Thickness tolerance and its effect on CRCA parts

CRCA thickness tolerance is the least glamorous line on a drawing and the one that decides whether a bend sequence closes. The drawing says 2.0 mm, the coil certificate says 2.0 mm, and the micrometer says 1.92 mm on one edge and 2.07 mm across the middle of the same sheet. Nothing failed inspection, because no one agreed what the thickness was allowed to be.

Thickness tolerance is the least glamorous line on a drawing and the one that decides whether a bend sequence closes. It moves the neutral axis, the flat pattern, the blanking load, the part mass and the weld schedule at the same time. On a part with one bend it is noise; on a frame with six it is the tolerance stack.

This guide gives the mill band a buyer can expect, the arithmetic that turns a thickness step into a bend error, and the inspection method that catches a coil change before the parts do.

What thickness band does a CRCA mill actually deliver?

Cold-reduced sheet is rolled to a nominal thickness and accepted against a tolerance table that depends on the ordered thickness, the width and the tolerance class. At 2.0 mm and a width near 1,250 mm, a normal commercial band is of the order of ±0.10 mm. Thinner sheet holds a smaller absolute band, around ±0.05 mm at 0.8 mm.

The blank load column is perimeter × thickness × shear strength, with a 600 mm perimeter and 300 MPa shear for a 400 MPa tensile steel. Read it as a trend, not a press selection: the point is that a tolerance band of ±5% moves the load by ±5%, and a press sized at 360 kN with no margin is a press that trips on the thick end of the coil.

Two conditions change the band. A restricted tolerance class costs more and narrows the range, which is worth paying for on a part where the flat pattern cannot be recalculated. A width at the top of the mill's range widens the band, because thickness control is harder across a wider strip. Neither is visible in the words "2 mm CRCA".

Crown and wedge across the strip are a separate variation from the across-lot band. A coil can be on nominal at the centre line and 0.02 mm thin at one edge, which is inside the band and still enough to change the bend on a part cut from that edge. That is why the inspection samples two edges and the centre, and why a blank nested across the full sheet width inherits the wedge.

How does CRCA thickness tolerance move a bend?

Bend allowance is the arc through the neutral axis: BA = π/180 × θ × (R + K·t). Hold the radius at 2 mm and K at 0.44, and vary only the thickness.

At 1.9 mm, BA = 1.5708 × (2 + 0.836) = 4.455 mm. At 2.0 mm it is 4.524 mm. At 2.1 mm it is 4.593 mm. The band is 0.138 mm wide, or ±0.069 mm around nominal for one bend.

The bend deduction moves further: BD = 2(R + t) − BA, which is 3.476 mm at 2.0 mm, 3.607 mm at 2.1 mm and 3.345 mm at 1.9 mm. The ±0.1 mm thickness band becomes a 0.26 mm bend deduction band, because the thickness term is counted twice and the bend allowance only partly compensates.

On a single bend that is invisible. On a four-bend box it is ±0.28 mm of flat length at the bend allowance and ±0.52 mm at the bend deduction. If the blank is cut from a flat pattern computed at nominal and the coil runs thick, the formed outside dimensions absorb the difference, and the last flange is the one that misses.

The practical rule is to compute the flat pattern from the measured coil thickness for the lot, not from the nominal on the drawing, and to hold the first article until the coil certificate and the micrometer agree. The bend allowance method shows the full flat-length arithmetic, and the bending tolerance guide covers the inspection side.

Springback moves less than the flat pattern. At a fixed 2 mm inside radius, a 5% thickness rise lowers R/t from 1.0 to 0.95 and reduces the springback angle by a few hundredths of a degree, which is well inside the 1–2° of tool correction a mild steel bend already carries. Thickness is a flat-length problem first and a springback problem second.

How does the band affect mass and purchasing?

Sheet is sold by weight and used by area, so a thick coil delivers fewer parts per tonne. At 2.0 mm, one tonne covers 1,000 ÷ 15.7 = 63.7 m². At 2.1 mm it covers 1,000 ÷ 16.485 = 60.7 m², 4.7% less area for the same invoice weight.

On a 12 kg fabricated part, a 5% thickness rise adds 0.6 kg. That is small on one part and material on a container load, and it is the reason freight and handling quotes move when a coil source changes without a drawing change.

The mass effect also reaches the coating quotation, because coating is priced by area and the area did not change. A heavier coil costs more in steel and the same in powder, so the per-part cost curve steepens at the thick end of the band. That is a purchasing argument for a restricted tolerance class when the part is weight-sensitive.

How does thickness variation affect fit-up and welding?

A stack of five 2.0 mm plates at ±0.10 mm each has a worst-case thickness spread of 5 × 0.10 = 0.50 mm. The statistical spread, treating each plate as independent, is √5 × 0.10 = 0.22 mm. The difference between those two numbers is the difference between a clamp that always closes and a clamp that closes most of the time.

