Stripper
The main function of the stripper is to strip the stock material off the punches after each stroke.
The stripper may act as a guide for the punches.
It also may hold the strip flat while the strip is being worked on.
Type of strippers
Strippers can be classified into two groups.
- Fixed strippers
- Travelling strippers
fixed strippers
Fixed strippers are easier and economical to manufacture.Fixed strippers are easier and economical to manufacture.Fixed stripper is positive in performance.It can exert strong stripping force.
Box stripper
box stripper is a type of fixed stripper.The stripper is clamped and located directly on the block.
In guide plate tools,the alignment of the punch and die is ensured by guiding the punch in the stripper.
In pillar guided tools,tragile punches are guided in the stripper to prevent breakage.
A channel is milled in the stripper to act as stock guide.the width of the channel is equal to the strip width plus adeqate clearance to allow for the variation in the width of the strip.The height of the channel should be sufficient to feed thestrip easily.It depends on the typ of the stop used(when a fixed pin stop is used the clearance should be more,because the strip has to be lifted over the stop for feeding)The back edge of the channel may serve as a back gauge to correctly position the strip.
Travelling strippers
Under the following conditions a fxed stripper may be impractical.
- when it is necessary to clamp the strip in addition to its stripping function.
- when it is necessary to keep the punches engaged in the stripper during the entire cycle.
- When piece parts are to be pressed from unit stock or secondary operations are performed increased visibility for locating and accessibility of space provided by the travelling stripper allows faster loading and unloading of piece parts,ther by increasing the production.
- Travelling strippers are actuated by compression springs,disc springs(Bellevelle washers) PUR springs or the hydraulic or pneumatic die cushions of the press.
Suspension of travelling strippers
Travelling strippers are suspended as well as limited in travel by screws known as limit belts.
Travelling stripper should project from the face of the punches by an amount of 0.1mm for light work and 1.5 mm for heavy work.
When the cutting punches are sharpened,they become shorter thereby increasing this distance.
The stripper has to travel more to facilitate the punch for cutting operation.This affects the smooth working of the tool.The design of the limit bolts should allow for the adjustment of the striper suspension.
When socket head screws are used,the stripper adjustment is attained by reducing the thickness of the compensator washer.
Stripping force
Stripping force for most of the operations range from 10 to 20% of the cutting force.
If the tool has more than one punch the stripping for that tool is the sum total of the stripping forces required for each punch.
Stripping for blanking and piercing
The following factors affect stripping force
- Stock material
- Materials which have a high friction value and materials which tend to cling are more different to strip.
eg:Silicon steel,Stainless steel etc.
Surface condition of side walls.
A punch which has a smooth finish on its side walls strip easily
- More stripping force is required when the area of the material remaining after punching is more.
eg:Figure shows two pice parts one larger than the other.The thickness and the type of stock material are identical.The blanked/pierced opening is of the same size in both parts.But the larger parts requires the greater stripping effort.
Reason:
The larger area of stock material surrounding the punch is stronger and causes the material to cling more tightly to the punch.
