板金曲げ加工: Complete Guide to 公差, Bend Allowance & DFM Tips

重要ポイント

• Bend allowance (BA) is the arc length of the neutral axis through a bend — the critical calculation that determines your flat pattern dimensions.
• K-factor (typically 0.33–0.50) defines where the neutral axis sits; it varies by 材料, 厚さ, and 曲げ半径.
• 最小値 曲げ半径 depends on 材料 type and 厚さ — 曲げ加工 tighter than 1× 材料 厚さ risks cracking in most steels.
• DFM rules — hole proximity (≥2.5× 厚さ from bend line), minimum flange length (≥4× 厚さ), and relief cuts — prevent part rejection before it happens.
• 曲げ加工 公差 for 精密 板金 typically fall within ±0.2 mm for bend angles and ±0.3 mm for flange lengths.
• 材料 choice drives bendability — 5052 アルミニウム bends far better than 6061; cold-rolled steel offers the best cost-to-formability ratio.

はじめに

板金 曲げ加工 is the backbone of modern manufacturing. From industrial enclosures to automotive ブラケット, nearly every fabricated metal part 含む at least one bend. Yet getting it right — consistently and cost-effectively — requires understanding a handful of engineering fundamentals that many 設計 engineers overlook.

This guide covers すべて you need: the bend allowance formula, how K-factor affects your flat pattern, minimum bend radii by 材料, achievable 曲げ加工 公差, and the DFM rules that separate production-ready parts from costly rework. Whether you are 設計ing a prototype or sourcing a production run, these principles will save you time, scrap, and supplier frustration.

ベンドアローアンスとベンドデダクション：基本の計算式

When 板金 bends, the outside surface stretches while the inside compresses. Somewhere in between sits the neutral axis — a theoretical line that experiences neither tension nor compression. The length of this neutral axis through the bend is your bend allowance (BA).

Bend Allowance Formula

BA = (π/180) × Bend Angle × (Inside Radius + K × 材料 厚さ)

Where:

• Bend Angle (θ): in degrees (e.g., 90°)
• Inside Radius (R): the radius after 曲げ加工, typically 0.5×–2× 材料 厚さ
• K-factor: dimensionless ratio (typically 0.33–0.50), defines neutral axis position
• 材料 厚さ (T): sheet gauge 厚さ

Example: For a 2 mm cold-rolled steel sheet bent 90° with R = 2 mm and K = 0.40:
BA = (π/180) × 90 × (2 + 0.40 × 2) = 1.571 × 2.8 = 4.40 mm

Bend Deduction vs Bend Allowance

While bend allowance tells you how much 材料 the bend consumes, bend deduction (BD) tells you how much to subtract from your total flat length:

• Outside Setback (OSSB) = tan(θ/2) × (R + T)
• Bend Deduction (BD) = 2 × OSSB − BA
• Flat Pattern 長さ = Sum of all leg lengths − Sum of all bend deductions

Most modern CAD software (SolidWorks, Fusion 360, FreeCAD) handles these calculations automatically once you set the correct K-factor for your 材料. But knowing the math helps you catch errors before they reach the shop floor.

最適な用途： Engineers creating flat patterns from scratch or verifying CAD output.
避けるべき場合： Your supplier provides CAM programming — they will handle bend calculations based on their specific tooling. 送信 the finished-part model instead.

Kファクター：正確な曲げ加工の核心

The K-factor is the ratio of the neutral axis distance from the inside bend surface to the 材料 厚さ. A K-factor of 0.50 means the neutral axis sits exactly in the middle of the sheet; 0.33 means it is closer to the inside radius — as happens with harder 材料s or tighter bends.

最適な用途： Production planning — use these K-factor ranges as your CAD starting point.
避けるべき場合： You need ±0.1 mm flat pattern クライアント連絡： — run physical test bends with your supplier’s exact tooling and batch of 材料.

