Week 7- Base Unit Design Challenges 2

Objective:

1. Design a Base Unit for the units designed for the above challenges with all weldments for the given Car Panel.

2. Design the complete Fixture assembly. Make all the 5 Panels given in the CAD data fully constrained with the 3-2-1 principle.

Base Unit:

Base unit is the unit of the fixture which is designed as such so that other units of the fixture can be mounted on it. Apart from these units, other necessary tools like pneumatic valve box, trunking cable path will also be mounted on the base unit.

Base unit is the fundamental componet 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.

Parts of the Base Unit:

• Base plate Pedestal
• Trunking channel
• Valve box
• Eye bolt
• Floor plate

• Welded Base Unit:

  base unit

  A typical Base unit and its parts

  The typical parts of the base unit are as follows:

  1)Base Plate: Base Plate is a machined plate made of steel or Aluminium of thickness 25mm to 40mm on which the holes are drilled to mount diffrent units like rest units, clamp units, pin units etc.

  base plate

  2)Pedestal: Pedestal is part used to connect the base plate with a floor plate. The pedestal can be a standard part or a make part, depending on the size and height of the fixture. Standard pedestals are available in different heights. Pedestals are also available in different inclinations. Depending on the size of the base plate, the number of a pedestal for a fixture can vary for load distribution.

  3)Trunking Channel: Trunking channel a duct or tube that carries all the electrical wire and pneumatic pipe connections for units of the different tool and units. The trunking channel includes all the pressure inlet cables going to the pneumatic parts of the fixture & protects them from damaging while operation is in process.
  

  truning channel

  4)Valve box: Valve Box is a standard part which contains all the valves for the pneumatic units. It is a kind of control unit The valve box is the box which includes different solenoid valves which is used to control the vales.

  Typical valve box

  5)Measuring hole cover : The measuring hole is drilled over the base plate which having exact cordinates and acts as reference for the other holes. Due to irregular shape of the car panel the holes coordinates are not nominal like 250.0, 310.0 But it like 250.62,310.84. These holes are covered by using some plate to protect against dust, damage etc.

  Eyebolt: Eye Bolts are the bolts that are used to lift the fixture for transportation by inserting hooks into them. For example, we usually use M20 bolts that are bolted on the base plate. Used for transportation purpose, sometimes forklift holes will be provided.

  Typical eye bolts

  6)Floor Plate: They are also known as foundation plate. They are grounded by using a fixed bolt. minimum of 10 mm clearance should be given between pedestal base plate and floor plate.

  7)Weld bar: Weld bars are used after the levelling is done by levelling screw. Weldbars are steel plates along the sides of the base plate of a pedestal. Weld bars are used to position the positions permanently.

• Points to be considered while designing Base unit:

  working height:
  • Working height 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 specific country will apply.
  • 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 and plate plate welded to tube.
  Depending upon the weight eye bolt 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 rest,clamp or location units
  Depending upon the component size and weight of the total units the base plate is fabricated or sometimes normal plate is used. Mostly fabricated bases are used made of square or rectangular tube with plate
  Most of the OEM makes 4 dowel holes at 4 corners and 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 dowel holes at 500mm distance

  The base unit of fixture must be strong enough so that deflection of the fixture should be minimum. This deflection of fixture is caused because of forces of welding, clamping of the workpiece. The Base unit of the fixture should have the mass to prevent vibration and chatter.

  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.

  The base unit should cover all the units with its outer periphery including opening. The base unit should be parallel to the floor, To adjust this there should be the provision of height adjusting mechanism.

• Design of Base unit and fixture assembly:
• 
• clamp units
• 
• REST UNITS
• 
• pedestals and square tubes
• Depending upon the component size and weight of the total units we can decide whether we need fabricated base or just plate.
• Mostly fabricated bases are used made of square or rectangular tube with plate.

  Selection of Eye Bolt:

  • As designer it is important to know how big base plate or bigger base welded structure will be handled during the fabrication and handling process.
  • Depending upon the weight eye bolt size is selected for lifting holes on the base plate.
  • Generally 4 eye bolt provision is made. We 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.
• 

BASE PLATE:

• There are few OEM’s, Line builders use only thick base plate. Few OEM's recommend thick base plate 40mm or 50mm for the child part assemblies (e.g BMW). 40/50mm plate makes a very heavy structure but some OEM's follow methodology.
• If we go for higher thickness it will not bend after machining and will have good flatness.
• General observation is if we go for 10-12 mm thick plate for large size for e.g. 2mx2m, after machining when we unclamp plate from milling machine the plate will not show good flatness.

Fundamentals of 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 BCS coordinates.
• A base plate should be drilled with datum holes/ measuring holes in line with a pitch of 100mm in general.
• Number of datum holes depends on the size of the base plate.
• Datum holes should be drilled at nominal values of coordinates and in terms of 50 and 100.
• Datum holes should be drilled with diameter 100mm away from other units.
• Datum holes should be covered with cover plates.

• Fixture Assembly:

  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

  The purpose of the Fixture is :

  1. Resting:

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

  2. 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.

  3. Orienting:

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

  4. 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 other 2 pin units restrict radial movement. As by the 3-2-1 method, intoducing a minimum of 1 clamp and 2 pins will restrict 6 degrees of freedom.

• Fixture Assembly:

  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

  The purpose of the Fixture is :

  1. Resting:

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

  2. 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.

  3. Orienting:

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

  4. 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 other 2 pin units restrict radial movement. As by the 3-2-1 method, intoducing a minimum of 1 clamp and 2 pins will restrict 6 degrees of freedom.