Practical Printing Series

Introduction

In this post, we will cover how to use a 3D printer to make cutting guides for parts cut with a hand-held plasma cutter. This technique can also work for other forms of cutting if you need to match a design perfectly.

What Is A Plasma Cutter

A plasma cutter is a tool that uses electricity to ionize gas particles and propels the gas particles at a high velocity using compressed air. Anything that is electrically conductive can be cut with a plasma cutter, but they are mainly used for metals. The plasma cutter is very effective, because its arc can be up to 45,000 °F (25,000 °C), and that is more than enough to instantaneously melt any metal.

Plasma cutters are used in home workshops and industrial applications where cutting metal is a frequent job. On the industrial scale, plasma cutters are often used with large cutting tables and Computer Numerical Controlled (CNC) devices. These can create complex patterns and shapes with ease and the proper programming.

For a hobbyist, the plasma cutter is usually a hand-held torch. The torch is used to cut basic shapes and different thicknesses of metal. The quality of the cut depends on the skills of the operator. The operator needs to move the torch in the correct pattern, at the correct speeds, and have the correct machine settings. The biggest factor to a quality cut is the steadiness of the operator’s hands. That’s where a cutting guide comes in.

Solutions

The simplest solution to making a good, clean, and repeatable cut is to use a guide. A cutting guide can be as simple as a block of wood or a ruler. In most cases, that works well, but what if precision or complex shapes are important? For these situations, it might be worth it to use a 3D printed cutting guide.

In a recent project, I used a 3D printed template to cut holes in 1/4″ thick aluminum, using my plasma cutter (see: Building A Tank). I needed to make ten holes in total and in a size that I didn’t have a hole saw. Not that a really wanted to cut ten holes in metal using a hole saw anyway. I decided to try a 3D print and my plasma cutter and it worked!

Know Your Torch

In order to make good cuts, you need to know a few things. First, If you are cutting a piece in half and you need both halves, you need to know the kerf of your torch. The kerf is the thickness of the material removed by the cutter. This thickness can be different based on the material thickness and how far the torch is from the surface. To find the kerf, you can make a short cut into some scrap metal and measure the gap left behind.

Another measurement you’ll need is the distance from the outside edge of the cutting hole on your torch to the outside edge of the torch housing. Keep in mind that most torches have steps of different diameters in their frames, so you need to know which one is the edge you’ll ride along. You need to make sure that the template is offset appropriately for the cut to be in the correct position.

Design For Success

Offsets

My torch has a drag cup (the silver part in the image above), which allows me to rest it on the material and keep it at the perfect height. When I am cutting a straight line, I use the drag cup to slide against the template because it is closer to the work piece (14 mm) and therefore means less 3D printing to get the right guide. The offset between the outside of the drag cup and the edge of the plasma nozzle is 10 mm.

For anything other than a straight line, I have to use the outside of the torch body. The drag cup is only solid on two sides, meaning that if I have to go around a circle, some of the circle will not have the drag cup resting against the template, leaving flats spots. The outside of the torch body is circular, so it doesn’t matter which direction or angle I am relative to the guide, it will be the same distance away. The offset distance for this portion is 12 mm, but the height is between 18 and 30 mm above the work surface. To use this portion of the torch, you need to print a taller fence.

Heat Problems

I’ll admit, my first attempt at using a 3D printed cutting guide on the aluminum tank, did not work well. Sure, it guided the torch just fine, but it also melted to the aluminum. Remember, the plasma can get as hot as 25,000 °C! Standard PLA, used in most 3D prints, begins to soften at around 60 °C and completely melts around 180 °C. You might think that you need some specialized engineering filament to handled the heat. You don’t. Besides, there is not a plastic on earth that can withstand 25,000 °C.

The goal with your cutting guide should not be to withstand direct contact with the plasma, or even continuous use of the plasma. The goal should be to not melt in the time it takes you to make one cut. This is actually a little easier than you might think.

The goal is not to minimize heat, but heat transfer. Heat transfer through conductivity (contact) is expressed by Fourier’s Law.

Most of the parameters in the equation are not things that we can change. The thermal conductivity is based on the material. The thickness is also based on the material, and the change in temperature will just be the difference between the glass transition temperature of the plastic (60 °C for PLA), and the ambient temperature.

Minimize Area

That leaves us one parameter to change – area. This is where the solution truly lies. The area is not necessarily the footprint of the cutting guide, it is the area that is in contact with the surface being cut. If we can minimize the contact between the two surfaces, we can minimize the heat transferred.

In the case of the aluminum I cut, this was important. Aluminum has high thermal conductivity, meaning that the heat travels through aluminum easily. In the time it took to cut just one hole, the local area would be very hot. And since, the pattern being cut was a hole, the template had to completely surround the cutting area. My first design, just sat on the aluminum and promptly melted on the first cut.

Heat Solutions

This is where I remembered two important things about heat transfer in everyday life. First, is the previously mentioned fact about the area. I knew that if I could reduce the area, I would reduced the heat transferred. Another fact is that air is a poor conductor of heat. It is considered an insulator. So my next design for the hole template, had grooves cut into the bottom (the area that contacted the metal) to minimize the surface contact area and to created pockets of air to insulate the piece.

I made three of these because I figured that I would still melt them after a use or two. However, it turned out to be the correct solution and I was able to use the same template to cut all ten holes in the aluminum.

For the gantry crane build, I needed to cut large square tubing at an angle that my bandsaw couldn’t handle. So I printed a guide that slipped over the end of the tube and allowed me to cut the angle perfectly on both sides. Since this guide had to stay on the pipe and couldn’t be moved easily, I had to make large channels on the inside face to minimize area and allow air to move.

Thick Steel

Important Update**: I recently used a hole template like the one above to cut a hole in the middle of 1/2″ (12 mm for my overseas friends) steel. In the time it took for the torch to pierce the steel to begin the cut, there was enough blow back and heat to melt the template. I did not have this problem with the aluminum because it was thinner and the torch easily penetrated the aluminum. However, with the thick steel it took a few seconds to cut through it. When cutting a hole in thick steel, mark you hole and pierce it before you install the guide. That way the plasma already has a hole to go through and doesn’t blow back on your template.

Design To Minimize Printing Time

On the other end of the same pipe, I had to make a notch to fit around a crossing tube of the same size. For this I printed two pieces (right and left) to cut the notch. In hindsight, I should have made it one piece to keep the two guides aligned with each other. My channels in these guides are slightly different. I had to make them triangles so that the printer wouldn’t try to print supports into the channels. Those are very hard to remove without breaking the part. This concession helped save print time and struggling to remove supports.

Conclusion

It is possible to use low temperature plastics in high temperature environments if the proper design is used and limitations are understood. Having a cheap guide that you can print directly from your CAD design of your project, minimizes errors and fitment issues. You do need to plan ahead, because printing times can be significant if it is a large guide.

Thank You

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