Get horizontal clearance wrong, and the line tells you inside one coil: burr on the strip edge, camber that only shows up at the recoiler, edge wave, knife marks, and tooling that dulls in a shift instead of a fortnight. It is also the setting most often blamed on the steel.
Most clearance charts you will find online, including the shorthand numbers elsewhere on this site, are written by material name: carbon steel this percentage, stainless that percentage, aluminium something else. That shorthand works until it does not, and it stops working the day you run 301 full hard on a line that was set for annealed 304. Both are stainless. Their tensile strengths differ by a factor of two and a half, and they need completely different clearance.
What follows is organised by tensile strength and gauge, because that is what the metal responds to. You get the horizontal clearance bands, the vertical overlap bands, the arithmetic, a millimetre chart you can set straight from, and a method for converging in three test cuts instead of thirty.
The short answer
Horizontal clearance is set as a percentage of strip thickness, and that percentage rises with the tensile strength of the material. Vertical penetration before fracture moves the opposite way: soft metals stay ductile and have to be driven halfway through before they separate, while high strength material cracks early and needs very little.
Slitter knife clearance chart by tensile strength
| Tensile strength (UTS) | Typical materials | Horizontal clearance (% of thickness) | Penetration at fracture (% of thickness) |
|---|---|---|---|
| Up to about 150 MPa | Dead soft aluminium (1050, 3003-O), copper, brass, tin, lead-coated | 3% to 6% | 40% to 50% |
| About 150 to 350 MPa | CQ and DDQ cold rolled steel, galvanised, tinplate, hard aluminium alloys, copper alloys | 6% to 10% | 20% to 30% |
| About 350 to 620 MPa | Hot rolled commercial and structural steel, annealed 304 and 430 stainless, CRGO and CRNO silicon steel | 10% to 14% | 12% to 18% |
| About 620 to 1350 MPa | HSLA and advanced high strength steel, temper rolled and full hard stainless (301, cold worked 316), spring steel, high carbon 65Mn | 14% to 25% | 4% to 8% |
These are starting bands, not settings. The setting that wins is whichever one your own line produces the best edge at, with your knives, at your running speed. Start from the band, cut a sample, read the edge, adjust.
Penetration at fracture is a material property, not a dial. It tells you how deep the knives must bite before the crack runs across, which is why the vertical setting behaves the way it does.
Silicon steel is the exception that proves the rule. CRGO and CRNO sit at the low end of the 350 to 620 MPa band on clearance, but their real problem is abrasion and coating damage, so they demand maximum wear resistance in the knife rather than an unusual gap.
The slitter knife clearance formula
Horizontal clearance = strip thickness x clearance percentage
Clearance is the gap at each cut, measured between the side face of the upper knife and the side face of the lower knife. It is one gap per cut, not two.
- 1.0 mm CQ cold rolled steel at 8%: 1.0 x 0.08 = 0.08 mm
- 2.0 mm hot rolled structural steel at 12%: 2.0 x 0.12 = 0.24 mm
- 0.5 mm full hard 301 stainless at 18%: 0.5 x 0.18 = 0.09 mm
- 1.5 mm soft aluminium at 4.5%: 1.5 x 0.045 = 0.068 mm
Where this catches people out is the stack. Each slit strip is bounded by two cuts, so the arithmetic that builds your top and bottom spacer stacks carries the clearance twice, once at each edge of the strip. Get one clearance wrong at one cut and you do not just spoil that edge, you shift the width of the two strips on either side of it. The stack arithmetic is covered in our guide to building a knife and spacer stack.
Why tensile strength is the right input, not material name
The cut is a controlled fracture. The knives compress the strip until the stress at the two cutting edges exceeds the material’s ultimate tensile strength, cracks initiate at both edges, and those cracks run toward each other and meet. Clearance is the setting that decides whether they meet.
