BIW Fixture Design Basics of Base, Dump And Slide Unit
Ans:
Q.1)
A base unit is the unit designed to mount all the 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.
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.for ex.shown in below fig.
fig.1
fig.2
fig.3
Fig.l :- shows fixed base unit which is fixed to the floor by using nut bolts.
Fig.2 :-shows movable base unit which can be moved from one place to another by using the wheels provided.
Fig.3 :-shows the rotating base unit. In this the base is fixed at two ends but the fixture can be rotate aroung its horizontal axis by using some rotating mechanism.
parts included in base plate are described below
1)Floor plate- It is the plate which join the fixture to the floor by using some nut bolts. This plate surface is machined for maintaining the horizontal position of the fixture.
2)Weld bars & pedestal- Pedestal is the base or support on which a The fixture is mounted.
3) Eyebolt- 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 use with angular loads up to 45°. 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.
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
4) Measuring hole cover - The base plate have some hole to mount various units like clamp, rest, pin etc. & every hole have some coordinates to define the exact location of the hole. Due to irregular shape of the car panel the holes coordinates are not nominal like 110.0, 150.0 But it like 110.62,150.84. The maesuring hole is drilled over the base plate which having exact cordinates so that all other holes can take reference from it. The measuring hole no. depends on the size of base plate. These holes are covered by using some plate to protect against dust, damage etc.
5) Valve box & trunking channel- 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 the pneumatic parts of the fixture & protects them from damaging while operation is in process.
6) Base plate- Base plate is the heavy plate of large thickness becuase it carries all the weight of the mounted parts. So it must be able to bear the load. Base plate is machined to maintain the parallelity within the tolerance. It also consist some drilled holes to mount the different units over
it.
Q.2)
Following points need to be taken into account whike designning the base unit
2)As designer its is important to know how big base plate or bigger base welded structure will be handeled during the fabrication and handling process. Special care needs to be taken care for flatness if the base size is very long.
6) The base unit of fixture must be strong enough so that deflection of the fixture is as minimum as possible. This deflection of fixture is caused because of forces of welding, clamping of the workpiece.
7) The Base unit of the fixture should have the mass to prevent vibration and chatter.
8) Base unit may be built from simple sections so that other units may be fastened with screws or welded whenever 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 the screws.
9) The base unit should cover all the units with its outer periphery including opening.
10) The base unit should be parallel to the floor, To adjust this there should be the provision of height adjusting mechanism.
Q.3)
- When we have some geometrical constraints then we can not mount normal units like clamp unit, pin unit etc. In this case the dump unit is used.
- Dump unit is the complex unit to design due to space constains there is less space avilable & have to adjust all the parts in available space.
- It consist of dump cylinder to perform the operation of clamping & unclamping car panel.
Q.4)
Following points are need to while designing Dump unit.
1) Dump units are used when we can not use normal units like clamp unit, pin unit etc. Due to some space constraints. The designed parts should be small in size but can do their functions effectively.
2) It contains the dump cylinder which will be the heavier part in all the unit, The riser & blade should have enough strength to bear the load of the cylinder & all the unit as well. The riser & blade thickness should be more in this type of unts.
3) The parts should be designed by considering the open condition of the unit. In the open condition of the unit the parts should not fall to another unit.
4) We can use any cylinder in this case which suits our requirements. In the below image of the dump unit Both clamp cylinder & hinge cylinders are used.
Q.5)
Slide unit are positioning units which are used to position the fixtures, weld gun, units, robots into position for further operations
they are as follows:
1)hardened guide ways
2)LM guide way
Slide units can be calssified into three types on the base of the prime mover used
1)Manual slides
2)Hydraulic/pneumatic slides
3)CNC slides
It may also be classified depending on application as
1) Horizontal slide
2)Vertical slide
3)Angular slide
Q.6)
Points needs to consider while designing the Slide unit
1) LM guide ways are preffered over hardened guide ways due to least co-eficient of friction
2) Slide unit essentially consist of LM guides, Linear cylinder ,front & rear stoppers, front & rear shock absorber, two proximity switches/reed switches for sensing the two end position, bellows or protection cover for rails.
3) Center trunnion/rear clevis mounted cylinders are ideally used for slide unit application as this allows for some float to take up mis-alignment.
4) Reverse stroke of 10mm to be considered in both forward & retracted condition.
5) Cylinder & LM guides to be validated for load & stroke.
6) LM guide rail length should be standard and should be more than the actual stroke.
7) Mechanical hard stop to be considered for both extened & retracted condition.
8) Shock absorber to be used for both extended & retracted condition to reduce impact load.
9) The two end limits to be sensed externally by a proximity switch.
10)The shock absorber should be mounted such that it is perpandicular to the pivot at mid stroke or within +/- 5degree.
Slide units are used when we have some space constraints. So we dont have space to design different special units & to assemble it. In those case we use sliding units.