Week 7:- Base Unit Design Challenges 1

Q-1 What is the base unit what are the typical types of the 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.
• Below are the some parts of a base unit in general.
• Trunking channel
• Base plate
• Valve box
• Measuring hole cover
• Eye bolt
• Pedestal
• Weld bars
• Floor plate

• Typical parts of the base unit are :-

1. Base unit: - base plate is a machined plate of 25 mm to 40 mm thickness made of steel and aluminum o to which the holes are drilled to mount the individuals units like clamp units, pin units etc. of a tool.

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

1. Pedestal: - A pedestal is a part which connects the base plate to the floor plate. A pedestal is a standard part in general, and in some cases it can be 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 distribustion. A pedestal are of different types with different inclinations.
2. Trunking channel : - Trunking channel is a standard part which will accommodate all the pneumatic and 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.

3. Valve box: - Valve box is a standard part which will contain all the valve for the pneumatic units. The valve box is the box, which includes different solenoid valve. The pneumatic fixtures work on air pressure provided to it. Therefore, 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 processs.

4. 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 then be tied to it. Machinery eye bolts are fully threaded and may have a collar, making them suitable for used for angular loads. Heavy forged eye bolts with a continuous eye may be forged with and integral shoulder, allowing their use for heavy off axis loads. These are used for the transporting of the too; by inserting hooks into them. Some, hooks or fork lift holes are also provided in some cases for transporting purpose.

5. Floor plate : - floor plate are grouted to the floor with 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 screw for, leveling the entire tool.

6. Weld bars : - weld bars are to be used to weld the positions permanently after leveling is done. A weld bar is plate along the sides of the base plate of the pedestal.

Q-2 what are the points need to account while designing a base unit?

• A points needed to account while designing a 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 850-1100 mm depending upon the working height of the worker standard of specific country will apply. If base structure is made with base plate 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 and 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 and handling process. Special care needs to take care of flatness. If the base size is very long.
• Base plate size should cover all the units with its outer preparatory including opening.
• A base plate is preferred to be designed with respect to BCS coordinates.
• 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. Ex. X=100, Y= 600, Z= 250.
• Datum holes should be drilled with diameter 100 mm away from the 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 considering the rope/belts used while shifting the structure or complete fixture are not damaging any 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 & put small aluminum plate marking XY&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.
• 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 section 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.

Q-3 what is a 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 can not 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 that one cylinder to perform such operation. the first cylinder dumps the assembly carrying clamp Mylar, pins and the 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 I 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 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 cylinders is checked against the weight of the load being carried. If cylinder force is more, it is validate or we use a more powerful cylinder.

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

• 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 don’t face problems at later stage.
• Points needed to consider while designing a dump unit:

1. 3-2-1 principle: -

We have designed the rest Mylars, pins and 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 on to 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 as less weight as possible without compromising on the strength of the assembly. This is because lesser the weight lesser will be the force required to push and pull the assembly. Here, we apply material as aluminum on cylinder and steel on the rest of the parts to calculate mass of the 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 possible 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 say 2 seconds to open then for that time will not be able to unload the panel which will increase the cycle tie resulting in more idle time.

4. Cylinder validation: -

• Cylinder validation is an important aspect of the dump unit or made any clamp unit design because should be able to push the load for the operation and then it pulls back after the operation is complete.
• Calculating the center of gravity of the moving assembly.
• Calculating the longest perpendicular distance from vertical line that passes through the center of gravity and the pivot center. The longest perpendicular distance could be in the work or in rest position or anywhere in between.
• Calculating the shortest distance between pivot center and cylinder line.
• We had all these values in the table along with cylinder bore and the results are shown whether the cylinder is valid or not according to the formula used in the table for pull push forces. If the calculated force is more than the required force the cylinder gets valid or else we change the cylinder.

5. Open condition:

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

Q-5 What is a slide unit?

• A slide unit can have a clamp plan or a pin or a rest unit attached to it with slides forward and performs the required operations. 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 slides 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 mechanisms, 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, weldgun, robots for further operations.

The above figure shows the slide unit in both working position and retracted position.

• Slides unit may be classified into two types based on the type of guide ways. They are

1. Hardened guide way
2. LM guide way

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

1. Manual slides
2. Hydraulic/pneumatic slides
3. CNC slides

• It may be further classified depending on the application as

1. Horizontal slide unit
2. Vertical slide unit
3. 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.

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

• Basic fundamentals of slide unit:

1. LM guide ways are preferred over hardened guide ways due to least coefficient of friction.
2. Slide unit essentially consists of LM guides, Linear cylinder
3. Centre trunnion/rear clevis mounted cylinders are ideally used for slide unit applications ass this allows for some float to take up misalignment.
4. Reverse stroke 10mm to be considered in both forward & retracted condition.
5. Cylinder & LM guides to be validate for load & stroke.
6. Master guide way should be constrained & the subsidiary guide way to be free.
7. Mechanical hard stop to be considered for both extended & retracted condition.
8. Shock absorber should be used for both extended & retracted condition to reduce impact load.
9. Slide to be locked/clamped in the working position.
10. Slide to be locked with a safety pin in the retracted position during transportation.
11. The two end limits to be sensed externally by a proximity switch.
12. The design should be checked for screw accessibility, ease of assembly, machinability of parts.
13. The LM guide & rails are integral assemblies and should be separated at any cost.
14. LM guide ways to be lubricated for long life.

• 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. Hence, we calculate the weight of our assembly and calculate the weight capacity of the rail. Hence, 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 damaged.

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 sensors 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.