Week 7:- Base Unit Design Challenges 1

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

A base unit is the unit designed to mount all the other units, such as the pin,clamp and rest units for example, of the tool and also the other components or parts necessary for a tool like the valve box, trunking cable path and such will be mounted on the base unit.

The typical parts of a base unit are as follows:

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

• Base Plate: Base plate is a machined plate of 25-40mm thickness made of steel and aluminum, onto which the required holes are drilled to mount the individual units like the clamp unit, pin unit, rest unit etc of a tool.

• Valve Box: Valve box is a standard part which will accommodate all the pneumatic valves for the respective units.

• Measuring Hole Cover: This is the plate that is used to cover or protect the holes that we use as our reference for the remaining holes on the fixture from dust and other foreign particles.
• Eye Bolt: Eye bolts are used for the transporting of the tool with the help of lifting hooks. Fork lift holes are provided in some cases to accommodate for that way of transportation.

• Pedestal: A pedestal is a part of the base unit that connects the base plate to the floor plate. A pedestal is a standard part in general and in certain cases it can be a welded frame integrated with the base unit. Pedestals can be of different sizes and height depending on the requirement as this depends on the size of the base plate and the load it will have to take. They can also come with different inclination for the respective use cases.
• Weld Bar: Weld bar is a plate along the sides of the base plate of the pedestal. Weld bars are to be used to weld the positions permanently once the levelling of the base plate is done.
• Floor Plate: The floor plate is grounded to the floor with fixed bolts on the floor. Floor plates are also called foundation plates and they have to maintain a clearance of 10mm between the pedestal base plate and the floor plate and this clearance will be used by the levelling screws to level the entire tool.

2) What are the points taken into account while designing a base unit?

The fundamentals of a base plate design are:

• 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 the 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 depends on the size of the base plate.
• Datum holes should be drilled at nominal value of coordinates and in terms or multiples of 50 and 100 in general.
• Datum holes should be drilled with a diameter of 100mm away from other units.
• Datum holes should be covered with a cover plate also known as measuring hole cover.

The other factors that need to be taken into account are:

a) Working Height:

- Working height depends on the nature of the manual operator or welding fixture robot. For a manual fixture we need to consider the working height of 850-1100mm, depending upon the working height of the worker standard of a specific region. In this case, the ergo study will come in handy.

-If the base structure is made with base plate having tubular structure welded below it, a height of 100-250mm can be achieved. In this case, an additional leg structure must be provided which may have a top and bottom plate welded to tube.

b) Depending on the weight, eye bolt size is selected for the lifting holes on the base plate. Generally, 4 eye bolt provisions are made. You also need to select the location of the eye bolt considering the rope/belts used while shifting the structure so as to not damage any units nearby.

c) Depending on the size and weight of the components of all the units the designer can decide if just a plate or a fabricated (welded) base. Mostly fabricated bases are made of square or rectangular with the plate.

d) Most OEM’s make 4 dowel holes at 4 corners and place a small aluminum plate marking the X, Y & Z coordinates of these dowel holes. If the base length is too long, some OEMs opt to have the holes at a distance of 500 mm from each other.

e) The base unit of the fixture must be strong enough that the deflection of the fixture must be kept as minimum as possible. This deflection of the fixture is caused by the various forces acting on it, such as the welding and clamping force on the panel. The base unit should have enough mass to prevent or negate the vibrations and chatter that occur on it.

f) The base unit must be built from simple sections so that the other units maybe fastened with screws or welded wherever necessary. Those parts of the fixture that remain permanently fixed are welded and the parts that require frequent removal or changing are fixed with the help of screws.

g) The base unit should cover all the units with its outer periphery including the opening. The base unit must be parallel to the floor and provision must be provided for a height adjusting mechanism.

3) What is a Dump unit?

A dump unit is a special type of unit that is specifically used when there are geometrical constraints that prevents the proper usage of a clamp and pin unit. Dump units are of complex designs and this is to achieve lack of availability of space and the need to have all the required parts available in that constrained space. The dump unit is comprised of a dump cylinder to perform the clamping and unclamping operation on the panel. The first cylinder dumps the assembly carrying clamping mylars, pins and then the required operation is performed on the panel. The one consideration to take is that, in the open position, we should be able to remove the panel without damaging it.

Functions of a Dump unit:

• To ensure the proper resting of the panel being welded and to restrict its motion properly.
• To ensure there is no rotation possible for the panel and to fix it either by pins or side blocks or magnets.
• Dump units should have the correct shimming for the adjustment of the panel.

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

The following points are to be considered when designing a dump unit to avoid any errors in its operation:

• 3-2-1 Operation: Once we have designed the rest mylars, pins, clamps and added a cylinder for clamping. We check the resting location and clamping of the panel being welded. This is important because the positional accuracy of the panel is of the highest priority while designing the dump unit.
• Mass Consideration: The assembly that is being dumped should have the least weight possible without the reduction in the strength of the assembly. This is because the lesser the weight, lesser will be the force required to move the assembly.
• 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 the assembly being dumped. We need to keep it to the minimum as as the time taken for open will be time wasted waiting to unload the panel and this will accumulate and increase the total cycle time.
• Cylinder validation: Cylinder validation is a very 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 center of gravity of the unit and the distances between the cylinder lines and end effectors.
• Open Condition: We need to check the open condition after the cylinder validation because the panel should be damaged during the removal process.

5) What is a Slide unit?

A slide unit is used in places which highly space constrained where separate units cannot be used to work on the assembly. The slide unit has a slide cylinder whose motion is controller with the help of guide rails. The assembly to worked on is mounted on the plate and the cylinder pushes the assembly forward. The sensors which are placed at both ends of the slide unit are used to determine the position of the slide. Once the sensors detects that the slide has reached the appropriate point, the stopper blocks will physically stop the slide unit at the correct point. Once it stops, the locking mechanism will hold the slide in place so that it does not move when the operation is being done on it so that the quality of the operation is not affected and the panel is not accidentally damaged.

There are different types of slide units that are used in the industry:

• Hardened Guide Ways
• Linear Motion Guide Ways

Slide units can be classified based on the type of prime mover used:

• Manual Slides
• CNC Slides
• Hydraulic Slides
• Pneumatic Slides

Slide unit can be classified depending on their application:

• Horizontal Slide
• Vertical Slide
• Angular Slide

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

Since the slide unit is a fairly complex unit when compared to the other units, there are some design considerations that must be followed:

• Weight Carrying Capacity: There are different types of guides rails in the market and each is rated for a different load capacity. So we must always calculate the weight of our assembly and use a the appropriately rated guide rail.
• Push and Pull Forces: We need to verify if the push and pull forces and check it if it enough to push and pull the assembly on the slide, especially if it is inclined.
• Locking Mechanism: The locking mechanism ensures that the slide does not move while it is in its open or retract position. If the slide moves while welding process is being done on the assembly, then this could lead to improper weld formation or it could even lead to a damaged assembly.
• Sensor Unit: The sensor unit is what check if the slide has reached the appropriate position. When the operator initiates the sequence, the slide moves forward or back and the sensor produces the signal that the assembly has reached that location to the PLC following which, the next operation starts.
• Stopper Blocks: The stopper blocks are used to the stop the slide or halt its motion at the required position. The slide unit has rest mylars and pins and the assembly needs to be stopped at the right position for proper resting and locating and without this the assembly could get damaged.
• Stroke: We need to select the length of the stroke based on the work area of the unit.
• Open Position: The only moving parts in this mechanism is the one that slides back and forth, we need to verify the fowling with a gun or another nearby unit. Only once this has been checked can we go ahead and approve the design.