Clearance correct. The two cracks are aligned on the same fracture plane and join in a straight line. You get a burnish band of consistent depth, a fracture face that is continuous rather than ragged, light rollover, and a burr you have to look for.
Clearance too tight. The cracks are offset. They do not meet on the first attempt, the material shears a second time, and you get a secondary shear band, a heavier burr, and knives that wear fast because they are doing work they should not be doing.
Clearance too wide. The material is bent and drawn out before it fractures rather than sheared. You get heavy rollover, a rough torn fracture face and a tall drawn burr on the underside.
Material name fails as an input because crack initiation stress depends on tensile strength, and tensile strength inside one material name varies enormously:
- Annealed 304 stainless runs roughly 515 to 620 MPa. Full hard 301 starts at about 1275 MPa. Same word on the coil tag, more than double the strength.
- Cold rolled CQ steel sits near 270 to 380 MPa. A DP780 dual phase from the same steelmaker is over 780 MPa.
- 1050 aluminium in O temper is under 100 MPa. 5052-H38 is close to 290 MPa.
This is also why published clearance charts appear to contradict each other. Some widely circulated charts quote 7% to 11% for stainless, because they are describing SUS 304 in the annealed condition on thin gauge. Slitting houses working temper rolled and high strength stainless quote 20% or more. Neither is wrong. They are describing different ends of the same curve.
Read the mill certificate, take the actual UTS, and pick the band. If you do not have a certificate, use temper and hardness as a proxy and start at the top of the band rather than the bottom.
Horizontal clearance chart in millimetres
This converts the percentages into gap values you can set. Values use the practical midpoint of each band, in millimetres, at the cut.
TSlitter knife horizontal clearance in mm, by strip thickness and material group
1. Overlap (also called penetration or depth)
Overlap is how far the male knife extends past the cutting plane of the female knife, measured vertically.
Typical starting ranges:
| Strip thickness | Soft non-ferrous (4.5%) | CQ steel, hard aluminium (8%) | HR steel, annealed stainless (12%) | High strength, full hard (18%) |
|---|---|---|---|---|
| 0.30 mm | 0.014 | 0.024 | 0.036 | 0.054 |
| 0.50 mm | 0.023 | 0.040 | 0.060 | 0.090 |
| 0.80 mm | 0.036 | 0.064 | 0.096 | 0.144 |
| 1.00 mm | 0.045 | 0.080 | 0.120 | 0.180 |
| 1.50 mm | 0.068 | 0.120 | 0.180 | 0.270 |
| 2.00 mm | 0.090 | 0.160 | 0.240 | 0.360 |
| 2.50 mm | 0.113 | 0.200 | 0.300 | 0.450 |
| 3.00 mm | 0.135 | 0.240 | 0.360 | 0.540 |
| 4.00 mm | 0.180 | 0.320 | 0.480 | 0.720 |
| 6.00 mm | 0.270 | 0.480 | 0.720 | 1.080 |
Look at the top rows. At 0.30 mm the entire usable clearance range across all four material groups spans 0.014 mm to 0.054 mm. That is 40 microns of total working range. Your knife thickness tolerance, your spacer tolerance and your arbor deflection all have to be an order of magnitude tighter than that spread, or the setting on the dial is fiction. This is the real reason thin gauge slitting is hard, and it is where cheap tooling shows up first.
Vertical overlap: the setting people over-apply
Vertical overlap is how far the cutting edge of the upper knife passes below the cutting edge of the lower knife. It is not the same thing as penetration at fracture, and confusing the two is what leads operators to run far more overlap than the job needs.
The relationship is inverse to clearance. Soft, ductile metal has to be driven a long way in before it will separate, so it needs positive overlap. High strength material cracks early, so it needs very little, and on heavy gauge high strength steel the correct setting can be zero overlap or even a small vertical gap, because the strip fractures before the two edges ever reach the same plane.
