Which brake
Where to buy| Machine | What it does | Limit in the Handbook |
|---|---|---|
| Bar folder | Turns edges; bend only as wide as the jaws are deep | Narrow work (H-31 p. 4-21) |
| Cornice brake | Folds sheet through the jaws front to rear without obstruction | 12 to 22 gauge, 3 to 12 ft long (p. 4-21) |
| Box and pan (finger) brake | As a cornice brake, plus boxes and pans: remove the fingers in the way | Check every remaining finger is seated and tight (p. 4-22) |
| Press brake | Heavier and more complex parts, hat channels, with gooseneck or offset dies | Cornice and box brakes top out near 0.090 in annealed aluminum, 0.063 in 7075-T6 or 0.063 in stainless (p. 4-22) |
H-31 = FAA-H-8083-31B, Airframe Handbook, Chapter 4. The brake's own capacity is set by its maker.
Make a bend
| Step | Action | What to do |
|---|---|---|
| 1 | Check the bend | Is the radius allowed for the alloy and temper? Check the bend radius chart and compute the flat length with the bend allowance calculator. |
| 2 | Mark the sight line | The line of the bend, drawn on the sheet. |
| 3 | Place under the bar | Sheet on the bed, sight line directly under the edge of the clamping bar. Bring the bar down to hold it firmly. |
| 4 | Set the stop | Set the stop at the right side for the angle, adding the spring-back: about 93 degrees to get 90. |
| 5 | Bend | Raise the bending leaf until it strikes the stop. |
| 6 | Next bend | Lift the clamping bar, move the sheet to the next position, repeat. |
| 7 | Check | Check the angle and the radius against the drawing, on a test piece first if the part is costly. |
Typical mistakes
| Mistake | What happens | Fix |
|---|---|---|
| Setting exactly 90 degrees | Part opens to 87 or 88 degrees | Set about 93 degrees (spring-back, p. 4-22) |
| Fingers not tight on a finger brake | Fingers shift during the bend | All unremoved fingers securely seated and tightened |
| Radius too tight for the temper | Cracks along the bend | Check the minimum bend radius chart |
| Thick stock on a light brake | Beyond capacity, damage | Capacity is set by the maker; use a press brake for thicker parts |
| Nose radius of the fingers too small | Wrong inside radius | Nose radius shims, made to the bend length (p. 4-22) |
Care
Keep the bed, bar and leaf clean and free of chips, or they mark the sheet. Lubrication, adjustment and calibration follow the machine maker's manual; the Handbook gives none.
Questions people actually ask
How much spring-back should I allow?
The Handbook's example: a cornice brake set for 90 degrees makes about 87 to 88 degrees, so set about 93 degrees (p. 4-22). Actual spring-back depends on alloy, temper and thickness; test a piece.
What is the difference between a cornice brake and a box and pan brake?
A box and pan brake has removable fingers instead of the solid upper jaw, so you can form boxes and pans a cornice brake cannot (p. 4-22).
Limits of this guide
What the sources support, and no further.
- Not approved data. The SRM or drawing gives bend radius, material condition and method.
- Spring-back. The 87 to 88 degree figure is the Handbook's example, not a rule for all metals.
Where to buy
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Distributors coming soon.
Sources
- FAA-H-8083-31B, AMT Handbook: Airframe (2023), Chapter 4 — pp. 4-20 to 4-22, "Bar Folder", "Cornice Brake", "Box & Pan Brake", "Press Brake"; Figs. 4-57, 4-58.
Educational reference only. not approved data. Always consult the applicable SRM, AMM, or AC 43.13-1B and your certifying authority.