Week 7:- Base Unit Design Challenges 1

QUE-1)What is the base unit? What are the typical part off the base unit?

A base unit is the unit design 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 will also be mounted on to a base unit.

Base unit is the fundamantal component of the fixture becouse all other componenet are mounted over it so every fixture have base unit .

The base unit design is different according to the panel , design of clamp system . locating system etc .

Typical parts of the base unit are given as fillows:

Base Plate:-

• The base plate is a plate of 25mm to 40mm thickness.
• The base plate has no. of holes drilled on it which will act as mounting points for clamp unit , pin and so on.

Pedestal:-

• It is used to connect the base plate with a floor plate
• The pesdestal can be a standard part or a make part , depending on the size and height of the fixture .
• standared pedestals are available in different heights.
• pedestals are also available in different inclinations.
• Depending on the size of the base plate , the number of pedestal for a fixture can vary for load distribution .

Trunking channel:-

• Trunking channel is duct that carries all the electrical and pneumatic connection (wires and pipes) for unit of the tool .
• It has a thickness of few millimetres.

Valve box:-

valave box is a standard part which contains all the valves for the pneumatic units. the valve box is the box which inlcudes different solinoid valves . the pneumatic fixture work on air pressure provided to it . so the arrangement of air pressure inlet & outlet is done with in this valve box . the trunking channel inlcude all the pressure inlet going to the pneumatic parts of the fixture & protects them from damaging while operation is in process.

Eyebolt:-

• Eye Bolts are the bolts that are used to lift the fixture for transportation by inserting hooking into them .
• For example . we usually use M20 bolts that are bolted on the base plate.
• Used for transportation purpose , sometime fork lift holes will be provied .

Floor Plate:-

• They are also know as foundation plate . They are grounded by using a fixed bolt .
• 10mm clearance should be given between pedestal base plate and floor plate.
• Weld bars need to use after the levelling is done by levelling screw
• weldbars are steel plates along the sides of the base plates of a pedestal .
• weld bars are used to postion the positiosn permanantly.

Ques:-2 What are the points need to account while designing a Base unit?

Working height:-

• Working height 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 specific country will apply.
• 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 same leg structure which may have top and plate welded to tube.

Depending upon the weight eye bolt size are selected for lifting holes on the 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 rest, clamp or location units.

Depending upon the component size and weight of the total units the base plate is fabricated or sometimes normal plate is used. Mostly fabricated bases are used made of square or rectangular tube with plate.

Most of the OEM makes 4 dowel holes at 4 corners and 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 dowel holes at 500mm distance.

Base unit may be built from simple sections so that other units may be fastened with screws or welded whenever necessary. Those parts of the fixture that remain permanently with the fixture that remain permanently with the fixture may be welded. Those parts that need frequent changing may be held with the screws.

The base unit should cover all the units with its outer periphery including opening. The base unit should be parallel to the floor, To adjust this there should be the provision of height adjusting mechanism.

Ques:-3 What is Dump unit?

When we have some geometrical constraints them it is difficult to place unit like clamp unit etc .
In this case the dump unit is used . dump unit is the complex unnit to design due to space constrains
there is less space available & have to adjust all the parts in available space . it consist of dump cylinder
to perform the operation of clamping & unclamping car panel.The first cylinder dumps the assembly carrying clamp mylars ,
pin and then this unit , we should make sure that in the open position of the open position of the panel , we should be able to remove the panel.

Function of the dump unit:-

• To ensure proper resting of the panel being welded and to restrict its motion.
• To ensure there are not rotation in the panel and to fix it 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 panel .

Consideration for the dump unit:-

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 , calculation are done force excerted 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

QUE:-4 What are the points you need to consider while designing a Dump unit?

Dump unit is a complex unit which 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 wastage of time and 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:

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 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 most important.

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 the 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 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. For this following calculations are necessary.

Calculating "the longest perpendicular 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 position or anywhere in between.

Calculating " the shortes distance between pivot centre and cylinder line."

We add 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.

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 Slide unit?

-Slide units are positioning units which are used to position the fixture, weld gun, units, robots into position for further operations.

-Slide units can be classified into two types on the base of the type of guide-ways used as

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

1)Horizontal slide

2)vertical slide

3)angular slide

-slide unit are used when we have some space constraints . so we don't have spce to design different special unit & to assembly it . in those case we use sliding units.

Ques:- 5What are the points you need to consider while designing a slide unit ?

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

1.Weight carrying capacity

There are different kind of guid rails in avialable in the market ,each having a different load capacity . hence we calculate the weight of our assembly and calculate the weight capacity of rail . hence , we choose rail based on its load - carrying capacity.

2.Push and Pull Forces

We need to check the puch and pull force and check if it is enough to push and pull the slide especially when the slide is at an inclination.

3.Locking Mechanism

Stopper blocks are used to stop the slides at the required position . it is important as if the slides 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.

4.Stopper Blocks:-

Stopper block are used to stop the slides at the required position . it is important as if the slides unit has rest mylor and 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 slide moves

forward , the sensor gives this signal of the presnece of the unit to the PLC which then performs the next operation.

6.Stroke

We need to select the length of stroke bassed 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.