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Flat patterns and bend allowances · Lesson 1 of 4

Flat patterns and bend allowances: principles and terminology

The geometry of one bend: mold line, tangent lines, inside radius, neutral axis and K, and the three numbers that turn a drawn profile into a flat blank.

You will be able to: Name the parts of a bend, define bend allowance, setback, bend deduction and the K-factor, tell the AFS order blank from a developed blank, and state the safety limits.

11 min · 12 slides · 8 questions

All lessons in this module
Slide 1 of 12

Where this module starts

The lesson Flat patterns and blank width shows how AFS sizes a blank: every leg to the mold line, plus the hem allowances.

This module goes further: the geometry of one bend, what the metal and the machine change, three working methods, and how a wrong flat pattern shows on the formed part.

Flat patterns and bend allowances at a glanceFive summary cards. Mold line first: AFS draws and orders every leg to the sharp corner. Three numbers, not one: bend allowance is the neutral-axis arc, setback runs from a tangent line to the mold point, and bend deduction is twice the setback less the allowance. Two blanks, two jobs: the AFS order blank is the legs plus hem allowances, the developed blank is a preliminary estimate. Where K comes from: published K is typical, so calibrate from three or more test pieces. Check the formed part: legs to the mold line, turns, radius, hems, coating and painted face, then record it.Flat patterns at a glanceBend allowances, blanks and checksMold line firstAFS draws and orders every legto the sharp corner, not tothe curve of the bend.Three numbers, not oneAllowance: the neutral-axis arcSetback: tangent to mold pointDeduction: 2 x setback - allowanceTwo blanks, two jobsOrder blank: legs plus hemallowances, as AFS quotes it.Developed: legs less deductions,a preliminary estimate.Where K comes fromPublished K is a typical value,never an AFS measurement.Calibrate from three or moretest pieces of the real metal.Check the formed partLegs to the mold line, turns,radius logged, hems, coating,painted face, then record it.

Five summary cards. Mold line first: AFS draws and orders every leg to the sharp corner. Three numbers, not one: bend allowance, setback and bend deduction. Two blanks, two jobs: the AFS order blank is the legs plus hem allowances and the developed blank is a preliminary estimate. Where K comes from: published K is typical, so calibrate from three or more test pieces. Check the formed part: legs, turns, radius, hems, coating and painted face, then record it.

The module in five cards. Each card is taught, with its source, in the lessons that follow.

Sources

Videos for this lesson

What this video shows: SendCutSend's education lesson on the two calculations behind a bend, bend allowance and bend deduction, and how radius, thickness and K-factor affect them.

Video: How to Calculate Bend Allowance and Bend Deduction - Education Chapter 5 Lesson 5 by SendCutSend, on YouTube. Chosen from the video's published description.

Glossary

Mold line
The line formed by extending the outside surfaces of the two legs of a bend; AFS measures legs to it.
Mold point
Where the extended outside surfaces of a bend meet: the theoretical sharp corner.
Bend tangent line
One of the two lines where the metal starts and stops curving.
Flat
The straight part of a leg between bend tangent lines.
Neutral axis
The layer of the sheet neither stretched nor squeezed by bending.
K-factor
The position of the neutral axis after bending, divided by the thickness.
Bend allowance
The arc length of a bend measured along the neutral axis.
Outside setback
The distance from a bend tangent line to the mold point: tan(turn/2) x (R + t).
Bend deduction
Twice the setback minus the bend allowance; what comes off the mold-line legs for a developed blank.
Turn angle
How far the metal turns from straight at a bend: 180 deg minus the included angle.