These are replacement "Side racks" or "End racks" for many common knitting machines. Studio, Artisan, Silver reed, Brother. They fit snugly and the carriage glides right over them, just like the original parts. If printed in Nylon, you will probably never have to replace them again. Recommended print settings: Nylon  (ABS will probably work fine too) 0.3mm Layer height 30% Infill Support - No