Week 7:- Base Unit Design Challenges 1

• What is a base unit? What are the typical parts of a 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 bax, trunking cable path will also be mounted onto a base unit.

Below are some parts of a base unit:

• Trunking channel
• Base plate Valve box
• Measuring hole cover
• Eye bolt
• Pedestal
• Weld bars
• Floor plate

Base plate:Base plate is 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.

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 1000mm in general.
• Number of datum holes is dependent on the size of the base plate.
• Datum holes should be drilled at nominal values of the coordinates, and in terms of 50 and 100.
• Datum holes should be drilled with dia 100mm away from other units.
• Datum holes should be covered with cover plates.

Typical parts of the base unit.

Pedestal: A pedestal is a part which connects the base plate to the floor late. A pedestal is a standard part in general and in some cases it can be a welded frame integrated with base plate.

• 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 inclinations.

Trunking Channel:

A trunking channel is a duct which accommodates all the pneumatic and electric hoses and wires for the units of the tool.

Valve box:

Valve box is a standard part which will accommodate the pneumatic valves for 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 a fixed bolt on the ground. Floor plates are also called foundation plates.

• There will be a clearance of 10mm maintained between the pedestal base plate and floor plate.
• This clearance will be used by levelling screws for levelling the entire tool.
• Weld bars are 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 pedesta
• 2) What are the points you need to consider while designing a base unit?

• Working Height:

• It depends on the nature of the manual or robotic welding fixture.
• For manual fixture working height of 850-1100mm is needed to be considered, depending upon the working height of the worker standard of a specific country will apply(in few countries human height is higher or lower eg: US Japan).
• If the base structure is made with a base plate having a tubular structure welded below it then we can reach the height of 100mm to 250mm. His case, to reach the working welding height we need to provide additional leg structure which may have the top and plate welded to the tube.
• Try to study the gun ergonomics properly then design the unit height and then base top height.

• Base plate or welded structure:

• Depending upon the component size and weight of the total units we can decide whether we need a fabricated base or just a plate.
• Mostly fabricated bases are made of square or rectangular tubes with plates.

1. Selection of eye bolt:

• As a designer it is important to know how big base plate or bigger base welded structure will be 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 eyebolt considering the rope used while shifting the structure or complete fixture are not damaging any units( rest , clamp or location units) nearby.

• Number of measuring holes required:

• We would need 3 holes to define the plane.
• Most of the OEMs make 4 dowel holes at 4 corners and put small aluminium plate marking XYZ coordinates of dowel holes.
• If the base length is too long, few OEMs follow the methodology of having dowel holes at 500mm distance.

• Base plate:

• There are few OEMs line builders that use only thick base plates. Few OEMs recommend a thick base plate 40mm or 50mm for the child part assemblies(eg: BMW). 40/50mm plate makes a very heavy structure but some OEMs follow the methodology.
• If we go for higher thickness it will not bend after machining and will have good flatness.
• The general observation is if we go for a 10-12mm thick plate for large size after machining when we unclamp the plate from the milling machine the plate will not show good flatness.

3) What is a Dump unit?

• When we have some geometrical constraints then we cannot mount normal units like clamp units, pin units etc. In this case the dump unit is used.
• Dump unit is a complex unit to design due to space constraint there is less space available and have to adjust all the parts in available space.
• It consists of a dump cylinder to perform the operation of clamping and unclamping car panels.

4) What are the points you need to consider while designing a dump unit?

The following are the points we need to consider while designing a dump unit.

• The dump units are used when we cannot 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 units. The riser and blade should have enough strength to bear the load of the cylinder and all the unit as well. The riser and blade thickness should be more in this type of unit.
• The parts should be designed by considering the open condition of the unit. In the open condition of the unit the parts should not fail to another unit.
• We can use any cylinder in this case which suits our requirements. In the below image of the dump unit both clamp cylinder and hinge cylinders are used.

5) What is a slide unit?

Slide units are positioning units which are used to position the fixtures, weld gun, units, and robots into position for further operations. They are as follows:

Hardened guideways

LM guide way

Slide units can be classified into three types on the base of prime mover used:

• Manual slides
• Hydraulic/pneumatic slides
• CNC slides

It may also be classified depending on application as

• Horizontal slide
• Vertical slide
• Angular slide

6) What are the points you need to consider while designing a slide unit?

Slide unit comes under the category of complex units, few points that need to be considered while designing a slide unit are:

Weight carrying capacity:

There are different kinds of guide rails available in the market each having a different load capacity. Hence we calculate the weight of our assembly and calculate the weight capacity of the rail. Hence we calculate based on its load capacity

Push and pull forces:

We need to check the push and pull forces and check if it is enough to push and pull the slide especially when the slide is at an inclination

Locking mechanism:

The locking mechanism ensures that the slide does not move while it is at open or retract position.If the slide moves while welding it can damage the panel as the positional accuracy will be hampered.

Stopper blocks:

Stopper blocks are used to stop the slide at the required position.It is important because, if the slide unit has rest mylars and pins,they need to be stopped at their respective position for proper resting and ;location. Without a stopper, it may dent a panel.

Sensor unit: This unit helps to check the position of the slide. When the operation is going to start, the operator pushes the button and slide moves forward, the sensor gives the signal of the presence of the unit to the PLC which then performs the next operation.

Stroke:

We need to select the length of the stroke based on the work area we want to get cleared.

Open position:

The only moving parts are the ones that slide, so we need to check the following with a gun or other unit nearby. Only after this has been checked that we can approve the design.