Strip piecing?
 

Ever time I strip piece the stitching pulls up the fabric and the whole thing gets a bow in it. We cut the strips with lengthwise grain as suggested by Judy Martin. I have Bernina Artista 640, a Bernina White Pearl, a Viking Mega Quilter, a vintage Singer Feathweight, and it's big sister a vintage Singer 301. I try to feed the fabric loosely and guide gently and have a fairly short stitch length. Same problem with every machine so it has to be something I'm doing wrong. WHAT IS YOUR SECRET??? I'm taking a class Saturday on a strip pieced pattern. And expanding the pattern to make a huge King Size quilt for my own bed. (I'm tired of having king size blankets not be wide enough. We have 2 twin adjustable sleep number beds shoved together to compose a king so we need extra width.)

Sew your first two strips then on the next strip start at the other end of your pieces on every strip..it was the first thing I was taught in a strip piecing class I took.

I tried cutting my strips parallel to the selvage.

Also is it your ironing/pressing? I was told when pressing strips to not put them horizontonally on the ironing board but vertically and press

I agree with Crqltr, alternate which end you start stitching from. That's how I was taught in a class too.

I agree on the alternate sewing. It is not anything you are doing wrong. This is a very common problem (the bowing of strips) in strip peicing. By switching directions of sewing every other strip you even out the bow somewhat. Many bargello patterns include this step in the directions.

Edited to add this link http://craftnectar.com/2011/06/30/ev...sewing-strips/

See, it is NOT just you! ;)

I don't feed fabric loosely. I hold it taut for about 6 or 8 inches while it is going thru the needle. Don''t know if that helps or not.

I agree with the alternate sewing, too. Also check your tension and stitch length. Too tight tension can cause all sorts of problems. Very short stitch length will also especially if you're not holding the fabric taut. Want to know how I know?

I'm wondering if the presser foot pressure is too high? I have a Bernina 640 as well, I'm surprised you have this while using the 640. When you find a solution let us know what worked?

I've not had issues with bowing but I'd suggest following the other suggestions of alternating your sewing directions. You might also want to lengthen your stitch length a tad. I'd think too short a stitch length would add to the problem. Not sure I'd adjust tension if your stitches are performing correctly. Try making just one adjustment at a time to see where the problem might really be. If you change everything at once you'll have a harder time correcting any given issue because you won't know where to start!