Master craftsmen in sheet metal factories can usually calculate this, and it is also covered in manufacturer training. Generally, the die clearance is selected based on the material.
The selection of die clearance (total clearance) is listed as follows:
Master craftsmen in sheet metal factories can usually calculate this, and it is also covered in manufacturer training. Generally, the die clearance is selected based on the material.
The formula is: Percentage × Material Thickness = Die Clearance
In my work, I always use the optimal clearance, which is excellent for die maintenance.
- Extends die service life
- Improves material stripping effect
- Minimizes burr formation
- Produces cleaner and neater holes
- Reduces the possibility of material sticking
- Ensures flat workpieces
- Enhances hole position accuracy
- Minimizes the pressure required for punching
The above content introduces the selection of double-sided die clearance. Below is an explanation of how to select single-sided die clearance.
The selection of CNC punching die clearance depends on factors such as the thickness of the sheet metal and the material grade.
For most iron plate materials with a thickness (T) of 0.5mm to 3mm, a universal calculation method is used—meaning the clearance is determined by multiplying the sheet thickness by a specific percentage.
-
Soft materials (copper, iron, aluminum):
- For thickness ≤ 1.0mm: Single-sided clearance = +4% of T
- For thickness 1.0–3.0mm: Single-sided clearance = +5% of T
- For thickness > 3.0mm: Single-sided clearance = +7% of T
-
Hard materials (stainless steel, high-carbon steel):
- For thickness ≤ 1.0mm: Single-sided clearance = +5% of T
- For thickness 1.0–3.0mm: Single-sided clearance = +6% of TNote: Remember to multiply the percentage by the material thickness.
Punching clearance is generally verified using two methods:
- Test punching with paper (to check for uniform force distribution)
- Measurement with a feeler gauge (to confirm actual clearance size)
Alternatively, the clearance can be evaluated by observing the bright band (smooth area) and burrs on the product. For bending operations, gaskets or feeler gauges are usually used to adjust and verify gaps.
Die clearance has a critical impact on the cross-sectional quality of stamped parts. Additionally, it affects:
- Die service life
- Stripping force and ejector force
- Punching force
- Dimensional accuracy of stamped parts
Thus, die clearance is an extremely important process parameter in both punching technology and die design.
The dimensional accuracy of a stamped part refers to the difference between its actual size and the basic design size—the smaller the difference, the higher the accuracy. This difference includes two types of deviations:
- Deviation of the stamped part from the punch or die size
- Manufacturing deviation of the punching die itself
The deviation of the stamped part from the punch/die size is mainly caused by elastic recovery of the material. When the part is pushed out of the die (for blanking) or removed from the punch (for punching), the material undergoes elastic recovery due to 挤压变形 (compressive deformation), 纤维伸长 (fiber elongation), and 穹弯 (doming). This recovery can result in either positive or negative deviations.
Key factors affecting this deviation include:
-
Punch-die clearance (the most significant factor)
-
Material properties
-
Workpiece shape and size
-
When clearance is large: The material is subjected to greater tensile forces during punching. After punching, elastic recovery causes the stamped part to shrink toward its solid core—resulting in blanked parts smaller than the die size and punched holes larger than the punch diameter.
-
When clearance is small: The material is subjected to greater compressive forces from the punch and die. After punching, elastic recovery causes blanked parts to expand (larger than the die size) and punched holes to shrink (smaller than the punch diameter).
The magnitude of dimensional change is related to material properties, thickness, and rolling direction:
- Soft steel has small elastic deformation, so elastic recovery after punching is minimal.
- Hard steel has large elastic recovery.
These effects are discussed under the premise of a certain die manufacturing accuracy. If the die cutting edge has low manufacturing accuracy, the accuracy of the stamped part cannot be guaranteed. Therefore, the manufacturing tolerance of the punch and die cutting edges must be determined based on the dimensional requirements of the workpiece.
In addition, the die structure (e.g., single-station vs. progressive die) and positioning method (e.g., pin positioning vs. guide rail positioning) also have a significant impact on the positional accuracy of holes.
Die service life is affected by a combination of factors, and clearance is one of the most critical. During punching:
- Friction occurs between the punch and the punched hole, and between the die and the blanked part.
- The smaller the clearance, the greater the compressive stress acting on the die, and the more severe the friction—this is extremely detrimental to die life.
Larger clearances, by contrast:
- Reduce friction between the punch side and the material
- Mitigate the adverse effects of uneven clearance (caused by manufacturing and assembly inaccuracies)
Thus, an appropriate larger clearance helps extend die service life.
