TECH

Making an Ultrasonic Cutter for Post-processing Tiny 3D Prints

An ultrasonic knife is a blade that vibrates a tiny amount at a high frequency, giving the blade edge minor superpowers. It gets used much like any other blade, but it becomes easier to cut through troublesome materials like rubber or hard plastics. I was always curious about them, and recently made my own by modifying another tool. It turns out that an ultrasonic scaling tool intended for dental use can fairly easily be turned into a nimble little ultrasonic cutter for fine detail work.
I originally started thinking about an ultrasonic knife to make removing supports from SLA 3D prints easier. SLA resin prints are made from a smooth, hard plastic and sometimes require a veritable forest of supports. These are usually done with flush cutters, or if they do not care about appearances, but sometimes the density of supports makes this process awkward, especially on small objects.
I imagined that an ultrasonic blade would make short work of these pesky supports, and for the most part, I was right! It will not effortlessly cut through a forest of support bases like a hot knife through butter, but it certainly makes it easier to remove tricky supports from the model itself. Specifically, it excels at slicing through fine areas while preserving delicate features. See It In Action
In the animation here you can see it slicing through some small supports on a tiny model. Cutting creates a puff of mist and particles, which a spritz of water helps control. The cutting edge of the tool shown is shaped like a chisel, which makes it easier to make smooth and clean cuts because the tip is very good. The usual method to remove is a good quality pair of flush cutters, but very small models and in dense support areas are neither maneuverable nor do they make it easy to see what one is doing.
Also, when cutting back with normal tools there is always some amount of stress put on the piece that's being cut. A knife forces its way through an object, and clippers shove anything between the blades apart which crack or shatter small pieces. A fragile part can break from the force of these tools, no matter how careful one is.
The advantage of cutting with an ultrasonic knife is that very little force is transferred to the part being cut. Additionally, the small tool tip means it is highly maneuverable and easy to see what one is doing. How It Was Made
We have shown an attempt to make an ultrasonic knife by harvesting parts from an ultrasonic cleaner. That project connected both the custom attachment and x-act knife blade to the transducer. Results were mixed, and it appears that a critical aspect of an ultrasonic tool is to match the shape and mass of the head to the transducer itself so that they work in harmony. By using an existing ultrasonic tool as my starting point, I hoped to avoid problems in this area.
I bought an ultrasonic dental scaler I bought for $ 125 from China off eBay, plus $ 10 for a set of steel tips which are blunt by design. Scalers are usually used to break up hard tartar deposits on teeth without damaging the teeth themselves. They are not designed for cutting, but I figured it might be the least used to test the concept. The unit has a screw attachment system, is handheld, and has adjustable power.
The first thing I did was ignore the water system. Normally, water flows through the cord and out of the hole in the tool to help cool the tip during extended use and flush away particles, but I did not want the mess and instead made of a small spray bottle.
To turn the tool into a cutter, I selected a few tips and sharpened them carefully on a very fine grinding wheel to make a knife edge. The one that worked the best was the one shown here: the heaviest one with the widest tip, which I sharpened like a chisel. Removing Delicate Supports From a 3D Print
The 3D model I used has small and intricate pieces, with several delicate areas from which it would be a challenge to remove with other tools. The thin tip of my ultrasonic cutter can easily reach these awkward areas at odd angles, and the small cutting edge makes it easy to see exactly what one is doing.
There was one particular spot that I intended to use whether the tool was a success or not: the antenna-like rangefinder on the helmet. It's a blocky assembly attached to the side of the helmet by a long stalk. The original model by [Printed Obsession] has it a separate piece, but I connected it to the rest of the helmet as a test. I knew that it would require support in order to print correctly, and I also knew that it would be fragile and unlikely to survive the stress of removing those supported by other methods.
Could my new ultrasonic cutter be used to slice through these anchor points without putting significant stress on the delicate part itself? The answer was yes! Embedded below is a video that begins with successfully removing the supports mentioned, and continues with removing supports from the rest of the piece.

Donald Papp