Week 7:- Base Unit Design Challenges 1

Q1. What is Base unit? What are the typical parts of the base unit?

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 be also be mounted on to a base unit.

Below are some parts of a base unit in general.

1. Trunking channel

2. Base plate

3. Valve box

4. Measuring hole cover

5. Eye bolt

6. Pedestal

7. Weld bars

8. Floor plate

Image : Typical parts of base unit

Image : Example of a welded base structure

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.

Fundamentals of base plate unit design :

• A base plate size should cover all the units within its outer periphery including opening.
• A base plate is preffered 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 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.

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.
• 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 accomodate all the pneumatic and electrical hoses and wires for the units of the tool.

Image : Trunking channel

Valve Box :

Valve box is a standard part which will accomodates the pneumatic valves for the units.

Image : Valve box

Eye Bolt :

Eye bolt are used for the transporting of the tool, using lift hooks. Fork lift holes will also be provided in some cases for transporting purpose.

Image : Eyebolt

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 the levelling screws, for levelling the entire tool.
• Weld bars are used to weld the positions permanently, after levelling is done.

Weld bar :

A weld bar is a plate along the sides of the base plate of the pedestal. Weld bars are used to weld the positions permanently, after welding is done.

Q2. What are the points need to account while designing a base unit ?

• Working height 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 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 base 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 welding height we need to provide additional some leg structure which may have top and plate welded to the tube.
• As a 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 needs to take care of flatness if the base size is very long.
• Depending upon the weight eyebolts size is selected for lifting holes on base plate. Generally 4 eye bolt provision is made. You also need to select the location of eyebolt considering the rope/belts used while shifting the structure or complete fixture are not damaging any units nearby.
• A base plate size should cover all the units within its outer periphery including opening.
• A base plate is preffered 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 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.
• Most of the OEM makes 4 dowel holes 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.
• The base unit of the fixture should have the mass to prevent vibration and chatter.
• 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.
• The base unit may be built from simple section so that other units may be fastened with screws or welded, if 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.
• The base unit should be parallel to the floor, to adjust this there should be provision of the height adjusting mechanism.

Q3. What is dump unit ?

A dump unit is a unit where you actually assemble a component which if you have certain constraints. 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, 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.

Image : Dump unit

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.

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.

Q4. What are the points you need to consider while designing a dump unit ?

Dump unit is a unit which has a lot of parts. Hence, we need to be extra cautious while designing a dump unit because any mistake can result in irreversible errors that can create wastage of time & money. Hence, we should check all the points that should be addressed so that we don't face problems at a later stage.

Points needed to consider while designing a dump unit ;

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 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 :

We need to make sure that the assembly being dumped should have as less weight as possible without compromising on the strength of the assembly. This is because the lesser the weight lesser will be the force required to push and pull the assembly. Here, we apply material as aluminium on cylinder and steel on the rest of the parts to calculate mass of moving assembly. We take mass a bit more than calculated for the factor of safety.

3. Time of opening and closing:

We also need to keep in mind that the time of opening and closing of the clamp should be least possible and this will depend on the power of the cylinder and the weight of the assembly being dumped. We need to keep it less because if it takes say 2 seconds to open then for that time 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 it should be able to push the load for the operation and then it pulls back after the operation is complete. Pivot point are important to lift the whole assembly by using the cylinder.

5. Open condition :

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

Q5. What is a slide unit ?

A slide unit is majorly used for difficult panels like (tunnel assembly) to be weld. 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, slide moves back.

Image : Slide unit

• It has a cylinder which 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 open state or close state to identify.
• It has a locking system which 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.

Guide rails should be lubriacted well.

The lubricants has the following roles,

1. Minimises friction in the travelling unit to prevent seizure and reduce wear.

2. Forms oil on the raceway to decrease stress acting on the surface and extend rolling form.

3. Covers the metal surface to prevent rust formation.

Q6. 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 consider while designing a slide unit are,

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 calculate 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 because, 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 a 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 slides moves forward, the sensor gives the 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 the ones that 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.