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BuyBend Allowance / Flat-Pattern Troubleshooting
Mode mixing, K-factor, bend vs included angle, and DXF import
Diagnose wrong flat blanks: mixing outside and inside dimensions, K-factor confusion, bend angle vs included angle, and DXF import problems with the minimal R12 line-only export.
Fix the inputs, then re-export
Most wrong blanks come from mixing dimensioning conventions, an unverified K-factor, or bend-angle confusion — not the math.
Open the Unfolder → Read the Guide →Mixing Outside and Inside Dimensions
The single most common error: dimensioning some legs to the outside/mold-line corner and others to the inside flat length on the same part. The unfolder deliberately locks one convention per part — the mode toggle applies to every leg. It has to: outside and inside legs differ by the outside set-back at each bend, so a part described half one way and half the other has no single correct blank. If a number looks wrong, first confirm the drawing's convention, set the mode once, and re-enter every leg to match. Do not "fix" one leg by switching its meaning.
K-Factor Confusion
The default K of 0.44 is a starting point, not a universal constant. K varies with material, temper, inside radius, and forming method (air bend vs bottoming vs coining). If your formed parts come out consistently long or short, K is the usual culprit. The tool flags K values outside the typical 0.30–0.50 range so you notice accidental entries. To dial it in: bend a test coupon at your real radius, measure the actual flat length, and adjust K until the tool's flat matches. Then reuse that K for the same material and setup.
Bend Angle vs Included Angle
The unfolder asks for the bend angle — how far the flange folds away from flat. A right-angle corner is a 90° bend (fold), which leaves a 90° included angle between the faces. A gentle 30° bend leaves a 150° included angle. It is easy to type the included angle (say 120°) when you meant a 60° bend. If a bend looks far sharper or flatter than intended in the side-profile schematic, you probably entered the included angle. Remember: 0° = no bend, 90° = square corner, 180° = folded flat back on itself.
DXF Import Issues
The DXF export is intentionally minimal R12, line-only — no arcs, no splines, no polylines — because straight LINE entities import cleanly across almost every CAD/CAM reader. You get two layers: OUTLINE (the blank rectangle) and BEND (dashed bend centerlines). If your software shows nothing on import, check that it is not filtering layers, and confirm units — the DXF carries raw numeric coordinates in the unit you built with (inch or millimeter), so set your CAD document units to match. Because bends are drawn as centerlines rather than arcs, treat the DXF as a layout/nesting aid, and rely on the PDF reference square for physical print verification.
Blank Comes Out the Wrong Length
If the formed part is off by a repeatable amount, work through this order: (1) confirm you did not mix conventions; (2) verify K against a test bend; (3) check every bend angle is the fold, not the included angle; (4) confirm thickness and inside radius are correct for the actual material; and (5) confirm you printed the PDF at 100% and the reference square measures true. The guide walks through each input in detail, and the single-bend Bend Allowance and Bend Deduction calculators help you sanity-check one bend in isolation.
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Recommended Bend Layout & Measuring Tools
Verify bend angles, transfer flat-pattern marks, and cut the blank — products from the TestTalkHQ affiliate catalog.
Klein Tools 935DGGP Digital Angle Gauge
- Confirm bend and included angles on formed parts
- Magnetic base sits on the flange or brake bed
- Check your assumed bend angle against the tool
- Zero/relative modes for reference-face reads
Starrett Electronic Slide Caliper (0–6")
- Measure material thickness for the K-factor model
- Read leg and flange lengths precisely
- Inch/mm switch matches either unit mode
- Verify flat-blank cuts before forming
Malco M2006 10" Left Offset Snips
- Offset left-cut for long flat-pattern edges
- Keeps hands clear of the sheet
- Shop standard for sheet-metal layout cuts
- Pairs with right-cut snips for corners
Malco AV73 Right Offset Snips
- Long-cut right offset for the opposite hand
- Follow the outline of the unfolded blank
- Aviation-style compound leverage
- Complement to the left-offset snips
Malco TS1 TurboShear Attachment
- Drill-powered shear for long blank cuts
- Faster than hand snips on large flats
- Useful after transferring the PDF layout
- 20-gauge sheet metal capacity class
Catalog gaps: No sheet-metal brake / press-brake products, and no bend-allowance reference charts or books, exist in the TestTalkHQ affiliate CSV at publish time — we do not substitute unrelated electrical conduit benders for those roles.
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