Always wanted to try plotter attachment, but I decided to make my own with few specific features in mind:

• High rigidity and good precision of lines.

• Spring loaded, so it provides more or less consistent force. And it works fine in case you have crooked bed.

• Universal pen holder that can hold anything from 6mm to 16mm in diameter.

It turned out really well and I'm happy with result. Despite it's a bit tricky to print properly, it is worth it

Bill of materials:

1. M2x6 with thin-head - 2 pieces

2. M2x4 (or another M2x6) with thin-head- 1 piece

3. M4x20 (not longer) - 1 piece

4. Spring from automatic pen (20-25mm long)

Important:

Originally designed to work with Neptune 3 Pro, so it'll work with other Neptune 3/4 printers with same extruder shell. In case you don't sure, print "mount.stl" and try to install it.

You'll need fine tuned ironing for guide and insert parts and place them as shown, otherwise you'll have issues with movement at lower end. Ironing settings in my case were:

• Speed 30mm/s

• Flow 20%

• Inset 0.21mm

• Spacing 0.15mm

I printed it with 0.4mm nozzle, 0.16mm layer height, PETG, 4 wall lines, 8 top/bottom layers, 20% infill, no supports - works fine.

Check this before full assembly:

There are 3 types of guide part with different gap. Start with 0.10mm and install it to rail. It must move freely, but not be loose. Too loose? Try 0.10mm_P version. Too tight? Probably 0.15mm will do. In worst case, you can loose screws a bit or cut corners of guide slightly - that will work too.

Assembly:

Install knob onto M4x20 and screw both parts of pen holder together with it.  Then install guide part to pen holder with M2x6, then insert them to rail and screw it to mount part with M2x4.  As final step - install spring.

Hint: you can lower spring force by carefully heating outer coils of the spring with lighter - it's a bit easier to control than just cutting coils. Or if you need just few more milimilers stretch spring a bit.

Few tricks:

I recommend to attach paper with masking tape to the smoothest side of your pei, you can also use clips if you know what you're doing (be careful pls)

Create new printer profile in slicer with diferent bed dimensions:

1. X = your original width - 50mm (or create additional extruder with -50mm X-offset)

2. Y axis almost alligned, so you can use original depth

Insert this as start G-code, it'll disable wiping and extruding:

M220 S100 M221 S0 G90 G28 M420 S1 Z5 G92 E0

In slicer set printing temperature and build plate temperature to 0. Enable retract, but set it to 0mm distance and 0.2mm retraction minimum travel, disable combing and enable z-hop with 1-1.5mm height.

Huge thanks to RemiRafael for M4 knob: