Slitter Arbor Runout and Spacer Tolerance: The Accuracy Nobody Specifies

Short answer: runout is not the same kind of number as a tolerance, because it does not move the cut once — it oscillates it once per revolution. A 0.01 mm TIR figure on a 1 mm job with 0.05 mm clearance swings the effective gap between 0.04 and 0.06 mm every single turn, so the knives alternate between too tight and too loose within one revolution. That is why intermittent burr is a runout signature while uniform burr is a clearance or wear signature. The slitter arbor runout tolerance for precision slitting in a purchase spec is almost always quoted for the bare shaft on a bench, which is not the number that reaches the strip. Ruilong specifies the assembled figure under torque instead, and section 2 gives the three grades.

1. Why does runout behave differently from an ordinary tolerance?

An ordinary tolerance shifts every cut by the same amount; runout shifts each cut by a different amount, cycling once per revolution. The consequence is a signature you can read without instruments. Uniform burr down the whole coil points at clearance or at knife wear. Burr that comes and goes along the length, or differs between strips cut by neighbouring knives in the same gang, points at runout.

Do the arithmetic on a common job. Annealed 1 mm cold-rolled runs roughly 5% clearance, so 0.05 mm total gap. Introduce 0.01 mm of total indicated runout at the knife and the gap cycles between 0.04 and 0.06 mm — a 20% swing each way, applied at rotational frequency. At 120 m/min with a Ø250 mm knife that is roughly 150 times a second. No chart setting survives that, and no amount of re-setting the gap will fix it, because the gap is correct on average and wrong at every instant.

Three separate sources feed the figure, and they are usually conflated:

Two of those three appear in most vendor literature. The third — the one that actually reaches the strip — almost never does, because it depends on the tooling the buyer supplies. A slitter built to 0.005 mm at the journal will still deliver 0.03 mm at the knife rim if the spacers sitting on it are held to generic import tolerance and the clamp nut is tightened by feel. Ruilong therefore measures the assembled figure on every line before it ships, rather than quoting the bare shaft.

✅ Bottom line: burr that varies within a single coil is a runout symptom, and no clearance setting will correct it because the gap is right on average and wrong at every instant.

2. What should you specify, and where does the error actually come from?

Specify the assembled tooling stack under clamp torque, and specify spacer thickness as the controlling figure. Spacers set every lane position in the gang, and their errors add in one direction down the arbor, so the spacer thickness tolerance for Slitting Line accuracy matters more than the grade of steel in the shaft.

Four things move the assembled figure after the tooling is bought, and three of them are free to control:

• Sorting beats grinding. Group every knife, spacer and stripper ring measured within ±1 µm into three bands — minus, zero, plus — then assemble so a plus part sits against a minus part. Ruilong ships spacer sets pre-sorted and labelled this way. Errors then cancel down the stack instead of accumulating. This is standard practice on shimless tooling sets and costs a morning with a micrometer.

• Torque to the published figure, not past it. On precision arbors, over-tightening the clamp nut distorts the tooling and the shaft and measurably increases runout. It does not add grip. A torque wrench belongs in the tool room.

• How much arbor deflection affects strip width tolerance comes down to one acceptance figure: ask the builder for the arbor deflection curve at maximum separating force and hold it under 0.08 mm per metre. Static measurements on the bench are taken at zero load, which is not where the cut happens.

• Hold the room steady. Arbor steel grows with temperature, so Ruilong tool rooms are held at 20–25 °C and tooling is left to stabilise before any clearance under 0.5 mm is set.

✅ Bottom line: buy the arbor for stiffness and buy the accuracy in the spacers, then sort them into plus and minus groups so the errors cancel rather than stack.

FAQ

Q: What slitter arbor runout tolerance for precision slitting should go into a purchase specification? A: Ask for assembled total indicated runout measured on the mounted knife rim, under clamp torque, with the spacer set you will actually run. A bare-shaft figure alone is not a specification, because the tooling contributes most of the error.

Q: Is ±0.01 mm spacer thickness tolerance for slitting line accuracy good enough? A: For commercial carbon work on gauges above 1 mm, yes. Below 0.5 mm, or on stainless and electrical steel, no — move to ±0.005 mm or ±0.002 mm, because at 0.3 mm foil the entire clearance window is around 40 µm.

Q: How much arbor deflection affects strip width tolerance on a line I already own? A: Request or measure the deflection curve at maximum separating force; anything approaching 0.08 mm per metre will show up as width drift between the first and last strip in the gang. If the figure is unknown, measure strip width at both edges of the coil at full load and compare it with a slow-speed sample.

CTA: Get an Arbor and Tooling Spec You Can Hold

Send Ruilong your gauge range, strip count and width tolerance. As a slitting line manufacturer in China, Tai'an Ruilong Machinery Co., Ltd. supplies the custom arbor and the matched OEM/ODM spacer set as one package, quotes factory-direct, and states the assembled runout figure in the contract — one tooling supplier accountable for the number that reaches the strip.