材質別の最小曲げ半径

曲げ加工 too tightly cracks the outer surface. The rule of thumb: minimum inside 曲げ半径 should be at least equal to 材料 厚さ (1T) for most steels. Below that, you risk structural failure. The table below shows recommended minimums.

曲げ加工の公差：実現可能な精度

精密 板金 shops can hold surprisingly tight 公差, but it varies by 曲げ加工 method and part geometry. Here are realistic numbers for 標準 air 曲げ加工 (the most common method):

板金曲げ加工のDFMルール

設計製造性（DFM） rules exist for one reason: to prevent parts that cannot be made, or that cost 5× more than they should. Follow these seven rules and your parts will be manufacturable at the first attempt.

1. Hole Proximity to Bend Line

Holes too close to a bend will distort during forming. 最小値 distance = 2.5 × 材料 厚さ + 曲げ半径. For a 2 mm sheet with R=2 mm, keep holes at least 7 mm from the bend line. If you cannot meet this, add a relief slot or drill after 曲げ加工.

2. 最小値 Flange 長さ

The flange must be long enough for the press brake tooling to grip. 最小値 = 4 × 材料 厚さ (or the die V-opening width, whichever is larger). For 2 mm sheet: 8 mm minimum. Shorter flanges require special tooling and increase cost.

3. Bend Relief Cuts

At the intersection of two bends or at the end of a bend flange, add a relief cut (typically 1.5× 材料 厚さ wide). Without relief, 材料 tears propagate from the bend corner. Relief cuts also prevent uncontrolled deformation at flange edges.

4. Consistent 曲げ半径

Use the same inside 曲げ半径 for all bends on a part. Mixing radii means tooling changes between bends — adding setup time and increasing the risk of operator error. 標準ize on 1× 材料 厚さ unless you have a specific reason to deviate.

5. Bend Sequence Accessibility

設計 parts so all bends can be reached by the press brake tooling in sequence. A U-channel with both flanges bent inward may require a gooseneck punch or two-stage tooling — plan the bend order during 設計, not on the shop floor.

6. Grain Direction

Always specify that bends run perpendicular to the rolling grain direction. 曲げ加工 parallel to the grain reduces the minimum achievable radius by roughly 50% and dramatically increases cracking risk. On your drawing, add a note: “Bend perpendicular to grain.”

7. Avoid 項目s on Bend Radii

Do not place holes, slots, or tabs directly on the 曲げ半径 — the 材料 deformation in this zone makes feature dimensions unpredictable. Place all features on flat flange faces, at least 3× 材料 厚さ away from the tangent point of the bend.

よくある曲げ欠陥とその防止方法

実例：筐体ブラケットの再設計

A German automation client submitted a ステンレス鋼 enclosure bracket with four 90° bends, all at R=1 mm on 2 mm 304ステンレス. The 設計 placed two Ø6 mm mounting holes only 4 mm from the bend line on each flange.

The result: First article 検査 showed hole distortion on all four flanges — the holes were ovalized by 0.3–0.5 mm after 曲げ加工. The tight 曲げ半径 (0.5T) also caused micro-cracking at two bend corners.

The fix: We increased 曲げ半径 to 3 mm (1.5T), moved holes to 9 mm from bend lines (meeting the 2.5T + R rule), and added 2 mm relief cuts at bend intersections. Second article: 100% pass rate, zero rework. Total re設計 cost: one hour of engineering time versus weeks of schedule delay.

まとめ

Mastering 板金曲げ加工 comes down to four fundamentals: calculate your bend allowance correctly, use the right K-factor for your 材料, respect minimum bend radii, and follow DFM rules from the start. Get these right in your 設計 phase, and you will eliminate the most common causes of part rejection — saving time, cost, and supplier relationships.

For engineers new to 板金 設計, the single most valuable habit is 送信ing your STEP file to your 板金 fabricator for a DFM review before finalizing 公差. A 10-minute review catches issues that cost days of rework later.