Quarter Inch Seam Test for Perfect Patchwork

Geometric patchwork quilt designs depend on accurate piecing to achieve their crisp look. If the piecing is askew, the blocks will look wonky. And if the blocks are wonky, the points and angles won’t line up.

patchwork points Night Day quilt kit
Perfect patchwork points depend on accurate 1/4″ seams.

And if that happens, it’s not the end of the world, but you won’t get the effect you were going for.

For accurate patchwork piecing, you not only have to measure and cut precisely, but your seam allowance needs to be consistent.

“Although 1/8″ (6 mm) off may not seem like much, when you add 1/8” across multiple seams, it really does add up!’ says quilter Nancy Purvis in her book Quilting from Every Angle.

In this excerpt, Nancy explains how to make sure your seams are exactly ¼” wide:

Most sewing machines come with a ¼” (6 mm) foot and markings on the throat plate, but I find that performing a simple test of my seam allowance helps me achieve the most accuracy. If you do this simple test, you will know exactly where to sew on your particular machine.

How to test your seam allowance by Nancy Purvis:

quarter inch seam guide1. Place a clear acrylic ruler under the needle and lower the presser foot. Manually lower the needle so that it gently touches the ¼” mark (Fig. 1).

2. Place four to six layers of washi or masking tape on your machine bed next to the ruler’s edge (Fig. 2). This will become your ¼” seam guide.

3. Cut two fabric scraps 2½” × 4″ (6.3 × 10 cm) with straight edges. Align one scrap on top of the other. Sew a ¼” seam, following the edge of the tape. Press the seam open. If the seam is accurate, the unit will measure 4½” × 4″ (11.5 × 10 cm) (Fig. 3). If it does not, check your needle and tape and reposition them as necessary. Test again until you achieve an accurate ¼” seam.

Over time, the tape will start to wear on the edges. I replace mine about every 5 to 6 quilts I make, but it just depends. Do not place tape over machine parts that need to be removed for cleaning, such as the throat plate. If your machine bed is removable, keep that in mind as well.

Note that even if you have an accurate mark or sewing foot, sometimes when you’re sewing, your hands may ever so slightly move the fabric to the left or right, causing the seam to be off by just a few threads. If you wonder whether that’s happening, place a ruler on the seam line after sewing a straight seam. If it is straight, it will run perfectly parallel to any of the marks on the ruler.

I find it also helpful to visualize the location of the fabric under the foot when it is fed through the machine at a perfect ¼”.

There you go! Try this test and see if your patchwork quilt tops don’t look more crisp. Do you have any tips for seam allowance accuracy? Share them with your fellow quilters below.

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