Week 7- Base Unit Design Challenges 2

1.BASE UNIT DESIGN

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.

                                   IMG 1- BASE UNIT

FUNDAMENTALS OF BASE PLATE DESIGN
• 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.

• Example - X=100,Y=-600,Z=250

 • Datum holes should be drilled with dia 100 mm away from outer units.

• Datum holes should be covered with cover plates.

                                            IMG 2- DESIGNED BASE UNIT

Below are the some parts of abase unit in general.

• Trunking channel

• Base Plate

• Valve box

• measuring Hole Cover

• Eye Bolt

• Pedestals

• Weld bars

• Floor plate

 Base Plate:

Base plate is used to mount all other units like pin units, rest units and clamp units.

Trunking channel:

Trunking channel is a duct that carries all the electrical and pneumatic connections like wires and pipes for units of the tool. It has a thickness of few millimetres.

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 cases 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.

VALVE BOX:
Valve box is a standard part which contains all the valves for the pneumatic units.

Valve Box:

• A valve box is basically a box which supplies air to all the other pneumatic cylinders. its its 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 COVERS:
Measuring Hole covers is on the base unit which is used to cover the datum holes. to safeguadr the datum hioles from damage, covers are used.

• 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.

EYEBOLTS:
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.

               Eye Bolt

• It is to drag the entire unit and relocated. (or)

• Eye bolts are the bolts that are used to lift the fixture for transportation by inserting hooks into them.

• For Example we usally use M20 Bolts that are bolted on the base plate.Used for transportation purpose and sometimes forklift holes will be provided.

                                                  Eye Bolts to lift the jig

WELD BARS:
Weld bars are used to mount the base plate to the floor.

FLOOR PLATE:
They are also known as foundation plate.They are grounded by using a fixed bolt 10mm clearance should be given between pedestal base plate and floor plate.
• Weld bars need to use after the levelling is done by levelling screw.

• Weld bars are steel plates along the sides of the base plate of a pedestal.

• Weld bars are used to position the positions permanently.

POINTS SHOULD BE TAKEN WHILE DESIGNING A BASE 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.
• Working height depends on the nature of the manual or robotic welding fixture and for manual fixture we need to consider the working height be 850 - 1100mm and its depending upon the working height of the worker or its about ergonomics.lt differs by countries.

• 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.

• Depending upon the Weight of the fixture design unit we need to select the eyebolt size for lifting the base plate.while shifting the fixture units from one place to another they will not damage or damaging any other units.

• Datum holes should be drilled at nominal values of coordinates,and in terms of 50 and 100 in general.
• Example - X=100,Y=-600,Z=250
• Depending upon the component size and weight of the total units you can decide whether you need a fabricated base or a plate.Mostly fabricated bases are used and made of square or a rectangle tube with the plate.

• Datum holes should be drilled with dia 100 mm away from outer units.

• Datum holes should be covered with cover plates.

• The base Unit of the fixture must be strong enough so that deflection of the fixture is as mimum as possible during welding,This Deflection is caused by the forces of welding and while clamping of the panel.

• should be robust.

3-2-1 PRINCIPLE
The 3-2-1 principle of location (six point location principle) is used to constrain the movement of workpiece along the three axes XX, YY, and ZZ. This is achieved by providing six locating points, 3 pins in base plate, 2 pins in vertical plane and 1 pin in a plane which is perpendicular to first two planes.

DEGREE OF FREEDOM CONTROLLED BY 3-2-1 PRINCIPLE
• 3-2-1 PRINCIPLE is the heart of the fixture design and world holding devices.

• In BIW FIXTURE design the panels are always fixed with the help of 3-2-1 .because it always minimizes the usage of unwanted components to hold the panels with the help of 3-2-1 principle we can arrest the six degrees of freedom which means six degrees of rotational and six degrees of translational axes .

