Some makers on sites like youmagazine or thingiverse give you permission to modify and redistribute their 3D creations. The problem is, a lot them only upload .stls, which require a bit or reworking before they can be modified.

In this tutorial I'll show you how to get your .stl files to the point where you can modify them as you like. I'll be using Google Sketchup Make for this. I can't make any promises that this will work with other 3D modeling software, so I'd advise you to follow along with Sketchup.

Before continuing to the next step, you should know how to download plugins for Sketchup and have the tool to import and export .stls. In my version of Sketchup, you go to window>extension warehouse. Then search for "SketchUp STL" by Sketchup Team and download and install it.

**Update: "Clean up" is a great tool by Thom Thom available in the Sketchup extension warehouse. It pretty much automates the following process, but I can't guarantee it'll work 100% of the time. So, if you do run into any problems with it, or you wonder what it does when you use it, follow along!

Step 1: The Problem With Not Cleaning a File Before Editing It

Here we've imported an .stl of a shepherds whistle I made. Notice all the black lines that cut across the whistles front. Our mission is to simplify this 3D model while maintaining it's solidness (making sure it remains water tight).

If we don't do anything and try to modify it, unwanted things might happen. For example, in the second image I'm trying to pull the vertical wall out with the push/pull tool, but I'm only able to pull out a small section of that wall. This is a simple example, but if you don't clean stuff up first, we can run into all sorts of problems.

Let's move on to get familiar with the tools we'll be using.

*note* You don't need to do this for some modifications (for example increasing or decreasing the scale of an object). This tutorial should really only be followed if you've tried to modify you 3D object and it keeps losing it's water tightness, or those black lines are getting in the way.

Step 2: Getting Familiar With the Tools We'll Be Using

While your pointer tool is selected (the hot key for this is space-bar). click your object once. If it gets highlighted in a blue box, that means it's grouped as an object. We need to either double-click it or right-click and explode it before we can rework it. The object should now be within a black dotted line or highlighted in blue without the blue box (if exploded).

When it comes to cleaning up stl files, the eraser tool is your best friend. you can select it by pressing 'e' on your keyboard or selecting from the list of tools.

You'll also want to have ctrl-z ready, as erasing the wrong line will happen often before you're comfortable with this.

Alright, now we're ready to get started with clean up! Move on to the next step.

Step 3: Cleaning Up Exterior Flat Surfaces

Flat surfaces are the easiest part to clean up. Did you notice all those lines that cut across the flat exterior wall of the whistle? Well, we can erase all of those! with your eraser tool click and drag over most of the lines. Stay away from the edges though. zoom-in with your mouse wheel or magnifying tool to get any lines that are close to the edges of your model. If you make a mistake and hit an edge, just undo it. Once your done, it should look like image 2 form this step.

Images 3 and 4 show two common mistakes: erasing an exterior edge and erasing the edge of an interior object.

Done? Ok, let's move on to clean up some curved surfaces.

Step 4: Cleaning Curved Surfaces.

Curved surfaces are a little more tricky. We need some of the lines that cut across them, so we can't get rid of them as freely as we did with those on flat surfaces.

But which ones to erase? From my experience, we want to keep the ones that are all parallel to each other (in this case parallel to the x-axis). We should get rid of any that deviate from this

In image 2 we start to get rid of the most obvious lines that aren't parallel to the x-axis (the ones that are one 45 degree angles). In image 3 we have some more difficult to spot lines we need to get rid of. Keep at it though, and eventually you'll get a curve that looks like image 4. If we get rid of any of the lines in image 4 our object will get no longer be water tight.

*quick tip: Sometimes lines look like they're necessary, but really aren't. When in doubt erase and ctrl-z (if you made a mistake).

Once you've cleaned up all the curved surfaces on your model, we can move on to the next step, cleaning up spheres!

Step 5: Cleaning Up Spheres

An easy way to think modifying spheres is to think of cleaning up consecutive curved lines.

In this case, it's almost like 2 curved surfaces followed by a flat exterior surface followed by another curved surface. Cleaning these up can be a bit tedious, but once you get the hang of it, it's not too bad.

With these three steps you should be able to clean up most if not all of your .stl. There are a couple of tricks to help with interior surfaces which I'll explain next though. So if you got your spheres all nice and clean, move along.

Step 6: Cleaning Interior Walls

Cleaning interior walls isn't as involved as curved surfaces or spheres, but if you're unfamiliar with making your objects see through and working with them that way, it can get a bit confusing, and you can make mistakes without realizing it. If you are comfortable doing it this way, it can save you time. We can clean up both interior walls at the same time. If you didn't clean up the exterior ones, you could do all 4 walls at the same time!

First let's make our object transparent.

Click on View>Face Style>Hard line and then

Click on View>Face Style> X-ray

Image 1: shows all those lines that cut across our flat surface that we want to get rid of. Maneuvering the camera is key here. We want to erase those lines that are part of the flat surface, but leave the ones that form the perimeters of the shapes we want to keep.

Image 2: Working away at getting rid of those lines.

Image 3: shows both interior walls cleaned up. Now there's just the interior curved surface left.

image 4: Shows what it looks like when you get rid of the lines we don't need for our interior curved surface.

Image 5 and 6 shows what happens when you erase the wrong lines.

Images 5:Erased part of the sphere on the other side of the whistle

Image 6: Erased part of the curved line inside of the whitsle

Step 7: Congratulations!

Congratulations! your 3D file should now be as simplified as possible while remaining water tight. Now it's time for you to modify it. You can adjust wall thickness, attach other 3D files to it easily and do many other things.

I hope you enjoyed this tutorial and it was helpful. While it may be a bit taxing on your time at first, once you get the hang of it, a model like this can be cleaned up in a few minutes.