The calculator
Runs in your browser. Inches or millimetres at the top. Fractions such as 1/16 work.
How the rule works
The FAA Airframe Handbook (FAA-H-8083-31, Chapter 4, Joggling): “The thickness of the material to be cleared governs the depth of the joggle.” Clearing a 0.032 in skin needs a 0.032 in joggle, plus any clearance the drawing adds.
Length. “In determining the necessary length of the joggle, allow an extra 1/16-inch to give enough added clearance to assure a fit between the joggled, overlapped part.” Length = overlap + 1/16 in.
Allowance is the distance between the two bends. The Handbook says it is normally called out on the drawing, and gives a rule of thumb: “four times the thickness of the displacement of flat sheets.” For 90° angles it must be slightly more, because of the stress at the radius while joggling. For extrusions it “can be as much as 12 times the material thickness”. The calculator applies 4 × depth for flat sheet, and for angles and extrusions shows 4 × depth only as a floor.
Worked example
A 0.040 in flat strap must lap 1 in over a 0.032 in skin.
- Depth = thickness to clear = 0.032 in.
- Allowance = 4 × 0.032 = 0.128 in between the bends.
- Length = 1 + 1/16 = 1.063 in.
Joggle 0.032 in deep, bends 0.128 in apart, 1.063 in long: unless the drawing says otherwise.
Limits — when NOT to use this
- The drawing governs. The Handbook says the allowance is normally called out on the drawing. Enter it, and the calculator uses it.
- Angles and extrusions. The Handbook gives no figure for 90° angles beyond “slightly more”, and up to 12 × thickness for extrusions. Do not form those to 4 × depth without the drawing.
- Hard tempers. The Handbook figures carry no alloy or temper. A tight joggle in a hard temper can crack at the bends: check the bend radius limits for the material.
Questions people actually ask
Is the depth the thickness of the joggled part or of the part it clears?
Of the part it clears. The Handbook: the thickness of the material to be cleared governs the depth.
How do I form one without joggle blocks?
On a straight flange or flat piece, in a cornice brake: bend up about 20° to 30° at the first line, turn the part over, clamp at the second line and bend until the joggle reaches its height. Then check it for dimensions and clearance.
Sources
- FAA-H-8083-31, AMT Handbook — Airframe, Volume 1, Chapter 4, Joggling — depth governed by the thickness to be cleared; an extra 1/16 inch on the length; allowance normally on the drawing, rule of thumb four times the displacement for flat sheets, slightly more for 90° angles, up to 12 times the material thickness for extrusions; brake and joggle-block procedures.