Week 7- Base Unit Design Challenges 2

Aim : To design a base unit for the pin units, clamp units, rest units and other units with all weldments for the given car panel.

Theory:

1.Base Unit :

A Base unit is a unit designed to mount all 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.

Image : Typical parts of base unit

Base plate :

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

Fundamentals of base plate unit design :

• A base plate size should cover all the units within its outer periphery including opening.
• A base plate is prefered 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). 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.

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 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 accomodate all the pneumatic and electrical hoses and wires for the units of the tool.

Image : Trunking channel

Valve box :

Valve box is a standard part which will accomodates the pneumatic valves for the units.

Image : Valve box

Eye bolt :

Eye bolt are used for the transporting of the tool, using lift hooks. Fork lift holes will also be provided in some cases for transporting purpose.

Image : Eye bolt

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 10 mm maintained between the pedestal base plate and the floor plate.

This clearance will be used by levelling screw, for levelling the entire tool.

Weld bars are used to weld the positions permanently, after welding is done.

Weld bar is a plate along the sides of the base plate of the pedestal, to weld the positions permanently, after welding is done.

Points to be considered while designing a base unit :

• Working height depends on the nature of the manual or robotic welding fixture. For manual fixture we need to consider the working height 850-1100mm, depending upon the working height of the worker standard of a specific countries human height is higher or lower e.g. (US, Japan)

• If base structure is amde with base plate having tubular structure welded below it we can reach to height 100mm to 250mm. In this case to reach the working height we need to provide additional some leg structure which may have top and plate welded to the tube.
• As a designer it is important to know how big base plate or bigger welded structure will be handled during the fabrication and handling process. Special care needs to take care of flatness if the base size is very long.
• Depending upon the weight eyebolts size is selected for lifting holes on base plate. Generally 4 eye bolt provision is made. You also need to select the location of eyebolt considering the rope/belts used while shifting the structure or complete fixture are not damaging any units nearby.
• A base plate should cover all the units within its outer periphery including opening.

• 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.
• Most of the OEM makes 4 dowel holes and put small aluminium plate marking X,Y and Z coordinates of dowel holes. If the base length is too long few OEM's follow the methodology of having holes at 500mm distance.
• The base unit of the fixture should have the mass to prevent vibration and chatter.
• The base unit of the fixture must be strong enough so that deflection of a 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 may be bulit from simple section so that other units may be fastened with screws or welded, if 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 screws.
• The base unit should be parallel to the floor, to adjust this there should be provision of the height adjusting mechanism.

Total weight of the base unit is 88kg.

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.

The axial degrees of freedom permit straight-line movement in both directions along the 3 principal axis shown i.e. +X, -X, +Y, -Y, +Z, -Z.

The radial degrees of freedom permit rotational movement, in both clockwise(CW) & anticlockwise(ACW) radial directions around the same three axis. i.e. X CW, X ACW, Y CW, Y ACW , Z CW, Z ACW.

To achieve 3-2-1 principle, the fixture consists of resting, locating, orienting and 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 seperation 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 movememts. Upward and downdard 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 restricts 6 degrees of freedom.

Clamp plan :

• In this assembly all the 5 panles are fixed by using 3-2-1 principle.
• A pin unit restricts 2 way translational motion of the panel thus restricting 3 DOFs
• Clamp units will restrict 1 translational motion and 2 rotaional motion of the panels thus restricting 3 DOFs
• Another pin unit inside a slot will restrict 1 rotaional DOFs
• Rest units are used wherever necessary to support the panels in a defined way.
• In this way, by using all these units the panel is locked firmly in a position.

Design methodology

The base unit is divided into 2 weld products

1. Weld 1

2. Weld 2

Weld 1 further consists of

a) Base plate

A base plate is provioded with 4 holes of diameter M24 at the corners to fix eyebolts.

And other mounting holes are provided on the base plate for mounting other units.

b) Square tubes

A square tube frame give support and strength to the base plate. Here we have used 5 tubes which are welded together as shown in the image above.

c) Plate

Support plates are provided below the square tube

Weld 2 further consists of

a) Square plates

A square plate which forms the top of the structure

b) Pedestal :

c) Ground plate :

Ground plate is a plate provided with some tapping for levelling purpose.

Thus finally a base unit is designed.

Structure of design tree :

• A process where all the assemblies and parts are named in a specific standard, different customers have different set of standards and the structure of 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 is done. And you have to follow that particular designing 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 particular unit. So this helps us to avoid errors during the design study, manufacturing assembly and inspection.
• It is an important process which 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 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 have that product in his iternary, but extra cost extra time will be taken. This is also known as 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 it will actually tell which unit or which assembly or which zone or in which station it actually belongs.
• It can help us to identify the quantity of items of same types or different types taht 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.

Design structure of different other units used to assemble the car panels in a fixture are as shown below;

Pin units : Pin unit is a unit designed to locate a car panel.

Image : Design structure of the fixed pin units locating a car panel

Image : Design structure of a fixed pin units

Clamp units : A clamp unit is a unit designed to clamp/hold the car panel with NC blocks/mylars/rest by using a pneumatic clamp cylinder.

Image : Design structure of a power clamp unit

Image : Design structure of a swivel clamp unit

Rest units :

Image : Design structure of a rest unit

Conclusion : Thus a base unit is designed with all the weldments for a given car panel and all the other remaining units have been designed for the given car panel and fixed its movement using 3-2-1 principle.

Image : Isometric view

Image : Top view

Image : Side view

Image : Front view