Challenge

Week 7- Base Unit Design Challenges 2

IImtiyaj Mathakiya29 Mar 2023

Base Unit Design Challenges 2

Aim:- Design of Base Units with all the weldments included. Design all the necessary Units for the fixture assembly.Design a Base Unit for the units designed for the above challenges with all weldments for the given Car Panel.the CAD data are fully constrained with the 3-2-1 principle.

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.

trunking channel
base plate
vlave box
measuring hole cover
eye bolt
pedestal
weld bars
floor plate

Base Plate:- Base plate is machined plate of 25 to 40mm thickness made of steel and aluminium,on to which the holes are drilled to mount be individual units like clamp unit,pin unit etc. of a tool.

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 case 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 size of the base plate, the number of pedestal varies for load distribution.
A pedestal are of different types with different inclinations.

Trunking Channel:- A trunking channel is a duct which accommodates all the pneumatic and electrical hoses and wires for the units of the tool.

Valve Box:- Valve box is a standard part which will accommodates the pneumatic valves for the units.

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 transportinig purpose.

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 plate and the floor plate.
this clearance will be used by the levelling screw, for levelling the entire tool.
weld bars are be used to weld the positions permanently,after levelling is done.
A weld bar is a plate along the sides of the base plate of the pedestal.

Fundamentals Of a Base Plate Design:-

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 BSC 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)
Ex: X=100,Y=-600,Z=250
Datum holes should be drilled with Dia 100mm away from other units.
Datum holes should be covered with cover plates

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.

3-2-1 Principle:

A workpiece free in space can move in an infinite number of directions. For analysis, this motion can be broken down into 12 directional movements or "degrees of freedom". All 12 degrees of freedom must be restricted to ensure proper referencing of a workpiece.

The 12 degrees of freedom all relate to the central axis of the workpiece. Notice the 6 axial degrees and 6 radial degrees of freedom. The axial degrees of freedom permit straight-line movement in both directions along the 3 principle axis shown as X, Y & Z.

The radial degrees of freedom permit rotational movement, in both clockwise & counterclockwise radial directions around the same three-axis.

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

X+

X-

Y+

Y-

ZCW

ZCCW

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

Resting

Locating

Orienting

Clamping

Resting:

In a fixture, the workpiece should rest in such a way that it doesn’t move or vibrate on the resting blocks.

Locating:

The fixture should locate the workpiece such that all the operations to be performed on it are performed without any problem and all of them are performed effectively.

Orienting:

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

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

Structure of Design tree:

A process where all the assemblies and parts are named in specific standards Different customers have different sets of standards and the structure of the design tree.
Every company, every customer has different standards of their own structural design tree.
So they will be giving a template on how the structuring and detailing of the design are done. And you have to follow that particular design or structuring of the tree.
In the design tree itself if we mentioned that particular name, wherever that part is we can easily know that this part belongs to this one or this station or this assembly on a particular unit. So this helps us to avoid errors during the design study, manufacturing assembly, and inspections.
It is an important process that helps us to identify a specific part or assembly as to which project, tool, it belongs to. Structuring a design tree is actually to avoid the errors, which might come during manufacturing assembly, or either in-house inspections as well.
If the part or assembly are not named properly, or the design tree is not structured properly, it will create confusion to understand the design and may create major possibilities of making an error.
Every part, every component should be named according to their procedures.
If we did a mistake or renamed the entire standard parts from the customer as a design part. Now the customer is going to manufacture the entire part again from the scratch, but he already has that product in his itinerary. but extra cost extra time and is feasible. This is also known as an error which is actually going to increase the cost of the entire assembly as well Naming a fixture or a unit or a part gives the information as to which project zone or assembly do they belong.
If we name the part properly or name the fixture properly ill actually tell which unit or which assembly or which zone or in which station it actually belongs
It can help us to identify the number of items of the same types or different types that are being used in an assembly. While starting of designing a fixture, The first unit will be the base unit.
In a Design tree, all the manufacturing parts should be first and the standard parts should be at the last. Design tree helps to study or understand the parts in the assembly easily.