When I sew something I haven’t done before I experiment first. It started when I saw a motorcycle seat by Roxan Jane. She has inserts sewn in the seat. I decided a half circle would be a good first step before trying one without an open end.
I found it harder then it looks. I still am working on technique. Maybe some of the more experienced trimmers can help me. First I did the layout.
Then sewed it together using the match marks.
The strangest thing was happening. When I was done they were offset( pic #3). After multiple attempts I tried something different.
The way I compensated was to lead piece #1 by the amount of offset I observed when I used the match marks. It works. There must be a better way to either sew or layout this half circle. Btw, the circle is a 2 1/2” radius.
Small radiuses like that are always going to be a challenge. When sewing circles I find it best to sew in two halves from the center point out. Relief cuts in the selvage help and cutting and sewing exactly 3/8" seam allowance is critical.
Hi Naseem- thanks for the reply. While I usually us 1/2” I did reduce to 3/8 for this. That helped. Most of my practice ones did have relief cuts. I tried a couple without cuts to see how it effected the problem. No change. As far as starting from the center I would agree it is a more practical way to approach it. Part of the challenge was to correctly lay it out so I could start sewing from one end to the other. I think the problem is in the way it is sewn to lay flat as opposed to boxing/riser. Since the center of the radius is on opposite sides of the needle it causes an offset if you line them up. Yes, small things are hard. This problem does not show up in our marine work due to the larger radius. (See below) Btw, the guy I work for says I over think things. But that is part of the fun.
It's always easier for me to sew the band (outer circle ) to the insert. You'll find that pulling very slightly on the band as you sew makes your marks line up better. Your picture shows it the opposite way. Like Naseem said, relief cuts, but more of them helps.
Summarizing what Jack, Naseem and Matthew said, the concave edge should be sewn to the convex piece edge. In other words the upper piece (concave edge) should be manipulated over the flat insert (convex edge). The relief cuts are very important to alleviate the sewing procedure at a slow pace.