Thank you all for your kind comments. Half the fun of building things is writing about here and getting feedback.

LBD, the bobbins she uses are inexpensive. She recently bought 100 for about $12 (USD).

987Ron, yarn would be a different ball of wax. These racks would need to be closet sized.

tinnman65, I bet your wife goes through a ton of thread. Do you ever go fabric shopping with her? I do (with my wife), and the quilting shops usually have several long arm quilting machines. They’re huge computer controlled machines that can sew intricate patterns. They’re fascinating to watch.

Birdseye49, my wife’s average cost per spool is much lower. She typically buys 250 yard spools, and they’re run about $4 at the local JoAnn Fabric store. The store almost always has coupons she can apply, and they’re frequently have buy-three-get-three-free promotions too. Some of my wife’s spools are very old. She was showing me some she got from her mom that were originally sold for 15 cents. 

Ryan, I never gave any thought to trying to sell these racks. I figured my wife’s requirements were unique to her situation. If she had a dedicated room, she probably would have wanted something wall mounted with fixed-angle spools. That would have been much easier to make. I suppose our design might appeal to people without dedicated spaces (like mobile tools and benches appeal to garage-shop woodworkers like me). If I were to consider making more, I’d really have to get my act together with respect to the tilt stops.

Splint, this thing does fold down more compactly than anything I’ve ever built. The weirdest thing for me while building was the ordering of the four racks. I numbered them 1-4 (left to right) in the closed position (and I cut them side-by-side from the same board). But, when the racks are unfolded, the inner and outer racks change position—the L-to-R ordering changes to 2-1-4-3. That definitely made me think I had drilled pivot holes in the wrong racks, etc.