Week 7:- Base Unit Design Challenges 1

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

Ans:-

Base unit:- It is a 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 a pneumatic valve box, trunking cable path will be mounted on to a base unit.

Fig 1: Base Unit

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

Parts of the Base unit

• Base Plate
• Pedestal
• Valve Box
• Measuring Hole Cover
• Eyebolt
• Floor Plate
• Weld Bar

Base Plate:- The base plate is a machined plate of 25 to 40 mm thickness made of steel & aluminum, on which the holes are drilled to mount the individual units like clamp units, pins, etc., of a tool.

Fig 2: Base plate

Pedestal:- A pedestal is a part that connects the base plate to the floor plate. A pedestal is a standard part in general & in some cases, it can be a welded frame integrated with the base plate. Pedestals are used for leveling the base unit

Fig 3: Pedestal

A standard pedestal can be of different heights & different sizes.

Depending on the base plate’s size, the number of pedestals varies for load distribution.

A pedestal is of different types with different inclinations.

Trunking Channel:- A trunking channel is a duct that accommodates all the pneumatic & electrical hoses and wires for the units of the tool. It has a thickness of a few millimeters.

Fig 4: Trunking Channel

Valve box:- Valve box is a standard part that will accommodate all the pneumatic valves for the units. The valve box is the box that includes different solenoid valves. The pneumatic fixtures work on air pressure provided to it. So the arrangement of the air pressure inlet & outlet is done within this valve box. The trunking channel includes all the pressure inlet cables going to pneumatic parts of the fixture & protects them from damage while the operation is in process.

Fig 5: Valve Box

Eyebolt:- Eye bolts are used for the transporting of the tool, using lift hooks, forklift holes will also be provided in some cases for transporting purposes. For example, we usually use M20 bolts that are bolted on the base plate.

Fig 6: Eyebolts

Floor plate:- Floor plate is grouted to the floor with a fixed bolt on the ground. Floor plates are also called 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 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.

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

Ans:-

Below points need to be taken into account while designing the base unit.

Working height:

• It depends on the nature of the manual or robotic welding fixture.
• For manual fixtures we need to consider the working height of 850-1100mm, depending upon the working height of the worker standard of a specific country will apply.
• If the base structure is made with a base plate having a tubular structure welded below it. We can reach a height of 100mm to 250mm. in this case to reach the working welding height we need to provide additional leg structure which may have top & plate welded to the tube.
• As a designer, it is important to know how big base plates or bigger base welded structures 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 eyebolts size is selected for lifting holes on the base plate. Generally, 4 eye bolt provision is made. You also need to select the location of the eye bolt considering the rope/belts used while shifting the structure or complete fixture are not damaging any units nearby.
• Most of the OEM makes 4 dowel holes & puts a small aluminum plate marking the X, Y, & Z coordinates of dowel holes. If the base length is too long few OEMs follow the methodology of having holes at a 500mm distance.
• The best unit of the fixture must be strong enough so that the deflection of the fixture is as minimum as possible. This defection of a fixture is caused because of forces of welding, and 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 units may be fastened with screws or welded as 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 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 concerning BCS coordination.
• A base plate should be drilled with datum holes / measuring holes in line with a pitch of 1000m in general.
• The number of datum holes depends on the size of the base plate.
• Datum holes should be drilled at nominal valves of coordinates 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?

Ans:-

Dump Unit:- It is used rarely when there is limited space for designing any unit like a clamp or pin unit. It consists of a dump cylinder used to perform loading or unloading action which is mounted on an unusual riser designed according to the space we have.

Fig 7: Dump unit

Functions of Dump unit

• To ensure proper resting of the panel being welded and to restrict its motion.
• A dump unit should have two cylinders one is to clamp the mylars at the localized point and the other cylinder is used to dump the whole unit.
• In the dump unit, one assembly is fixed and that whole fixed assembly will mount on the dump unit cylinder.
• Programming is the main part of to function dump unit. Because when the two cylinders move at the same time it causes distortions to the panel. So at that 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 the force exerted by the cylinder is checked against the weight of the load being carried. If the cylinder force is more, it is validated or we use a more powerful cylinder.

