Week 7:- Base Unit Design Challenges 1

1.Base unit

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 valave box,trunking cable path will also be mounted on to a base unit.

Typical Parts

• 1.Trunking channel - A duct which accomodated all the pneumatic and electrical hoses and wires for the units of the tool. 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.

• 2.Base plate - Should cover all the units with its outer periphery including opening. Preferred to be designed with respect to BCS coordinates. 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 boxplate. 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.

• 3.Valve box - Valve box is standard part which will accomodates the pneumatic valaves for the units. It is kind of control unit. The unit valave box is the box which includes different solenoid valaves which is used to control the valaves.

• 4.Measuring hole cover - The base plate has some drills to mount various units like clam,rest,pin etc. and every drill have some coordinates to define the exact location of the hole.Due to irregular shape of the car panel the drilled holes coordinates are not like 110.0,150.0 but like 110.62,150.84. The measuring hole is drilled over the base plate which having exact coordinates so that all other holes can take reference from it.The measuring hole number depends on the size of base plate.These holes are covered by using some plate to protect against dust,damage etc.

• 5.Eye bolt - Eye bolts are used for the transporting of the tool using lift hooks, fork lift holes willl also be provided in some cases for transporting purpose. We usually use M20 bolts that are bolted on the base plate, used for transportation purpose,sometimes forklift holes will be provided.

• 6.Pedestal - A pedestal is apart which connects the base plate to the floor plate. A pedestal is a standard part in general and in some cases it can be welded frame integrated with the base plate. A standard pedestal can be of different height and different size. Depends on the size of the base plate, the number of pedestal varies for load distribution. A pedestal are of different types with different inclination.

• 7.Floor plate - They are also known as foundation plate. They are grounded by using a fixed bolt. 10 mm 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 plate of a pedestal. Weld bars are used to position the positions permanently.

2). Points to account while designing a Base Unit

• a) Working height: It depends on the nature of the manual or robotic welding fixture. For manual fixture we need to consider the working height 850-1100mm depending upon the height of the worker stanadard of a specific country will apply( in few countries human height is higher or lower eg.(US, Japan). If base unit 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 working height we need to provide additional some leg structure which may have to and bottom plate welded to the tube.
• b) 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 need to take care of flatness if the base size is very long.
• c) Depending upon the weight eyebolt size is selected for lifting holes on the base plate. Generally four eye bolt provision is made. You also need to select the location of eye bolt considering the rope/belt used while shifting the structure or complete fixture are not damaging any units ( rest,clamp or location units) nearby.
• d) Only base plate or welded structure.Depending upon the componenet size and weight of the total units you can decide whether you need a fabricated base or just plate. Moatly fabricated bases are used made of square or rectangualr tube with the plate.
• e) Most of the OEM makes 4 dowel holes at 4 corner 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.
• f) The best unit of the fixyure must be strong enough so that deflection of the fixture is as minimum as possible. The deflection of a fixture is caused because of forces of welding,clamping of the work piece.
• g) The base unit of the fixture should have the mass to prevent vibration and chatter.
• h) The base unit may be built from simple section 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.
• i) The base unit should be paralled to the floor, to adjust this there should be provision of the height adjusting mechanism.

3). Dump unit

It is the complex unit to design due to space constrains there is less space available and have to adjust all the pats in available space.It consists of dump cylinder to perform the operation of clamping and unclamping car panel. 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,pin and 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 panelFunction of dump unit

• To ensure proper resting of the panel being welded and to restrict its motion.
• To ensure there 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.

Considerations 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,calculations are done and force exerted by the cylinder is ched against the weight of the load being carried.
• If the cylinder force is more,it is validated or we use more powerful cylinder.

4).Points to consider whlle 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 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 utmost importance.
2. Mass Consideration: Weight of dump unit must be at lesses side. Less the weight of unit less will be the force required to operate this unit. While taking such consideration of mass, strenght will be at maximum side.
3. Time of opening and closing: 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.
5. Open Condition: It is one of the basic point which is consider in every moning unit like movable pin unit or clamp unit or slide unit, etc. Open condition is check to determine whether the car panel is load-unload proper or not.

5). Slide Unit

Slide Unit is used for different purposes like clamping or resting or may be as pin unit. Sliding action is performed with the help of pneumatic or hydraulic cylinder or may be by using manual toggle clamp.

This unit has built on a plate which is guided on rail. Stopper block or plate is mounted at the end position to stop the plate on which whole unit is build. This type of unit is used where space available is less.

Classification of Slide Unit:

1. According to the guide ways used:

1. Hardened Guide way
2. LM Guide way

2.According to the type of operation used for sliding:

1. Manual
2. Pnematic or Hydraulic
3. CNC

3. According to the Applications:

1. Horizontal
2. Vertical
3. Angular

6). Points to consider while designing a Slide Unit.

1. Weight Carrying Capacity: Weight carrying capacity of rails are one of the important point while selecting proper guide rails which perform sliding action without any problem. Different rails have different capacity to carry load of unit assebly.
2. Push and Pull Forces: Pull and Push force should to be less as much as possible. It is majorly consider when slide unit is in inclination.
3. Locking Mechanism: Locking Mechanism is necessary to stop sliding unit at perticular position. While doing operation the slide unit must be locked at some position hence this provision is made is the fixture.
4. Stopper Block: At start and end of stroke of sliding unit stopper block is provided to hold the plate in guide rail on which whole assembly is mounted.
5. Sensor Unit: Basically to check the postion of slide unit sensor unit is designed.
6. Stroke: Stroke is decided while designing this type of unit according to the space availble and some other factors.
7. Open Position: Open position is check to ensure that slide unit is collide with other unit in fixture or to welding gun.