Week 7- Base Unit Design Challenges 2

Base unit:- It 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 a pneumatic valve box, trunking cable path will be mounted on to a base unit.

Fig 1: Base Unit

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

Parts of the Base unit

• Base Plate
• Pedestal
• Valve Box
• Measuring Hole Cover
• Eyebolt
• Floor Plate
• Weld Bar

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

Fig 2: Base plate

Fundamentals of base plate design:

• A base plate size should cover all the units with its outer periphery including the opening.
• A base plate is preferred to be designed concerning 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 a diameter 100mm away from other units.
• Datum holes should be covered with cover plates.

Pedestal:- A pedestal is a part that connects the base plate to the floor plate. A pedestal is a standard part in general & in some cases, it can be a welded frame integrated with the base plate. Pedestals are used for leveling the base unit

Fig 3: Pedestal

A standard pedestal can be of different heights & different sizes.

Depending on the base plate’s size, the number of pedestals varies for load distribution.

A pedestal is of different types with different inclinations.

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

Fig 4: Trunking Channel

Valve box:- Valve box is a standard part that will accommodate all the pneumatic valves for the units. The valve box is the box that includes different solenoid valves. The pneumatic fixtures work on air pressure provided to it. So the arrangement of the 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 damage while the operation is in process.

Fig 5: Valve Box

Eyebolt:- Eye bolts are used for the transporting of the tool, using lift hooks, forklift holes will also be provided in some cases for transporting purposes. For example, we usually use M20 bolts that are bolted on the base plate.

Fig 6: Eyebolts

Selection of Eye Bolt:

• As a designer it is important to know how big base plates or bigger base welded structures 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 the 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.

Floor plate:- Floor plate is grouted to the floor with a fixed bolt on the ground. Floor plates are also called 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 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.

Fig 7: Base Unit (all views)

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

Working height:

• It depends on the nature of the manual or robotic welding fixture.
• For manual fixtures we need to consider the working height of 850-1100mm, depending upon the working height of the worker standard of a specific country will apply.
• If the base structure is made with a base plate having a tubular structure welded below it. We can reach a height of 100mm to 250mm. in this case to reach the working welding height we need to provide additional leg structure which may have top & plate welded to the tube.
• As a designer, it is important to know how big base plates or bigger base welded structures will be handled during the fabrication & handling process. The 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 the base plate. Generally, 4 eye bolt provision is made. You also need to select the location of the eye bolt considering the rope/belts used while shifting the structure or complete fixture are not damaging any units nearby.
• Most of the OEMs make 4 dowel holes & put a small aluminum plate marking the X, Y, & Z coordinates of dowel holes. If the base length is too long few OEMs follow the methodology of having holes at a 500mm distance.
• The best unit of the fixture must be strong enough so that the deflection of the fixture is as minimum as possible. This defection of a fixture is caused because of forces of welding, and clamping of the workpiece.
• The base unit of the fixture should have the mass to prevent vibration & chatter.
• The base unit may be built from a simple section so that other units may be fastened with screws or welded as 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 the provision of the height adjusting mechanism.

Fig 8: Base Unit (height)

Fundamentals of Base Unit:

• A base plate size should cover all the units with its outer periphery including the opening.
• A base plate is always preferred to be designed concerning BCS coordination.
• A base plate should be drilled with datum holes / measuring holes in line with a pitch of 1000m in general.
• The number of datum holes depends 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.

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 :

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:

Sufficient clamp units, pin units & rest units were used to achieve the 3-2-1 principle

Fig 9: Clamping

Fig 9: Clamping, pin & resting units

Pin Unit:- Pin unit is a unit designed to locate the car panel using Pin. The pin unit is used to lock the rotational position of the panel.

Fundamentals of pin unit design

• it is necessary to have 2 pins to locate a car panel & here one pin must arrest 4 ways (4 translational) & other must arrest 2 ways (i.e. 2 rotational) of a car panel.
• The axis of the car panel hole & the axis of the pin should be the same (in general)
• The pin tolerance for a geo pin will be of closer tolerance i.e. -0.15mm & for a re-spot pin is -0.28mm
• Pin should engage 3 to 5 mm from the car panel entry surface.
• All pins should be accommodated with 2-way adjustable shims irrespective of the direction of arresting.
• Round pins should be used if the hole is round in case of 4-way arresting of the car part.
• Round pins can be used for 2 way arresting of car parts only if the hole is slot.
• Diamond pins should be used in case of 2-way arresting of car part if the hole is round
• Diamond pins should be used in case of 4-way arresting of the car part if the hole is slotted.

There are mainly 2 types of pin units

• Fixed Pin Unit
• Retractable Pin Unit

Fig 9: Pin Units

Swivel clamp unit

Clamp unit: - A clamp unit is a unit designed to clamp/hold the car panel with NC blocks/ Mylar/ rest by using a pneumatic clamp cylinder & below are different parts of a clamp unit in general. Usually clamp open position is 90°. Fingers are attached to the clamp Mylar. In a clamp unit shimming to one side is more than enough because it will restrict translation motion in upward & downward directions only. There is another type of clamp unit known as the swivel type. There is a pivot pin around which the clamp arm rotates.

CLAMP PLAN:

1. The customer provides the clamp plan which consists of the position of the clamp, pins, rests and dump, etc.
2. It has the exact coordination of these components and in addition, it also has the hole’s dimensions and tolerances.
3. As this is critical input data, it must not be modified.
4. In case the spot for weld and the clamp position is nearby and it may create a problem later and there is a need to change the position of the clamp, the same must be done so with the permission of the customer as it will be easy for the customer to make changes from his side also.

Fig 11:- Clamp Plan

Factors should be considered while designing a clamp unit:

• There should be adequate space between the cylinder & other parts like the riser, or car panel because the cylinder moves or swing while opening & closing
• Pin clamp unit used where two panels or components are located for welding.
• Clamping is made in such a way that it clamps two car panels at once
• All parts should snap & butt properly to each other.
• Make sure that parts of the clamping unit are 10 to 12 mm away from the car panel so they do not damage the car panel
• Avoid any clash. It will not arise an issue at assembly.

Fig 12:- Swivel Clamp units

Fig 13:- Clamp units

REST UNIT:

• A rest unit is a unit that is designed to support or rest the car panel on it.
• Its sole purpose is to rest the panel onto it and thus no force or clamping is done.
• But as it is a simple design, it can be combined with other units to reduce the cost of the overall project.
• For example it can be used along with locating unit to locate and rest the panel or with a clamp unit to clamp and rest the panel.
• Or the rest unit, locating unit, and clamp unit can be used together in one single project.

Fig 14:- Rest units

Fig 15:- Car panel with the Base unit, Clamp unit, Pin unit & rest unit

Fig 16:- Clamp unit, Pin unit & rest unit on Base Unit

Fig 17:- Clamp unit, Pin unit & rest unit on Base Unit with Car panel

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 and every customer has different standards for their 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, or tool, it belongs. 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 is 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 and 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 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 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 they belong.
• If we name the part properly or name the fixture properly ill tell which unit or which assembly or which zone or in which station it 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. When 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.