Spot welding is more sensitive than fit-up. Weld current, electrode force and cycle time are set for a stack resistance, and resistance follows thickness. A 5% thicker stack at the same schedule runs hotter and can expel; a thinner stack runs cold and produces an undersized nugget. The weld schedule is validated on the actual coil band, and the coil number belongs on the weld parameter sheet.

Lap joints and hemmed edges absorb thickness variation in the joint gap, not in the finished dimension. A 0.1 mm step under a hem shows as a visible line after painting, which is a cosmetic defect that no amount of clamping removes.

How does the band affect stamping and laser cutting?

Die clearance is set as a percentage of thickness, normally 8–12% per side for a sheared edge. At 2.0 mm, 9% is 0.18 mm per side. A 2.1 mm blank at the same die clearance runs at 8.6%, still inside the normal window, but the fracture zone lengthens and the burr grows. The part passes, and the deburr time rises.

A draw die is less forgiving because the gap is fixed by the punch and die. A gap set at 2.2 mm for 2.0 mm sheet, which is 0.10 mm per side, closes to 0.05 mm per side on a 2.1 mm blank. That thins the wall, raises the punch load and turns a clean draw into a split on the third coil.

Laser cutting is the least sensitive of the three. Focus position is set to the sheet surface, and the depth of focus on a 12 kW fibre laser at 2 mm is wide enough that a 0.1 mm thickness step barely moves the kerf. At 1 mm the focus window is tighter, and a coil at the thin end of the band cuts with a different dross pattern unless the focus is re-set. The laser cutting tolerance guide covers the cut-side number, which is set by the machine and not by the coil.

How should a quality team inspect thickness?

Measure the bare blank with a micrometer or a calibrated thickness gauge, at three points: two edges and the centre, keeping the anvil at least 20 mm clear of the cut edge and off any burr. Record the coil number and the mill certificate with the result.

Track the coil mean and range rather than the individual reading. If the drawing band is ±0.10 mm and the process standard deviation is 0.03 mm, the capability index is Cp = 0.20 ÷ (6 × 0.03) = 1.11. If the coil mean sits 0.02 mm high, Cpk falls to 0.08 ÷ (3 × 0.03) = 0.89, which is a process that will produce out-of-band parts even though every individual measurement looks close.

The gauge itself needs a check. A micrometer with 0.001 mm resolution and a flat anvil is adequate; a caliper with 0.02 mm resolution is not, because that resolution is a tenth of the band. A gauge repeatability and reproducibility study should keep the measurement variation under 10% of the tolerance, which is 0.02 mm on a 0.20 mm band.

What does Ganesh run for thin-gauge CRCA?

CRCA parts run on mechanical presses from 20 T to 300 T with 250 T punching, and on the 12 kW fibre laser with a 40 ft × 16 ft bed for blanks and short runs. Formed parts run on the 200 T, 5-axis press brake with a 4,450 mm bed and Delem control to 10 mm. Formed tolerance is ±0.5 mm typical, tighter on critical features by agreement, and first article inspection or PPAP documentation is available.

The inspection route records the coil certificate, the measured thickness band and the critical dimensions on the first article. Where a part is thickness-critical, the flat pattern is computed from the measured lot thickness rather than the drawing nominal, and the change is recorded against the drawing revision.

Ganesh has manufactured since 1992 under ISO 9001:2015 and ISO 14001:2015, with five units in Mumbai (Vasai) and Nagpur.

When thickness tolerance does not matter

On a flat part with no bends, a ±0.10 mm band changes mass and load but not geometry, and a standard commercial coil is fine. On a part where the thickness is set by a machined pad or a bearing seat, the mill band is irrelevant because the feature is machined after forming, and the tolerance that matters is the machining one.

It also stops mattering when the design has enough adjustment. A slotted bracket, a shimmed joint or an oversized hole with a floating fastener absorbs a 0.2 mm stack without a drawing change. The expensive cases are the ones with a fixed stack and no adjustment.

What to put on the drawing

• Nominal thickness with the tolerance class and the standard it comes from.
• The measurement condition, if it differs from the standard: clear of the edge, bare, at a stated temperature.
• Which features are thickness-critical, and which are set by a machined or formed operation.
• The bend radii and the flat pattern basis, nominal or measured lot.
• The stack-up dimensions that must close, with the tolerance that matters.
• The weld schedule reference and the coil record requirement for welded assemblies.
• The general tolerance class from ISO 2768-1, plus any tighter geometric callout.

Send the drawing and the measured thickness band to request a quote. The materials and grades hub covers the grade and finish side, and the stamping capability page shows the press and inspection route behind a thin-gauge part.