Short answer: punch-to-die clearance is the gap between the punch and die opening in a tā tool, typically 5-15% of material thickness per side. Correct clearance produces a clean shear zone with minimal burr. Too-tight clearance causes excessive tool wear and breakage. Too-loose clearance creates large burrs and a poor edge finish.
This guide is for tooling engineers, tā die designers, and quality engineers who need to specify or verify punch and die clearance for blanking, piercing, and notching operations. The right clearance affects edge quality, burr height, tool life, press tonnage, and part dimensional accuracy.
Tukuna your part material, thickness, and tolerance requirements through the RFQ form. For related edge quality standards, see tā konganuku tolerances guide and tā konganuku burr control guide.
Clearance by material type
The recommended clearance depends primarily on material type, thickness, and temper. Softer materials need less clearance; harder materials need more.
| Rawa | Clearance per side (% of thickness) | Typical burr height |
|---|---|---|
| Aluminum (soft) | 3-5% | 0.03-0.08 mm |
| Brass (half-hard) | 4-6% | 0.03-0.08 mm |
| Copper (soft) | 4-6% | 0.03-0.08 mm |
| Cold-rolled steel (low carbon) | 5-8% | 0.05-0.10 mm |
| Cold-rolled steel (high carbon) | 6-10% | 0.05-0.12 mm |
| Stainless steel (300 series) | 8-12% | 0.08-0.15 mm |
| Spring steel | 10-15% | 0.10-0.20 mm |
| Silicon electrical steel | 6-10% | 0.03-0.08 mm |
| Copper alloys (phosphor bronze) | 5-8% | 0.05-0.10 mm |
| Titanium (commercial pure) | 6-10% | 0.05-0.12 mm |
For very thin materials under 0.3 mm, clearance ratios may increase to 10-15% because the punch engagement depth is small relative to the clearance zone. For thick materials over 5 mm, clearance ratios may decrease slightly to limit rollover depth. For more on material-specific settings, see tā konganuku material selection guide.
Effect of clearance on edge quality
The stamped edge consists of four zones: rollover, shear, fracture, and burr. Clearance controls the proportion of each zone:
- Optimal clearance (5-8% for steel): shear zone is approximately 50-60% of material thickness. Fracture zone is clean and angled inward from the shear. Burr height is minimal and uniform. Rollover depth is 5-10% of thickness.
- Tight clearance (under 4% for steel): shear zone increases but secondary shear appears at the die exit. Tool wear accelerates due to metal-to-metal whakapā on punch and die sidewalls. Punch breakage risk increases, especially for small-diameter piercing punches.
- Loose clearance (over 12% for steel): shear zone decreases and fracture zone increases. Burr height grows significantly and is uneven along the cut edge. Rollover depth increases. For thick materials, the edge may show a visible secondary break. Wāhanga dimensional accuracy decreases because the fracture angle shifts outward.
For edge quality inspection standards, see tā konganuku first article inspection checklist.
Calculating clearance for specific punch sizes
Clearance should be calculated for each punch station individually, not as a single value for the entire die. Rules of thumb:
- For small piercing punches under 3 mm diameter, use clearance at the lower end of the range (5% for steel) to maintain punch strength and alignment.
- For large blanking punches over 50 mm, use clearance at the higher end of the range (8-10% for steel) to reduce press tonnage and die deflection.
- For obtuse or irregular contours, use the smallest radius section of the profile to determine clearance. Tight corners need tighter clearance to avoid cracking.
- For progressive dies, clearance should account for the strip lift during each station. Excessive clearance on pilot holes can cause strip positioning errors in downstream stations.
For die design guidelines, see mate ahu whakamua aratohu whakataurite.
Clearance and press tonnage
Clearance directly affects the cutting force required. Tighter clearance increases the peak cutting force because a larger proportion of the thickness is sheared rather than fractured. The relationship is approximately:
- 5% clearance: 100% of calculated cutting force
- 8% clearance: 85-90% of calculated cutting force
- 12% clearance: 75-80% of calculated cutting force
- 15% clearance: 65-70% of calculated cutting force
If press tonnage is limited, increasing clearance by 2-3 percentage points can reduce cutting force enough to fit the available press capacity. However, the edge quality tradeoff must be acceptable for the application. For press selection guidance, see tā konganuku production wā tuku guide.
Measuring and verifying clearance
Clearance should be verified during die tryout and periodically during production:
- Shim check: insert a feeler gauge or shim of known thickness between the punch and die. This is the simplest method but only works for accessible stations.
- Edge quality inspection: measure burr height and shear zone percentage on the first stamped part. If shear zone is below 40% or above 70% of thickness, clearance is likely outside the optimal range.
- Light band method: shine a light from behind the punch-die interface. The visible gap should correspond to the specified clearance when viewed from the correct angle.
- CMM measurement: for precision dies, measure the die opening and punch dimensions separately. Clearance = (die opening – punch diameter) / 2.
For quality system references, see tā konganuku kaiwhakarato quality audit checklist.
RFQ checklist for clearance specification
- Rawa type, grade, temper, and nominal thickness (with tolerance).
- Desired edge quality: maximum allowable burr height and shear zone percentage.
- Critical dimensions: which features have tight tolerances that clearance affects.
- Secondary operations: whether edge quality affects subsequent bending, welding, or plating.
- Press tonnage available if the tooling will run on an existing press.
- Expected die life between regrinds: tighter clearance reduces regrind interval.
- Wāhanga volume: if volume is very high, clearance may be set slightly tighter than optimal to improve edge finish, accepting more frequent sharpening.
Submit your drawing with material and tolerance information through the RFQ form. For tolerance specifications, see the tā konganuku tolerances guide.