Week 7:- Base Unit Design Challenges 1

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

Ans: A Base unit is a unit designed to mount all the other units(Clamp, pin, rest units) of the tool, also other parts necessary for a tool like pneumatic valve box, trunking cable path will be mounted on to the base unit.

Parts of Base Unit:

1. Trunking channel: All the pneumatic, electrical hoses and wires of the units are accommodated in the trunking channel.
2. Base plate: It is a machined plate of 25 to 40 mm thickness made of steel and aluminium, on to which the holes are drilled to mount the individual units.
3. Valve box: It is a standard part which accommodates the pneumatic valves for the units.
4. Measuring hole cover: It is the cover of the datum hole.
5. Eyebolt: It is used for the transporting of the tool, using lift hooks, Fork lift holes will also be provided in some cases for transporting purpose.
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. Weld bars: These are the support structures between the pedestal and floor plate.
8. Floor plate: It is grouted to the floor with fixed bolt on the ground. Floor plates are also called as foundation plates.

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

Ans:

• A Base unit should cover all the units with its outer periphery including openings.
• 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 dependent on the size of the base plate.

• Datum holes must be drilled in nominal values of coordinates and in terms of 50 and 100.
• Datum holes should be drilled with diameter 100 mm away from other units.
• Datum holes should be covered with cover plate.
• There must a minimum space for the trunking channels.

• The weld gun position and the weldment have to be checked. A minimum of 5 mm gap must be present between the gun and the units.

3. What is a Dump unit?

Ans: Sometimes, It happens that there are some hard to reach panels and which cannot be fixed by simple units, in such cases, to fix them, we use a 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 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.

Functions of the dump unit:

• Have to check the correct resting position of the panel being welded and to constrain its motion.
• The panel rotation must be constrained with all DOF
• It must have correct shimming for adjustment of the panel.

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

Ans: Dump unit is a complex unit. The design of 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 Dump unit should be light weighted. In the same time must be hard enough to carry the panel with constraints in all DOF This is due to the lesser the weight, lesser will be the force required to push and pull the assembly. Here, we apply material as aluminium on cylinder and steel on the rest of the parts to calculate the mass of 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 least possible and this will depend on the power of the cylinder and the weight of 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 removal.

5. What is a Slide unit?

Ans:

• Slide units are positioning units which are used to position the car part, fixture, weld gun robots into position for further operations.
• Slide units may be classified into two types based on the type of guide ways used as hardened guide way and LM guide way.
• 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.
• 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 mis-alignment.
• 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 guiderail 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 absorbers 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.