Cutting Clearance

This is the space between one side of the punch and the corresponding side of the die opening when the punch is inserted into the die.Cutting clearance should always be specified as the amount of clearance on each side.
Proper cutting clearance is essential for extending tool life and ensuring the quality of the punched parts.Visual inspection of the punched components can reveal whether the punch and die have:
    ✔ Optimal cutting clearance OR
    ✔ Excessive clearance OR
    ✔ Misalignment

Optimum Cutting Clearance:
When the cutting clearance is optimal, a small edge radius forms due to the plastic deformation that occurs during the initial shearing stage. This results in a highly burnished cut band that is roughly one-third the thickness of the stock material. The remainder of the cut is characterized by a break from the final fracturing stage.

Excessive Cutting Clearance: 
When the gap between the punch and die is too large, the stock material behaves more like it's being formed than cut due to the increased initial pressure. This results in a larger edge radius that does not blend smoothly with the cut band. Consequently, the cut band appears smaller, and the break exhibits greater irregularities. Heavy burrs can also be observed along the cut contour, resulting from the material being dragged during the cutting process.

Insufficient Cutting Clearance: 
When the cutting clearance is slightly too tight, the width of the cut band increases. If the clearance is excessively reduced, multiple cut bands may form. The steeper angle between the punch and die edges increases the material's resistance to fracture, causing the initial fracture to occur at the clearance rather than at the cut edges. Additionally, compressive forces may create burrs along the cut.

Misalignment Between Punch and Die: 
The cutting characteristics can reveal if the punch and die openings are correctly aligned. Misalignment leads to uneven clearance, with one side increasing and the other decreasing, which causes noticeable variations in the cut band of the component.

Burr Side: 
The burr side is located adjacent to the break and should be minimal if the cutting clearance between the punch and die is optimal and the cutting edges are sharp. For a blank or slug, the burr side always faces the punch (since the die initiates shearing), whereas for a punched opening, the burr side faces the die opening (as the punch initiates shearing). The characteristics of the burr side in both cases are inversely related.





















When measuring pierced or blanked piece parts, measurements are taken at the cut band.

Size of Die and Punch Profiles
In a blanking operation, the die opening's edge cuts the blank or slug, so the die opening size determines the size of the blank or slug. In a piercing operation, the punch performs the actual cutting of the opening in the stock material, so the punch size dictates the size of the punched opening.

For Blanking Operation,
✔ Blanking Die Size = Blank Size
✔ Blanking Punch Size = Blank Size - Total Clearance  
















For Piercing Operation,
✔ Piercing Punch Size = Pierced Hole Size  
✔ Piercing Die Size = Pierced Hole Size + Total Clearance
















Cutting Clearance (C) calculation 








Where,
c is a constant (0.005 for high-precision components and 0.01 for standard components.)
t is sheet thickness in mm,
τmax is shear strength of the steel material in N/mm2


Cutting clearance in percentage of sheet thickness


Examples

Q-1 Calculate cutting clearance for a mild steel sheet with the following specifications:
       a) Sheet Thickness (t): 2 mm
       b) Material: Mild Steel
       c) Shear Strength (τ): 400 N/mm²
      
Solution :
Cutting Clearance = 0.01 x 2 x (40)1/2 
                                    = 0.02 x 6.324
                              = 0.13 mm/side

Q-2 Calculate cutting clearance for a low carbon C.R. steel sheet with the following specifications:
       a) Sheet Thickness (t): 1.5 mm
       b) Material: low carbon C.R. steel
       c) Shear Strength (τ): 276 N/mm²
      
Solution :
Cutting Clearance = 0.01 x 1.5 x (27.6)1/2 
                                    = 0.015 x 5.25
                              = 0.078 mm/side

Q-3 Calculate cutting clearance for a mild steel sheet with the following specifications:
       a) Sheet Thickness (t): 2.0 mm
       b) Material: Mild Steel
       c) Cutting clearance in percentage of sheet thickness : 2.5%
      
Solution :
Cutting Clearance = Cutting clearance in % of sheet thickness x Sheet thickness
                              = 0.025 x 2
                                    = 0.05 mm/side

Q-4 Calculate size of die bore for a piercing a Ø 25 mm with the following specifications:
       a) Sheet Thickness (t): 2 mm
       b) Material: Mild Steel
       c) Cutting clearance in percentage of sheet thickness : 2.5%
      
Solution :
A. Cutting Clearance = 0.025 x 2
                                          = 0.05 mm/side        
B. Punch Diameter = Diameter of pierced hole = Ø 25 mm   
              
B.  Die Bore = Punch Diameter + Cutting Clearance
                     = Ø 25 + (2 x 0.05)
                     = Ø 25 + 0.1
                     = Ø 25.01 mm

Q-5 Calculate diameter of punch for a blank of  Ø 40 mm with the following specifications:
       a) Sheet Thickness (t): 2 mm
       b) Material: Mild Steel
       c) Cutting clearance in percentage of sheet thickness : 2.5%

Solution :
A. Cutting Clearance = 0.025 x 2
                                          = 0.05 mm/side        
B.Die Bore Diameter = Diameter of blank = Ø 40 mm   
              
B. Punch Diameter = Die bore Diameter - Cutting Clearance
                                = Ø 40 - (2 x 0.05)
                                = Ø 40 - 0.1
                                = Ø 39.90 mm

Q-6 Calculate size of die for a piercing operation, a component shown  in figure with the following specifications:
       a) Sheet Thickness (t): 1.5 mm
       b) Material: Aluminium
       c) Cutting clearance in percentage of sheet thickness : 1.5%







      








Solution :
A. Cutting Clearance = 0.015 x 1.5
                                          = 0.0225 mm/side        
B. Punch Size = Size of pierced profile



  
              







B.  Die Size = Punch Size + Cutting Clearance
                    












Q-7 Calculate size of punch for a blank operation, a component shown  in figure with the following specifications:
     a) Sheet Thickness (t): 2 mm
     b) Material: Mild Steel
     c) Cutting clearance in percentage of sheet thickness : 2.5%

      
Solution :
A. Cutting Clearance = 0.025 x 2
                                          = 0.05 mm/side         
B.Die Size = Size of blank profile   
             



















B. Punch Size = Die Size - Cutting Clearance