I hear so many say they use their 1/4" foot on their sewing machine so it must be accurate and right. NOT!!!! It can be but I'll bet it isn't. Mine sure wasn't! And the 1/4" foot isn't the only thing that comes into play when trying to get an accurate 1/4" seam. In reality, it's not really about the accurate 1/4" seam, it's about getting an accurate seam allowance that when pressed, results in the needed size block. Believe it or not, the thread you sew with, the thickness of the fabric you are sewing, and how sharp you press the seam all play a role in getting the desired results.
This example below is an example of what my seam allowance is using my 1/4" foot on the top one and my adjusted seam allowance on the bottom. There isn't much difference but it's enough to wreck havoc with the fun of putting together a quilt when the pieces won't fit.
This hows the two pressed open. See how much difference that becomes when you multiply it by the two blocks.
So how do you get an accurate 1/4" seam? It's really very simple. Take two identical pieces of fabric and sew them together using what you think is 1/4" seam allowance. Press the seam allowance open and then measure the block to see if it adds up to the needed size.
In my case, I'm using a 3 1/2" block. Two sewn together should measure 6 1/2". In my example above, the incorrect one using the 1/4" foot measured 6 1/4". That 1/4" too short, which isn't much. I more than likely could ease in 1/4" and make it fit. But what if that were a 1 1/2" square being sewn together. Now I have to ease in 1/4" in a 2 1/2" space instead of 6 1/2" space. Not so easy to do in a shorter space. See what I mean?
So if you can't get an accurate 1/4" seam allowance using the 1/4" foot for your sewing machine, how do you fix it? You have a couple of options. In my case, I move my needle position over one notch on my sewing machine settings and it fixes my problem. If you can't move your needle position, take a piece of tape (I like to use that wonderful 3M painters tape and the King likes it too when we do. wink wink.) and place it where it's needed for your guide to stitch the seam.
Remember that example above... I made a 9 patch block example below. The one on the right is sewn with the correct seam allowance to get a 9 1/2" unfinished block. The one on the right is sewn using the 1/4" foot. Now that 1/4" difference just became 1/2".
And look what happens when I try to sew two 9 patch blocks together.... They're off 1/2" and the seams don't match.
And what happens when I try to sew a 9 patch block to a solid block that is cut 9 1/2". It should fit but it doesn't. Again, a disaster waiting to happen.
Since I pretty much use the same thread on all my piecing projects, and I've figured out my needle position, I rarely check my 1/4" seam unless I notice that my fabric is a definite difference in weight (and even though it's all "quilt shop quality" fabric, there IS a difference in weight sometimes). I will almost always check it if I'm working with really small pieces also. By doing this, I can switch from one sewing machine to another on the same project and still have happy results.
I hope the Tips, Tricks, and Tuts Tuesday on 1/4" seams help.... and may all your 1/4" seams be accurate and your bobbins be full!