible without compromising on the strength of the parts. If we design the unit very little in weight there will be more possible to break the whole assembly.
• So design as much as possible to maintain the strength and reducing the weight . Because mass has an important role to play inn the dump units.

Fig shows: Mass of the part in a dump unit

Time of opening and closing:

• We also need to keep in mind that the opening & closing of the clamp will cause damages to other units or assembly. The timing of the cylinders in the dump unit should also vary or must not be simultaneous.
• Because the panel may get damaged while having same opening or closing times.

Cylinder validation:

• Cylinder validation is the most important thing to create a dump unit. Because the cylinder should sustain the various motions of the arms and to be long lasting. It should be validated for proper functioning on the pivot joints. Pivot points are important to lift the whole assembly by using the cylinder.

Calculating the centre of gravity of moving assembly:

• Calculating 'the longest perpendicuar 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 psition or anywhere in between.
• Calculating 'the shortest distance between pivot centre and cylinder line'.
• By adding 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.

Open condition:

• We also need to 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?

Answer:

• 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.

Fig shows: Slide unit

• It has a cylinder that pushes the assembly forward.
• Assembly is mounted on a plate, which is attached to guide rails.
• There are sensors on the extreme end of the slide to sense the position of the slide whether it is in the open state or closed state.
• It has a locking system that ensures the slide does not move from that place and it gives welding quality.
• Slide units are commonly used at places where there are space constraints. The opening and closing of the slide should not fowl.

Fig shows: Slide units

Fig shows: Slide unit with weld gun

Guide rails should be lubricated well:

The lubricants have the following roles,

• Minimizes friction in the traveling unit to prevent seizure and reduce.
• Forms oil on the raceway to decrease stress acting on the surface and extend rolling.
• Covers the metal surface to prevent rust.

Slide units are classified on the basis of type of guide way used:

1. Hardened guide ways
2. LM guide way

Slide units are classified on the basis of prime mover used:

1. Manual slides
2. Hydraulic/pneumatic slides
3. CNC slides

Slide units are classified on the basis of application:

1. Horizontal slide
2. Vertical slide
3. Angular slide

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

Answer:

A sliding 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 differnt load capacity. Hence, we calculate the weight of our assembly and calculate the weight capacity of the rail. Hence, we chose rail based on its load carrying capacity.

2. Push and Pull forces:

• We need to check the push and pull forces and check if its 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 doesnot 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 bent the panel.

5. Sensor unit:

• Sensor unit helps to check the pposition of the slide. When the operation is going to start, the operator pushes the button and the slide moves forward, the sensor gives 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 following with a gun or other unit nearby. Only after this has been checked that we can approve the design.