Here are a few tricks to stitch a perfect quarter inch
First, be sure to always cut your pieces as accurate as possible. Sometimes simply the width of a marking pencil is enough to cause inaccuracy. When using acrylic rulers and tools for cutting be sure to include the line in measuring. If you are cutting a two inch strip -- the previous cut edge of the fabric should include the two inch line in your strip.
Purchase a special quarter inch piecing foot at your sewing machine dealer. Take some scraps of fabric and ask if you can try it out on their demonstration machines. One style might meet your needs more than another. I have a "generic" patchwork piecing foot that I prefer to the one that was supplied with my brand name computerized sewing machine.
If you need to use a presser foot you already have and it isn't quite accurate here are some things you might try.
Machine setting -- If your machine has settings for different needle positions, simply set your needle to the left or right so your stitching line is exactly a scant 1/4" from the edge of the presser foot. Program this into your machine memory so you can touch a button every time you turn on your machine. If your machine doesn't have a memory, write your special piecing setting on an index card or sticky note and keep it near your machine.
Make a raised area to guide the fabric -- Place the quarter inch line of a small ruler under the presser foot with 1/4" showing at the right of the needle. Turn the hand wheel on the machine until the needle touches the 1/4" mark on the ruler. Move the ruler back and forth as necessary until the 1/4" mark is exactly below the point of the needle. Cut a piece of masking tape about two inches long and place it on the bed of the sewing machine butted up to the right edge of the ruler. Place additional layers of tape to build up a thickness so you have a raised area to guide the fabric accurately. The same could be done with a stack of paper sticky notes.
An alternate to masking tape and sticky notes would mole foam -- a foot care product available in the pharmacy. Cut a piece about 1/4" wide and two inches long. Peel the protective paper and stick the mole foam on the machine bed next to the ruler.
Stitching pieces together -- Begin stitching squares or strips by placing them right sides together under the presser foot. Line the cut edge of the fabric to the edge of your foot or your masking tape or sticky note line. Recommended stitch length for machine piecing is 10-12 stitches per inch. If you have a metric machine set your stitch length at 2.5 to 3.0. After stitching a few seams you may want to change your stitch length slightly to your preferences.
Begin stitching and stitch carefully guiding the fabric through the machine. Simply guide the fabric -- don't push or pull. When the seam is stitched, do not cut the thread. Continue adding pairs of patches until you have a string of patches hooked together with a small amount of thread between. Cut them apart. This is called chain piecing and will help you piece quickly as you don't have to stop and cut thread each time and you don't have to start a new seam.
Sometimes when stitching the first seam, the thread will jam in the machine. To avoid this, pull out about seven inches of both top and bobbin thread and hold it toward the back left of the machine. This will keep the threads from going down into the throat plate and jamming. A little extra thread is worth a lot in frustration.
For piecing blocks I work in rows if possible. This works for blocks and also for larger quilts. First I make small units as the pattern specifies. Then I stitch all the vertical seams first following the lengthwise grain of the fabric. The lengthwise grain of the fabric has no stretch whereas the crosswise grain has a slight stretch when it is pulled.
Line up all your patches with the lengthwise grain of fabric in position to be stitched. I stitch the first vertical seam by chain piecing. I don't cut the threads between pieces. I have several rows of two squares stitched together with threads between. Next, I stitch the second vertical seam. Now I have several rows of three squares stitched together with threads between. Continue until all vertical seams are stitched.
Then turn the quilt a quarter turn and start stitching the horizontal seams. Now if all your patches were stitched on the lengthwise grain in the vertical seams, all your horizontal seams will be crosswise grain seams. The crosswise seams will have that small amount of stretch that will work for you. I don't clip the threads from the vertical seams. I just fold the seam allowances in opposite directions. The seam allowance on the bottom facing machine bed is turned toward me and the seam allowance on the top toward the machine foot is turned away from me. Begin stitching. As the top seam allowance turned away comes toward the seam allowance that is turned toward you the machine foot actually pushes that seam allowance together and locks them together.
Stitching the vertical seams on the lengthwise grain of fabric first and then the horizontal seams on the crosswise grain next -- works well both when stitching simple squares into a large quilt, stitching complete blocks together, or actually the piecing of blocks.
Careful pressing is also important to accurate piecing. If the pieces are not pressed carefully some inaccuracy can occur. A lot of patterns call for a scant quarter inch seam simply because a tiny amount of the seam allowance is taken up in the fold when the fabric is pressed.
It is usually recommended to press seam allowances toward the dark fabric, but with machine piecing this is not always possible or necessary. When piecing four squares into a four-patch it is sometimes better to press one seam allowance in one direction and the other in the opposite direction. Then when you are stitching across the seam the bulk is distributed evenly.
It is OK to press to the light if it is necessary to keep the build up of numerous intersecting seam allowances less bulky. If the dark color shows through slightly, you might want to trim a slight amount from the dark seam allowance allowing the light color seam allowance to cover the darker seam allowance.
A good rule to follow especially in strip piecing is to press in the direction stated in the pattern. Most pattern designers carefully plan pressing directions for ease of future assembling so you would want to follow their directions. That way when you are working on the next step you will have your seam allowances pressed in the direction required to continue the piecing process as the designer planned.
When piecing individual blocks and the block has a center area it is usually recommended to press seam allowances toward the outside although this is not an absolute rule. Also if several seams are intersecting from one direction and the other direction has fewer seams, press in the direction of the fewer seams.
At other times you might want to press seam allowances toward the piece you want to "come forward" or stand out. By pressing seam allowances toward these particular pieces they become prominent because the pieces surrounding them tend to recede.
Pressing seams open
Did you know it is OK to press seams open? When we rotary cut our pieces our seam allowances are a standard width so there is no reason to be required to press to the dark or one side.
Today's quilter has a lock-stitch sewing machine that makes a very firm and tight seam. Sewing machines are available to most quilters. Many quilters began sewing by making garments. We used to stitch the back seam of our trousers and press the seams open! Then we wore them for several years and sat on that seam and gained weight in those pants and still the seam didn't split open. All men's dress trousers have the back seam pressed open. It is hard for me to imagine why quilters do not press seams open as they will lay much flatter. These rules have been carried over from hand piecing.
Many professional quilters press all their seams open. Machine quilting of our modern day quilts help stabilize the stitching of the piecing. Our machine made quilts are stronger than the hand made quilts of yesteryear.
The direction you press your seams is also determined by how you will be quilting. If you will be machine quilting "in the ditch" you will want to press seams away from where you want to quilt. This way you are quilting only through the three layers of the quilt "sandwich" rather than the additional layers of the seam allowance.
If you will be "outline" quilting (1/4" from seam) it really doesn't matter the direction of pressing for quilting purposes because you will not be quilting in the area where the seam allowance is.
If you find your seam allowances are "twisting" remember that once some of the seams in your block are pressed you will need to be careful to not turn them in the opposite direction when stitching a new seam.
Look at your block carefully and decide the direction you wish your seam allowances to be pressed rather than following any particular rule. As you become more experienced at piecing, pressing will become easier and you will know the direction to press each seam allowance.
Rotary cutting, machine piecing, and pressing should be as accurate as possible making your final pieced top very flat. A very flat top will be much easier to quilt and will look great when it is finished.