Bending wood
Kerf spacing, depth and count for a radius (with the deflection test), bent laminations with ply thickness from each species' strength and stiffness, springback and form compensation, steam bending times and the tightest radius with and without a strap, and arcs from chord and rise for laying out forms — all drawn to scale.
Added
Works with
- Wood species Woodworkingbending ratings and the stiffness and strength figures behind the lamination and steam limits
- Woodworking angles and fractions Woodworkingfraction arithmetic and the batten-offset method for long arcs
How to use it
Four ways to make wood curve, in inches (with fractions) or millimetres — the units are shared with the other woodworking tools.
How to use it
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Kerf bending. Board thickness, the inside radius (the face you kerf), your blade’s kerf width, the web of uncut wood to leave on the outside, and the bend angle. You get the kerf spacing, how many kerfs, their depth, the length of the kerfed section, and the reading to expect from the deflection test, with the bent board drawn to scale. Warnings appear when the web is thick enough to crack or the kerfs would almost touch.
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Bent lamination. Choose a species, the finished thickness and the inside radius. The tool works out the thickest ply that bends without breaking, the number of plies, the springback when the clamps come off, the radius you will actually get, and the form radius that compensates for it.
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Steam bending. Species, thickness, and optionally the radius and angle you want: time in the steam box, the tightest radius with a strap and without, the strap length, and whether your bend is realistic, with a table of limits for the good bending species.
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Arcs & forms. Any two of chord, rise and radius give the third, the arc angle and length, and the heights above the chord at equal stations for drawing a long curve with a batten.
How it works
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Kerfs. Closing a kerf of width w and depth d turns the board by θ = 2·atan(w ÷ 2d) about the web. The outside face (radius R + t) must turn by θ every (R + t)·θ, which is the spacing — and exactly the rise the deflection test measures at a distance R + t from a closed kerf.
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Plies. A ply of thickness t bent to radius R stretches its surface by t ÷ 2R; it breaks near the strain MOR ÷ MOE (strength over stiffness). So R ÷ t must be at least MOE ÷ (2·MOR), times a safety factor. For white oak (MOE 1.78 million psi, MOR 15,200 psi) that is about 59.
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Springback. Bent separately, each ply carries a moment in proportion to its own stiffness; glued, the stack is n³ times as stiff as one ply, so it straightens by 1/n² of its curvature: two plies 25%, four 6%, eight under 2%. The form is made that much tighter. The tests check the 1/n² result and that the compensated form springs back to the target radius.
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Steam. About an hour per inch of thickness for air-dried wood (longer for kiln-dried). Steamed hardwood compresses by a quarter or more but stretches only 1–2%. A strap on the outside face keeps it from stretching, so the inside compresses by t ÷ (R + t); without a strap the neutral axis stays central and the outside stretches by (t/2) ÷ (R + t/2). The strain limits by species rating are my simplification of the published guidance.
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Arcs. R = (c²/4 + h²) ÷ 2h; angle = 2·asin(c ÷ 2R); length = R × angle.
Limits
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Real boards vary: knots, run-out and short grain break long before these limits. Test a piece.
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The lamination limit uses dry Wood Handbook strengths at 12% moisture content; it is conservative for steamed or wet plies and for thin veneers.
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Springback also comes from glue creep and moisture change, which the 1/n² rule leaves out; urea-formaldehyde and epoxy creep less than PVA.
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Steam limits are planning figures, not guarantees.
Sources: USDA Forest Products Laboratory, Wood Handbook (FPL-GTR-282), Tables 5-3 and 5-4 (MOE and MOR) and the chapter on bent wood members; E. C. Peck, Bending Solid Wood to Form, USDA Agriculture Handbook 125; W. C. Stevens and N. Turner, Wood Bending Handbook (HMSO).
Linking to an example
Share link copies a URL with your settings. Lengths in links are in millimetres — for example
?lsp=ash-white<=19.05&lr=254.
| Parameter | Meaning |
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Kerf bending: thickness, inside radius, kerf width, web (mm), angle (°) |
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Lamination: species id, thickness, inside radius (mm), safety factor, thinnest ply (mm) |
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Steam: species id, thickness, inside radius (mm), angle (°), kiln-dried |
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Arc: mode ( |
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Species ids are those of the wood species reference, e.g. oak-white,
ash-white, hickory-shagbark, beech-european.
Presets (?example=<name>):
example= |
Shows |
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