From the entry face downward they are rollover, burnish, fracture and burr, and they form in that order as the knives rotate through the strip. Rollover is plastic flow under the compressive load as the knife first touches the metal. Burnish is the polished band where the knife flank slid against it. Fracture is where the metal finally tore. Burr is the tensile tail left at the exit. How to inspect slit edge quality under magnification is in practice just the discipline of telling the four apart.
| Band | Formed by | What it reports | Responds to |
| Rollover | Plastic flow under the entry compressive load | Clearance magnitude | Gap size and strip support |
| Burnish | Knife flank sliding on the metal | Whether clearance suits the grade | Grade, hardness, gauge |
| Fracture | Tensile separation | Ductility and clearance direction | Clearance, material temper |
| Burr | Tensile tail at the exit side | Knife sharpness | Regrind state, overlap depth |
The burnish-to-fracture split varies by material family, and the trend is not noise:
Stronger material separates earlier in the cut, so the tear starts higher up the edge and the polished band gets shorter. AHSS grades push this further and fracture with very little rollover or burr at all, which is exactly why the AHSS community stopped judging edge quality by burr height alone and moved to examining the transition between the burnish and fracture zones.
Every Ruilong line is signed off on the four-band reading rather than on a burr-height number alone.
✅ Bottom line: treat the four bands as four separate measurements, because the ratio between them is the signal and any single band read alone can point two ways.
Rollover tracks how much clearance you have; secondary shear tracks whether it is too tight; they are different features, and only one of them is what the standards call rollover. This is where published diagnostic tables genuinely disagree. One widely circulated slitting guide lists "large rollover, crushed edge" as evidence that clearance is too small. Shearing data published by the Australian Steel Institute says the opposite, and says it with numbers.
| Total clearance | Rollover | Burnish | Fracture | Fracture angle | Burr |
| 2–5% | 2–5% | 50–70%, in two bands | 25–45%, in two bands | — | Large |
| 10–15% | 4–7% | ~50% | 35–50% | 6–11° | Medium |
| 15–20% | 6–8% | 25–40% | ~50% | 7–11° | Small |
| 20–25% | 8–10% | 15–25% | 60–75% | 8–11° | Small |
| 35–40% | 10–20% | 10–20% | ~75% | 14–16° | Large |
Rollover rises monotonically with clearance, from 2–5% of thickness at the tightest setting to 10–20% at the loosest. What the "too tight" row is really describing sits at the top of that table: burnish appearing in two separate bands with fracture between them. That is secondary shear. It occurs at minimum rollover, and it is the one appearance that overrides everything else — if you see two burnish bands, open the clearance by 2–3 points regardless of what the rollover looks like.
The sequence Ruilong uses at acceptance runs in a fixed order, because reading the bands out of order is what produces the wrong correction:
On magnification, two mistakes are common. A 2–3× hand lens is not enough: a peer-reviewed cut-edge study puts it bluntly, calling naked-eye evaluation with a 2–3× magnifying glass "largely insufficient" and adequate at best for a quick burnish-to-fracture estimate. At the other extreme, a metallographic cross-section is the only way to measure local clearance and fracture angle, but it is destructive — take it at set-up, not on production samples. A 50× USB digital microscope sits in between and covers daily shop-floor work without sectioning, and Ruilong ships one with every line along with the sectioning kit for set-up checks.
✅ Bottom line: hunt the second burnish band before you judge rollover, because secondary shear is the only appearance in the cross-section that names its own cause.
Q: What should the burnish to fracture ratio on slit steel edge be for mild steel? A: Around 40–60% burnish and 40–60% fracture on annealed low-carbon coil. A burnish band running past roughly 70% usually means clearance is tight for that gauge, and a rough torn fracture with burnish under 25% usually means it is loose.
Q: How to inspect slit edge quality under magnification without building a metrology lab? A: A 50× USB digital microscope with a calibrated reticle covers daily work. Keep a sectioning kit for set-up verification only, since a cross-section is the only way to confirm fracture angle and it destroys the sample.
Q: What is the rollover depth on slit edge acceptable limit for general work? A: Below about 15% of material thickness for general slit coil, and below 10% where the edge is a functional surface. Rollover grows with clearance, so a rising rollover reading on a stable job usually means the gap has opened, not that the knives have dulled.
Send Ruilong your grades, gauge range and the downstream operation each strip feeds. As a Slitting Line manufacturer, Tai'an Ruilong Machinery Co., Ltd. returns a custom acceptance sheet with the four-band limits for every job you run, ships the matched OEM/ODM knife and spacer set as one package, and quotes factory-direct — one tooling supplier instead of three.
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