Week 7- Base Unit Design Challenges 2

AIM:

• Design a Base Unitfor 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 Panelsgiven in the CAD data are fully constrained with the 3-2-1 principle

Base Unit:

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.

Below are the some of the parts a Base unit in general.

• Floor plate
• Weld Bars
• Pedestal
• Measuring hole cover
• Valve Box
• Base Plate
• Trunking channel

Typical Parts of Base unit:

1.Base Unit:

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

There are three key benefits to a base Plate:

• They Can help to quickly and accurately tram in fixture and vises.
• They can hold large, complicated, or Odd-Shaped parts
• They protect the cast iron machine table.

Fundamentals of Base Plate Design:

• A base Plate Size Should cover all the unit with its outer periphery including opening
• A Base plate is preferred to be design 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 depended on the size of the box plate.
• Datum hole should be drilled at nominal value of coordinates and in terms of 50 & 100 in general Ex. X= 100, Y=-600, Z=250.
• Datum holes should be drilled with diameter 100mm away from the other unit.
• 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 is general and, in some cases, it can be a welded frame integrated with the Base plate.

Fundament of Pedestal design:

• A standard Pedestal can be of different height and different sizes.
• Depending on the size of the base plate the number of pedestals varies for load distribution.
• A Pedestal are of different types with different inclination.

2.Trunking Channel

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

4.Valve Box:

Valve Box is a standard part which will contain all the valves 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 and outlet is done within this valve box. The trunking channel includes all the pressure inlet cables going to the pneumatic parts of the fixture and protects them from damaging while operation is in process.

5.Eye Bolt:

An eye bolt is a bolt with a loop at one end. They are used to firmly attach a securing eye to a structure, so that ropes or cables may then be tied to it.

Machinery eye bolts are fully threaded and may have a collar, making them suitable for use with angular loads up to 450. Eye bolts without a shoulder should not be used for angular loads. Heavy forged eye bolts with a continuous eye may be forged with an integral shoulder, allowing their use for heavy off-axis loads.These are used for the transporting of the tool by inserting hooks into them. Some, hooks or fork lift holes are also provided in some cases for transporting purposes.

6.Floor Plate:

Floor plate are grouted to the floor with fixed bolt on the ground. These 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 screws, for levelling the entire tool. Weld bars to be weld the positions permanently after levelling is done.

The Important Parameter to be considered while designing base Unit:

• Thickness of base plate.
• Trunking channel shape and size.
• Eye bolt size.
• Leg height with respect to working height

Constrained With the 3-2-1 Principle:

3-Locators/Supporters

2-Locating pin

1-Clamp Unit

The 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.

Locating a part to be machined involves mainly three steps: Supporting, Positioning, and Clamping.

Two main intentions when placing a job on a jig/fixture are:

Precisely positioning the part at the desired coordinates. Curbing all six degrees of movement so that the part cannot budge. An extensively used method for obtaining these objectives is the 3-2-1 principle or six degrees of freedom for part location.

The 3-2-1 method is a work-holding principle where three pins are located on the 1st principal plane, i.e., either XY, YZ, ZX. And two pins are located on the 2nd plane perpendicular to the 1st plane, and at last, one pin on the plane is mutually perpendicular to the 1st and 2nd planes. The aim is to constrain the movement of the workpiece along all three axes.

BASIC NEEDS TO DESIGN A BASE UNIT:

• Working height: It depends on the nature of the manual or robotic welding fixture.
• For manual fixture we need to consider the working height 850-1100mm, depending upon the working height of the worker standard of a specific country will apply.
• If base structure is made with base plate having tubular structure welded below it. We can reach to height 100mm to 250mm. 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.
• 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 case 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 eye bolt considering the rope / belts used while shifting the structure or complete fixture are not damaging any units nearby.
• Most of the OEM makes 4 dowel holes & 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 holes at 500mm distance.
• The best unit of the fixture must be strong enough so that deflection of the fixture is as minimum as possible. This defection of a fixture is caused 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 section so that other units may be fastened with screws 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.
• The base unit should be parallel to the floor, to adjust this there should be provision of the height adjusting mechanism.

Fundamentals of Base unit:

• A base plate size should cover all the units with its outer periphery including opening.
• A base plate is always preferred to be designed with respect to BCS coordinated.
• A base plate should be drilled with datum holes / measuring holes in line with a pitch of 1000m in general.
• Number of datum holes is depend on the size of the base plate.
• Datum holes should be drilled at nominal valves of coordinates and in terms of 50 and 100. Example X=100, Y= -600, Z=250.
• Datum holes should be covered with cover plates.

Design Methodology:

We have to consider welding gun to designing a base unit. Because the welding gun has to flow through between panel and base plate. So first we have to measure the height between panel and base plate. The minimum space between the welding gun and base plate is 50 mm to 100 mm. because this is why we are creating the fixture. The base plate has to be strong and rigid to hold the entire fixture tool for welding process. In some cases the base unit is mounted on a trolley or it will fix to a slide bases unit.

Front View:

Back View:

Top View:

Isometric View:

Tree Structure:

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 the 3-2-1 Principle.