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 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 on to a base unit.

Below figure shows example of welded base structure.

Base unit is the fundamental component of the fixture because all other components are mounted over it so every fixture have base unit. The base unit design is different according to the panel design of clamp system, location system etc.

Typical parts of base units are given as follows,

a) Base plate:

The base plate is 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 which connects the base plate to 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 height & different sizes.
• Depends on the size of the base plate, the number of pedestal varies for load distribution.
• A pedestal is of different types with different inclinations.

c) Trunking Channel: A trunking channel is duct which accommodates all the pneumatic & electrical hoses and wires for the units of the tool. It has thickness of few millimeters.

d) Valve box: Valve box is standard part which will accommodates all the pneumatic valves for the units. The valve box is the box which 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 damaging while operation is in process.

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

f) 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 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.

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

Answer:

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

a) Working height:

It depends on the nature of the manual or robotic welding fixture.

For manual fixture we need to consider the working height 850-1100 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 base structure is made with base plate having tubular structure welded below it we can reach to height 100 mm to 250 mm. 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.

b) 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 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.
• Number of datum holes is 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 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.

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 plate. Mostly fabricated bases are used made of square or rectangular tube 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 work piece.

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 provision of the height adjusting mechanism.

3. What is a dump unit?

Answer:

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.

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 cautious while designing a dump unit because any mistake in design can result in irreversible errors that can create wastage of time & money. Hence, we should check all the points that should be addressed so that we do not face problems at a later stage. Points to be considered are mentioned below.

a) 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 important because it is the reason why we are creating a dump unit. The positional accuracy of the panel is of at most importance.

b) Mass:

We need to make sure that the assembly being dumped should have as less weight as possible without compromising on the strength of the assembly. This is because the lesser the weight lesser will be the force required to push & pull assembly. Here, we apply material as aluminum on cylinder & steel on the rest of the parts to calculate mass of moving assembly. We take mass a bit more than calculated for the factor of safety.

c) Time of opening & closing:

We also need to keep in mind that the time of opening & closing of the clamp should be least possible & this will depend on the power of the cylinder & the weight of the assembly being dumped. We need to keep it less because if it takes say 2 seconds to open then for that time we will not be able to unload the panel which will increase the cycle time resulting in more idle time.

d) Cylinder validation

Cylinder validation is an important aspect of the dump unit or any clamp unit design because the cylinder should be able to push the load for operation & then pull it back after the operation is complete. For this following calculations are necessary. Calculating C.G. of moving assembly.

Calculating the longest perpendicular distance from vertical line that passes through C.G. & pivot centre. The longest perpendicular distance could be in the work or rest position or anywhere in between.

We add all these values in the table along with the cylinder bore & the results are shown whether the cylinder is validated or not according to the formula used in table for push & pull forces. If the calculated force is more than the required force the cylinder gets validated or else we change the cylinder

e) 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 can have a clamp or a pin or a rest unit attached to it which slides forward & perform the required operation. On completion of the operation, slides moves back.

It has cylinder which pushes the assembly forward. For precision operation ball screw with servo motor is used to move slide forward & backward. Also at the both ends shock absorber are provided.

Assembly is mounted on a plate which is attached to guide rails.

There are sensors on the extreme ends of the slide to sense the position of the slide

It has stop docks on either side of the slide to stop the cylinder at required position.

It has locking mechanism which ensures that the slide does not move in particular position as it may affect the welding quality & may damage the panel.

Slide units are used in places where there is space constraint.

The opening & closing of slide should not fowl.

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

Slide units are used when we have some space constraints. So we don’t have space to design different special unit & to assemble it in those case we use sliding unit. Sometimes there are too many small panels near each other & there is not enough space as each panel requires resting, location using 2 pins, clamping & sensing. Thus becomes more difficult when the panel is at angle. To solve this problem slide unit mounts rest, clamp or pin unit or all of them on slide using guide rails, guide block, a cylinder, locking mechanism, stop blocks, sensors. When particular panel is to be welded, then this slide moves forward & after completion moves back. & get locked at its position.

Guide rails should be lubricated well.

The lubricant has following function.

a) Minimizes friction in travelling unit to prevent seizure & reduce wear.

b) Forms oil on the face way to decrease stress acting on the surface & extend rolling life.

c) Cover the metal surface to prevent rust formation.

Figure shows slide in working & in retracted position.

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 considered while designing a slide unit are

Weight carrying capacity:

There are different kinds of guide rails available in the marke each having a different load capacity. Hence we caliculate the weight of our assembly & calculate the 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 is enough to push & pull the slide especially when the slide is at an inclination.

Locking mechanism:

The locking mechanism ensures that the slide doesnot move while it is at open or retract position. If the slide moves while welding it can damage panel as the positional accuracy will be hampered

Stopper block:

Stopper blocks 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 & location. Without a stopper, it may dent the panel.

Sensor unit:

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

Stroke:

We need to select length of stroke based on work 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.