Week 7:- Base Unit Design Challenges 1

What is the Base Unit? What are the typical parts of the Base Unit?

A base unit is the unit designed to mount all the 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.

The parts of a base unit are:

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

Pedestal:

• A pedestal is a part which connects the base plate to the floor plate.
• A pedestal is a standard part in general, and in some cases it can be a welded frame integrated with the base plate.
• A standard pedestal can be of different height and different sizes.
• Depends on the size of the base plate, the number of pedestal varies for load distribution
• A pedestal are of different types with different inclinations.

Trunking Channel: A trunking channel is a duct which accommodates all the pneumatic and electrical hoses and wires for the units of the tool.

Valve Box: Valve box is a standard part which will accommodates the pneumatic valves for the units.

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.

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 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 are be used to weld the positions permanently, after levelling is done.
• A weld bar is a plate along the sides of the base plate of the pedestal.

What are the points need to account while designing a Base unit?

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

• 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 (In few countries human height is higher or lower e.g. US, Japan).
• As designer its is important to know how big base plate or bigger base welded structure will be handled during the fabrication and handling process. Special care needs to be taken care for flatness if the base size is very long.
• 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 units(rest, clamp or location units) nearby
• Depending upon the component size and weight of the total units you can decide whether you need fabricated base or just plate. 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 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 dowel holes at 500mm distance
• The base unit of fixture must be strong enough so that deflection of the fixture is as minimum as possible. 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 provision of height adjusting mechanism.

What is a Dump unit?

A dump unit is a unit where you can actually assemble a component if you have certain constraints.

If you have particular constraint and if you cannot assemble it in a normal way so you can use this dump unit to assemble it in different ways again.

Dump unit has dumb cylinder, open position, closed position and it has clamp cylinder unit too.

If you have any geometrical constraints, you can use this dump unit to constrain your components.

What are the points you need to consider while designing a Dump unit?

Points need to consider while designing Dump unit:

• Dump units are used when we can not use normal units like clamp unit, pin unit etc. Due to some space constraints.The designed parts should be small in size but can do their functions effectively.
• It contains the dump cylinder which will be the heavier part in all the unit, The riser & blade should have enough strength to bear the load of the cylinder & all the unit as well.The riser & blade thickness should be more in this type of unts.
• The parts should be designed by considering the open condition of the unit. In the open condition of the unit the parts should not fall to another unit.
• We can use any cylinder in this case which suits our requirements.

What is a Slide unit? Mention where Slide units are commonly used.

Slide units are positioning units which are used to position the fixtures,units,robots,gun at their position for various operations.

Sliding units are basically used in places where you have a lot of constraints or space constraints, when you cannot use different types of units to design the entire assembly. So, here you will use this sliding unit.

Sliding unit consists of:

• Slide Cylinder
• Guide Rail
• Guiide Block
• Position Locking Unit - Position locking unit which will lock the position at that particular point doesn’t allow the entire plate to move.
• Sensor Unit - Sensors need to sense whether this entire component has reached here or not.
• Stopper Blocks - Stopper blocks are used to control the entire moving and it’s a moving part.

What are the points you need to consider while designing a Slide unit?

Points needed to be considerd while designing the Slide unit:

• LM guide ways are preferred over hardened guide ways due to low co-efficient of friction
• Reverse stroke of 10mm to be considered in both forward & retracted condition.
• Cylinder & LM guides to be validated for load & stroke.
• Centre trunnion/rear clevis mounted cylinders are ideally used for slide unit application as this allows for some float to take up misalignment.
• LM guide rail length should be standard and should be more than the actual stroke.
• Mechanical hard stop to be considered for both extended & retracted condition.
• Shock absorber to be used for both extended & retracted condition to reduce impact load.
• The two end limits to be sensed externally by a proximity switch.
• The shock absorber should be mounted such that it is perpendicular to the pivot at mid stroke or within +/- 5degree.