I now have about 8 empty 1 kg filament spools.  While it is possible to transfer filament from coils to spools by spinning the filament off of the coils and onto the spools, that operation seems needlessly difficult; it is especially difficult with 3 mm filament, which is quite stiff.  It would be much easier to just pop one of the edge retainers off the spool, drop the coil in place, and then restore the retainer.

I created a jig that allows me to hold a spool firmly in place so that I can use my cutoff band saw to cut one side off of a spool.  The cut is not perfectly uniform, but it is good enough.  I try to leave about 5 mm on the smaller part.  I then printed a small ring that I use to rejoin the two parts of the spool.

Jig to allow me to mount the spool so I can cut it

 

 

Spool in place, ready to be cut.  Note the wedges in the vise to ensure that the cut is more or less parallel to the spool edge.

 

 

Blade positioned for the cut.

 

 

Mid cut

 

 

Spool after the cut

 

 

Spool re-joined using printed retaining ring.