williamskay wrote:
I think I'm really dumb! I've been using paper pieces to cut HST triangles just to see what works. I do want to do HST triangles around each block, but when I use the formula Barbara suggests adding 7/8" to the finished 6 inch size, (making the square I cut from 6 and 7/8th), the resulting HST has 1 and 1/2 inch hang over on each side--too big! Alex's 5 and 1/4 inch square works, but why? It's not the "magic" formula. And I guess the real question is--will the 5 and 1/4 HST work on all the blocks (on point)?
When using the add 7/8" rule, you are adding the 7/8" to the length of the short side of the triangle. If you look at the half-square triangle (HST), there are two short sides and one long side. When you cut the HST from a square, the short sides are the outside edges of the square and the long side is the diagonal that you cut. When adding HSTs to a square to put that square on point, it's the long side of the HST (so the length of the diagonal that you cut) that ends up being sewn to the square. In this case, the blocks are 6" finished, so your HST needs to have the long side be 6" finished, not the short sides. To calculate the finished length of the short sides (all math in quilting must be done using finished lengths, not cutting lengths) from the finished length of the long side, we go back to geometry, which tells us that the length of the long side of a HST is 1.414 times the length of the short sides. So, the finished length of the short sides is 6"/1.414 or 4.243, which rounds up to 4.25" finished. Now we know we need HSTs with 4-1/4" finished short sides, so can use the 7/8" rule to figure out that we need to cut 5-1/8" to get the needed HSTs. Cutting the squares at 5-1/2" gives you a little more wiggle room to either trim the blocks to size or let the points float a small amount, if you have perfect seam allowances.
Alex's method is to use graph paper and draw the block (again, at finished size) then draw the HST, add the 1/4" seam allowance with a ruler and then measure the short side to determine the cutting measurement.