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Thread: Pinwheel math question

  1. #1
    kbiederman's Avatar
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    Hi there! I have a pinwheel math question for you. I used two Moda charm packs to make "fast and easy pinwheels" from the Missouri Star Quilt company. If you have not seen it, it is ingenious. You lay two charms together, light and dark, and sew 1/4 inch around the entire charm. Then you just cut them diagionally from one corner to other, both ways. Sew the four pieces back together and what I am left with is about a 5 3/4 inch pinwheel. They turned out pretty well, only a few points that didn't match up as planned.

    What I would like to do is sew one same fabric triangle, to every side of each of my blocks so that I will have a bigger block, and the pinwheel would be on point. If my blocks are all about 5 3/4 inches, how big do I need to make the triangles, so that 4 triangles sewn together will be the size of my pinwheel block? Is hard to keep it from getting all wonky?

    Thank you in advance!
    ~Karen

  2. #2
    Super Member BKrenning's Avatar
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    If you lay another 5 3/4 inch square face down on top and sew around all 4 sides then carefully cut an X on the square only (not the pinwheel) up to the stitching line, that should work. It's a technique called Exploding Pineapple but it should work for your pinwheels as well. There was a tute on this board but it was called something like Spanish pineapples in my Shampoo or something to keep it from being confused with a certain famous quilt designer & author. Not to stir up anything but a technique cannot be copyrighted!

    Your edges are going to be on the bias, though so starch, starch, starch those squares first and be gentle when handling the edges.

  3. #3
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    One difficulty is your statement above that says "what I am left with is about a 5 3/4 inch pinwheel. To make successful pinwheels the measurements should not be "about x inches". So. suggest you measure all and cut them to be consistent. If 5 3/4 is the smallest then make sure all are that size. If not, then recut size to 5 1/2". There are formulae to determine the side triangles but I can never find them. Therefore, what I would do is to draw a square on graft paper the size the finished pinwheel should be and then draw the side triangles - again the finished size. Then add your 1/4" seam allowance and determine the size of the side triangles. If you leave the unfinished size at 5 3/4 then the finished size of the pinwheel block will be 5 1/4 which is an awkward size to work with. Cutting them to 5 1/2 would be easier for me to work with. I found that accuracy and consistency in seam allowances on pinwheels is most important.

  4. #4
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    Quote Originally Posted by Holice
    One difficulty is your statement above that says "what I am left with is about a 5 3/4 inch pinwheel. To make successful pinwheels the measurements should not be "about x inches". So. suggest you measure all and cut them to be consistent. If 5 3/4 is the smallest then make sure all are that size. If not, then recut size to 5 1/2". There are formulae to determine the side triangles but I can never find them. Therefore, what I would do is to draw a square on graft paper the size the finished pinwheel should be and then draw the side triangles - again the finished size. Then add your 1/4" seam allowance and determine the size of the side triangles. If you leave the unfinished size at 5 3/4 then the finished size of the pinwheel block will be 5 1/4 which is an awkward size to work with. Cutting them to 5 1/2 would be easier for me to work with. I found that accuracy and consistency in seam allowances on pinwheels is most important.
    I agree about the "about/approximate" measurements.

    IF the edges are 5-3/4 inches (finished size of the unit would be 5-1/4 inches) I would cut squares approximately 4-5/8"

    (5.25 x 5.25) / 2 = the square of one side = 13.78125

    The square root of 13.78125 = (approximately) 3.7123

    3.71 + .75 = 4.585 = (approximately 4-5/8)
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  5. #5
    kbiederman's Avatar
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    Thank you so much for your help, I have been busy trying to get it together...I'll post pics when I get it done!!

  6. #6
    Super Member AnnaK's Avatar
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    Good advice. I usually cut the half square triangles a little bigger so that I have enough room to square up without cutting off my points.

  7. #7
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    Quote Originally Posted by AnnaK
    Good advice. I usually cut the half square triangles a little bigger so that I have enough room to square up without cutting off my points.
    Me, too.

  8. #8
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    I usually use the enlargement factor. Example: 4.5 divided into 5 = 1.11 and multiply all the other dimensions by that factor. ...and I thought High School Math would be useless in real life. HA! lol

  9. #9
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    Quote Originally Posted by MadQuilter
    I usually use the enlargement factor. Example: 4.5 divided into 5 = 1.11 and multiply all the other dimensions by that factor. ...and I thought High School Math would be useless in real life. HA! lol
    The enlargement factor works well for the "finished" part of the block - but if one enlarges the entire thing, then the seam allowances get too big.

    I use the pythagorean theorem a lot when trying to figure out triangle sizes. I also use graph paper a lot. Sometimes the graph paper is easier!

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