CUTTING CLEARANCE

CUTTING CLEARANCE

Cutting clearance is the gap between the side of the punch and the corresponding side of the die opening on one side of the edge, when the punch is entered into the die opening. It is expressed in the amount of clearance per side.

IMPORTANCE OF CUTTING CLEARANCE:

Proper cutting clearance is necessary to:
Aid the life of the die.

Increase the quality of the piece part.

Improve the characteristics of piece part.

Reduces undue stress and wear on the cutting edges of the tool.

OPTIMUM CUTTING CLEARANCE:

Fig shows the blank or slug made under optimum cutting conditions.

The edge radius (die roll) is the result of initial plastic deformation, which occurred during the first stage of plastic shear action.

Highly burnished cut band results from the second stage (penetration) of shear action. 
The width of the cut band is approximately 1/3rd of the thickness of stock material. 
The balance of the cut is the break, which results from the third stage (fracture) of the shearing action.

EXCESSIVE CUTTING CLEARANCE:

In this the large gap between the punch and die cutting edges allows the stock material to react to the initial pressure on a manner approaching that of forming rather than cutting.  Therefore the edge radius becomes larger and the cut band becomes smaller.

INSUFFICIENT CUTTING CLEARANCE:

     

When the cutting clearance is slightly less the condition can be identified by greater width of the cut band. Because of steeper angle between the punch and die cut edges the resistance of the stock material to fracture is increased. In case of excessive clearance the burr results from dragging of the material. While insufficient clearance compressive forces cause the burr.

BURR SIDE:

        

The burr side is the adjacent to the break. The burr side is also called because of a noticeable burr developed in this side. Burr should be practically very little if the cutting clearance between the punch and die is correct and if the cutting edges are sharp.
The burr side of the blank or slug is always towards the punch (die starts shearing) the burr side of the punched opening is always towards the die opening.

DETERMINATION OF PUNCH AND DIE SIZE:

For Piercing:

       Pierced hole size =Piercing punch size

       Die = Hole size + 2 clearance.

For Blanking:
       Blanking punch = Blanking size - 2 clearance.
       Blanking size = Die size

For finding the cutting clearance following formula to be used.

Clearance  = c X s X √ (Tmax/10) 

for above 3 mm clearance = (1.5x s) x (s-0.015) x √ (Tmax/10)

  

Where ,

s = sheet thickness in mm

c = constant= 0.005

T max = shear strength = 80% Ultimate tensile strength in N/mm2

If we take c as 0.005 we get a Clarence which yields a better and cleanest work piece but required a higher cutting force and considerably more energy. If we take c as 0.01 cutting force energy as its minimum but finish will be bad.  The usual practice however is to take c as 0.01

Applying Clearance:

Given diagram illustrates how to apply clearance to obtain correct size of hole and blank.

When the metal is punched out from the functional part and the metal around the opening is scrap, the die is made to desired part size and the clearance is subtracted from (applied to) the punch size as shown in figure A. 

when the slug is discarded and the punched opening is functional, the required clearance is applied (added to) the die opening as shown in figure B.  

PROBLEMS:

1. Calculating the clearance for punching a 2mm sheet. Tmax to be assumed to be 300 N/mm2.

Clearance  = c X s X √ (Tmax/10)

                 = 0.01 X 2 X√ (Tmax/10)

                  = 0.02 X 

                  = 0.12 mm/side 

Therefore clearance on one side = 0.12 mm 

2. Determine the punch and die dimension for the component Given below. Sheet thickness 0.5mm, stainless steel sheet, T max is 400 N/mm2. C = 0.01

Clearance  = c X s X√ (Tmax/10)

              = 0.01 X 0.5 X√ (400/10)

              = 0.03 mm/side 

Blanking punch:        

     

Blanking die dimension is the same as that of component dimension.

Piercing punch size is the size of the pierced hole. 

Piercing Die: