This 4 mm mandrel is made from high grade 316L Stainless Steel. It has a screw thread on one end. Screw our M4 sleeves onto this end of the mandrel and simply wind the glass on the sleeve to make your own knobs .
We suggest putting a little dot of bead release to seal the end just to prevent any glass entering the thread.
M4 Sleeves for these mandrels can be found here
I also have a Knob making kit with a tutorial