Week 7:- Base Unit Design Challenges 1

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 like 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.

FUNDAMENTALS OF BASE PLATE UNIT

• A base plate size should cover all the units with its outer periphery inclusding 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 1000mm in general.
• Number of datum holes is depend on the size of the base plate.
• Datum holes should be drilled at nominal values of coordinates,and in terms of 50 and 100 in general.
• Datum holes should be drilled with dia 100 mm away from outer units.
• Datum holes should be covered with cover plates.

TYPICAL PARTS OF THE BASE UNIT

TRUNKING CHANNEL

• Trunking channel is a duct which accommodates all the pneumatic and electrical hoses and wires for the units of the tool.

BASE PLATE

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

VALVE BOX

• Valve box is a standard part which contains all the valves for the pneumatic units.
• A valve box is basically a box which supplies air to all the other pneumatic cylinders.
• it's a device which includes a lot of other solenoid valves, so it will have one inlet and one outlet and from that one inlet, all the other solenoid valves will be attached to it.
• And each solenoid valves will have the input, and then again supplied to the separate cylinder, then again the output will be collected, while this solenoid valves, and again given back to the valve box.

MEASURING HOLE COVER

• Measuring Hole covers is on the base unit which is for measuring purpose.
• 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 and these holes are covered by using some plates to protect against from dust damage etc.

EYE BOLT

• Eye bolts are used for the transporting of the tool using lift hooks, Fork Lift holes will also be provided in some cases for transporting purpose.
• It is to drag the entire unit and relocated.
• Eye bolts are the bolts that are used to lift the fixture for transportation by inserting hooks into them.
• Example we usally use M20 Bolts that are bolted on the base plate used for transportation purpose and sometimes forklift holes will be provided.

PEDESTAL

• A pedestal is a part which connects the base plate to the floor plate.A pedestal is a standard part in general and in some cased it can be a welded frame integrated with the base plate.
• A standard pedestal can be of different height and different sizes.
• Depends on the sizes of the base plate,The number of pedestal varies for load distribution.
• A pedestal are of different types with different inclinations.

WELD BARS

• Weld bars are used to position the positions permanently.
• Weld bars are steel plates along the sides of the base plate of a pedestal.

FLOOR PLATE

• Floor plate is grouted to the floor with fixed bolt on the ground.
• Floor plates are also called as foundation plates.
• There will be a clearance of 10mm maintained between the pedestal base and the floor plate.

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

• 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.
• 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.
• 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.
• 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.
• 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
• 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.
• The Base unit of the fixture should have the mass to prevent vibration and chatter.
• 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.
• The base unit should cover all the units with its outer periphery including the opening.
• 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 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 NEED TO BE CONSIDER

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 most 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 Centre of gravity of moving assembly.

• 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 shortest 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 a Slide unit?

• A slide Unit can have a clamp or a pin or a rest unit attached to it and which slides forward and performs the required operation then it slides backward.
• It has a cylinder which pushes the assembly forward and which pulls the assembly.
• Slide unit contains Sensor unit,stopper,guide block,locking units,slide cylinders etc.
• Sensnor units in the extreme ends of the slide unit to sense the position.
• Opening and closing of the slide should not fowl.
• Sliding units are basically used in places where you have a lot of constraints or space constraints, when you cannot use different types of units to design the entire assembly. So, here you will use this sliding Only.
• Sliding cylinder has One open position and One closed position, which has to guide rails direction to the entire panel is going to move.
• Position locking unit which will lock the position at that particular point doesn't allow the entire plate to move.
• Sensors need to sense whether this entire component has reached here or not.
• Stopper blocks as I said initially to control the entire panel moving and it's a moving part.
• Slide units are used at where there are space constraints and places where there is less accessibility.
• Sometimes there is too small panels near each other and there is not enough space as each panel requires resting location using clamping,sensing.
• This becomes more difficult when the panel is at angles.To solve this problem,A slide unit mounts rest,Clamp or pin unit or all of them on a slide using guide rails,Guide BLocks,A cylinder,Locking mechanis, Stoppers and sensors.
• When the panel is to be welded,This slide unit moves forward,unload the panel or place the panel at one place or it completes its operation then it slides to backward.
• After that it locked at its position similarly same is done for other remaining panels.

1) Slide units can be calssified into two types on the base of the type of guide-ways used

• Hardened guide ways
• LM guide way

2) Slide units can be calssified into three types on the base of the prime mover used

• Manual slides
• Hydraulic/pneumatic slides
• CNC slides

3) It may also be classified depending on application

• Horizontal slide
• Vertical slide
• Angular slide

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

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.