To make curved seams lie flat when stitched it needs to be treated in a special way and it differs for concave and convex curves.
For concave (inward) arch slit seam allowance 3/4 way to the stitching line. It will let the fabric to spread without pulling and tagging.
Sewing convex (outward) curve you will have to cope with an extra fabric creating a bulge. Notch small triangles out of seam allowance, do not go deeper then 3/4 of seam allowance width to avoid cutting through the seam. Use small pointed scissors. The sharper the curve the more notches you will have to make. Don't be tempted to put a bigger notches, as it will most likely result in a nibs and bumps when turned right side out.
Smaller stitch length on your sewing machine allows you a better control over the fabric and insures a smoother curve.
Examples of curved seams:
- Facing the neckline and armholes.
- Attaching the lining to the pockets.
Sewing two convex shapes together try to alternate notches on fabric layers .
- Sewing the waist band to a circle skirt.
- Creating complex designs.
When you are turning two convex shapes (lined pocket for example) right side out run the blunt pointed object along the seam. Don't apply to much pressure, as you don't want to reap the seam or poke a hole through the fabric. Your goal is to define the edge and smooth away any possible bumps. It is important to do before pressing, after all you don't want to enhance imperfections and fix them in place.
Every seam needs to be iron, and the curved seams are doubly so. It will set the fabric into shape. The pieces should look like they grew together without any pulling or stretching, lumps or bumps. So, press and steam your curved seam carefully in order to insure a sleek professional look.
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