I like the bearing supported spool holder design, but wanted it to be easier to change spools quicker and easier. I have my printer in a cabinet that didn't really leave lots of room to pull the threaded rod out to the left or right. It was also a tight space to put the spool, bearing cones, etc. on and thread on the nuts. With this change, you can put the spool, cones, inserts, and nuts on the threaded rod, tighten it down, and have it ready to just drop into the arms. The arms hold the spool rod insert securely, without any risk of it falling out, yet it is easy to slip in and out.
I also added a small spool cone that works better with the spools with a small center opening. It is design to use 608 bearings. If you don't use a spool cone, you are missing out on the very low resistance a bearing supported center mount spool holder can provide.
The purpose of the round insert is so you can quickly and easily thread the nuts onto the threaded rods and tighten (not too tight!) the spool cones into the spool, then just drop the full assembly onto the arms. You can skip the inserts if you prefer to put the nuts between the arms and the spool cone, but I like how tight and secure it keeps everything.
The arm has holes on each side of the bottom for the arm shim but I only use the ones in front. I like the hole on the front for easy access - the hole is just on both sides so the same part can be used facing to the left or the right. The shims are not t-nuts, and are loose enough in the slot to allow you to slide the arms left and right to accommodate any spool thickness quickly, without tools, but still holds the arm on securely, keeping it from tipping over or coming off.
I'm using this on a FolgerTech 2020 Prusa i3. I love the very low resistance you get with a center bearing supported spool holder directly above the extruder. Pretty much eliminates any potential quality issues related to filament feed resistance.
I used 0.3 just because it was quicker, and high resolution isn't required, but 0.2 works fine too.
To put the rod together, put on the spool, and a bearing cone on each side, then one of the inserts against the bearing, with the longer end towards the bearing. Thread the nuts on, and you are ready to drop the spool into the holders. The supports mount the same as the previous design, using the 2020 inserts included in that project to hold the uprights in place, while still letting it move side-to-side.
When mounting the arms, you just need one shim on the front. The holes are on both sides so you can face the arm either direction. Using just one hole will keep the arm on securely and prevent any tipping over. Use the shim rather than a T-nut so you can freely slide the arm left and right, to fit spools of different thickness.