Inspired by asc2 asking on my homemade tamper instructable if my bottomless portafilter was a DIY project I grabbed a spare portafilter and headed to techshop to see if I could successfully lop the bottom off. This is a pretty simple project if you've got access to a metal lathe. I don't have one at home so I made it at techshop.
Step 1: Disassemble the Portafilter
Start by clamping the portafilter in a vise and unscrewing the handle. The factory was nice enough to use loctite or something similar so it was a pain to disassemble it. I was worried that I would ruin the handle so I brought along a rosewood handle that I made a few months ago. I think if the plan was to keep the same handle I would clamp the body of the portafilter in the vise with a scrap of wood on each side to prevent it from getting all banged up.
Step 2: Start Cutting
Mount the portafilter on a lathe with a three jaw chuck, be sure everything is square and securely in place before turning the thing on. Presumably a better machinist would measure where they need to cut but being a big fan of just winging it I eyeballed it and started cutting a groove with a carbide cutting tool. After a little while the bottom of the portafilter will fall in, stop the lathe, pull the center out, and see how much more cutting you need to do to finish the job. I ended up off by about 1/16" so I did one more pass cutting away the last of the bottom then cleaned up the cut edge so it wouldn't be sharp.
Step 3: Reassemble and Go Make Yourself Some Coffee
Take the portafilter off the chuck, put the spring back in, drop the basket in, screw the handle on, and call the project done.
The only difference between my purchased bottomless portafilter and this DIY one is that I ended up with a nicer curve on the bottom of the DIY one. Aside from having yet another thing I use every day that I made myself.