Base Unit Design 2
AIM: To design the base unit and all the necessary fixtures to the given component
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 onto a 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.
Typical parts of base units are given as follows,
a) Baseplate:
The base plate is a 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 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.
A standard pedestal can be of different heights & different sizes. Depending on the size of the base plate, the number of pedestals varies for load distribution. A pedestal is of different types with different inclinations.
c) 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
few millimeters.
d) Valve box: The valve box is the 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 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.
e) Eyebolt: Eyebolts are used for the transporting of the tool, using lift hooks, forklift holes will also be provided in some areas for transporting purposes. For example, we usually use M20 bolts that are bolted on the base plate.
f) Floorplate: The floorplate is grouted to the floor with a 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
Considerations for designing a Base unit
a) Working height:
It depends on the nature of the manual or robotic welding fixture. For manual fixtures 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).
If the base structure is made with a base plate having a tubular structure welded below it we can reach to the height of 100 mm to 250 mm. 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.
b) As a 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.
- 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.
- The number of datum holes 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 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 (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 a plate. Mostly fabricated bases are used made of square or rectangular tubes with the plate.
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 workpiece.
g) The base unit of the fixture should have the mass to 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 the provision of the height adjusting mechanism.
3-2-1 principle achievement:
3-2-1 principle is the 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 the part plane. Three locators or supports are placed under the workpiece. Three locators are usually positioned at primary locating surfaces. This restricts two translation motions i.e. upward & downward & four rotational motions ( i.e. clockwise or anticlockwise). Together with tree locators with clamps restrict six degrees of freedom.
(2) Round locating pin in a round hole that defines the location in four directions (4 ways) perpendicular to the plane previously established. This restricts four axial movements.
(1) means restricting one degree of freedom i.e. rotational motion about z-axis clockwise or anticlockwise. Another diamond pin in a hole is sufficient to prevent rotational motion around Z-axis. The below figures show how the 3-2-1 principle is achieved. Three clamps restrict 6 degrees of freedom & two-pin unit restrict 6 degrees of freedom
STRUCTURING THE DESIGN TREE:
