[sleepybrett]

The adjustable mechanisim on the axidraw is nothing but a screw. Essentially there is a right-angled channel you press he pen into and then you tighten a set screw that pushes the pen into the corner.

This design is simple to produce however it causes registration problems if you attempt a multi-pen (multicolor, say) plot, where the pens have different diameters. In the past when faced with this i've 'fattened-up' the narrower pen with tape wraps.