Sewing Upholstery Basics
Before starting this project please study Basic
Cutting and Sewing, How to Install
a Zipper in a Pillow and Howto Sew a Boxed and Welted Cushion.
Also if you don't want to have a zipper studyHow to Hand sew.
Sewing round shapes presents a few special problems I'll try to address in this instruction segment. The main thing to contend with in any curved sewing is the fact that outer part of a seam needs to become longer than the inner (stitched) part. As you can see below in the photos where the welt has been sewn to the end pieces, the welt fabric has flared open where I've snipped it to become longer on the outer edge.
If you don't have bolster foam you can buy it on our Supplier's Page. In a pinch (I've done it) you can make the filling by cutting, say, a 4" rectangle piece of foam, trimming off the sharp edges with shears, and wrapping it in bonded polyester (Dacron) until the right size. If you use several layers of Dacron it should finish 1 1/2' or more bigger than the cover so it doesn't squish the first time it's used.
I decided not to put a Dacron (poly) wrap on the foam filling but I still wanted a nice tight fit so I made my finished measurements a 1/2" smaller than the filling. You can find the distance around a circle by multiplying the diameter x pi(3.14). Add for your seam allowance or zipper. I used my foam for a pattern for the round end pieces but you could use a compass. Cutting to the outside of my chalk line gave me the finished measurement I wanted.
Notch the top and bottom of the end piece and use it for a pattern for the other end. Mark the top on both pieces.
Sew fabric to one side of zipper as per previous instructions. Then turn and sew the other side to the zipper. If sewing the zipper this way is too bulky, too awkward or just makes you crazy then, try making a separate zipper boxing. Just don't make it so wide it can be seen when finished. Or sew in a 1-2" seam on each end (backstitch) and handsew it closed when you are done filling the cushion.
Fold and notch what will be the top of this cushion.
Sew the welt cord to the end piece just as you would for a square pillow except snip the fabric every inch or so to release the fabric to make the turn. Off-set your starting point so that you don't finish right at the same spot where the zipper will be and cause a bump from too many layers of fabric..
End piece with welt cord sewn to it.
Line up the top notch of the end piece on top of the top notch of the main cushion piece. Get the sewing machine needle between the cording and the stitching. As you sew check a few times to make sure the bottom notch of the end piece is going to lineup with the center of the zipper. You can either go ahead and sew all the way around or flip the whole cushion over and start again from the top notch and sew the plate fabric to the the end piece. Turn right side out and make sure no stitching shows.
Other round sewing:It would seem a simple task making a round cushion for a round dining room chair seat but this can be one of the most difficult projects to get to look right. One problem is with the filling the other with the sewing. Factory made foam fill is often molded and shaped to fit. If the foam is in good condition sometimes it can be steamed a little to expand it back to it's original fullness. Or if it needs a wrap sew 2 half layers together of bonded Dacron and turn the seams to the inside then put the foam in. You can also do this with a new piece of 1" foam. And/or you can make a ticking out of a lightweight fabric and put the fill in there first then into new cover. You can glue instead of sew if you make a ticking as it will help hide the Dacron's edge from showing through the new cover.
Top stitching and installing a welt can both add "body" to a round cushion helping it to retain it's shape. The ends to the above bolster could also have been top stitched instead of welted. On round or shaped cushions (like ones that go around rungs on dining chairs) sometimes it's simply easier to do them with a boxing if you want a nice tailored look.