Week 7:- Base Unit Design Challenges 1

1. What is the base unit & what are the typical parts of the base unit?

ANSWER:

A base unit is the 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 on to a base unit. And below are the some parts of a base unit in general.

They are trunking channel, base plate valve box, measuring hole cover, eye bolt, pedestal, weld bars and floor plate.

Fig . Shows example of welded base structure.

TYPICAL PARTS OF BASE UNIT:

BASE PLATE:

The base plate is a plate of 25mm to 40mm thickness.

The base plate has no of holes on it which will act as mounting points for clamp unit, pin unit, etc of a tool.

PEDESTAL:

It is 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.

TRUNKING CHANNEL:

Trunking channel is a duct that carries all the electrical and pneumatic connections (wires and pipes) for units of the tool.

It has a thickness of few millimeters.

Fig . Trunking channel

VALVE BOX:

Valve box is a standard part which will accommodates all the pneumatic valves for the unit. The valve box is the box which includes different solenoids valves. The trunking channel includes all the pressure inlet cables going to pneumatic parts of the fixture & protects them from damaging while operation is in process.

Fig . Valve box

EYE BOLT:

Eye bolts are the bolts that are used to lift fixture for transportation by inserting hooks into them.

For example, usually we use M20 bolts that are bolted on the base plate.

Used for transportation purpose, sometimes forklift holes will be provided.

Fig . Eyebolt

FLOOR PLATE:

Floor plate is grounded 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 leveling screw, 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.

2. What are the points need to account while designing a base unit?

ANSWER:

Following 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 fixture we need to consider the working height 850-1100mm, depending upon the working height of the worker standard of a specific country will apply.
• If base structure is made with base palte having tubular structure welded below it. We can reach to height 100mm to 250mm. in this case to reach the working welding height we need to provide additional some leg structure which may have top & plate welded to the tube.
• As designer it is important to know how big base plate or bigger base welded structure will be handled during the fabrication & handling process. 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 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 nearby.
• Most of the OEM makes 4 dowel holes & 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 holes at 500mm distance.
• The best unit of the fixture must be strong enough so that deflection of the fixture is as minimum as possible. This defection of a fixture is caused because of forces of welding, clamping of the work piece.
• The base unit of the fixture should have the mass to prevent vibration & chatter.
• The base unit may be built from simple section so that other units may be fastened with screws 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.
• The base unit should be parallel to the floor, to adjust this there should be provision of the height adjusting mechanism.

Fundamentals of Base unit:

A base plate size should cover all the units with its outer periphery including opening.

A base plate is always preferred to be designed with respect to BCS coordinated.

A base plate should be drilled with datum holes / measuring holes in line with a pitch of 1000m in general.

Number of datum holes is depend 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. What is dump unit?

Dump unit is mostly used on difficult panels where the welds are not achieved by easily. Sometimes, it happens that there ae some hard to reach panels and which cannot be fixed by simple units, in such case, fix them we use a dump unit.

The first cylinder dump the assembly carrying clamp mylars, pins and then this assembly performs the required operation. In this unit, we should make sure that in the open position of the panel, we should be able to remove the panel.

Fig . Dump unit

Functions of Dump unit:

To ensure proper resting of the panel being welded and to restrict its motion.

Dump unit should have two cylinders one is for to clamp the mylars at the localized point and other cylinder used to dump the whole unit.

In dump unit one assembly is fixed and that whole fixed assembly will mount on the dump unit cylinder.

Programming is the main part to function dump unit. Because when the two cylinders move at a same time it cause distortions to the panel. So at that time programming will play the role.

Cylinder validation is more important in case of a dump unit as the weight can be too heavy for the cylinder to dump. In such case, calculations are done and force exerted by the cylinder is checked against the weight of the load being carried. If cylinder force is more, it is validated or we use a more powerful cylinder.

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

ANSWER:

Dump unit is a complex unit which has a lot of parts. Hence, we need to be extra cautions while designing a dump unit because any mistake has happen in design can result a lots of errors that can create wastages of time and money. Hence we should check all the points that they are clearing all areas for all operations. Most probably we should check it on with the simulation software like ROBCAD, DELMIA V5, and PROCESS SIMULATE if there any clash are hiding on catia these software will show the particular area of clashing.

Fig .Pivot point

3-2-1 Principle:

Once we have designed the rest mylars, pins & clamps &added a cylinder for clamping. We will check the resting location & clamping of the panel being welded. This will be done by the assembly being dumped onto panel to operate. It is very important to create a dump unit. The position of the panel orientation, pivot point and PLP points are important to check while designing.

Mass:

We need to ensure that the whole assembly being dumped should have less weight as possible without compromising on the strength of the parts. If we design the unit very less in weight there will be more possibility to break the whole assembly. So design as much as possible to strength the design and more like reduce the weight. Because mass is important role to play in dump unit.

Fig . Mass

Time of opening & closing:

We also need to keep in mind that the opening and closing of the clamp will cause damages to other units or assembly. The timing of the cylinders in dump unit should always vary. Because if the timing will be same the panel will get damage.

Cylinder Validation:

Cylinder validation is most important thing to create a dump unit. Because the cylinder should be able to push the load from one place to another and then pull it back after the operation has done. Pivot point are important to lift the whole assembly by using the cylinder.

Open condition:
We need to also check the open condition after cylinder validation because the panel should not fowl with the unit while removal.

5. What is a slide unit?

ANSWER:
A slide unit is majorly used for difficult panels like( tunnel assembly) to be weld. Slide unit can have a clamp or a pin or a rest unit attached to it which slides forward and performs the required operations. On completion of the operation, slide moves back.

Fig . Slide unit

• It has a cylinder which pushes the assembly forward.
• Assembly is mounted on a plate, which is attached to guide rails.
• There are sensors on the extreme end of the slide to sense the position of the slide whether it is in open state or close state to identify.
• It has a locking system which ensures the slide does not move from that place and It gives welding quality.
• Slide units are commonly used at places where are space constraints. The opening and closing of the slide should not fowl.

Guide rails should be lubricated well.

The lubricants has the following roles,

1. Minimizes friction in the travelling unit to prevent seizure and reduce wear.
2. Forms oil on the raceway to decrease stress acting on the surface and extend rolling life.
3. Covers the metal surface to prevent rust formation.

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

ANSWER:

Slide unit comes under the category of complex units, few points that need to be consider 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 & calculate he weight capacity of the rail. Hence, we choose rail based on its load carrying capacity.

Push & Pull Forces:
We need to check the push & pull forces & check if it enough to push & pull the whole assembly 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 close position. If the slide moves while designing it can damage panel as the positional accuracy will be hampered.

Stopper Block:
Stopper block are used to stop the slide at the required position. It is important as if the slide unit has rest mylars & pins, they need to be stopped at their respective position for proper resting & locations or else it may dent the panel

Stroke:
We need to select length of stroke based on our requirements we want to get cleared.

Open position:

The only moving parts are that one the slides, so we need to check fouling with gun or other unit nearby. Only after this has been checked then we can approve the design.