Concave Curve Template, Quilting Ruler & Hourglass

A rectangular template with a curve bitten into any edge you like. Three different curves and a straight edge on the default, or set your own.

Files

• STL: concave_curves_mw.stl. Plain mesh, any slicer.

• 3MF: concave_curves.3mf. Bambu Studio project with my tested settings (PrusaSlicer opens the geometry).

• SCAD: arc_slab_mw.scad. The parametric source, for any other size.

What it is for

Quilting first. Lay it on the fabric, draw round the curve you want with a marker, cut along the line. There is no seam allowance in it and no offset applied, so the edge you draw against is exactly the line you get. Three different curves and a straight edge on one piece means one template instead of four, which matters when they live in a drawer.

Away from sewing the same shape earns its keep as:

• an hourglass or bow-tie silhouette for a sign, a tag or wall art

• a cotton reel or spool outline

• a waisted coaster, trivet or luggage tag blank

• a curve template for marking concave cuts on timber, card or leather

• a French-curve substitute for gentle arcs, which is what it effectively is

Need a different curve? Customise in OpenSCAD

Download the .scad file and open it in OpenSCAD (free). Every option below is in its Customizer panel.

Each of the four edges takes its own arc:

• Arc diameter. Stated as a diameter, the way curve templates are sold. Zero leaves that edge straight.

• Depth. How far the curve reaches. Positive bites in, negative bulges out.

• Position. Where along the edge the curve centres, half way by default.

For the whole piece: width and height in inches, thickness (1/4 in by default, matching shop-bought templates), a hanging or finger hole, corner radius and a chamfer on the top face.

Why it is shaped the way it is

The bottom edge is square and vertical, and the chamfer is on the top face only. If the underside is rounded, a pen tip creeps beneath the edge and you draw inside the true line, so every piece comes out slightly small.

It refuses bad settings rather than printing them. Ask for an arc with more chord than the edge has, a bite deeper than half its own circle, a slab that pinches to a hinge, or a hole with no material around it, and it tells you what is wrong instead of handing you something broken.

Pairs with

• Crescent Templates: long, gentle arcs for borders and swags

• Circle Templates: rings for 4, 6, 8 and 10 in circles

• Quarter Circle Template: drunkard's path and quarter-circle blocks

All of them are in my Quilting & Sewing Tools collection.

Print settings

• Material: PLA, or PETG if it will get knocked about

• Layers: 0.2 mm, or 0.12 mm for the smoothest drawing edge

• Walls: 4 to 5, because the edge is the functional surface

• Infill: 15% gyroid. The walls make the edge, so more infill only adds weight

• Supports: None, flat on the plate as oriented

• Elephant foot compensation: 0, and no first-layer chamfer

• Wall order and seam: Outer wall first and a scarf seam, so the drawing edge stays clean and true. The 3MF here already has both

The design decisions and the testing here are mine. The OpenSCAD was written with Claude, and every version was rendered and measured before it went up.

If it is useful, a like and a make with a photo really help. I read every comment.

Also on MakerWorld: https://makerworld.com/en/models/3323882