Week 7:- Base Unit Design Challenges 1

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

Answer 1. Base Unit- A base unit is the unit designed to mount all the other units 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.

Parts of a base unit in general

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

The typical Parts of a base unit are-

1. Base Plate- Base Plate is a machined plate of 25mm to 40mm thickness made of steel and aluminium on to which the holes are drilled to mount the individuals units like clamp unit, pin unit, etc.

There are three key benefits of a Base plate-

• They can help to quickly and accurately tram in fixtures and vices.
• They can hold large, complicated and odd-shaped parts.
• They protect the cast iron machine table.

2. Pedestal- A pedestal is a part which connects the base plate to the 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 heights and different sizes.

Depending on the size of the base plate, the number of pedestal varies for load distribution.

A pedestal are of different types with different inclinations.

3. Trunking Channel- Trunking channel is a standard part which accommodates all the pneumatic, electrical hoses and wires for the units. The trunking channel include all the pressure inlet cables going to the pneumatic parts of the fixture and protects them from damaging while operation is in process.

4. Valve Box - Valve Box is a standard part which will contain all the valves for the pneumatic 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 and outlet is done within this valve box. The trunking channel includes all the pressure inlet cables going to the pneumatic parts of the fixture and protects them from damaging while operation is in process.

5. Eye Bolt- An eye bolt is a bolt with a loop at one end. They are used to firmly attach a securing eye to a structure, so that ropes or cables may tied to it.

Machinery eye bolts are fully threaded and may have a collar, making them suitable for use with angular loads up to 45⁰. Eye bolts without a shoulder should not be used for angular loads. Heavy forged eye bolts with a continuous eye may be forged with an integral shoulder, allowing their use for heavy off-axis loads.

These are used for the transporting of the tool by inserting hooks into them. Some, hooks or fork lift holes are also provided in some cases for transporting purposes.

6. Floor Plate- Floor plate are gronded to the floor with fixed bolt on the ground. These 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 screws,for leveling the entire tool.

7. Weld Bars-

Weld bars are to 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.

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

Answer 2. Points needed to account while designing a base unit-

The fundamentals of a base plate design are-

• 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.

• 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.
• 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 x 100. (In general) ex. X=100, Y 600, Z-250.
• Datum holes should be drilled with diameter 100 mm away from other units.
• Datum holes should be covered with cover plates.
• Depending upon the weight, eye bolt 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.
• 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.
• Most of the OEM makes 4 dowel holes at 4 corners and put small aluminum plate marking X, Y and Z coordinates of dowel holes. If the base length is too long few OEMs follow the methodology of having holes at 500 mm distance.
• 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 called 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 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.
• The base unit should be parallel to the floor to adjust this there should be provision of the height adjusting mechanism.

Question 3. What is a Dump unit?

Answer 3. Dump unit- Dump unit is where you assemble component if you have certain constraint. Here you will have clamp unit, pin unit attached panel will be assembled. So if you have certain constraint you cannot assemble in normal way, dump unit uses it in different way. It consists of dump cylinder to perform the operation of clamping & unclamping car panel. A dump unit can have more than one cylinder to perform such operation. The first cylinder dumps the assembly carrying clamp mylar, pins and then this assembly performs the required operation in this unit, we should make sure that in the open position of panel, we should be able to remove the panel.

Functions of 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 or by side blocks or by 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 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 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.

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

Answer 4. Dump unit is a unit which has a lot of parts. Hence, we need to be extra cautious while designing a dump unit because any mistake can result in irreversible errors that can create wastage of time and money. Hence, we should check all the points that should be addressed, so that we do not face any problems at a later stage.

Points needed to consider while designing a dump unit-

1. 3-2-1 principle-

Once, we have designed the rest mylars, pins, clamps and added a cylinder for clamping, we will check the resting location and 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.

2. Mass-

We need to make sure that the assembly being dumped should have less weight if 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 the assembly. Here, we apply material as aluminium on cylinder and 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.

3. Time of opening and Closing-

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

4. Cylinder Validation-

Cylinder validation is the 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 points are important to lift the whole assembly by using the cylinder.

5. Open Condition-

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

Question 5. What is a Slide unit?

Answer 5. Slide unit- A slide unit can have a clamp unit, a pin unit or a rest unit attached to it, which slides forward and performs the required operation. On completion of the operation, slide moves back. 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 ends of the slide to sense the position of slide. It has stop docks on either sides of the slide to stop the cylinder at required position. It has a locking mechanism which ensures that the slide unit does not move in a particular position as it may affect the welding quality and may damage the panel. Slide units are commonly used at places where there are space constraints.

Slide units are used at places where there are space constraints and places where there is less accessibility. Sometimes, there are too many panels near each other and there is not enough space as each panel requires resting, locating 2 pins clamping and sensing. This becomes even more difficult when panel is at angles.To solve this problem, a slide unit mounts rest, clamp or pin unit or all of them on a slide using guide rails, guide blocks a cylinder, locking mechanism ,stop blocks and sensors. When a particular panel is to be welded, this slide moves forward, fixes the panel and on completion of the operation, slide moves back and gets locked at its position. Similarly,the same is done for other panels. Slide units are positioning units, which are used to position the car part or panel, units, weld gun, robots for further operations.

Slide units may be classified into two types based on the type of guide ways. They are-

• Hardened guide way
• LM guide way

Slide units may further classified into three types based on the prime mover. They are-

• Manual slides
• Hydraulic/pneumatic slides
• CNC slides

It may be further classified depending on the application as

• horizontal slide unit
• Vertical slide unit
• Angular slide unit

Guide rails should be lubricated well

The lubricant has following function-

• Minimizes friction in travelling unit to prevent seizure and reduces wear.
• Forms oil on the face way to decrease stress acting on the surface and extend rolling life.
• Cover the metal surface to prevent rust formation.

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

Answer 6. Basic fundamentals of slide unit-

• LM guide ways are preferred over hardened guide ways due to least coefficient of friction.
• Slide unit essentially consists of LM guides, Linear cylinder.
• Centre trunnion/rear clevis mounted cylinders are ideally used for slide unit applications as this allows for some float to take up misalignment.
• Reverse stroke 10mm to be considered in both forward & retracted condition.
• Cylinder & LM guides to be validate for load & stroke.
• Master guide way should be constrained and the subsidiary guide way to be free.
• Mechanical hard stop to be considered for both extended & retracted condition.
• Shock absorbershould be used for both extended & 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 design should be checked for screw accessibility, ease of assembly, machinabilityof parts.
• The LM guide & rails are integral assemblies and should not be separated at any cost.
• LM guide ways to be lubricated for long life.
• The design to be checked for screw accessibility, ease of assembly, machinability of parts.
• 

Slide unit comes under the category of complex units. Few points that needed to be considered while designing a slide units are-

1. Weight carrying capacity-

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

2. Pull and push forces-

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

3. Locking mechanism-

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

4. 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 and pins, they need to be stopped at their respective position for proper resting and location. Without a stopper, it may dent the panel.

5. Sensor Unit-

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

6. Stroke-

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

7. Open position-

The only moving parts are that one of 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.