Ask two experienced toolmakers what clearance to run on 2 mm mild steel and you may get "10%" from one and "20%" from the other. Both are correct. They are quoting the same gap in two different conventions, and the moment someone applies one convention's number with the other's arithmetic, the tool ends up with double the clearance it should have - rolled edges, heavy burr, and a die that never produces a clean part.
What Clearance Actually Means
Clearance is the gap between the punch and the die opening. For a round punch:
- Total clearance = die opening diameter − punch diameter. It is the gap counted across the whole diameter, both sides at once.
- Clearance per side = half of that. It is the gap you would measure at any one point around the punch.
So per side = total ÷ 2, always. They are not competing recommendations; they are the same number stated two ways.
The trap: turret-punching tooling charts almost always quote total clearance, while tool-and-die practice and die-component catalogues usually quote per side. A chart saying "20% for mild steel" and a toolmaker saying "10% for mild steel" describe an identical gap. Take the chart figure as per-side and you will make the die opening twice as big as it should be.
Per Side vs Total
Before using any clearance figure, establish which convention it is in. Two quick tests:
- Is the number for mild steel above about 12%? It is almost certainly total clearance.
- Does the source talk about "die clearance" for a turret or CNC punch press? Total. Does it talk about "clearance per side" for a compound or progressive die? Per side.
If a figure is genuinely ambiguous, work it through: on 1 mm mild steel, a sensible die opening for a Ø10 mm punch is around Ø10.15 mm. That is 0.15 mm total, 0.075 mm per side - 15% total, 7.5% per side. Any interpretation that gives you Ø10.3 mm is wrong.
Clearance by Material
Stated both ways, so there is nothing to convert:
| Material | Total clearance | Per side | Typical starting point |
|---|---|---|---|
| Aluminium (soft) | 10 – 15% | 5 – 7.5% | 10% total (5% per side) |
| Brass / copper (½ hard) | 8 – 16% | 4 – 8% | 12% total (6% per side) |
| Mild steel (CR / HR) | 15 – 20% | 7.5 – 10% | 15% total (7.5% per side) |
| Stainless steel | 20 – 25% | 10 – 12.5% | 20% total (10% per side) |
Percentages are of material thickness. Thin stock sits at the low end of the range, thick or work-hardening stock at the high end. Per-side figures are simply half the total - they are the same recommendation stated two ways.
Two adjustments on top of the table:
- Thickness: thin stock sits at the low end of the range, thick stock at the high end.
- Hardness: harder and more work-hardening material wants more clearance. Spring steel and hardened grades run above the stainless figures.
There is also a modern school of thought that deliberately runs more clearance than the traditional figures - up to around 20% per side - accepting a rougher edge in exchange for markedly lower stress on the punch and longer tool life. If tool life is your problem rather than edge quality, it is worth testing.
Which Member Is Cut to Size
This is the part that gets reversed, and it is not a detail - it decides whether your part comes out on size.
The cut feature always takes the size of the member that forms it:
- Blanking - you want the slug, and the blank takes the size of the die opening. So make the die to size and take the clearance off the punch.
- Piercing - you want the hole, and the hole takes the size of the punch. So make the punch to size and add the clearance to the die.
| Operation | You want | Made to size | Clearance applied to |
|---|---|---|---|
| Blanking | The slug (the blank) | Die opening | Punch (made smaller) |
| Piercing / perforating | The hole | Punch | Die (made larger) |
Worked Example
Piercing a Ø10 mm hole in 2 mm mild steel:
- Mild steel, 2 mm: take 15% total clearance from the table - mid-range for a moderate thickness.
- Total clearance = 15% × 2 mm = 0.30 mm. Per side = 0.15 mm.
- This is piercing, so the punch is made to size: Ø10.00 mm.
- The die opening gets the clearance: 10.00 + 0.30 = Ø10.30 mm.
Blanking a Ø10 mm disc from the same material reverses it: die opening Ø10.00 mm, punch Ø9.70 mm.
Reading the Slug and the Edge
The slug tells you what the clearance is doing better than any measurement. A cut edge has four zones - rollover, burnish (the shiny band), fracture, and burr - and their proportions are a direct readout:
- Correct clearance: the burnish band is roughly a third of the thickness, the fracture zone is clean and at a consistent angle, and the burr is minimal. The fracture lines from punch and die meet cleanly.
- Too little clearance: a wide burnish band, sometimes with a second band - the fracture from the punch side misses the fracture from the die side and the material shears twice. This is secondary shear, and it drives cutting force, heat and punch wear up sharply.
- Too much clearance: heavy rollover at the top, a short burnish band, a rough, tapered fracture and a large, ragged burr.
A burr that grows steadily over a production run is a wear signal, not a clearance signal - the punch edge has gone dull and is tearing rather than cutting. Time to regrind.
When Clearance Is Wrong
| Symptom | Likely cause |
|---|---|
| High cutting force, punch running hot | Clearance too small |
| Double burnish band on the slug edge | Secondary shear - clearance too small |
| Chipped or galled punch point | Clearance too small, or misalignment |
| Large burr with heavy rollover | Clearance too large |
| Tapered, rough fracture zone | Clearance too large |
| Slug pulling back up with the punch | Clearance too large, or no slug retention - consider a jector pin or vented punch |
| Burr growing over a production run | Punch edge wear, not clearance - regrind |
| Clearance uneven around the punch | Punch and die not concentric - check alignment and guidance |
Slug pulling deserves a mention of its own: a slug that rides back up the die and onto the strip will be punched into the next part and can wreck the tool. Generous clearance makes it more likely, which is one of the practical limits on simply opening the clearance up to save punches.
If you are unsure which convention a figure came from, or you want a punch ground to a specific point geometry, send us the material, thickness and hole size and we will work the clearance with you.
Need HSS Punches?
Ground HSS punches from Ø0.6 mm upward in 50, 70, 80 and 100 mm lengths - ex-stock Mumbai.
View Punch RangeFrequently Asked Questions
Is punch and die clearance per side or total?
Both conventions are in use, which is why sources appear to disagree. Total clearance is the die opening minus the punch diameter; per side is half of that. Turret-punching charts usually quote total, while tool-and-die practice usually quotes per side - so "20% total" and "10% per side" describe exactly the same gap.
What clearance should I use for mild steel?
Around 15 to 20% of material thickness as a total figure, which is 7.5 to 10% per side. Thin stock sits at the low end, thicker stock at the high end. On 2 mm mild steel, 15% total gives 0.30 mm total clearance, or 0.15 mm per side.
Do I make the punch or the die to size?
It depends on which piece you want. When blanking, the blank takes the size of the die opening, so the die is made to size and clearance comes off the punch. When piercing, the hole takes the size of the punch, so the punch is made to size and clearance is added to the die.
What happens if the clearance is too small?
Cutting force rises sharply, the punch runs hot and wears fast, and the slug shows a wide or doubled burnish band - secondary shear, where the fracture from the punch side fails to meet the fracture from the die side and the material shears twice. Chipped and galled punch points are the usual end result.
What causes slug pulling?
Excessive clearance is the most common cause, because the slug is not gripped in the die opening as the punch retracts. Vacuum between punch face and slug, a worn punch point and a die land that is too short all contribute. Remedies include reducing clearance, a jector pin in the punch, a vented or bevelled punch face, and keeping the die land correctly sized.
How do I read the cut edge to check clearance?
Look at the proportions of the four zones - rollover, burnish, fracture and burr. With correct clearance the burnish band is roughly a third of the material thickness, the fracture is clean and consistent, and the burr is small. A wide or doubled burnish band means too little clearance; heavy rollover with a large ragged burr means too much.