A "naked" portafilter is one that has the bottom of the basket exposed. This:
- allows the coffee to come out without hitting additional metal, and so is supposed to keep at a more consistant temperature.
- preserves more of the crema as the bubbles don't burst when hitting the spigot.
- allows you to better see the extraction process and so get better / more consistent shots.
- looks really cool!
After spending a long time trying to find the perfect naked portafilter for our machine (Uniq Rumba) I ended up deciding to just make one. This is documented below.
Start off by removing the plastic handle, metal bar handle and spigot. This can by difficult as they are dissimilar metals and so may have corroded together. I used a bunch of penetrating oil and then had to heat the spigot with a pencil torch.
We want to remove the entire center section, so drill a bunch of close / overlapping holes. I did not have a centering bit, so I just placed them as close as possible to each other and then broke away the material between them with an old screwdriver bit (a bunch was too large).
To make this simpler / faster you can make a jig out of a piece of wood and a bolt though the spigot hole - this will allow you to drill, rotate, drill rotate and get all of the holes nicely lined up. I didn't do this, but really should have!
Now place the portafilter (without the metal handle!) on a lathe and machine away the excess material. I used a Central Machinery 7" X 10" Precision Mini Lathe. Something larger / more powerful would have made the process faster, but this worked ok. Running some fine sandpaper (on a dowel or similar) around the machined edge (while turning on the lathe) makes short work of tidying up the sharp edges, removing burrs and polishing the work.
Now simply reasemble