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

A dump unit is a complex unit that 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 a waste of time & 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.

3-2-1 principle:- Once we have designed the rest mylars, pins & clamps & added a cylinder for clamping. We will check the resting location & clamping of the panel being welded. This will be done by the assembly being dumped onto the 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 at most importance.

Fig 8: 3-2-1 principle

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 & pull the assembly. Here, we apply material such as aluminum on the cylinder & steel on the rest of the parts to calculate the mass of the moving assembly. We take mass a bit more than calculated for the factor of safety.

Time of opening & closing:- We also need to keep in mind that the time of opening & closing of the clamp should be the least possible & this will depend on the power of the cylinder & 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 we will not be able to unload the panel which will increase the cycle time resulting in more idle time.

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 operation & then pull it back after the operation is complete. For this following calculations are necessary. Calculating C.G. of moving assembly.

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

5. What is a Slide unit?

Sliding unit:- It is majorly used for difficult panels like( tunnel assembly) to be welded. A 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 9: Sliding unit

A slide unit can have a clamp or a pin or a rest unit attached to it which slides forward & perform the required operation. On completion of the operation, slides move back.

• It has a cylinder that pushes the assembly forward. For precision operation ball screw with a servo motor is used to move the slide forward & backward. Also at both ends shock absorbers are provided.
• Assembly is mounted on a plate which is attached to guide rails.
• There are sensors on the extreme ends of the slide to sense the position of the slide
• It has stop docks on either side of the slide to stop the cylinder at the required position.
• It has a locking mechanism that ensures that the slide does not move in a particular position as it may affect the welding quality & may damage the panel.
• Slide units are used in places where there is a space constraint.
• The opening & closing of the slide should not fail.
• Slide units are positioning units that are used to position the fixture, weld gun units, and robots into position for further operations.

Slide units are used when we have some space constraints. So we don’t have space to design different special unit & assemble it in those cases we use the sliding unit. Sometimes there are too many small panels near each other & there is not enough space as each panel requires resting, location using 2 pins, clamping & sensing. Thus becomes more difficult when the panel is at an angle. To solve this problem slide unit mounts rest, clamp or pin unit or all of them on the slide using guide rails, guide block, a cylinder, locking mechanism, stop blocks, and sensors. When the particular panel is to be welded, then this slide moves forward & after completion moves back. & get locked at its position.

Guide rails should be lubricated well.

• The lubricant has the following function.
• Minimizes friction in the traveling unit to prevent seizure & reduce wear.
• Forms oil on the face way to decrease stress acting on the surface & extend rolling life.
• Cover the metal surface to prevent rust formation.

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

Slide unit comes under the category of complex units. A few points that need to be considered while designing a slide unit are

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 & calculate the weight capacity of the rail. Hence, we choose rail based on its load-carrying capacity.

Push & pull forces:- We need to check the push & pull forces & check if it is enough to push & pull the slide especially when the slide is at an inclination.

Locking mechanism:- The locking mechanism ensures that the slide does not move while it is in an open or retracted position. If the slide moves while welding it can damage the panel as the positional accuracy will be hampered

Stopper block:- Stopper blocks are used to stop the slide at the required position. It is important as if the slide unit has rest Mylars & pins, they need to be stopped at their respective position for proper resting & location. Without a stopper, it may dent the panel.

Sensor unit:- Sensor unit helps to check the position of the slide. When the operation is going to start the operator pushes the button & the slide moves forward, the sensor gives this signal of the presence of the unit to the PLC which then performs the next operation.

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

Open position:- The only moving parts are on the slides, so we need to check fouling with guns or other units nearby. Only after this has been checked then we can approve the design.