This is a printable piccolo trombone. It's got a few problems, though.
-It plays much lower than it should
-It's not airtight unless you oil the slide
-It requires quite a bit of post-processing
Removing the Built-In Support Material
If you've used the versions of the parts with the built-in support material, you start by removing the plates at the bottom of the inner and outer slides. First, put your index finger underneath one of the tubes, and grab the plate with your thumb near the base of the tube. Next, sort of peel off the plate. It should separate from the tube fairly cleanly, maybe needing a little sanding to clean up some stubborn bits. Repeat this until both plates are completely removed from the slides.
You might still have some support material for the grips. If so, then grab the supports near the grip, and snap it off. Sand down the last little bit left behind.
For the supports at the top of the outer slide, you'll need to cut them off. Take a utility knife or razor blade and cut the support material off near the base. Remember to take care not to cut yourself; cut away from yourself. Sand down any remaining support material. Repeat this process for Bell 2.
Once you've sanded away any remaining support material, you're going to turn your attention to the inner and outer slides. Sand down the inner slide until you can't feel the individual layers. Remember to sand 'with the grain,' around the tubes. This will probably take a while.
Next, roll up your sandpaper and use it to sand the inside of the outer slide. Every minute or so, blow into one side of the slide to get out the dust. You don't need to do this for as long as you sanded the inner slide, just long enough to knock off the roughest edges.
If you're printing this in ABS, you can probably get good results with an acetone vapor bath.
Assembling the Trombone
There are a couple places, namely both ends of Bell 2, where you need to create an airtight seal between the parts. With PLA, I've had good results with Loctite super glue. With ABS, you can use acetone to fuse parts. I also like using a soldering iron to fuse parts, which is what I did with this.
Insert the Bell into the wide end of Bell 2, and fuse the parts.
One of the pegs on Inner Slide is slightly thinner, to be inserted into the narrow end of Bell 2. If you're having trouble, try the other peg. After inserting the peg, fuse the two parts.
Slide the Outer Slide onto the Inner Slide. It should slide on easily and smoothly even without lubricant.
- Insert the Mouthpiece into the remaining peg of the Inner Slide. Twist it to get it in farther and a better seal.
Stuff to Make it More Airtight
These steps will make it more airtight, but require additional materials.
Use a viscous lubricant to oil the slide. Personally, I've had good results with mineral oil, but Monster brand valve oil would probably work well, too. You don't need to worry about a lot of the same things you'd worry about with a brass trombone, so see what works for you. But don't use a cooking oil, as those can go rancid.
You can do this by wrapping the mouthpiece in something to prevent air from escaping around it. I like to use plumber's thread tape. Although, if you don't mind not being able to remove the Mouthpiece again, you can also just fuse the mouthpiece to the Inner Slide.