PRYM Knitting Mill Midi - BlueMidi Knitting Mill by Prym (624168) Ideal tool for creating knitted tubes Easy to use with instructions Perfect for beginners Made from plastic & metal Suitable for wool and yarn for needle size 2mm 4. 5mm. Thick or fibrous wool can damage the knitting mill. The knitted tube should form and work it's way through the bottom of the mill and can be worked on once you've reached the desired length