What is punch and die clearance？
The punching and die clearance of the CNC punching machine and the CNC turret punching machine refers to the gap between the punch and the lower die when the punch is inserted into the lower die, which is usually referred to the total clearance.
Why use the correct die clearance?
◆ Extend the service life of the mold
◆ Good material returning effect
◆ The generated burr is small
◆ Get a cleaner and tidy hole
◆ Reduce the possibility of sticking
◆ Workpiece leveling
◆ The position of the hole is more accurate
◆ The minimum punching force is required for blanking
The optimum clearance between the punch and die
Can make the shear crack joint, balanced blanking force, good punching quality, long service life of the mold.
Recommended punch and die clearance table (see Table 1 below)
The harm of too small clearance
- Secondary shearing occurs
- The punching force increases
- The life of the mold is shortened
Determination of the punch and die clearance value
For the determination of the die clearance value, empirical formulas and charts are generally used.
Because of the experience, naturally, a person who has worked as a stamping die fitter and a stamping die design for a long time will understand and know a variety of different products, including its material, size and appearance accuracy requirements, as well as how to design the mold to successfully produce qualified products, how to reduce mold repair, repair times, etc.
When they get the product drawings, they naturally think of the structure of the mold and the various values.
The gap between the punch and die has a great influence on the quality of the stamping part and the life of the stamping die.
Therefore, when designing the stamping die, we must choose a reasonable gap to ensure the cross-section quality and dimensional accuracy of the stamping parts in order to meet the requirements of the product.
In addition, the required punching force is small, thereby increasing the life of the stamping die.
However, the reasonable clearance determined from the requirements of quality, punching force, stamping die life, etc. are not in the same value, but close to each other.
In view of the deviation in the manufacture of stamping dies and the wear in use, it is usually to select an appropriate range as a reasonable clearance in actual production.
As long as the clearance is within this range, a good part can be punched out.
In the production process, the stamping die will wear continuously to increase the gap.
Therefore, the minimum reasonable clearance value should be adopted when designing and manufacturing the new stamping die.
According to the experience of the factory masters in the research and improvement of molds for many years, the parts with high dimensional accuracy and high cross-section perpendicularity requirements should use smaller clearance values.
However, for those stamping parts with low cross-section perpendicularity and dimensional accuracy requirements, large gap values can be used to reduce the punching force and improve the service life of the stamping die.
So how to determine the punch and die clearance?
Generally, there have two methods – by experience and by formula.
Determine the punch and die clearance by experience
The percentage of the die clearance to the material thickness:
◆ Min service life of dies: 15%
◆ Optimal clearance: 20~25%
◆ Long service life of dies: 30%
◆ Heavy duty die clearance: 30%
(Table 1) Recommended die clearance table
|Thickness (mm)||Aluminum (mm)||Mild Steel (mm)||Stainless Steel (mm)|
Determine the punch and die clearance by formula
Its value can be calculated according to the following punch and die clearance formula:
- Material thickness t < 1 mm, blanking clearance c = (3% ~ 4%)t
- t = 1 ~ 3mm, c = (5% ~8%)t
- t = 3 ~ 5mm ,c = (8% ~10%)t
- t < 1mm,c = ( 4% ~5% )t
- t = 1 ~ 3mm, c = ( 6% ~8% )t
- t = 3 ~ 8mm, c = ( 8% ~ 13%)t
Theoretical calculation of punch and die clearance
The following is the theoretical knowledge in the book, the main basis is to ensure that the upper and lower cracks meet, in order to obtain a good section.
According to the relationship of the triangle ABC, the clearance value c can be obtained as:
c =( t – h0 ) tanβ = t (1-h0/t) tanβ
In the formula,
- h0 – Punch penetration depth;
- Β – The angle between the maximum shear stress direction and the vertical direction
From the above equation, the gap c is related to the material thickness t, the relative penetration depth h0/t and the crack direction β. While h0 and β are related to the nature of the material, the harder the material, the smaller the h0/t.
Therefore, the main factors affecting the clearance value are material properties and material thickness. The harder or thicker the material, the larger the die clearance value.