Use a shorter stitch
If each stitch is a straight line, it stands to reason that shorter lines can shape a smoother curve. This is true in every type of curve, from convex to concave, structural to topstitching. Shortening your stitches will help you craft clearer quilting designs and smoother curves of decorative stitching. In garment making, curves are also frequently stress points—think of sleeves and armscyes, princess seams, and collars. In these cases, shortening your stitch length helps distribute that stress more evenly, preventing split seams, fraying, and other wear and tear.
Mark your seam allowances flat
Curves are tricky to sew in part because they must, necessarily, lie in part on the bias. This makes them unusually stretchy in places, and easy to warp out of shape. Before you start stitching, take time to measure your seam allowances and clearly mark your seam lines. As you manipulate the fabric through stitching the curve, you’ll always have a clear reference point for where your needle should land. If your fabric is particularly soft or if you will be manipulating it quite a bit before sewing, you will want to add a line of stay stitching. This is a simple line of stitching set into the seam allowance of each curve. As your stitching is not as stretchable as the bias of your fabric, stay stitching will keep your curved pattern pieces from distorting while you work.
Pivot on tight curves
Planning to sew tight curves, like scallops or the outlines of applique shapes? Practise using this technique to keep these seams crisp and clear. When a curve is too tight to comfortably follow by shifting the fabric, stop your machine with your needle down, raise your presser foot, and readjust your fabric so that the seam is now headed in the proper direction. If you plan to do lots of work like this, it can be helpful to use a machine with a programmable needle up-down function (meaning it automatically stops in the position you choose), and a knee-lift lever, which allows you to raise and lower your presser foot without taking your hands off your work.
Use narrow seam allowances
Nothing throws off the smooth line of a curved seam like bulky, lumpy seam allowances. While your pattern may call for consistent seam allowances throughout your project, you may want to trim down the seam allowance inside your curves once you’ve stitched them. This may remove your stay stitching, if you used any, but once your seam is in place, there’s no need for it any more. Don’t cut too close to your seam line; a quarter inch seam allowance should be narrow enough to avoid fraying as well as lumpy seams.
Clip and notch your curves
This is one of the most basic tips to preparing your curves once they’ve been stitched. Convex curves should be notched, removing tiny triangles of fabric to keep the seam allowance from bunching when it’s turned. Don’t get too close to your stitch line, or you’ll risk snipping a stitch and fraying your seam. With garment making, wait until your fit is perfect before you notch your seam allowance. Though notched seams can be taken in relatively easily, they are very difficult to let out
For concave curves, instead of notching, all you’ll need are simple slits in the seam allowance. For a stronger seam, however, you should clip only one side of the seam allowance at a time. Angle your clips so they are not directly perpendicular to the line of your seam; this decreases the tension placed on the clip when the curve is turned right side out.
Probably the single most important trick for getting curves to look their best is pressing them well. Of course, different kinds of curves require different kinds of pressing. Garments with curves, like dresses and fitted bodices, should be pressed using atailor’s ham, which is a soft of firm pillow that provides the appropriate shape for the curves while you’re pressing them. If you tried to press them into shape against your ironing board, they’d end up squashed flat! For some curves, though, such as quilt blocks with curved shapes, that’s exactly what you’re going for. Press all your curved seams using plenty of steam and a spritz of water if your fabric can tolerate it to help the fabric lay perfectly.
Keep the correct side up
When joining convex and concave curves, you will have to ease the fabric of the concave half around the convex side. While the seam lines should line up, you’ll have extra fabric just inside the seam line. This is easier to deal with if it’s not hidden on the underside of your work. Always stitch these curves with the convex side down so you can carefully manipulate the extra fabric and keep in out of the way. Work slowly! Catching extra fabric in your seam will result in puckers and lumps.
Curves are tricky, with fabric pulling this way and that. If you find yourself wishing you had extra hands to hold everything in place, reach instead for your pins! Curved seams benefit from much more dense pinning than regular straight seams, as it’s important to keep your seam lines in place even when the edges of your fabric don’t necessarily line up. Long, sharp pins are best since they can easily be placed and removed, and pinning perpendicular to your seam, rather than along it, will help you remove pins as you sew to avoid hitting any with your sewing machine.