9 patch blocks
 

I'm trying to make an Irish chain quilt for a friend...easy right? Yet, I've never had so much trouble getting blocks to come out the same size, and my brain is on a tropical vacation somewhere!! What should 3 2 1/2" blocks sewn together equal?? And what method do you use to make sure you have an accurate 1/4" seam allowance. It appears I'm going to rip everything apart and start over again.
I'm using a thicker thread than the normal 50wt that I usually use for the piecing....but still, if I'm cutting right and sewing consistently shouldn't I get the same size blocks??
Thanks in advance!!

I think 6.5"
2.5-.25+2.5-.25-.25+2.5-.25=6.5 unfinished.

Thread definitely affects size of blocks. For me, it also affects accuracy. Thinner thread such as Aurifil 50/2 works much better for me than more traditional 50/3 ply thread.

I use strips of adhesive moleskin on the bed of my machine to provide a physical guide for me seam allowance. I use my favorite ruler with fine lines, place it under the pressed foot, lower the needle until it touches just to the right of the 1/4" line, lower the pressed foot to hold the ruler in place, and check that the ruler is running straight from front to back. I cut strips of moleskin with my rotary cutter and a ruler, peel off the backing on a strip, then butt he moleskin up against the ruler to position. I usually place another strip of moleskin on top.

Sew three 2.5" strips together, iron, and they should measure 6.5" across the width.

Yes, in a 9-patch you should be getting consistently sized blocks if your seam allowance is consistent - and if your patches are all the same size, you're using the same thread throughout, and you are pressing them all flat. So check your cutting and your pressing. Like Prism, I've switched to thinner threads for piecing. I use Bottom Line, a 60/2 thread. I also like that the bobbin holds more with a thinner thread, so I don't run out as often.

Fabric and thread weight will affect your seam so it's a good idea to check at the beginning of every project. Here's how to check:
http://www.quiltingboard.com/tutoria...ce-t89997.html

What is the issue you are having? Too big? Too small? Every thing in between? Are you just off a thread or you can still see it on a galloping horse? Making 9 patch is an excellent way to judge your accuracy, every time!

I make and use a lot of 9 patches and I have learned that most of my non-matching problems come with the pressing. I seem to be prone to pressing in a lot of little extra folds on my seam lines.
I used to always press from the back first, then from the top, and that is where so many of my pressing errors occurred. Now I press the front first, making sure there are no tucks or folds at my seam lines and my accuracy in matching squares and seams has improved at least 75%.

How frustrating. Size of block 6 and half inches.
By the time you solve the problem you will be extremely accurate at 1/4" seams.