CustomPartNet
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August 10, 2026
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Updated August 10, 2026
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A flat pattern that's off by even a few hundredths of an inch can turn a simple bracket into a scrapped part. Before you send a sheet metal design out for quotes, it helps to know that your dimensions already account for how the material actually behaves when it's bent. Suppliers can spot a flat pattern that hasn't been checked against bend allowance and springback, and it's one of the fastest ways to add rounds of revision to a quoting process. Browse our supplier network on the right to compare fabricators experienced with precision sheet metal work, review sample parts, and request quotes once your design is dialed in.
Every bent sheet metal part begins life as a flat blank. Getting from that flat blank to a finished bracket, chassis, or enclosure means accounting for two things that happen the moment metal starts to bend.
First, the material stretches. When a flat sheet is forced around a bend radius, the outer surface stretches while the inner surface compresses. Somewhere in between sits the neutral axis, a layer that neither stretches nor compresses. That neutral axis is longer than the flat sheet was before bending, and figuring out exactly how much longer is what a bend allowance calculation solves.
Second, the material springs back. Once the forming force is removed, the elasticity built into the metal causes it to relax slightly, opening back up from whatever angle the punch and die pushed it to. Ignore this and your finished angle won't match your drawing.
CustomPartNet's Bend Allowance and Bending Springback widgets handle both problems, and they work best used together: one for laying out an accurate flat pattern, the other for making sure the final angle actually lands where you designed it.
The Bend Allowance Calculator determines the arc length added to a part's flange lengths by the bend itself, which lets you work backward to the correct flat blank size. It also reports the bend deduction, a related figure that describes how much the outside of the sheet has stretched relative to the mold line dimensions on your drawing.
You'll need four inputs:
Sheet thickness
Inside bend radius
Bend angle
K-factor (the position of the neutral axis relative to the sheet thickness, which depends on your material and bending method)
From these, the widget returns the bend allowance, bend deduction, and setback, giving you everything needed to convert flange lengths on a finished drawing into a flat pattern length a fabricator can actually cut and bend.
The Bending Springback Calculator picks up where bend allowance leaves off. Once you know your flat pattern is correct, you still need the part to close to the right angle after it leaves the press.
This widget takes your sheet thickness, K-factor, initial bend radius, and initial bend angle, along with the material's yield strength and modulus of elasticity, and estimates how far the part will relax after the load is removed. The output includes a springback factor, along with the resulting final bend radius and final bend angle. Since springback always opens the angle back up and increases the radius slightly, the practical use of this calculator is figuring out how far to overbend in the die so the finished part still meets spec.
Say you're designing a simple L-bracket with two 90 degree flanges, each measured to 2" and 3" along the mold lines, formed from 0.090" mild steel with a 0.125" inside bend radius and a K-factor of 0.33.
Step 1: Bend allowance. Plugging those numbers into the Bend Allowance Calculator gives a bend allowance of roughly 0.243". After accounting for the setback at each flange, the two leg lengths come out to about 1.785" and 2.785". Add the bend allowance and your flat pattern length lands at approximately 4.81".
Step 2: Springback. Mild steel at this thickness with a yield strength around 50,000 psi and a modulus of elasticity near 29,000,000 psi typically shows modest springback, on the order of a few degrees for a 90 degree bend. Running those material properties through the Bending Springback Calculator might show the part relaxing to around 92 to 93 degrees if it's formed to a true 90 degree die angle. To land on a finished 90 degree bracket, you'd need to overbend the punch to roughly 87 to 88 degrees.
That two-step check, flat pattern first, then angle compensation, is exactly the sequence a fabrication shop runs before cutting steel, and it's a good habit to build into your own design process before parts ever reach a quote.
Bend allowance and springback calculations show up constantly in:
Electrical and electronics enclosures
HVAC ductwork and equipment panels
Structural brackets and mounting hardware
Automotive and appliance chassis components
Architectural and building fixtures
Custom equipment housings and covers
Any part with a folded flange, and most sheet metal parts have at least one, depends on getting these two numbers right.
Bend allowance is the arc length added along the neutral axis by the bend. Bend deduction describes the difference between the mold line lengths on a drawing and the actual flat pattern length. They're two ways of describing the same underlying stretch, just measured from different reference points.
K-factor depends on your material, bend method, and bend radius to thickness ratio. A K-factor around 0.33 to 0.50 is a common starting point for air bending mild steel, but your fabricator's shop-specific values, based on their tooling and process, will always be more accurate than a general estimate.
No. Springback is driven by yield strength and modulus of elasticity, so higher-strength materials like stainless steel and spring steel typically show more springback than mild steel, while softer aluminum alloys often show less. That's why the calculator asks for material properties specifically rather than assuming a fixed value.
It's tempting on non-critical brackets, but even a couple of degrees of springback can throw off a part that needs to mate with another component or fit inside an enclosure. It's a quick check that's worth running any time tolerance matters.
Not exactly. Shop-specific bend tables account for that fabricator's particular tooling, press, and material lot, and will generally be more precise for production work. These widgets are best used early in design, to get a solid working estimate before you're locked into a specific supplier's setup.
Bend allowance and springback cover flat pattern layout and angle accuracy. CustomPartNet also has widgets for V-bending force, wipe bending force, and deep drawing force, which help you estimate the tonnage needed to actually form a part on a press. We'll be covering each of those in upcoming posts.
Once your flat pattern and bend angles are dialed in, get competitive quotes from fabricators experienced with precision sheet metal bending. Upload a CAD file, drawing, or flat pattern and receive quotes from qualified suppliers matched to your project requirements.
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