Vertical overlap starting points for metal coil, subject to a test cut
| Strip thickness | Typical vertical overlap |
|---|---|
| Under 0.50 mm | 0.05 to 0.15 mm |
| 0.50 to 1.50 mm | 0.03 to 0.10 mm |
| 1.50 to 3.00 mm | 0 to 0.05 mm |
| Above 3.00 mm | Zero, or a small vertical gap on high strength grades |
The governing principle is simple: run the shallowest overlap that still separates the strip every time. Excess overlap does not improve the cut. It multiplies the separating force on the knives, which deflects them sideways, which changes your horizontal clearance at exactly the moment you least want it to change. Excess vertical overlap is the usual cause of edge wave, crossbow, deep knife marks on the strip surface, and premature knife wear that gets misdiagnosed as a steel grade problem.
There is one more trap. Horizontal and vertical settings are not independent. If horizontal clearance is wrong in either direction, the strip will resist separating, and the operator’s instinct is to compensate by adding overlap. That works, briefly, and then you are running heavy overlap on a wrong clearance and both settings are now wrong. Fix the horizontal setting first, always, then set the minimum overlap that finishes the cut.
How to converge in three test cuts
Cut one. Set clearance from the chart at the band midpoint for your material. Set minimum overlap. Run a short sample and cut a cross section, or inspect the edge under 10x magnification.
Read the edge. You are looking for four features from top to bottom: rollover at the entry side, a burnish band where the knife slid against the metal, a fracture face where the metal broke, and a burr at the exit side. On a correct setup the burnish band is present but does not dominate, the fracture face is continuous, the line between the two is straight rather than wavy, and burr height is under 10% of material thickness.
Cut two. Adjust based on what you saw.
Slit edge diagnosis and corrective action
| What the edge shows | Diagnosis | Action |
|---|---|---|
| Burnish band occupies most of the edge, secondary shear step visible, burr present | Clearance too tight | Open clearance by 2 to 3 percentage points |
| Thin burnish, rough torn fracture, heavy rollover, tall soft burr | Clearance too wide | Close clearance by 2 to 3 percentage points |
| Burr on one edge of the strip only | Unequal clearance across the knife set | Check spacer stack and knife thickness at that station |
| Edge wave, crossbow, deep surface marks | Excess vertical overlap | Reduce overlap, do not touch horizontal |
| Clean edge on cut one, burr returning after 30 minutes | Edge wear, not setup | Wrong grade or wrong hardness for the material |
Cut three. Confirm and record. Write the clearance, overlap, gauge, grade and UTS into a setup sheet for that job. The second time you run that coil you should be setting from your own record, not from a chart.
Burr troubleshooting beyond clearance, including camber, crossbow and slit width drift, is covered in depth in our guide to fixing burr and edge defects in coil slitting.
What destroys clearance accuracy even when the setting is right
You can dial a perfect number and still get a bad edge, because clearance at the cut is the sum of everything in the stack, not the number you intended.
Knife thickness tolerance
A knife held to plus or minus 0.01 mm is fine on 3 mm plate and useless on 0.3 mm foil, where the whole clearance range is 40 microns. Precision slitter knives should be held to within a few microns on thickness and parallelism, and that tolerance should be stated on the certificate, not implied.
Face flatness and parallelism
A knife that is not flat does not sit square against the spacer. It cocks, and it introduces runout that varies the clearance once per revolution. This is why the faces are lapped rather than only ground. The difference between the two processes, and the surface finish figures that matter, are set out in grinding versus lapping slitter knives.
Spacer stack-up
Every spacer carries its own tolerance. Twenty spacers each 5 microns out in the same direction is 100 microns of accumulated error, which is more than the entire clearance budget on thin gauge. This is the whole argument for shimless, metal-to-metal tooling, and for spacers held to the same tolerance class as the knives.
Arbor deflection
Separating force runs to thousands of pounds at every cut, and a full gang multiplies it. That force deflects the arbor and the knives, and the deflection opens clearance under load, exactly where you cannot measure it. Deflection rises with excessive vertical overlap and with knives that are too thin for the gauge. A common rule of thumb is a knife thickness of three to five times the heaviest gauge you intend to run.
