I started learning Fusion 360 a couple of weeks ago and was looking for a cool project to test what I had learned, when I came across the Plastic Bottle Cutter. I saw a bunch of designs, some commercial, others ones diy solutions but all had the same objective, to reuse the standard plastic bottle by cutting into thin stripes of filament that could then be used to tie, weave and even tow cars.

So I took that basic idea went on to set the goals for this design, it needed to be portable, so that I could take with me when camping, it had to be safe so that a child could use it, so no sharp edges or exposed blades, it should not depend on a base or other means of torque besides my hands, so it had to be ergonomic, it should be easy to adjust the thickness of the stripes and finally it should be mostly printed, this was simply because I wanted to hone my skills.

So let's start the design.

Step 1: Basic Shape

I started by sketching the shape I though would be most comfortable while minimizing the amount of plastic I would have to use. Once I had the main shape (the center one) I offset it two times, one by 0.4mm and one by 3.4mm. The interior shape will become the main body of the design and I extruded it by 155mm, this accommodates space for the hand to hold it and space for the blade and its adjustment. The outer shape will become, shortly, a cover that will protect the blade, the users and provide and easy way to adjust the size of strips of plastic.

Step 2: Ergonomy

With the basic shape extruded I went on to make a more ergonomic grip. So I sketched four 20mm circles, placed side by side and used them to cut the main body, leaving a bit of space between each cut to accommodate the shape of the fingers. Next added a small 1mm fillet around the cut area to remove the sharp angles and make it more comfortable.

Step 3: Lanyard

At this moment I realized that I should add a hole for a lanyard, so I did just that. A small 4mm hole, that I then chamfered, this removes the sharp angles that could cut the lanyard. Notice that I apply a chamfer not a fillet simply due to the angles, if I applied a fillet there would be small parts of it that would have a angle superior to 45º, these can be tricky to print, so to avoid them I apply chamfer instead of a fillet when these occur .

Step 4: Bottle Slot

Now I needed a slot for the plastic bottle to slide through, so I made a 3mm slot with 55.10mm of height. These values were eyeballed to what I thought would work and in the end worked really well. I then proceeded to chamfer body all around by 1mm except in the slot I had just cut. In fact on my first pass I actually also chamfered the slot, but afterwards I had some issues with chamfers in other places, so I went back and removed them from the slot and applied them later in the design

Step 5: Blade Receptacle

The crucial part of a plastic bottle cutter is the blade and I needed a place to put it in, so I created a small rectangle with 0.8mm of height that I then used to cut into the body, until the hole was almost at the end of the curved wall, I stayed roughly 11mm away from base. I chamfered this and the previous slot, by 0.5mm, as these were going to be hidden by the cover I was going to put on top of it, I really didn't need much more.

Step 6: Cover

The main body was almost done, so I went begun making the cover, I extruded the initial design and cut a 4mm rounded slot through it. This is were the plastic bottle will be placed, and the height at which the cover is placed will determine the thickness of the strips

Step 7: Adjustment Screws

Next I toyed with a couple ideas before settling with a design that required 3D printed screws, so I made slots on the sides of cover to accommodate the screws I would have to design. Once again I just eyeballed the dimensions, if these didn't work I could easily return and adjust them, something I had to do a couple of times, before I settled with the ones that you see here, so props to Fusion 360 history and parametric design it's my first parametric design and I don't I will ever be able to go back.

It's time smooth out some parts of the cover, so I applied a 2.5mm fillet to the places were the plastic bottle would pass through and a 1mm chamfer to the to the side slots, notice that I intend to print this part standing up, so it should no require any supports.

Step 8: Screws

The screw was a real pain, Autodesk you read this please, please give us a better way to make custom shape threads.

At this point I wasn't confident that I could design a working screw that didn't require supports and was strong enough, so I created a new design just to experiment with it. In the end, after many, many tries I settled on the following approach.

First I created a coil with the dimensions that you can seed in the screenshot, I then sketched a bigger triangle on the base of the coil, the purpose of this is to create a design that does not need support for printing. I then applied a sweep of that triangle using the initial coil as my path and guide rail. The shape wasn't perfect but with correct tolerances it should (and it did) work. So I had a new coil with a 90º angle that could be easily printable, so I hid the original one and then placed a cylinder at the center of the one I had just created, thus creating the screw, now I needed a tap to cut material from the main body. I duplicated the screw and proceeded to scale it by 1.15 in the X and Z axis (attention this was modeled with the default modeling orientation Y up, so if you are replicating this and your orientation is Z up, the axis you will to scale should be X and Y),this gave the tap I need, I just cut a bit of material from bottom and from the top to ensure that I had the correct shape for the ends of the tap.

Step 9: Thumbscrew

With the tap done, I proceded to finish the screw, by making a thumbscrew, adding a cylinder to ensure that it would press against the cover I had designed and finally cutting the excess from the top.

Step 10: Taping the Thread

As I had made this tap on another design file I had to transport it to the design file were I had the main body and here I encountered an unexpected difficulty, I could not copy paste it, in fact it took me quite a while and bit of reading to find a way to import the tap on to the another project. This involves making the tap a component copying it, and the finally being able to paste it onto to the other project, so once again Autodesk, could you make this less cumbersome? Pretty please?

So I managed to import the tap, I then rotated it and duplicated it(this is important, you need to duplicate and not mirror or else you will need two different screws one left threaded and one right thread).

With them roughly in place, I used them to cut the main body.

Step 11: Print It

And with that final step, it's done.

I printed in the design in PLA at 205ºC, 3 top and bottom solid layers,3 perimeters and 30% infill (maybe a bit of overkill but it works).

To use the design simply place a blade from a small box cutter inside the slot, slide the cover so that the open slots are facing up and thread the screws into their holes, cut the bottom of a plastic bottle, place the bottle in the cutter and start turning the bottle while forcing it down, it should start cutting into the plastic, as soon as a strip of plastic appears, simply grab it and start pulling it until you've cut the whole bottle. To adjust the size of the plastic strips, unscrew the two thumbscrews slide the cover to the correct height and tighten the thumscrews.

So here is the design for the screw and for the plastic bottle cutter, you only need to download them and print them:

Screw