Week 7:- Base Unit Design Challenges 1

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

Base Unit:

The base unit is the unit of the fixture which is designed as such so that other units of the fixture can be mounted on it. Apart from these units, other necessary tools like a pneumatic valve box and trunking cable path will also be mounted on the base unit.

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

Parts of the Base Unit:

• Base plate Pedestal
• Trunking channel
• Valve box
• Eyebolt
• Floor plate

Welded Base Unit:

1. BASE PLATE:

A base plate is a machined plate of 25 to 40mm thickness made of steel and aluminum on which the holes are drilled to mount individual units like clamp unit, pin unit, rest unit, etc.

2. PEDESTAL:

A pedestal is a part that 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 heights and different sizes.
• Depending on the size of the base plate, the number of pedestals varies for load distribution.
• Pedestals are of different types with different inclinations.

3. Trunking Channel:

The 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 a few millimeters.

4. Valve Box:

The valve Box is a standard part that contains all the valves for the pneumatic units. The valve box is the box that includes different solenoid valves. The pneumatic fixtures work on air pressure provided to it. So the arrangement of the air pressure inlet & outlet is done within this valve box.

The trunking channel includes all the pressure inlet cables going to the pneumatic parts of the fixture & protects them from damage while the operation is in process.

5. Eye Bolts:

Eye bolts are used for the transporting of the tool, using lift hooks, and 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. 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.

2: What are the points that need to be accounted for while designing a Base Unit?

1. Working Height:

• It depends on the nature of the manual or robotic welding fixture.
• For manual fixtures, a working height of 850-1100 mm needs to be considered, depending upon the working height of the worker standard of a specific country. (In a few countries human height is higher or lower e.g., the US, and Japan).
• If the base structure is made with a base plate having a tubular structure welded below it then we can reach to height of 100 mm to ~250 mm. In this 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.

2. 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.

3. Selection of Eye Bolt:

• As a designer, it is important to know how a big base plate or bigger base welded structure will 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 the 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.

4. 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 aluminum plates marking the X, Y &Z coordinates of dowel holes.
• If the base length is too long few OEMs follow the methodology of having dowel holes at a 500mm distance.

5. Base Plate:

• There are few OEMs, Line builders use only thick base plates. Few OEMs recommend a thick base plate of 40mm or 50mm for the child part assemblies (e.g. BMW). 40/50mm plate makes a very heavy structure but some OEMs follow the methodology.
• If we go for a higher thickness it will not bend after machining and will have good flatness.
• The general observation is if we go for a 10-12 mm thick plate for a large size e.g. 2mx2m, after machining when we unclamp the plate from the milling machine the plate will not show good flatness.

Fundamentals of base plate design:

• A base plate size should cover all the units with its outer periphery including the opening.
• A base plate is preferred to be designed concerning BCS coordinates.
• A base plate should be drilled with datum holes/ measuring holes in line with a pitch of 100mm in general.
• The number of datum holes depends on the size of the base plate.
• Datum holes should be drilled at nominal values of coordinates and in terms of 50 and 100.
• Datum holes should be drilled with a diameter 100mm away from other units.
• Datum holes should be covered with cover plates.

3: What is a Dump Unit?

Dump Unit:

The dump unit is used when we have any constraints and we cannot use our normal units like clamp unit and pin unit etc. In such cases, to fix them, we use a dump unit. A dump unit can have more than one cylinder to perform such an operation. The first cylinder dumps the assembly carrying clamp mylars, and 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.

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

Functions of the dump unit:

• To ensure proper resting of the panel being welded and to restrict its motion.
• To ensure there is no rotation in the panel and to fix it either by pins side blocks or magnets.
• To ensure there is enough clamping force in the cylinder so that it doesn't move while welding.
• The dump unit should have correct shimming for adjustment of the panel.

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

Ans: The dump unit is a complex unit. The design of the dump unit must be done with almost care and preciously.

• The clamp unit and pin unit which are designed in the Dump unit have to be checked whether the clamp and pin positions are correct. There should be no interference between the parts. All the parts in the dump unit must be independent.
• The assembly of the Dump unit should be lightweight. At the same time must be hard enough to carry the panel with constraints in all DOF This is due to the lesser the weight, the lesser the force required to push and pull the assembly. Here, we apply material such as aluminum on the cylinder and steel on the rest of the parts to calculate the mass of the moving assembly. We take mass a bit more than calculated for the factor of safety.
• The time of opening and closing of the clamp should be the least possible and this will depend on the power of the cylinder and the weight of the assembly being dumped.
• 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 and then pull it back after the operation is complete.
• Check the open condition after the cylinder validation because the panel should not coincide with the unit while removing.

5. What is a Slide unit?

Ans:

• Slide units are positioning units that are used to position the car part, fixture, and weld gun robots into position for further operations.
• Slide units may be classified into two types based on the type of guideways used as hardened guideway and LM guideway.
• Slide units may further be classified into three types based on the prime mover used as manual slides, Hydraulic/pneumatic slides, and CNC slides.
• It may be further classified as Horizontal, Vertical, and Angular slide units depending on the application.
• LM guides are preferred over the hardened guide ways due to the least coefficient of friction.
• The slide unit essentially consists of LM guides, linear cylinder, front and rear stopper, front, and rear shock absorbers, two proximity switches/reed switches for sensing the two end positions, bellows, or protection covers for rails.

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

Ans:

• Center Trunnion / 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 is to be considered in both forward and retracted conditions.
• 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 guides the way to be constrained and the subsidiary guides the way to be free.
• Mechanical hard stop to be considered for both extended and retracted conditions.
• Shock absorbers are to be used for both extended and retracted conditions 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 are 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 is to be checked for screw accessibility, ease of assembly, and machinability of parts.