Week 7- Base Unit Design Challenges 2

Aim :-

Design a Base Unit for the units designed for the above challenges with all weldments for the given Car Panel.

Design the complete Fixture assembly (the units designed in the previous challenges can also be used). Make sure that all the 5 Panels given in the CAD data are fully constrained with the 3-2-1 principle.

Design :-

           

                 

                            

Software Used :- CATIA V5

Introduction :- 

Base Unit :  A base unit is the 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 component of the fixture because all other components are mounted over its so every fixture have base unit. The base unit design is different according to the panel. Design of clamp system, location system etc.

                                            

The typical parts of the Base Unit

Base plate:- 

• The base plate has number of holes drilled on it which will act as mounting points for clamp unit, pin unit and so on.

 Pedestal: -

•  It is used to connect the base plate with a floor plate.
• The pedestal can be standard part or a make part, depending on the size and height of the fixture.
• Standard pedestals are available in different inclinations.
• Pedestal 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.

Trunking channel:-

• Trunking Channel is duct that carries all the electrical and pneumatic connections (wires and pipes) for units of the tool.
• It has a thickness of few millimeters.

                                              

                               

Valve Box:-

• valve box is a standard part which contains all the valve 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.

                                                  

Eyebolt: - 

• Eye Bolts are the bolts that are used to lift the fixture for transportation by inserting hooks into them.
• For e.g. we usually use M20 bolts that are bolted on the base plate.
• Used for transportation purpose, sometimes forklift holes will be provided.

                                             

 Floor Plate: -

•  They are also known as foundation plate. They are grounded by using a fixed bolt.
• 10mm clearance should be give between pedestal base plate and floor plate.
• Weld bars need to use after the levelling is done by levelling screw.
• Welded bars are steel plates along the sides of the base plate of a pedestal.
• Weld bars are used to position the positions permanently.

Fundamentals Of a Base Plate Design:

The fundamentals of a base plate design are:

• As designer it is important to know how big base plate or bigger base welded structure will be handled during the fabrication & handling process. Special care needs to take care of flatness if the base size is very long.
• A base plate size should cover all the units with its outer periphery including 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 1000 mm in general.
•  Number of datum holes is depends on the size of the base plate.
• Datum holes should be drilled at nominal values of coordinates, & in terms of 50 x 100. (In general) ex. X=IOO, Y 600, Z-250.
•  Datum holes should be drilled with dia 100 mm away from other units.
• Datum holes should be covered with cover plates.
• Depending upon the weight, eye bolt size is selected for lifting holes on 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.
• Depending upon the component size & 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 & put small aluminum plate marking X Y&Z coordinates of dowel holes. If the base length is too long few OEM' s follow the methodology of having holes at 500 mm distance.
• The best 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 called because of forces of welding, clamping of the work piece.
• The base unit of the fixture should have the mass to prevent vibration & chatter:
• The base unit may be built from simple sections so that other units may be fastened with screw or welded necessary. Those parts of
• the fixture that remains permanently with the fixture may be welded &Those parts that need frequent changing may be held with screws.
• The base unit should be parallel to the floor to adjust this there should be provision of the height adjusting mechanism.

Points to be considered while designing a base plate: -

1) Working Height: -

                               It depends on the nature of the manual or robotic welding fixture. For manual fixture working height 850-1500mm, depending upon the working height of the standard of a specific country will apply (in few countries human height is higher or lower e.g. (US, Japan)). If base structure is made with base plate having tubular structure welded below it we can each to height 450mm to 550mm. In this case to reach the working welding height we need to provide additional some leg structure which may have top & plate welded to the tube. The total working height of this unit is 1492mm. The dimensions of the base plate are obtained as 1800x1500x40mm.

                            

                                  

                                   

                              

                        

                        

2). The base unit of fixture must be strong enough so that fixture is as minimum as possible. This deflection of fixture is caused because of forces of welding clamping of the work piece.The base plate is 40mm in thickness, which is machined to maintain the parallelism which can easily sustain the load of units without deflection.The weldment support and leg assembly part are made by using 80x80x550mm thick seamless pipe which can withstand the forces caused by welding and clamping of the workpiece.

3.) 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 required frequent changing may be the held with screws. The permanent parts of the base units are welded together and other parts have the facility to be fastened to the base unit. All the rough locators & the units can be fastened by using the threaded bolts. Base, plate is designed to assemble these units without being failed to each other.

4.) The base units should cover the all units within its outer periphery including opening.

 Fig. Shows Top View

                         

Fig. Shows Bottom View 

                         

5.) The base unit is parallel to the floor, to adjust this there should be provision of height adjusting mechanism. The height adjusting mechanism is provide to maintain the parallelly of the floor. At the time of adjusting we can provide spacers in order to maintain the parallelism with the floor.

3-2-1 Principle:

                        A work-piece free in space can move in an infinite of direction. For analysis, this motion can be broken down into 12 directional movements or "All 12 degrees" of freedom must have been restricted to ensure proper referencing of a work piece.

                       The 12 degrees of freedom all relate to the central axis of the work-piece. Notice the 6 axial degrees of freedom axial degrees of freedom permit straight-line movement in both directional along the 3-principal axis shown as X, Y&Z. 

                       The radial degrees of freedom permit rotational movement, in both clockwise & Counterclockwise radial directional around the same three axis.

 For a rigid body in plane has 6 degrees of freedom (i.e.) The motion of a ship at sea has the 6 degrees of freedom.

• X+
• X-
• Y+
• Y-
• ZCW
• ZCCW

Now try to understand 3-2-1 principle of fixture. The purpose of fixture is 

• Resting
• Locating
• Orienting
• Clamping

Resting:

In a fixture the work piece should rest in such a way that it dosen't move or vibrate on the resting blocks.

Locating:

The fixture should orient the work piece such that all the operations to be performed on it are within the each of the work piece.

Orienting:

The fixture should orient the work piece such that all the operations to be performed on it are within the reach of the work piece.

Clamping:

A clamp is a fastening device used to hold or secure objects tightly together to prevent movement or separation through the application of inward pressure. There are many types of clamps available for may differently purposes. Some are temporary, as used to position components while fixing them together, others are intended to be permanent.

Achievement of 3-2-1 Principle:

                      This requires the minimum of three locator blocks to establish the part plane. Three locators or supports are placed under the work piece. Three Locators are usually positioned at the primary locating surface.Clamp unit restricts 2 Axial movements upward and downward and other 2 pin units restrict radial movement.As by 3-2-1 method, by introducing a minimum of 2 clamp and 2 pins will restrict 6 degrees of freedom (DOF).

                         

                         

                         

Clamp Plan:

• In this assembly all the 5 Panel are fixed by using 3-2-1 principle.
• The Front side panels are fixed by using a pin clamp unit with a two-way clamping clamp mylar and a retractable pin unit with a pin.
• The back side panels are fixed by using a pin clamp unit with a two-clamping clamp mylar and retractable or fixed pin unit.
• The center panel is locked by using two rest units at the bottom, two pin units into the holes to lock rotation movement and 4 swivel cylinders in the front and three power clamp cylinders at the back.
• In this way by using all these units the panel is locked by using these units.