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Thread: Math help - half square triangles

  1. #1
    Super Member LucyInTheSky's Avatar
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    Math help - half square triangles

    I'm working on a pattern where the center of the block (see 1st picture) looks like that. Each piece gets cut at 3 7/8" square to finish at 3", making that whole piece 6" when it's completely finished.

    But, I want the center piece to be a single piece of fabric (see 2nd picture), so rather than doing 4 sets of small HST units (especially since each outside wing will be a different color), I want to make the center into 1 big block. My plan is to do a big square and then cut the little wings into squares, sew them on top, and flip them up (rather than cutting the center center to exactly its size and working with it as a diamond ... I'd rather have a little bit of waste).

    So my thinking is that the center square should be cut to 6.5". Does that seem right? Then each wing still gets cut to 3 7/8"... or would it be 3.5"? I think it would be 3.5.... help! Thanks
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  2. #2
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    sounds right. to make certain, make one just to see how it works for you. that way, after only one, you can cut the rest according to exactly what you need to get the effect you want.

  3. #3
    Power Poster amma's Avatar
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    Your center block needs to be smaller than 6 1/2"
    Draw this on a piece of paper, measure the distance from the top point to the bottom one.
    You will notice that this measurement is significantly longer than the side measurements of the inner square

    Start out with a 6 1/2" square, draw your square on point in the center, cut it apart and then add your seam allowances
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    Power Poster joyce888's Avatar
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    I'm thinking you winged pieces would be cut at 3 7/8". Cut 2 @ 3 7/8"; cut once on the diagonal; match up your centers and if you've got a 1/4" overhang then you know it's right.
    Joyce

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  5. #5
    Super Member LucyInTheSky's Avatar
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    I'm confuzzled... wouldn't the distance from the top point to bottom be 6 1/2" (or maybe 6, without the seam allowance?) If I'm doing a big square and adding squares to the sides for the triangles (a la snowball method), wouldn't I need it 6 1/2"? I know the finished sides will be 6 / sqrt (2), but I don't plan to cut it as a diamond.

    Quote Originally Posted by amma View Post
    Your center block needs to be smaller than 6 1/2"
    Draw this on a piece of paper, measure the distance from the top point to the bottom one.
    You will notice that this measurement is significantly longer than the side measurements of the inner square

    Start out with a 6 1/2" square, draw your square on point in the center, cut it apart and then add your seam allowances
    Nothing's a mistake. It's a learning experience. Some experiences, you learn more than others.

  6. #6
    Super Member LucyInTheSky's Avatar
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    Quote Originally Posted by joyce888 View Post
    I'm thinking you winged pieces would be cut at 3 7/8". Cut 2 @ 3 7/8"; cut once on the diagonal; match up your centers and if you've got a 1/4" overhang then you know it's right.
    Awesome, thanks. It was how much on the overhang I was missing, which, duh, it should be 1/4"
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  7. #7
    Super Member GrannieAnnie's Avatar
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    Quote Originally Posted by LucyInTheSky View Post
    I'm working on a pattern where the center of the block (see 1st picture) looks like that. Each piece gets cut at 3 7/8" square to finish at 3", making that whole piece 6" when it's completely finished.

    But, I want the center piece to be a single piece of fabric (see 2nd picture), so rather than doing 4 sets of small HST units (especially since each outside wing will be a different color), I want to make the center into 1 big block. My plan is to do a big square and then cut the little wings into squares, sew them on top, and flip them up (rather than cutting the center center to exactly its size and working with it as a diamond ... I'd rather have a little bit of waste).

    So my thinking is that the center square should be cut to 6.5". Does that seem right? Then each wing still gets cut to 3 7/8"... or would it be 3.5"? I think it would be 3.5.... help! Thanks
    just remember that the wings are going to be cut on the bias and not too easy to sew evenly------or your straight sides of the wings will be on the bias. at any rate--------quite do-able, just remember you're dealing with some bias cuts.
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  8. #8
    MTS
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    Quote Originally Posted by LucyInTheSky View Post
    I'm confuzzled... wouldn't the distance from the top point to bottom be 6 1/2" (or maybe 6, without the seam allowance?) If I'm doing a big square and adding squares to the sides for the triangles (a la snowball method), wouldn't I need it 6 1/2"? I know the finished sides will be 6 / sqrt (2), but I don't plan to cut it as a diamond.
    Please allow me to add to your confuzzilation.

    The square in your diagram is ON POINT - so the DIAGONAL of the square (point to point) is what would be be 6" FINISHED, not the sides of the square - they would need to be smaller.

    How much smaller?
    The relationship is 1.414.
    For example, a 6" FINISHED block ON POINT will measure ~8.5" across FINISHED.
    6" x 1.414 = 8.5"

    For your block, you need to work backwards:

    6" FINISHED block divided by 1.414 = a 4.25" SQUARE plus .5" SEAM ALLOWANCE = 4.75" starting square for the center of the block.

    And if each side of the square is 4.25" FINISHED when it's on the DIAGONAL, then you'd need a 3" FINISHED square (3.5" UNFINISHED) square to use as the snowball.

    4.25" divided by 1.414 = 3" FINISHED + .5" SEAM ALLOWANCE = 3.5" STARTING SNOWBALL Square.

    Since you're going to be sewing ON the diagonal, you don't need the extra 3/8" for the seam allowance like you do for HST blocks (as there aren't 2 equal seam allowances to worry about).

    There is a slight bit of rounding in the above calculations.

    Too convoluted? Try this other option - no-waste Square-in-a-Square method:
    http://www.hgtv.com/video/super-fast...deo/index.html

    I'd test one out first, starting with 4.75" squares. If you aren't married to the 6" FINISHED - meaning you're not combining this block with different blocks - then you could start with ANY size you wanted. Charm packs of 5" squares would render a slightly larger FINISHED block, but not much at all. And that method makes the sewing a BREEZE!
    Last edited by MTS; 12-19-2011 at 01:11 PM.

  9. #9
    Super Member LucyInTheSky's Avatar
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    Quote Originally Posted by MTS View Post
    Please allow me to add to your confuzzilation.

    The square in your diagram is ON POINT - so the DIAGONAL of the square (point to point) is what would be be 6" FINISHED, not the sides of the square - they would need to be smaller.
    I understand what you're saying, but that's not how I'm doing it (or rather, did it). Rather than cutting the center to exactly its size, I used the snowball method, where you make the back piece the full size (6.5") then sew stuff on top of it. So it did need to be the 6.5". I see how to calculate it with the way you're talking about it, with the dividing by sqrt 2 (which is approx 1.414), but I wanted to deal with a full sized finished piece, rather than bias edges on the wings + matching up with my approximated square
    Nothing's a mistake. It's a learning experience. Some experiences, you learn more than others.

  10. #10
    Super Member LucyInTheSky's Avatar
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    And as an update - finished the block. The big square needed to be 6.5". The little guys were 3.5" if they were sewn on as squares, but I ended up sewing them on as cut HST because of the fabric I had (BOM), so they were 3 7/8".
    Nothing's a mistake. It's a learning experience. Some experiences, you learn more than others.

  11. #11
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    Quote Originally Posted by LucyInTheSky View Post
    And as an update - finished the block. The big square needed to be 6.5". The little guys were 3.5" if they were sewn on as squares, but I ended up sewing them on as cut HST because of the fabric I had (BOM), so they were 3 7/8".
    Wasn't that hard, after all, was it?

    Good job on figuring it out.

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