2X Extruder Upgrade

This is a remix of 256754, and credit should go to the author of that thing.

Here in the UK we can't get the recommended spring from the US suppliers (at least not without jumping thru hoops!), and whilst other alternatives are available this modification provides a means of applying pressure to the filament via small M4 bolts.

If you need springs for this design take a look at the following:

http://e3d-online.com/RepRap-Spares-Mechanical/Compression-spring-21x7.5mm

This update has installed holes in the arms (for M4 bolts) together with hex cutouts (for M4 nuts). You can still use the springs as needed, however for problem filaments such as nylon more pressure might be required - and that pressure can be applied by tightening the bolts.

You require 2 M4 bolts (minimum 30mm long) and 2 M4 nuts.

One thing to be wary of is that the bolts could collide with the left side of the 2X frame when the X axis reaches its left side limit, so do check that before you commit the i