This guide gives you the working numbers: the clearance formula, a thickness-by-thickness gap chart for mild steel, stainless steel, and aluminium, and a practical method for dialling it in on the machine. It also explains how to read the sheared edge so you can diagnose a clearance problem in seconds, without guessing.
What is shear blade clearance?
Shear blade clearance is the horizontal gap between the upper and lower blades of a guillotine or swing-beam shear as they pass each other during the cut. It is set as a percentage of the thickness of the material being cut.
That gap is what allows the metal to fracture cleanly rather than tear, fold, or crush. When clearance is correct, the cracks that start at the upper and lower cutting edges run toward each other and meet in the middle of the sheet. The plate then separates with a clean break, usually after the blade has penetrated roughly one-third of the way through the thickness. Set the gap wrong in either direction and those cracks no longer meet cleanly, which is where every common shearing defect comes from.
Quick answer: shear blade clearance by material
Clearance is always expressed as a percentage of the material thickness, and it changes with the metal you are cutting. Softer, more ductile metals need a tighter gap. Harder, higher-strength metals need a wider gap.
| Material | Recommended Clearance (Per Side) | Why |
|---|---|---|
| Aluminium (soft alloys) | 4% to 6% of thickness | Soft and ductile, smears and burrs if the gap is too wide. |
| Mild / Carbon Steel (A36, S235, IS 2062) | 5% to 10% of thickness | Cuts cleanly at moderate clearance, the baseline setting. |
| Stainless Steel (304, 316) | 8% to 12% of thickness | Higher tensile strength and work-hardening need more room. |
| High-Strength / Abrasion-Resistant Steel (AHSS, HSLA) | 10% to 16%+ of thickness | Fractures early, needs the widest gap for a clean break. |
A safe general rule when no manual or chart is available: set the clearance at about 7% of material thickness and fine-tune from a test cut.
The shear blade clearance formula
The calculation is simple:
Clearance (per side) = Material thickness × Clearance percentage
Worked examples:
- 6 mm mild steel at 8%: 6 × 0.08 = 0.48 mm clearance per side. A practical working window is 0.30 mm to 0.60 mm (5% to 10%).
- 3 mm stainless steel at 10%: 3 × 0.10 = 0.30 mm clearance per side. Working window 0.24 mm to 0.36 mm (8% to 12%).
- 2 mm aluminium at 5%: 2 × 0.05 = 0.10 mm clearance per side. Working window 0.08 mm to 0.12 mm (4% to 6%).
One point that trips people up: clearance is the gap on each side between the two blades. Some machine adjustment systems and charts state the value per side, others state a combined figure. Always confirm against your machine manual which convention the controller or adjustment screws use, and use the same scrap-test method below to verify the real-world result.
Shear blade clearance chart by thickness
This chart converts the percentages above into actual gap values you can set with a feeler gauge. Values are clearance per side, in millimetres, using practical midpoint percentages for each material.
| Thickness | Mild Steel (5%–10%) | Stainless Steel (8%–12%) | Aluminium (4%–6%) |
|---|---|---|---|
| 1 mm | 0.05 – 0.10 mm | 0.08 – 0.12 mm | 0.04 – 0.06 mm |
| 2 mm | 0.10 – 0.20 mm | 0.16 – 0.24 mm | 0.08 – 0.12 mm |
| 3 mm | 0.15 – 0.30 mm | 0.24 – 0.36 mm | 0.12 – 0.18 mm |
| 4 mm | 0.20 – 0.40 mm | 0.32 – 0.48 mm | 0.16 – 0.24 mm |
| 6 mm | 0.30 – 0.60 mm | 0.48 – 0.72 mm | 0.24 – 0.36 mm |
| 8 mm | 0.40 – 0.80 mm | 0.64 – 0.96 mm | 0.32 – 0.48 mm |
| 10 mm | 0.50 – 1.00 mm | 0.80 – 1.20 mm | 0.40 – 0.60 mm |
| 12 mm | 0.60 – 1.20 mm | 0.96 – 1.44 mm | 0.48 – 0.72 mm |
| 16 mm | 0.80 – 1.60 mm | 1.28 – 1.92 mm | 0.64 – 0.96 mm |
| 20 mm | 1.00 – 2.00 mm | 1.60 – 2.40 mm | 0.80 – 1.20 mm |
Treat these as a verified starting point, not a fixed law. The exact figure inside each range depends on the specific grade, its hardness and tensile strength, the rigidity of your shear, the rake angle, and the condition of the blade edge. Always confirm with a test cut.
Why each material needs a different clearance
The reason clearance changes by material comes down to how each metal fails under the blade.
Aluminium and soft metals are highly ductile. They flow before they fracture. If the gap is too wide, the blade pushes the metal sideways and folds it rather than shearing it, leaving a heavy burr and a rolled edge. A tight clearance forces a clean separation, so soft alloys get the smallest percentage.
Mild steel is the easy middle ground. It is ductile enough to shear cleanly and strong enough that it does not smear. This is why 5% to 10% is treated as the baseline, with most shops settling near 6% to 8% for general plate work.
Stainless steel is the harder case. It has a much higher tensile strength than carbon steel and it work-hardens during the cut, meaning the metal gets tougher exactly where the blade is trying to separate it. The higher force deflects the blades slightly and demands a wider gap so the fracture can complete cleanly without overloading and chipping the edge. This is also why a tougher blade grade matters for stainless, not just a wider gap.
High-strength and abrasion-resistant steels take this further still. They fracture early with very little plastic flow, so they need the widest clearance of all to give those early cracks room to meet. Run AHSS at a mild-steel gap and you will chip blades fast.
How to set shear blade clearance step by step
- Identify the material and thickness. Confirm grade and gauge before you touch the adjustment. A 6 mm mild steel setting is wrong for 6 mm stainless.
