STRIPPERS

STRIPPERS:-

The main function of the stripper is to strip the stock material off the punches after each stroke. In addition the stripper may act as a guide for the punches, as well as hold the strip flat and tight, while the strip is being worked on.

---

STRIPPER CATEGORIES AND TYPES

Strippers can be classified into 2 groups,
• Fixed strippers.
• Traveling strippers.

Fixed strippers are simple and easier to make than the traveling strippers. Fewer components are required in the construction of fixed strippers when compared to the equivalent traveling strippers, Therefore the fixed strippers are economically desirable as far as the die construction cost is concerned, Mechanically a fixed stripper are solid (positive) in performance. This is an advantage where strong stripping force is necessary.

But,

In some situations a fixed stripper may be impracticable. i.e.
When it is necessary to clamp the strip in addition to it’s stripping function.
When it is necessary to keep the punches engaged in the stripper during the entire press cycle.
A traveling stripper permits the operator to observe the work while the tool is operating.

---

BOX STRIPPER:-
A typical box pin stripper is shown in figure.
The overall dimensions E and D are made the same as the die block.  Strippers are generally left soft. How ever when required the back gauge should be hardened.

---

The tunnel dimensions are as fallows:-
The tunnel width X can be determined as  X = W + F
W = Stock strip width at maximum tolerance.
F = Desired horizontal feeding clearance.
for the average progressive die, assuming there are no other specific requirement, Clearance   F may be 0.3 per 100mm tunnel length.
Tunnel height H=S+G , G is the required vertical feeding clearance, G may be= 0.5s for flat work cutting dies with short tunnel length. Or it may be several times larger than the ‘S’. 
Lead angle L must be provided at the tunnel entrance. The angle facilitates starting the stock strip in to the tunnel and is very necessary for practical operation.  As a principle hard materials will have more clearance than soft materials. If the clearance is too large it will lead to stripping distortion.     

---

A fixed stripper is a steel plate with a clearance slot to allow the part material to pass under it. This plate is mounted to the die plate in a fixed position, Clearance holes are cut through the stripper plate letting the punches extend through without interference. At withdrawal the part material is prevented from lifting as punches are retracted. The part material is stripped off of the end of the punch or punches.
Fixed strippers have several drawbacks. These strippers do not hold the stock strip flat and lack the ability to absorb impact and snap through shock. The result is poor part flatness and premature punch failure. Fixed strippers are not generally recommended for high volume or high precision applications.

---

HOOK STRIPPER:-

Hook pins are made from cold drawn steel rod. The function is as shown in figure.

---

PRESSURE PAD STRIPPERS:-

Pressure pad strippers hold the material during cutting and strips it from the punch in the upward stroke. They may be actuated by the springs, rubber cushions or hydraulically.

---

Travelling strippers offer superior performance.
The main advantage is that as the die closes the stripper holds the stock strip or part flat and in place while the perforating and stripping operation take place.The stripper prevents the part material from Lifting or hanging upon the punches.Because the stripper lifts away from the part material after each stroke, visual monitoring of the die performance is made easy. Stripping force varies based on part material type and thickness as well as punch and die clearance. This force can range from nearly zero to as much as 25°/0of the force required to perforate the initial hole. Most applications do not exceed 10°/0of the perforating force.

---

SPRING STRIPPERS-

Spring stripper is a pressure pad stripper. They are used when it is necessary or desirable to hold the stock material flat (or very nearly flat), or to provide better visibility and access when the tool is mounted on the press. Inverted dies have stationary punches & therefore require traveling pressure pad strippers. Pressure pad strippers are also used for push back applications.

---

CLAMPING SPRING STRIPPERS:-

They are true pressure pads. They bears against the stock material, applying pressure to it. The material is clamped between stripper and die. Clearance must be large enough to ensure clamping.

---

NON CLAMPING SPRING STRIPPER:-

     These kinds of strippers are used when the material is not to be clamped. There will be clearance between the stock strip and the strip for obtaining good flatness, clearance within 0.05 to 0.4mm is recommended. The pilot registers the stock strip. In most cases, spring stripper are an effective device for producing good flat piece parts.

---

PILOTING THROUGH SPRING STRIPPERS:-

It is often necessary to employ pilots in conjunction with spring strippers. If the stripper is a clamping stripper, it cannot be used  to strip the pilot completely. This is because the pilot should register the stock strip before the strip contacts the material.
To strip the material from the pilot, the guide rails are used.  If the pilots however are too far away from the hooking action of the guide rail legs, the stock material may pull up, bowing the strip.

Even if the stock material doesn’t pull out of the rail confinement, there will be bowing action. It can cause excessive pilot wear, seriously deteriorate the quality of the pierced opening and adversely affect the ultimate flatness of the pierce part.  When the pilot position is too far away a non clamping stripper are applied, so that the stripper strips the stock strip also from the pilot.

---

COMPENSATING WASHER:-

When cutting punches are sharpened they become shorter. In many applications, the springs  are compressed a little more and are not always desirable. A practical method to eliminate this is to install the cylindrical washer as shown in the figure. Each time the punches are sharpened the washer is reduced for the amount

---

SPRING AROUND THE STRIPPER BOLTS :-
Such a construction is shown figure. This construction has desirable features and undesirable features

Desirable features are:-
The  bolt retains the stripper at center of spring pressure
The bolt acts to confine the spring in location so that the double spring pocket can be eliminated

Undesirable features are:-
The assembly needs considerable vertical space often than available

---

STRIPPER BOLT SUSPENSION :-

Bolt hole is drilled larger than shoulder diameter (clearance hole is provided ).When the die fully closed and the stripper bolt is at its maximum travel position must be sufficient to assure adequate punch grinding life.
            Normally, a space should exist between the end of the stripper bolt and the stripper.To ensure stripping a spring stripper should over travel a distance equal to sheet thickness, when the stripper is at its extended position. The over travel is between 0.1 for every light work to 1.5 for heavy work. In any case each time the punch is sharpened, the over travel increases. This should be corrected from time to time by inserting the compensator under striper bolt head as shone in figure.

        

---

GUIDING STRIPPERS :-

Two typical stripper guide pins arrangements are shown in figure. The drawings are self explanatory.    

---

STRIPPING FORCE :
Stripping force for most operations range from 10 to 20% of the cutting force. If the die has more than one punch the stripping force for that die is the sum of stripping force required for each punch.

STRIPPING FORCE FOR THE BLANKING AND PIERCING :-
The following factors affects stripping force,
1 Stock material : Material, which have high friction, value and material, which tend to cling, are more difficult to strip.
2. Surface condition of side walls : A punch, which has smooth finish on its side, wall strip more easily than punch, which is not as smooth.
3. Distance between punches: More effort is required to strip punches that are close together.
4. Area of the stock material to be stripped : Figure shows two piece parts one larger than other. The thickness and the type of stock material. The pierced   opening is the same size in both parts . The cutting is the same for both the parts . But the larger piece part requires the greater stripping effort.                                        
                 The larger area of the stock material surrounding the punch is stronger and causes the material to cling more tightly to the punches.