Bearing play and cleanliness
Worn arbor bearings put a variable component into a setting you believe is fixed. A single chip trapped between a spacer and a knife face changes the axial position of everything outboard of it. Clean, dry, undamaged faces are not housekeeping, they are part of the tolerance chain.
The setting sheet
- Read the mill certificate. Record actual UTS, not the material name.
- Pick the clearance band from the tensile chart. Take the midpoint.
- Multiply by strip thickness. That is your horizontal clearance at each cut.
- Build the stack with clean, undamaged knives and spacers, dry, no shims.
- Set the minimum vertical overlap for the gauge.
- Run a sample. Inspect the edge. Measure burr height against 10% of thickness.
- Adjust horizontal clearance first, in steps of 2 to 3 percentage points. Only touch overlap once horizontal is right.
- Record the final numbers against the job. Reuse them next time.
Choosing Male Knife Material
Every number on this page assumes the knife is the thickness it says it is, the faces are flat and parallel, hardness is uniform through the section, and the edge holds its geometry through the run. When any of those fail, the setting on the machine stops describing what is happening at the cut, and no amount of adjustment will recover it.
Maxwell Slitter Industries has manufactured slitting line tooling since 1976. Our rotary slitter blades and slitter knives are vacuum hardened for uniform through-hardness, ground and lapped for flatness and parallelism, and inspected on a CMM with results documented on the certificate that ships with the order. Send us your gauge range, grades and tensile figures and we will quote the knife, spacer and stripper ring package to hold clearance across your full job list.
Frequently Asked Questions
For cold rolled commercial quality steel, start at 6% to 10% of strip thickness at each cut. For hot rolled and structural grades, start at 10% to 14%. For advanced high strength and high carbon grades, start at 14% to 25%. Take the actual tensile strength from the mill certificate rather than working from the material name, then confirm with a test cut.
It depends entirely on temper. Annealed 304 and 430 at roughly 450 to 620 MPa sit around 10% to 14% of thickness. Temper rolled and full hard grades such as 301 at 1000 MPa and above need 18% to 25% or more. Setting a full hard stainless at annealed clearance is one of the most common causes of heavy burr and rapid knife wear.
Dead soft aluminium needs the tightest gap of any coil material, around 3% to 6% of thickness, because a ductile metal at wide clearance folds and smears instead of fracturing. Harder aluminium alloys move up into the 6% to 10% band. Aluminium also needs the deepest penetration before it separates, so it tolerates more vertical overlap than steel.
In coil slitting there is one clearance gap at each cut, measured between the side face of the upper knife and the side face of the lower knife. A strip is bounded by two cuts, so the spacer stack arithmetic accounts for the clearance twice, once at each edge of the strip.
The cracks starting at the upper and lower knife edges are offset and do not meet cleanly. The material shears a second time, producing a secondary shear band and a burr, while the knives absorb work they were not meant to and dull noticeably faster. Very tight clearance on hard material also risks edge chipping.
The strip is bent and drawn before it fractures rather than sheared. The result is heavy rollover on the entry side, a rough torn fracture face, and a tall drawn burr on the exit side. Wide clearance is the most common single cause of burr in coil slitting.
It was the industry default for decades and it is now a poor default. On thin gauge it hands you a gap several times wider than the metal wants, and on high strength material it falls short of the 15% to 25% those grades need. Use it only as a rough sanity check on mid-range carbon steel, and set from the tensile band instead.
Run the shallowest overlap that still separates the strip every time. Typical starting values are 0.05 to 0.15 mm under 0.5 mm gauge, falling toward zero as thickness increases, with zero overlap or a small vertical gap on heavy high strength material. Excess overlap causes edge wave, crossbow and knife marks, and it increases knife deflection which then corrupts your horizontal clearance.