- Calculate the target gap. Use the formula and chart above to get your starting clearance per side.
- Set the gap with feeler gauges. Adjust the blade clearance using the machine’s screw or lever adjustment (or CNC input on modern shears), and check the gap with a feeler gauge along the full length of the blade, not just at one end. The gap must be even end to end.
- Test on scrap first. Always cut a scrap piece of the actual material before running production. Never adjust against finished stock.
- Adjust in small increments. Move the gap by no more than about 0.025 mm (0.001 inch) at a time. Small changes make a large difference at the cutting edge.
- Read the cut edge and refine. Inspect the sheared face, compare it to the edge-quality guide below, and nudge the clearance until the cut is clean and the burr is minimal.
How to read the sheared edge
Every sheared edge tells you whether your clearance is right. A correctly cut edge has four distinct zones, and their proportions are your diagnostic readout.
- Rollover: the slightly rounded, deformed lip at the top where the blade first pushed into the sheet. A small rollover is normal.
- Burnish: the smooth, shiny, vertical band where the blade slid against the metal before it fractured. On a good cut this is a clean, even strip.
- Fracture: the rougher, angled zone where the metal finally cracked and separated. It should be smooth and continuous, with no secondary tearing.
- Burr: the thin lip of metal pushed out on the bottom trailing edge. The goal is to keep this as small as possible.
On a correctly set shear, you see a defined burnish band, a clean fracture, a small rollover, and a minimal burr, with the break completing after roughly one-third penetration. A common practical target is a burr height below about 10% of the material thickness.
Troubleshooting: clearance problems and their symptoms
| Symptom on the Cut | Most Likely Cause | Fix |
|---|---|---|
| Large burr on the underside, rounded top edge | Clearance too wide | Reduce the gap in small steps |
| Edge folds or bends instead of cutting cleanly | Clearance far too wide (more bending than shearing) | Tighten clearance significantly, recheck full-length evenness |
| Blades chip or wear rapidly, blade sticks mid-cut | Clearance too tight, or wrong blade grade | Open the gap slightly, verify blade grade suits the material |
| Rough, torn fracture zone with secondary shear marks | Clearance too tight | Increase clearance, check blade sharpness |
| Burr growing over time at a previously good setting | Dull or worn blade edge | Sharpen or replace the blade, then reset clearance |
| Bow, camber, or twist in the cut strip | Rake angle too high for the job | Reduce rake angle, balance against available tonnage |
The role of rake angle
Clearance is not the only geometry that controls edge quality. The rake angle, the slight tilt of the upper blade so it cuts progressively across the width rather than all at once, controls the cutting force and the flatness of the cut.
A larger rake angle reduces the tonnage needed to cut thick or hard plate, but it can introduce bow, camber, or twist in the cut piece. A smaller rake angle gives a flatter, straighter cut but demands more force. There is a real interaction with burr too: increasing rake on thin stainless can noticeably reduce burr height because the angled edge pushes material ahead of the cut instead of folding it back. Set rake and clearance together, not in isolation.
Clearance is only half the job: blade material matters
You can set perfect clearance and still chip blades if the blade grade is wrong for the material. The two settings work together. Wider clearance helps stainless and high-strength steel fracture cleanly, but the blade itself also has to survive the higher impact and abrasion without chipping or wearing.
At Maxwell, shear blade grade is keyed to the material being cut and the duty of the application:
- D2 for general-purpose shearing of carbon and mild steel, with high wear resistance.
- H11 and H13 hot-work tool steels where toughness and shock resistance matter, including impact-heavy and elevated-temperature work.
- HSS M2 and HSS M35 for high wear resistance on demanding, abrasive cuts.
- PM ASP2030 powder metallurgy steel for the toughest, highest-wear applications such as stainless and high-strength grades, where edge retention and chip resistance are critical.
Matching the right grade to the cut, then setting clearance correctly, is what delivers a clean edge and long blade life together.
Frequently Asked Questions
For mild and carbon steel, set clearance between 5% and 10% of the material thickness, with most general work falling around 6% to 8%. For a 6 mm mild steel plate, that is roughly 0.30 mm to 0.60 mm per side.
Stainless steel needs more clearance than mild steel, typically 8% to 12% of thickness, because of its higher tensile strength and tendency to work-harden during the cut. For 3 mm stainless, that is about 0.24 mm to 0.36 mm per side.
Aluminium and other soft metals need a tighter gap, around 4% to 6% of thickness, to prevent the ductile metal from folding over and burring. For 2 mm aluminium, that is roughly 0.08 mm to 0.12 mm per side.
Too little clearance forces the blades together against full material resistance. It causes accelerated blade wear, chipped edges, secondary shear marks on the fracture zone, higher cutting force, and blades that can stick in the workpiece mid-cut.
Too much clearance lets the metal fold and bend instead of fracturing. You get a heavy burr on the underside, a rounded and rolled top edge, angular distortion, and in extreme cases the sheet bends rather than cuts.
Calculate the target from material thickness, set it with feeler gauges checked along the full blade length, test on scrap, and adjust in increments of about 0.025 mm until the cut is clean.
Clearance is conventionally the gap on each side between the upper and lower blade. Charts and machine controls vary in how they state it, so confirm the convention in your machine manual and verify with a test cut.
Correct clearance gives you a clean edge only when the blade behind it is built for the job. Maxwell has manufactured precision shear blades since 1976 and exports to more than 50 countries, producing blades up to 5000 mm in length with heat treatment matched to the material being cut. From D2 for general steel work to PM ASP2030 for stainless and high-strength applications, every blade is ground to tight accuracies for a sharp edge and superior surface finish.
To specify the right grade and geometry for your shear and your material, contact the Maxwell technical team for a recommendation tailored to your application.