The drawing
Where to buy
Step by step
| Step | Action | What to do |
|---|---|---|
| 1 | Get the radius | From the drawing; if it is not there, the SRM minimum bend radius chart. See the bend radius chart. |
| 2 | Fit the radius bar | Swap the bar, or build up a smaller one with nose radius shims. The Handbook example: a 0.063 in shim round a 1/16 in nose gives 1/8 in. |
| 3 | Set the clamp | Sheet of the same thickness under the bar. Pull the lever: firm, not springy, reaching its stops. Test strips 3 in from each end and one in the centre: it must stop them slipping. |
| 4 | Set the nose gap | Hold the leaf at the finished angle. Gap = one sheet thickness, checked with a scrap as a feeler, even along the length. |
| 5 | Test two strips | Clamp a strip 3 in from each end, bend 90°, lay one on the other. They should match. If not, ease back the end with the sharper bend. |
| 6 | Prepare the sheet | Deburred, relief holes drilled, grain across the bend if you can, sight line drawn one radius inside the bend tangent line that goes under the bar. |
| 7 | Position | Slide the sheet under the clamp. Looking straight down the nose, move it until the sight line is directly below the edge of the radius bar. Clamp. |
| 8 | Bend | Raise the leaf. The bend starts exactly on the bend tangent line. Check the angle and the flats against the drawing. |
Tools you need
| Tool | What for | Buying guide | Catalog |
|---|---|---|---|
| Cornice or box brake | Straight bends | Bending brakes | — |
| Radius and fillet gauges | Check the bar and shims | Bending brakes | — |
| Deburring tool | Edges before bending | Deburring tools | Edge tool |
The three settings
| Setting | Right when | Adjust with |
|---|---|---|
| Radius bar | Equals the required radius | Interchangeable bars or shims |
| Clamp pressure | Firm pull to the stops, no sponginess, no slipping | Clamping pressure nut, both sides |
| Nose gap | One thickness, even along the part, at the finished angle | Nose gap knobs at the rear of the upper jaw |
FAA-H-8083-31B pp. 4-68 to 4-71.
Quick check
Where does the sight line go?
- One radius inside the bend tangent line that goes under the brake
Best fit. Yes: the Handbook puts it there, one radius from that line.
- In the middle of the bend allowance
Not this one. The Handbook names this as the common mistake.
- On the bend tangent line
Not this one. That line must end up where the bend starts, not under the edge of the bar.
On a real job, the SRM decides.
What to check when it is done
| Look at | Good | If not |
|---|---|---|
| Radius | Matches the drawing; check with a gauge | Reset the bar |
| Outside of the bend | No cracks | Reject: radius too small or edge rough |
| Angle | As drawn, even along the length | Check the nose gap |
| Flats | To the drawing | Recheck setback and sight line |
| Corners | Relief holes in place | Remake the part |
Questions people actually ask
Why did it crack when the radius looked fine?
Check the grain, the temper and the edge. Stress left by too tight a radius can fail a part in service even if it does not crack on the brake.
How do I get a radius my brake does not have?
Nose radius shims, bent from sheet of a known thickness. Check with radius gauges.
Limits of this guide
What the sources support, and no further.
- The manual governs. Where the SRM, the drawing or the fastener maker says otherwise, follow them; this page explains general practice.
- Flat pattern maths is not here. It is in the bending guide.
Where to buy
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Distributors coming soon.
Sources
- FAA-H-8083-31B, Aviation Maintenance Technician Handbook: Airframe, Chapter 4 — p. 4-58, grain; p. 4-66, sight line, Fig. 4-131; pp. 4-68 to 4-71, using a brake, Figs. 4-135 to 4-144.
- FAA AC 43.13-1B CHG 1, Acceptable Methods, Techniques and Practices — par. 4-55, bending metal.
Educational reference only. not approved data. Always consult the applicable SRM, AMM, or AC 43.13-1B and your certifying authority.