FIRST PLANE
This is the '3" in 3-2-1. So, three specific points are used to define the first plane. Fewer than three points cannot define a plane, and in the real world dimensional tolerances mean that four or more points will not be coplanar. A real-world, less than ideally perfect part placed on four or more reference points will, in fact, rest on only three of the points due to its less than perfect surface. Different parts may rest on different combinations of three points, resulting in variation between finished parts. A stool can be used to illustrate this concept. A two-legged stool would certainly be unstable. A three-legged stool sits rock-solid. A four-legged stool is often found to rock. In the illustration, a three dimensional part, represented by a cube, is placed on a datum plane defined by three support points. The part's six degrees of freedom have now been reduced to three. It can still move along the X or Y axes, and it can still be rotated about the Z axis. (The part cannot move along the Z axis because it is held against the plane by clamping force.
TIPS
• Use the largest surface of the part for the first ("primary") reference plane. • Position the three support points as far apart as possible. • If more than three support points are required to prevent deflection, make the additional points adjustable.

SECOND PLANE
A second plane, if it is perpendicular to the first, can be defined by two points, the '2" in 3-2-1. The part is now constrained to one degree of freedom: movement along the Y axis. (The part cannot move along the X or Z axes because it is held against the planes by clamping force.

TIPS
• Use the largest surface of the part for the first ("primary") reference plane. • Position the three support points as far apart as possible. • If more than three support points are required to prevent deflection, make the additional points adjustable.

THIRD PLANE
A third plane, if it is perpendicular to both of the first two planes, can be defined by one point, the "1" in 3-2-1. The part is now entirely constrained. It cannot move along or rotate about the X, Y, or Z axes. (Remember that the part is held against each of the three planes by clamping force).

What is the working height?
It depends on the nature of the manual or robotic welding fixture. For manual fixture you need to consider the working height 850-1100 mm , depending upon the working height of the worker standard of 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 you can reach to height 100 mm to —250 mm. In this case to reach the working welding height you need to provide additional some leg structure which may have top and plate plate welded to tube.

HOW MANY EYE BOLTS NEEDED?
Depending upon the weight eye bolt size are selected for lifting holes on the base plate. Generally 4 eye bolt provision is made. we 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) .

How base plate or base structure handeled during fabrication, machining or during final installation?
As designer its is important to know how big base plate or bigger base welded structure will be handeled during the fabrication and handling process.

2. Design all the other remaining Units required for this Fixture assembly
In this fixture design they are five panels,For each panel a pin unit and one clamp unit is for holding the panel and fix the panel or to restict 6 degree of freedom by using 3-2-1 principle.

Fixture Design Fundamentals
Fixture design consists of a number of distinct activities: fixture planning, fixture layout design, fixture element design, tool body design, etc. They are listed in their natural sequence, although they may be developed in parallel and not necessarily as a series of isolated activities in actual execution.

Fixture design deals with the establishment of the basic fixture concepts:

•Fixture layout is an embodiment of the concepts in the form of a spatial configuration of the fixture.

•Fixture element design is concerned with the concrete details of the locators,clamps and supports.

• Tool body design produces a structure combining the fixture elements in the desired spatial relationship with the machine tool.

FUNCTIONS OF FIXTURE DESIGN
• The function of fixture is a holding device of work piece in proper aligns during manufacturing of product. Fixture is supporting, clamping the work piece, its checking, positioning, individual marking and non-uniform quality in manufacturing of product is eliminated by fixture. This increase productivity and reduce operation time. Fixture is basically uses in the industry to increasing production because of feature and advantages.

• To locate and immobilize workpieces for machining, inspection, assembly and other operations fixtures are used. The tool is uses in during of inspection of panel assemble.

• Fixture is not guide, but is always fixed to production line, various type of machining operation of product is suite on fixture.

• The car body is made up of more than of hundreds of stamped sheet metal components which are joined together by spot welding process, accurate production of the car body is essential if the automated assembly line is to fit within the required tolerance, therefore we use the welding fixture.

• We can use a different type of units and they have different function in welding fixture design, based on clamping plan, location plan, and simulation.