Week 7:- Base Unit Design Challenges 1
Q-1 What is the Base Unit? What are the typical parts of the Base Unit?
A base unit is the unit designed 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 path will also be mounted on to a base unit. Base unit is the fundamental componet of the fixture because all other components are mounted over it.so every fixture have base unit. The base unit design is different according to the panel, Design of clamp system, location system etc.
Related image Related image
Typical parts of the base unit are given as follows:
1 Base Plate: -
Th e base plate is a plate of 25 mm to 40 mm thickness.
The base plate has no of holes drilled on it which will act as mounting points for clamp unit, pin unit and so on.
2 Pedestal: -
It is used to connect the base plate with a floor plate.
The pedestal can be a standard part or a make part, depending on the size and height of the fixture.
Standard pedestals are available in different heights.
Pedestals are also available in different inclinations.
Depending on the size of the base plate, the number of a pedestal for a fixture can vary for load distribution.
3 Trucking 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 contains 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 & outlet is done within this valve box. The trunking channel includes all the pressure inlet cables going to the pneumatic parts of the fixture & protects them from damaging while operation is in process.
5 Eyebolt: -
Eye Bolts are the bolts that are used to lift the fixture for transportation by inserting hooks into them.
For example, we usually use M20 bolts that are bolted on the base plate.
Used for transportation purpose, sometimes forklift holes will be provided.
6 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.
- What are the points need to account while designing a Base unit?
Following points need to be taken into account while designing the base unit
1 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 the base structure is made with a 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 and plate welded to the tube.
2 As a designer its 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 be taken care of flatness if the base size is very long.
3 Depending upon the weight eyebolt 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 units(rest, clamp or location units) nearby.
4 Only Base plate or welded structure
Depending upon the component size and 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.
5 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
6 The base 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 caused because of forces of welding, clamping of the workpiece.
7 The Base unit of the fixture should have the mass to prevent vibration and chatter
8 The 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 may be welded. Those parts that need frequent changing may be held with the screws.
9 The base unit should cover all the units with its outer periphery including the opening.
10 The base unit should be parallel to the floor, To adjust this there should be the provision of the height adjusting mechanism.
3.What is a Dump Unit?
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, 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.
Functions of the dump unit:
To ensure proper resting of the panel being welded and to restrict its motion.
To ensure there are 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.
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.
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 utmost importance.
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.
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 a 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 calssified 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 calssified into three types on the base of the prime mover used
1)Manual slide
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 units are used when we have some space constraints. So we dont have space to design different special units & to assemble it. In those case we use sliding units.
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.