PencilCADSheet metal flat patterns on iPad (bends, K-factor, DXF)
A flat pattern is a bent part unfolded: the outline that gets cut, with a line wherever it folds. On iPad you can draw one with Apple Pencil, give every bend its angle, radius and K-factor, check it, and send SendCutSend or OSH Cut a flat DXF with the bend lines on their own layer.
1. Set the sheet to metal
Choose Cut in Settings → Tools or More, tap Sheet and turn on Sheet metal. Set the material, thickness and alloy; every bend works from the thickness.
2. Draw the flat
Draw the part unfolded, at true size, with holes where they sit on the flat. The parametric sketching guide covers constraints, and iPad sketch to DXF covers clean outlines.
3. Make bend lines, or tap an edge for a flange
Make any line a bend with Bend Up… or Bend Down…, or tap an edge for a Flange. Add flange grows the outline and leaves a bend line on the old edge.
4. Set angle, direction, radius and K-factor
Set each bend’s angle, direction, inside radius and K-factor, matching the radius to the shop’s tooling. The label reads both ways, such as “60° bend (120° inside)”.
5. The bend maths
BA is the neutral axis through the bend; OSSB runs to where the outside faces meet. For angle θ, inside radius R, thickness t and K-factor K:
BA = θ·π/180·(R + K·t)
OSSB = (R + t)·tan(θ/2)
BD = 2·OSSB − BA
| Number | Worked out | Result |
|---|---|---|
| Inputs | t = 2 mm, R = 2 mm, K = 0.4, θ = 90° | – |
| Bend allowance, BA | 90·π/180·(2 + 0.4·2) | 4.40 mm |
| Outside setback, OSSB | (2 + 2)·tan 45° | 4.00 mm |
| Bend deduction, BD | 2·4.00 − 4.40 | 3.60 mm |
6. Run the bend checks
Tap Check: with the usual preflight, it flags a bend too near an edge or a hole. Fix those before you export.
7. Pick the shop rules
SendCutSend and OSH Cut use their own radius and K-factor; Generic covers anyone else, your own brake included. Still read the shop’s own bending guidelines before you order.
8. Send the flat DXF to the shop
Send hands SendCutSend, OSH Cut or Ponoko a full-size DXF in millimetres, outlines on CUT and bends on BEND, through Files, AirDrop or iCloud Drive. To bend it yourself, Export for → Generic writes SVG or DXF R2000; for flat parts see the laser cutting guide, sketch to G-code for a CNC router, Cricut cut files and the features page.
Questions
What is a sheet metal flat pattern?
It’s the part unfolded flat: the outline that gets cut, with bend lines where it folds. Its size allows for the metal that stretches and compresses round each bend, so the folded part comes out at the right size.
What K-factor should I use?
Use your shop’s value if they give one. The K-factor says where the neutral axis sits in the thickness, as a fraction measured from the inside of the bend, and for most bends it falls between 0 and 0.5. The worked check on this page uses 0.4.
How do I calculate bend allowance?
BA = θ·π/180·(R + K·t), where θ is the bend angle in degrees, R the inside radius, K the K-factor and t the thickness. For a 90° bend in 2 mm sheet with a 2 mm inside radius and K = 0.4, BA is 4.40 mm.
Can I send the flat pattern to SendCutSend or OSH Cut?
Yes. Tap Send and pick the shop: PencilCAD writes the flat DXF, with the outlines on a CUT layer and the bend lines on a BEND layer, and hands it off through Files, AirDrop or iCloud Drive. You upload it on their site.
SendCutSend, OSH Cut and Ponoko are trademarks of their owners; PencilCAD isn’t affiliated with them.