I found the basics of this technique here.....
https://www.youtube.com/watch?v=alReWeIynGw and she explains it very well and opens up all her seams for perfectly flat blocks.
Being really lazy, I shortened a couple of the steps and also attempt to explain in this tutorial how to use a 3 1/2" ruler and rotating mat to make it go even faster. Even if you plan to trim by hand, you can still use the sewing method.
So, here goes.
I am making a 3 1/2" 4 patch, so I will be using a 4 1/2" square for each four patch and trimming. There is some waste, but I can never get them right otherwise. So, always add 1" to whatever size four patch you are making.
For this method, cut a 4 1/2" strip WOF of both fabrics that you want to use for your four patch.
Place them right sides together and sew a 1/4" seam down either side to make a tube.
Put the tube on your cutting mat and cut the tube every 4 1/2". This will create 4 1/2" squares.
Take one square and turn it so it is on your cutting mat with one opening facing you. The 1/4" sewn seams on either side should line up with the main printed "inch" lines on your cutting mat. (See the picture) (Note: Not sure this works with every size, so you should measure with other sizes. You want to cut right down the centre.)
Cut between the sewn seams exactly in the middle. (At the 2" line) You will create two rectangular pieces with both of your fabrics facing. Iron them open, seams to the darkest fabric.
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Take your opened rectangles and place two together, opposites facing. Because they were all ironed to the dark side, it will be easy to get the points to match by nesting the two seams together. Do this on both ends to get them matched up as well as possible. Sew across the first side, so you are sewing across the seam...chain piece the rest. Don't be too concerned if the open sides of the fabric don't match perfectly...it will work out.
When you get to the end, snip them all apart, and go up the other side, again nesting the the two seams together.
I never used pins or anything else and got 99% good points.
Snip them all apart and take them to your cutting table.
Again, take one square and turn it so it is on your cutting mat with one opening facing you. The 1/4" sewn seams on either side should line up with the main printed "inch" lines on your cutting mat. (See the picture) Cut between the sewn seams exactly in the middle at the 2" line. (For the 4 1/2" size, otherwise measure exactly in half) You will end up with two four patches.
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Then it's time to trim. The easiest way is with a 3 1/2" ruler and a rotating mat. It took me awhile to figure out the lines on the ruler so I'll write it out in case someone else has the same problem.
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In the first picture, there is a big "right angle "L" you can see in the very centre (near the hole) That is the very centre of the block. The point of that L needs to go on the point of your block and the arms on the lines of your block. If you look past the L, you will see more lines that continue out, those lines need to line up with the seam lines of your block. Don't worry about what is going on fabric-wise around the outside of the plastic, (See picture 2) as long as the whole block covers fabric. As long as you have those lines lined up with the seam lines, the block will come out square. Once you have cut all around the block, put down your cutter and pull the bits of fabric away from the plastic so that if anything isn't quite cut through, you have a second chance to make a cut without disturbing anything.
Ta da! A perfect 3 1/2" square and they are fast!
And, check out the points!
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I hope this helps..
Watson