Week 7:- Base Unit Design Challenges 1

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

Ans: A Base unit is the 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 pneumatic valve box, trunking cable path will also be mounted on to a base unit.

Below are the some of the parts a Base unit in general.

• Floor plate
• Weld Bars
• Pedestal
• Measuring hole cover
• Valve Box
• Base Plate
• Trunking channel

Welded Base Structure

Typical Parts of Base unit:

1.Base Unit:

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

There are three key benefits to a base Plate:

• They Can help to quickly and accurately tram in fixture and vises.
• They can hold large, complicated, or Odd-Shaped parts
• They protect the cast iron machine table.

Fundamentals of Base Plate Design:

• A base Plate Size Should cover all the unit with its outer periphery including opening
• A Base plate is preferred to be design 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 depended on the size of the box plate.
• Datum hole should be drilled at nominal value of coordinates and in terms of 50 & 100 in general Ex. X= 100, Y=-600, Z=250.
• Datum holes should be drilled with diameter 100mm away from the other unit.
• 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 is general and, in some cases, it can be a welded frame integrated with the Base plate.

Fundament of Pedestal design:

• A standard Pedestal can be of different height and different sizes.
• Depending on the size of the base plate the number of pedestals varies for load distribution.
• A Pedestal are of different types with different inclination.

2.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 will contain 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 and protects them from damaging while operation is in process.

5.Eye Bolt:

An eye bolt is a bolt with a loop at one end. They are used to firmly attach a securing eye to a structure, so that ropes or cables may then be tied to it.

Machinery eye bolts are fully threaded and may have a collar, making them suitable for use with angular loads up to 450. Eye bolts without a shoulder should not be used for angular loads. Heavy forged eye bolts with a continuous eye may be forged with an integral shoulder, allowing their use for heavy off-axis loads.

These are used for the transporting of the tool by inserting hooks into them. Some, hooks or fork lift holes are also provided in some cases for transporting purposes.

6.Floor Plate:

Floor plate are grouted to the floor with fixed bolt on the ground. These 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 by the levelling screws, for levelling the entire tool. Weld bars to be weld the positions permanently after levelling is done.

2) What are the point need to account while designing a Base unit?

Ans: Point need to account while designing a base unit:

The fundamentals of a base plate design are:

• Working Height, it depends on the nature of the manual or robotic welding fixture. For manual fixture we need to consider the working height 850-1100 mm, 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 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 & plate welded to the tube.
• 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. Special care needs to take care of flatness if the base size is very long.
• 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 1000 mm in general.
• Number of datum holes is depending on the size of the base plate.
• Datum holes should be drilled at nominal values of coordinates, & in terms of 50 x 100. (In general) ex. X=100, Y 600, Z-250.
• Datum holes should be drilled with dia 100 mm away from other units.
• Datum holes should be covered with cover plates.
• Depending upon the weight, eye bolt size is selected for lifting holes on base plate Generally, 4 eye bolt provision is made You 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.
• Depending upon the component size & weight of the total units you can decide whether you need a fabricated base or just plate. Mostly fabricated bases are used made of square or rectangular tube with the plate.
• Most of the OEM makes 4 dowel holes at 4 corners & put small aluminium plate marking X Y&Z coordinates of dowel holes. If the base length is too long few OEM' s follow the methodology of having holes at 500 mm distance.
• The best 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 called because of forces of welding, clamping of the work piece.
• The base unit of the fixture should have the mass to prevent vibration & chatter.
• The base unit may be built from simple sections so that other units may be fastened with screw or welded 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.

3) What is Dump Unit?

Ans: 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.

Function of Dump Unit:

• To ensure proper resting of the panel being welded and to restrict its motion.
• To ensure there I no rotation in the panel and to fix it either by pins or by side blocks or by 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 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 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.

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

Ans: The following are the points to be considered while designing the dump unit otherwise it results in errors and waste of time and money

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 into 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 utmost importance.

2) Mass consideration: The assembly being dumped should have as less weight as possible without reduction in the strength of the assembly. This is because the lesser the weight, lesser will be the force required to push and pull the assembly. We have to consider factor of safety as well.

3) 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 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.

4) 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 and then pull it back after the operation is complete. The centre goes gravity of the unit and the distances between the cylinder centre lines and end effectors.

5) Open Condition: We need to also 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?

Ans: A slide unit is basically used in space constrains in assemblies a part. This unit is also a complex unit. It contains sliding bed, cylinder, open position, close position, guide box, position locking unit, sensor unit, stopper block. Slide units are positioning units which are used to position the fixture, weld gun, units, robots into the position for further operation.

• A slide unit can have a clamp or a pin or a rest unit attached to it which slides forward and perform the required operation. On completion of the operation slide moves back.
• It has cylinder pushes the assembly forward. For precision operation ball screw with servo motor is used to move slide forward and backward also at the both ends shock absorber 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 required position.
• It has locking mechanism which ensures that the slide does not move in particular position as it may affect the welding quality and may damage the panel.
• Slide units are used in places where there is space constraint.
• The opening and closing of slide should not fowl.
• Slide units are positioning units which are used to position the fixture, weld gun units, robots into position for further operations.

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

There are different types of slide units:

1. Hardened guide ways
2. LM guide way

Slide units can be classified into three types on the base of the prime mover used:

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

It may also be classified depending on application as follows:

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

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

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 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.

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.