Week 7:- Base Unit Design Challenges 1

A.NO-1

-A base unit is the unit design to mount all the other units(Clamp unit,Pin unit,Rest unit etc.)of the tool,also oter parts like pneumatic valve box,trunking cable path will also be mounted in a base unit.

-As name suggested base,all the fixtures have must a base unit and the unit design is different according to the panel,Clamp system,Location system etc.

Different types of base unit as shown in below picture-

Parts that are used to create base unit are belows:

• Base plate
• Pedestal
• Trucking channel
• Valve box
• Eye bolt
• Floor plate

Base plate:

-It consists of 25-40mm plate thickness.

-The base plate has no.of drilled holes are done for mounting purpose of different units.

Pedestal:

-It is used to connect base plate with a floor plate.

-The pedestal can be a standard part or a make part,depending on size and height of the fixture.

-Pedestal are available in different height and different inclinations.

-Depending on the size of a base plate,for load distribution no of pedestal can be used for a fixture.

Trucking channel:

-For carrying all the electrical and pneumatic connections i.e wires/pipes for units of tool.

Valve Box:

-Valve box is a standard enclosure part which contains all the valves for a pneumatic units.Inside the box there are different types of solenoied valves are used for provide air pressure,so that air inlet and outlet done inside it.The box protect inside parts while operation is perform.

Eyebolt:

-These are the bolts that are the used to lift fixture for transportion by inserting hooks into them,usually M20 bolt is provided in base unit for lift,it can be varries according to load.

-Some times forklift holes are provided for transportion purpose.

Floor plate:

-They are also knows as foundation plate.It is grounded by a fixed bolt.

-10mm clearance should be given between pedestal base plate and floor plate.

-Weld bar is needed to used after levelling is done by levelling screw.Weld bars are along the sides of the base plate of a pedestal.

-Wel bars are also used to position permanently.

A.NO-2

While designing base unit below points are taken to consideration i.e

-First working height should be taken consideration as nature of manual work or Robot work.For manual fixture working height should be 850-1100mm taken to consider or it depends upon the height of the worker.

If base unit structure made with base plate having tubular structure welded below it we can reached the height 100-250mm in this case to reach the working height we need to provide some addtional leg structure which may have bottom plate welded to the tube.

-As a designer it is impotant to know how big the base plate welded structure will handle in working condition or fabrication.Special care need to taken for flatness if plate is bigger one.

Plate design fundamentals:

• A base plate should cover all the units with its outer periphery including opening.
• A base plate is preferred to designed with respect to BCS cordinates.
• A base should be drilled with datum holes/measuring holes in line with a pitch of 1000mm in general.
• No of datum holes depends upon the size of boxplate.
• Datum hole should be drilled at nomial value of co-ordinates and in terms of 50 and 100 in general.X=100, Y=-600, Z=250.
• Datum holes should be drilled with diameter 100mm away from other units.
• Datum holes should be covered with cover plate.

-Depending upon the weight eyebolt size is selected for lifting holes on the base plate.Generally four eyebolt prvision is given for lifting. Again eyebolt location should be considered for rope/belt used, while lifting hole structure they are not damaged at any condition.

-Depending upon the component size and weight of the total units you can decide whether it can be fabricated or simple plate taken.

-Most of the OEM are taken 4 dowel holes at 4 corner and put small aluminium plate with X,Y and Z co-ordinate marking of dowel holes.If the base plate is too long they can follows the methodology having holes at 500mm distance.

-The base unit fixture should be strong enough so that defelection of fixture is as minium as possible.The deflection of a fixture should cause because of the welding and clamping forces of the workpiece.

-The base unit of the fixture should have the mass should prevent vibration and chatter.

-It built in simple section so that other unit can fasten easily with screw/welded necessary.Those parts need permanentely in fixture that should welded and those need frequiently change need to be fasten.

-The base unit should be parallel to floor,to adjust this there should be provision of the height adjusting mechanism.

A.NO-3

Dump unit is a complex unit to design due to space constraints there is less space available and have to adjust all the parts in available space.It consists of dump cylinder to perform the operation of clamping and unlamping of car panel.Sometimes there is 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 myler,pins and then this assembly performs required operatio.In this unit ,we should make sure that in 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 restric its motion.
• Ensure that there are 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 does not move while welding.
• Dump unit should have correctshimming 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,claculation 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.

