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Engineering
Grade 10-12
Sheet Metal Design Reference Cheat Sheet
A printable reference covering bend allowance, bend deduction, K-factor, minimum bend radius, hole spacing, hems, and common forming operations for grades 10-12.
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Sheet metal design uses flat patterns, bends, cutouts, and forming operations to turn thin metal sheets into useful parts. This cheat sheet helps students connect part drawings to manufacturable designs. It focuses on the geometry and design rules needed for accurate bend layouts, clean features, and fewer fabrication problems. Students can use it as a quick reference when sketching, modeling, or checking a sheet metal part.
Key Facts
- Bend allowance estimates the arc length added by a bend and can be calculated with BA = angle in degrees x pi/180 x (inside radius + K-factor x thickness).
- Bend deduction helps find the flat pattern length and can be calculated with BD = 2 x outside setback - bend allowance.
- Outside setback for a 90 degree bend can be calculated with OSSB = inside radius + thickness.
- Flat length for a part with one bend can be calculated with flat length = flange 1 length + flange 2 length - bend deduction.
- K-factor is usually between 0.30 and 0.50 and represents the neutral axis location as K = distance from inside face to neutral axis / material thickness.
- A common starting rule is minimum inside bend radius = 1 x material thickness, but harder or thicker materials often need larger radii.
- Holes should usually be placed at least 2 x material thickness plus bend radius away from a bend to reduce stretching and distortion.
- Relief cuts near bend ends should be at least as wide as the material thickness and long enough to reach past the bend tangent line.
Vocabulary
- Bend allowance
- The length of material along the neutral axis that is used up by a bend in a flat pattern.
- Bend deduction
- The amount subtracted from the total outside flange lengths to find the correct flat pattern length.
- K-factor
- A ratio that describes where the neutral axis lies through the thickness of the sheet during bending.
- Neutral axis
- The layer inside the sheet metal that does not stretch or compress during bending.
- Bend radius
- The inside radius formed at a bend, measured from the inside surface of the sheet.
- Relief cut
- A small cut added near a bend or corner to prevent tearing, bulging, or unwanted deformation.
Common Mistakes to Avoid
- Using outside dimensions as the flat length without bend deduction is wrong because the material stretches and compresses around the bend.
- Placing holes too close to a bend is wrong because the bending process can stretch the hole into an oval or tear the edge.
- Choosing a bend radius smaller than the material can safely form is wrong because it may cause cracking, especially in hard materials.
- Ignoring grain direction is wrong because bending parallel to the grain can increase the risk of cracking in some sheet metals.
- Forgetting relief cuts at tight corners is wrong because the metal needs space to move during forming without buckling or tearing.
Practice Questions
- 1 A 90 degree bend uses thickness = 2.0 mm, inside radius = 2.0 mm, and K-factor = 0.33. Calculate the bend allowance using BA = angle x pi/180 x (inside radius + K-factor x thickness).
- 2 A sheet metal bracket has two outside flange lengths of 40 mm and 60 mm. If the bend deduction is 3.5 mm, calculate the flat length.
- 3 A designer wants to place a hole near a bend in 1.5 mm sheet metal with an inside bend radius of 2.0 mm. Using the rule distance = 2 x thickness + bend radius, what minimum distance from the bend should be used?
- 4 Explain why a part with the same outside dimensions may need different flat pattern lengths when made from different materials or thicknesses.