Coil winder adjustable bobbin holder

[retailer]

A short while back I posted about an Aumann coil winder and mentioned I was working on an adjustable bobbin holder - I was using wooden mandrels to hold bobbins/winding tubes for transformer winding, a mandrel should be made with sufficient accuracy so the bobbin doesn't wobble about it's axis - I have a collection of wooden, mandrels it is very rare to find one that will fit any new transformer that comes my way so usually a new one needs to be made each time, and with enough accuracy so the bobbin doesn't wobble on it's axis.
To get around having to make the mandrels I came up with this adjustable bobbin/winding tube holder - machined from aluminium - similar principle to a 4 jaw chuck but with internal 'jaws'. The 'jaws' can be adjusted for width and locked in place with a clamp screw, I've made 7 sets of jaws to cope with bobbin widths from 15mm up to 55mm - 4 jaws in each set. One of the pics shows it in use on a good-sized mains transformer I just finished winding the transformer central core on this is 45mm wide - the two output transformers that go with it are monsters - nearly 3Kg of wire in each.