Week 7:- Base Unit Design Challenges 1

Base Unit Design Challenges 1

Q.1:What is the Base Unit? What are the typical parts of the Base Unit?

Base Unit:

The base unit is the main unit of fixture, it provides the support to all other units mounted on it.

The base unit is mounted over the pedestal on the weld bar. The shape, size and the strength of base unit is depending upon the design of clamp system, position system,size and weight of workpiece.

Typical parts of the Base Unit:

Base plate

Weld bar

Valve box

Floor panel

Eye bolt

Pedestal

1.Base plate:

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

Holes are provided on this plate to mount all units, eye bolts.

2.Weld bar:

It is the above or over to the floor plate.It is welded to pedestal.

3.Floor plate:

The 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.

4.Valve box:

It is nothing but a control unit of BIW Fixture consist of no. of solenoid valve which is used control valve.It is operated under certain pressure and operated to hold and release the workpiece with the help of cylinder having appropriate stroke length.

5.Eye bolt:

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.

When installing lifting eye bolts it is critical to ensure the face is in firm contact with the mating surface. These lifting rings are intended for linear pulling loads in the direction of the thread.

They are not intended for use where the load is perpendicular to the thread. This is because in these instances the bolt could unscrew and move or it could be twisted and break

6.Pedestal:

It is the part which connects the base plate to the floor plate. It is the standard part in general, and in some cases, it can be a welded frame integrated with the base plate.

7.Trunking channel:

A 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.

Q.2:What are the points need to account while designing a 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 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 the 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 plate or bigger base welded structure 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 OEM makes 4 dowel holes & put small aluminum plate marking 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 deflection of the fixture is as minimum as possible. This defection of a 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 & chatter.

The base unit may be built from a 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 the provision of the height adjusting mechanism.

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 with respect to 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.

Q.3.What is a dump unit?

Sometimes it happens that there are some hard to reach panels & which cannot be fixed by simple units. In such cases, to fix them we use dump unit. A dump unit can have more than one cylinder to perform such operation. The first cylinder dumps the assembly carrying clamp mylars, pins & then this assembly performs the required operation. In this unit, we should make sure that in the open position, we should be able to remove the panel.

Function of the dump unit:

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

To ensure there is no rotation in the panel & to fix it either by pins or slide block or magnets.

To ensure there is enough clamping force in the cylinder so that it doesn’t move while welding.

Dump unit should have correct shimming for adjustment of the panel.

A dump unit can have a slide cylinder or a clamp cylinder attached to it. Cylinder validation is 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 & force exerted by the cylinder is checked against the weight of the load being carried. If the cylinder force is more, it is validated or we use a more powerful cylinder.

Q.4. What are the points you need to consider while designing a Dump Unit ?

ANS. The points you need to consider while designing a Dump Unit are as below:

3-2-1 Principle: This is a basic principle while designing any fixture used to locate any car panel in fixture proper.

Mass Consideration: Weight of dump unit must be at-least side. Less the weight of unit less will be the force required to operate this unit. While taking such consideration of mass, strength will be at maximum side.

Time of opening and closing: Time of opening and closing will be less as much as possible.

Cylinder Validation: It is one of the important point which is consider while designing such unit. Cylinder is selected such that it is able to take load of unit while operation.

Open Condition: It is one of the basic point which is consider in every moving unit like movable pin unit or clamp unit or slide unit, etc. Open condition is check to determine whether the car panel is load-unload proper or not.

Q.5) What is a Slide unit?

- Slide units are positioning units which are used to position the fixture, weld gun,units,robots into position for further operations.

- Slide units can be classified into two types on the base of the type of guide-ways used as

1)hardened guide ways

2)LM guide way

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

1)Manual slides

2)Hydraulic/pneumatic slides

3)CNC slides

- It may also be classified depending on application as

1)Horizontal slide

2)Vertical slide

3)Angular slide

- Slide units are used when we have some space constraints. So we don’t have space to design different special units & to assemble it. In those case we use sliding units.

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

Center Trunion / rear clevis mounted cylinders are ideally used for slide unit application as this allows for some float to take up misalignment.

Reverse stroke of 10 mm to be considered in both forward and retracted condition.

Cylinder and LM guides to be validated for load and stroke.

LM guide rail length should be standard and should be more than the actual stroke.

Master guide way to be constrained and the subsidiary guide way to be free.

Mechanical hard stop to be considered for both extended and retracted condition.

Shock absorber to be used for both extended and retracted condition to reduce impact load.

Slide to be locked/clamped in the working position.

Slide to be locked with a safety pin in the retracted position during transportation.

The two end limits to be sensed externally by a proximity switch.

The LM rails should always be covered to protect against weld spatter and dust by a bellow or sheet metal cover.

The design to be checked for screw accessibility, ease of assembly, machinability of parts.