Plasma cutting is incredibly useful. Paired with bending and welding it is often the fastest and cheapest way to build any kind of mechanical structure or enclosure. Results that might take a day on a 3D printer or on a CNC mill can be achieved in minutes on a plasma cutter. Not to mention the cost of sheet metal is a lot cheaper than 3D printer materials or billet for machining. I'm constantly making folded and welded structures with my plasma cutter and every time I have the same problem. I will design a complicated folded part and then when I go to bend it I can't place the bends accurately. I'll spend a lot of time trying to measure them out but the plasma cutter can make any arbitrary shape which can make measuring difficult.
To solve this problem I designed and built a pen plotter head for my plasma cutter. It allows me to first draw all of the bend lines then cut out the part. This gives me perfectly registered bend locations that make folding the parts really easy.
One of the things that I really wanted to achieve with this tool was for the workflow to be fluid and simple. If I have to spend a lot of time generating the toolpaths or setting up the machine or aligning things then I'm not going to use it. I achieved this by arriving at a setup where the marker head is just considered another tool so that toolpaths can be generated from pretty much any cam system all in one setup. Mechanically it is always mounted to the machine with a known tool offset which allows automatic switching between the torch and marker. To keep the marker from drying out, the system can automatically open and close the marker as well as raise and lower it to draw.
Everything is pneumatically actuated which makes integration into the existing electronics simple and avoids dealing with electrical noise issues which can be a big challenge with plasma cutters. An air cylinder normally only has two defined positions of opened and closed. To make this work I needed to get three positions out of the air cylinder. To achieve this I have a pair of opposing springs which center the air cylinder when no pressure is applied. This is so that the "up" position can be reserved for opening and closing the sharpie, the "down" position can be used for drawing and the "middle" position can be used to keep the sharpie clear of the work when I'm cutting. Without the middle position I would always have to lift the sharpie up to clear the work which would mean the tip would be pushed out and exposed when cutting.
I'm very happy with the results. The marker is sensitive to the height that you draw at so the main thing that I want to improve is to add a spring in series with the marker to give more tolerance. Currently I probe with the torch before I draw which works well, but adds a lot of time. The other big improvement is either getting a finer point marker or potentially moving to some kind of scribe. I shyed away from scribes since I was afraid of moving the part around, but I hear that there are low force blunt diamond scribes. I'll be looking into this.
This setup is also great for making art and signs which is a very common use of plasma cutters. You can imagine how adding in paint pens or other scribes can let the machine make some really cool pieces.
Full details of the design and build are in the video. Enjoy!
I've gotten several requests for marker dimensions. Here's a step file with the marker reference geometry that I used. The stroke to actuate the marker is right around 20.8mm.