What is it about Half-square triangles that make them so useful yet so frustrating? I have a few thoughts on that conundrum.
First of all, you can do so much with them. The traditional Churn Dash, Shoofly and so many other blocks utilize these versatile units. Second, a simple change in color placement can create secondary designs like pinwheels and squares on point.
However, frustration can arise when the bias edges get distorted or seams aren't sewn perfectly straight. If two triangles are sewn together to create a HST, then the pointed corners can get pulled down into our machine and cause a mess of tangled thread and wadded up fabric. If the seam isn't sewn perfectly, the HST may not end up the size needed.
So what can be done to ease or eliminate the frustration factor of this common unit. I have some suggestions. Some of these require special tools are supplies but most are doable with a simple square ruler at least as big as the unfinished size of the unit and a pencil.
To make a HST, the simplest way is to sew two triangles that are half of a square together along their long sides. The trick is know how big to cut the original square so that the finished size of the unit is correct. The answer is 7/8". So, for example, if the finished size desired for a HST is 3", the fabric used should be cut into squares that are 3-7/8". Each of the squares would then be cut in half along a diagonal from one corner to another. Here is where it can get tricky. The long sides of the resulting triangles are bias edges. That means they are going to be stretchy. It is very easy to distort the shape of these pieces.
Fortunately, there is a way to minimize the stretch. Use plenty of starch. Starch will stabilize the fabric and make both cutting and piecing easier and more accurate. Now, when I say starch, I don't just mean regular starch. I am using it as a generic term that would also include such products as Best Press. Whatever product you choose to use, read the directions for it and follow them for best results.
Once you have your 2 triangles prepared, simply layer one on the other with right sides together. Sew them together using an accurate or scant 1/4" seam. Note that a scant seam means to sew inside the seam allowance by 1-3 threads. The thicker the fabric you are using, the further away from the exact 1/4" you should be. Be sure to sew straight without wobbling the seam.
Once sewn, set the seam and then press it toward the darker fabric. Be sure to press on the right side to be sure that you haven't created a pleat. Your HST will have dogears that you can snip off with scissors or trim with your rotary cutter.
Check the size of your HST using a ruler with a 45 degree line. That marking should be lined up with the seam in your unit. Is the unit the correct size? I sure hope so. It will make the rest of your piecing easier and quicker.
If it isn't correct, is it too large? If so, trim it down keeping that seam line on the 45 degree marking. This also means that you have sewn too far from the exact 1/4" for the thickness of your fabric. Or is it too small because you sewed the seam too large? In this case, you may need to rip out the seam and try again or set this unit aside and make a new one.
Well, enough for this post. I'll be back with other methods to make HSTs soon.