How to Get an Accurate Quarter Inch Seam

  from www.About.com

•  Put the quarter-inch or standard presser foot on your sewing machine.
• Align a 2"x4" light strip with a 2"x4" dark strip, right sides together and edges carefully matched.
• Sew the patches together lengthwise, feeding them through the machine with the presser foot's right edge aligned exactly with the right edge of the patches
• Sew another light strip to the opposite edge of the dark strip.
• Press seams allowances towards the dark strip.
• Use a rotary ruler to measure the dark center strip.  It should be exactly 1-1/2" wide along it's entire length. The outer strips should be exactly 1-3/4" wide along their entire lengths.
• If the dimensions of your pieced unit are accurate - congratulations! If the size is off a bit, move on to the next step...

Inspect the Unit and it's Seams

• Were seam allowances pressed adequately? It's easy to 'lose' width in patches that aren't fully pressed. Press again and recheck.
• Make sure aligned strip edges didn't shift away from each other when you sewed the seam. If they did, try placing a very warm iron on top of the patches before sewing to help them stick together or secure edges with very fine straight pins before sewing. Remove pins as the needle approaches.
• Are the seams irregular, wide in somes spots and narrow in others? Sew more slowly to improve accuracy. Perhaps you can set the sewing machine to run on half speed. Sew another set of strips, then press and measure. If dimensions are still not accurate, don't worry, you'll get there with a few more changes. Cut several more 2" wide strips of fabric to use in test units...

Change Needle Position

• If your sewing machine allows it, change the needle position.  Move the needle a notch to the right if you must shorten the seam allowance. Move to the left if you must increase the allowance.
• Sew another test set, again aligning the fabric with the right edge of the presser foot. Press and measure. If seems are still not accurate, you can place a seam guide on your sewing machine's throat plate.
• Before you add the extra guide, look at the grooved lines on the throat plate to see if there's one located at 1/4" to the right of the needle. Sew another test strip, aligning the edges to that line.  Press and measure. Some sewing machines come with a seam guide that can be positioned to the right of the needle, where it forms a ridge to butt the fabric against as you sew.

Here's What To Do if Your Sewing Machine Doesn't Come with a Guide

• Place a rotary ruler under your needle, positioning it's first 1/4" mark just below the needle's tip. Check position by lowering the needle slowly by hand until it just touchs the mark on the ruler.
• Make sure the ruler is positioned in a straight line on the machine - compare placement with nearby grooves in the throat plate.
• Place a 1" piece of masking tape on the throat plate, it's left edge flush with and aligned against the right edge of the ruler. Remove the ruler.
• Sew another test unit, guiding the right edge of the fabric along the left edge of the tape as you sew. Press and measure as before.
• If seams are accurate, stack additional pieces of masking tape on top of the first to build up a ridge of fabrics to butt against, or obtain height quickly by rotary cutting a piece of adhesive-backed moleskin and applying it to the tape.
• If your seams are still off, move the tape and try again. Continue tweaking and testing until seams are accurate.