Drawing CNC Plasma Cut Replicas:
This page explains in some detail, how CNC plasma cut replicas in 2D can be manually traced from a photograph using a full featured drawing program. CorelDraw is used here, but there are many other programs out there with similar features. We are not going to cover importing clip art, copying the work of others, or drawing per se, but rather how to create shapes using photographs as the basis.
If you are going to invest the time and money to build a CNC plasma table, the cost of good software is a relatively small additional expense, but necessary. It should have ability to do what is shown below at a minimum. Below are the steps that were involved in drawing a CNC plasma cut replica of a motorcycle in two dimensions (a silhouette).
The first step is to locate a photograph on-line that shows a good profile of the object straight-on. Any angle in the viewing point creates a distortion that is undesirable in a 2D replica (silhouette). A contrasting background, preferably white, is also helpful. The photos at right illustrate this.
The first photo shows the bike on an angle, with a poorly contrasting background. The second photo is preferable.
When we think of tracing the outline of an object, we may be under the impression that we need a steady enough hand to accurately follow a path around the shape. Or we may think that we must place hundreds of dots around the shape and then connect them. We don’t have to do either.
In CorelDraw you draw a rough line around the periphery of the object. This results in something like you can see in the third image — a rough line with a series of dots (called nodes) along its length.
If we highlight all the nodes between the two at the ends of a particular section and delete them, we are left with only the two at the ends. If we highlight either of them, two small arrows (handles) appear that can be manipulated with your mouse to produce a precise outline. The fourth image shows the outline after the handles have been tweaked using the mouse.
You can draw the rough line around the entire shape, or you can do a section at a time. If drawn in sections, the end nodes in the sections can be joined later. In other words, you can create a single outline from a series of short lines. You can move, add, and delete nodes at any time. Join the two end nodes of a contoured line to create a shape that will fall out when cut.
This ability to edit nodes is the key to making a relatively easy task out of what would otherwise be an extremely tedious tracing process.
Since you can’t include every feature of the original photo in your traced image, you must be selective in choosing the features you want to include. In the case of the motorcycle, the gas tank, seat, exhaust pipes, cylinder fins, timing cover, engine cases, kick starter, gear shift lever, carburetor, frame, wheels, forks, and oil tank cover were among the features we considered essential. We emphasized those, with other parts being more or less secondary, where there was room for them.
Since plasma produces a kerf (cut width) about .050″ wide, a space must be left between lines and internal shapes. This space must be sufficient to prevent cuts from overlapping each other or parts falling away unexpectedly. Our rule of thumb for this is to leave a .150″ gap between all parts of the shape. We do this by drawing a circle with .150″ diameter and making sure it will fit between all parts.
Note that not all the cylinder fins can be drawn — just as many as will fit given the necessary .150″ clearance. Some parts must be shifted slightly to get the clearance. See illustration at right.
When creating your traced drawing, some features must appear to be in front of others. Painting helps create this illusion, but cut lines can do this as well. Cut lines provide the added advantage of making it easier to mask off sections of the shape to be painted in different colors.
When a part is supposed to appear in front of another part, the illusion can be helped by cutting the lines of the front item across the rear item. It the image at right, note how the kick starter is intended to appear in front of the fiberglass side cover. The kick lever is fully drawn and will fall out of the plate. The outline of the side cover has gaps on either side of the lever to leave the required .150″ clearance. When painted the gaps can be made to almost disappear.
When you have finished your drawing and deleted the photo, it is possible (at least in CorelDraw) to see a preview of what the cut piece will look like. This also shows errors such as incomplete interior shapes, omissions, etc.
This is done by highlighting everything but the exterior outline and bringing it to the front of the page. Next, you must change the color of all interior lines and shapes to white and increase the line width.
Then, making sure the exterior outline is at the rear of the page (behind everything else), change its fill to black. You will end up with something like the image at bottom right. Save this image under a separate file name, as these changes to the drawing are for preview purposes only and will not be present in the file you export.
The last step in the drawing process is to export the shape as a dxf (drawing exchange format) file that your machine’s cut software understands. This is covered on another page.