A.NO-4

Dump unit is a complex unit which has lot of parts.Hence we need to extra caution while design a dump unit because any mistake can cause 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 any problems at a later stage.

Points to be considered are given below.

3-2-1 Principle:

The resting location of the panel being welded is checked in accordance with the designed mylers,Pins and Clamps.This will be done by assembly being dumped onto the panel to operate.It is important because it is the essential reason for creating a dump unit.

Mass

The assembly being dumped should have a little weight as possible without compromising strength.Less weight means less force is required to push and pull the assembly.

Time of Opening and Closing

The time of opening and closing of the clamp should be less.This basically depends on the capacity of cylinder and the weight of the assembly being dumped.More time will result in an increase the cycle time leading to more idealtime.

Cylinder validation

Cylinder validation is an important aspects of dump unit or any clamp uunit design because the cylinder should be able to push the load for the operation and pull it back after the operation is complete.The center of gravity of the unit and the distance between the cylinder center line and end efforts.

Calculating the center of gravity of the moving assembly.

Calculating the longest perpendicular distance from a vertical line that passes through the CG and Pivot center The longest perpendicular distance could be in the work or in rest position or anywhere in between.

Calculating the shortest distance between pivot center and cylinder line.

We add all the values in table along with the cylinder bore and the results are shown whether the cylinder is validated or not according to the formula used in table for push and pull force.If the calculated force is more than the required force the cylinder is validated or we can change cylinder.

Open condition

We also need to check open position after cylinder is validate because the panel should not foul with the unit while removed.

A.NO-5

A slide unit is basically used in space constraints 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 position 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 forword and perform the required operation.Once complete the operation it slides back.
• It has a cylinder which pushes assembly forward.For precision operation a ball screw with servo motor used to move slide forward and backward also at the both ends shock absober are provide.
• Assembly is mounted on a plate which is attached in a guide rail.
• There are sensors in 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 in required position.
• It has locking mechanism which ensure that the slide does not move in perticular position as it may effect the welding quality and may damage the panel.
• Slide units are used in places where there is space constraints.
• The opening and closing of slide should not foul.
• Slide units are positioning units which are used to position the fixture,Weld gun units,robots into the position for further operations.
• While designing sometimes there are too many small panels near each other and there is not enough spaces as each panels requires resting location using two pins,clamping and sensing.It become more difficult when the panel is at angle,to assemble it those cases we use sliding unit due to space constraints.
• Guide rails should be lubricated well.

The lubrication has the following functions:-

-Minimizes the friction in travelling unit to prevent seizure and reduce wear.

-Forms oil on the faceway to decrease stress acting on the surfaces and extend rolling life.

-Cover the metal surface to prevent the rust position.

Slide units classified as follows-

It classified two types based on the type of guide-ways used in base.

-Hardened guide-ways

-LM type guide ways

It classified three types based on the prime movers used in base.

-Mannual slides

-Penumatic/Hydrolic slides.

-CNC slides.

It also classified depending on the applicatios.

-Horizontal slide

-Vertical slide

-Angular slide

Components of slide units are-

Sliding cylinder:Sliding cylinder has one 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 perticular point doesnot allow the entire plate to move.

Sensor unit:Sensor need to sense whether this entire component has reached here or not.

Stopper blocks:It is used to control the entire panel moving and its a moving parts.

A.NO-6

A slide unit come in category of complex units,Few points are needed to consider while designing slide units as belows,

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 how much weight of load that bear a guide rail.So that we choose rail based on its load carrying capacity.

Push and pull forces:We need to check push and pull forces,also check it is enough to push and pull the slide especially when the slide isn at an inclination position.

Locking mechanism:

The locking mechanism ensure that the slide does not move while it is an open/retracable position.If the slide moves while welding it can damage the panel as the position accuracy can hampered.

Stopper Blocks:

Stopper blocks are used to stop the slide at the required position.It is important as if the slide unit has the rest mylers and pins,they need to stop at their respective position,for the proper resting and location without a stopper it may dent the panel.

Sensor unit:

Sensor unit helps for check the position of the slide.When the position 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 perform the next operation.

Stroke:

We need to select the length of the stroke based on the work area we want to get cleared.

Open position:

The only moving parts are that on 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.