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 knife edge minor superpowers. It gets used much like any other blade, but it becomes far 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.

Cheap ultrasonic scaler. The blue disk is for adjusting power. Foot switch not shown.

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 can sometimes require a veritable forest of supports. These supports are normally removed with flush cutters, or torn off if one doesn’t 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 won’t 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

Model by [Printed Obsession] hosted on MyMiniFactory. Printed on a Formlabs Form 2 SLA printer at 25% scale.

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 controllable.

The usual go-to method for removing supports is a good quality pair of flush cutters, but on very small models and in dense support areas they are neither maneuverable nor do they make it easy to see what one is doing.

Also, when cutting supports 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 can 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 once featured an attempt to make an ultrasonic knife by harvesting parts from an ultrasonic cleaner. That project connected both a custom attachment and x-acto 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.

Sharpening into a chisel tip preserves the original shape and orientation of the tool as much as possible.

I modified 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 normally used to break up hard tartar deposits on teeth without damaging the teeth themselves. They are not designed for cutting, but I figured it could at least be 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 a hole in the tool to help cool the tip during extended use and flush away particles, but I didn’t want the mess and instead made do with 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

This would be a challenging piece from which to remove supports cleanly, and therefore a good test.

The 3D model I used has small and intricate pieces, with several delicate areas from which supports 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 aimed to use to decide 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 as a separate piece, but I connected it to the rest of the helmet as a test. I knew that it would require supports in order to print correctly, and I also knew that it would be fragile and unlikely to survive the stress of removing those supports 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.

What Did Not Work Out?

There were a few dead ends in the process of creating this DIY ultrasonic cutter. Sharpening the dull factory tool ends was not my first idea; I originally tried to attach a small blade to one of the tool heads. An attempt to braze them together failed, and a later and much less confident attempt to join them with epoxy failed shortly after it even began.

I then decided to sharpen the tool ends instead of adding a blade, but it still took a few tries before I hit paydirt. Here are some of the attempts:

Attempts at sharpening the (blunt) factory tool ends to give them cutting edges. Left to right are: straightened knife edge, curved knife edge, sharpened chisel tip.

Of these three, the chisel tip (rightmost) was the clear winner. I suspect it is no coincidence that it also best preserves the original orientation and design of the tool. The others were sharpened on their sides, like knives. The middle one is in the original shape, and the leftmost one has been straightened. All will cut, but it was clear during testing that the tip is the “business end” of the tool. The sides cut very poorly in comparison, even at full power.

How Well Did It Succeed?

With the right modifications, an inexpensive tool was successfully turned into a precision DIY ultrasonic cutter. It handled removing the tricky supports on my test model beautifully.

Early on, I had fantasized that it might allow me to slice through wads of plastic like they weren’t even there. Sadly it doesn’t work that way, but it is outstanding for detail work.

While it performs wonderfully at the niche application I tested it for, are there other tasks a tool like this could be useful for? Would anything perhaps improve its performance? Let us know your ideas in the comments.

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