Week 7- Base Unit Design Challenges 2

Aim/ Objective: Design a Base Unit with all weldments for the given Car Panel & Design all the other remaining Units required for this Fixture assembly.

Theory & report:

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

Below figure shows designed base unit with all other unit mounted on it.

Below figure shows base unit without panel & other unit.

Typical parts of base units are given as follows,

a) Base plate:

The base plate is machined plate of 25 to 40 mm thickness made of steel & aluminum, on to which the holes are drilled to mount the individual units like clamp units, pin etc., of a tool.

b) Pedestal: A pedestal is a part which connects the base plate to floor plate. A pedestal is a standard part in general & in some cases it can be a welded frame integrated with the base plate.

• A standard pedestal can be of different height & different sizes.
• Depends on the size of the base plate, the number of pedestal varies for load distribution.
• A pedestal is of different types with different inclinations.

c) Trunking Channel: A trunking channel is duct which accommodates all the pneumatic & electrical hoses and wires for the units of the tool. It has thickness of few millimeters.

d) Valve box: Valve box is standard part which will accommodates all the pneumatic valves for the 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 pneumatic parts of the fixture & protects them from damaging while operation is in process.

e) 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 transporting purpose. For example, we usually use M20 bolts that are bolted on the base plate.

f) 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 & floor plate.
• This clearance will be used by the leveling screws, for leveling the entire tool.
• Weld bars are be used to weld the positions permanently after leveling is done.
• A weld bar is a plate along the sides of the base plate of the pedestal.

Points need to account while designing base unit.

Following points need to be taken into account while designing the base unit.

a) 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). Here in this design maximum height is around 1370 mm.

If base structure is made with 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 & plate welded to the tube.

Below figure shows base plate with sq. tube welded at bottom. The sq. tube is of size 50mm x 50mm x 2 mm.

The legs are used to increase the height of base unit. The sq. tube used for leg is of size 80mm x 80mm x5mm. The total height of leg is 440mm.

so total height of base unit is 530mm which is enough.

b) 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.

Fundamentals of base plate design:

• A base plate size should cover all the units with its outer periphery including opening. Below figure shows top view of base plate covering all units which are mounted on it. There is enough space left from outer side of units. Also no unit will be interfering with each other as shown in below figure.

• In this design, the base plate is of size 1800mm x 1200mm x 20mm thk.
• 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 & 100. (In general) ex. X=100, 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.

c) Depending upon the weight eyebolt 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.

d) Only base plate or welded structure

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.

In this unit, 20 mm thickness base plate is used. The rectangular sq. tube with size of 50 x 50 x 2mm is used for fabricated base & sq. tube 80 x 80 x 5mm is used to support welded base.

e) 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.

f) 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 caused because of forces of welding, clamping of the work piece.

g) The base unit of the fixture should have the mass to prevent vibration & chatter. The designed base unit have mass 421Kg which enough to good prevent vibration & chatter.

h) 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 remain permanently with the fixture may be welded. Those parts that need frequent changing may be held with screws.

i) The base unit should be parallel to the floor, to adjust this there should be provision of the height adjusting mechanism. Leveling stud is used at each leg to adjust the height of base unit. In this design M24 bolt is used for leveling.

3-2-1 principle achievement:

3-2-1 principle is basis of any fixture design. Holding fixtures used in assembly operations often follow a 3-2-1 locating scheme to position parts. Under this scheme, three locators position a part in a primary plane or direction (e.g., high/low).

(3) minimum: Three locator blocks to establish part plane. Three locators or supports are placed under the work piece. Three locators are usually positioned at primary locating surfaces. This restrict two translation motion i.e. upward & downward & four rotational motion ( i.e. clockwise or anticlockwise). Together tree locators with clamp restrict six degrees of freedom.

(2) Round locating pin in a round hole that defines location in four directions (4 ways) perpendicular to the plane previously established. This restricts four axial movements.

(1) means restricting one degrees of freedom i.e. rotational motion about z axis clockwise or anticlockwise. Another diamond pin in hole is sufficient to prevent rotational motion around Z-axis.

Below figures shows how 3-2-1 principle is achieved. Three clamps restricts 6 degrees of freedom & two pin unit restrict 6 degrees of freedom.

Structuring a design tree:

Structuring a design tree implies process where all the assemblies & parts are named in specific manner. Different customers have different set of standard like how to structure a design tree? So they will be give standard template based on that design tree structured, which helps us to identify specific part of assembly. It specifies the Project number, station number, unit number; part number of each component so that it is easy to identify the parts.

It is an important process which helps us to identify a specific part or assembly as to which project, tool or unit, it belongs to.

This helps us to avoid error during design study, manufacturing, assembly, inspection etc.

If parts or assemblies are not named properly or the design tree is not structured properly, it will create confusion to understand & create major possibilities of making an error.

Naming of a fixture, a unit or a part gives the information as to which project or zone or assembly they do belong.

It can help us to identify quantity of items of same type or different types that are being used in assembly for e.g. bolt. It actually tells you how many bolts are particularly required in that assembly or unit.

For e.g. as in below figure highlighted in blue

P001 stands for project no 1.

Z01 stands for zone no 1.

ST006 stands for station no 6.

01 stands for unit no. e.g. Swivel clamp unit.

01 stands for part no. e.g. Rest mylar.