How to cut line art with CNC machines

Generating G-Code for line art is a simple process that can be perfected with a little practice and the appropriate software. CNC machines are spectacular for cutting line art, whether you are doing a one-off piece or mass production. Line art can mean anything from actual line art, to simple mechanical components that can be easily drawn with a line, such as a gear or wheel. CNC machines can replace machines like a scroll saw, and make part to precision. This article will discuss exactly how to create G-code from line art.

Step 1: Get a good digital image of what you need to create

If you have line art on physical paper, such as a scroll saw pattern you purchased, you need to first digitize the image. A digital camera won’t do, since even the slightest tilt will result in a stretched image. What you really need is an orthographic view of the page, and a digital camera can’t do that for you. You will need a scanner with at least 600 DPI scan resolution. To my knowledge, this is about as high as most scanners go, and is a common number. Scanners are cheap if you’d like to purchase one, just make sure if has this scan resolution. Otherwise, you can use a FedEx Express if you only have a few pages that need scanning.

Step 2: Cleaning up the digital image

CNC machines

Scanning in the image is not enough. You must clean it up before you can use it to create G-Code. Your goal is to produce a nearly black and white image with very crisp lines. To the left is an example of the clean up process before and after. Note, the image is fairly large, but you can see all the details if you click on the image.

There are a couple things you should notice. First, the image is very large, especially for a gear that is about 3 inches across. This is because many CNC machines are accurate to a few microns, so you need to keep everything in the original resolution of 600 DPI. This will produce good, accurate G-code later on. In the original image, notice that you can see the other side of the page leak through, as well as one of my hairs that apparently got on the paper before scanning. These can both be easily removed.

To perform the image cleanup, you’ll need a copy of Photoshop. If you don’t own a copy since it’s expensive, Adobe recently started a model where you can rent Photoshop for a month for thirty dollars or so. That puts it well within reach for many CNC hobbyists. To clean up an the edges of all the lines as well as eliminate the bleed through from the back of the page, go to Image->Adjustments->Levels, and play around with the arrows under the histogram until you have nice, crisp lines that are very well defined. Don’t go too far, however, since you don’t want any breaks in any of the lines. To get rid of other defects such as hair or text, you can use the small eraser in Photoshop and zoom in very far. This will let you remove any defects with ease.

Step 3: Generate G-code

To generate G-code for you line art, you’ll need just one more program. I use a program called VCarve Pro. Frankly, I wasn’t expecting too much from VCarve Pro at first, but it soon exceeded all of my expectations. Unfortunately, it’s a bit expensive, at $600. If you can get access to it by TechShop, or a university for a day, that will be the more sensible solution for you. They also have a free trial version so you can see what it’s capable of, but it won’t make G-code for you. It takes maybe an hour to learn how to use it proficiently. You’ll need to go through two basic phases. First, define your material size and thickness. Then load an image…a PNG image you exported from Photoshop. Note that you should use PNG format instead of JPEG because PNG is lossless. Yes, it’ll take more hard drive space and more memory to run your programs, but it’s worth it to have no compression artifacts. After you do this, you’ll need to draw lines in VCarve Pro using your image as a reference only. VCarve Pro does have some features, especially line tracing, which will automatically create lines for you based on your image. This is extremely useful. However, it is prone to have holes in it if your lines in the original image aren’t defined very well. Make sure the lines VCarve Pro generates are exactly correct. If you see holes or other problems, go back to Photoshop, fix it, export it, and load it into VCarve Pro and try again.
Once you have all your toolpath lines defined, You can switch to toolpath creation mode, and define all your toolpaths one by one. Finally, once you define all your toolpaths, you absolutely must simulate your design to make sure the toolpath is correct, and wont damage your CNC machine by going through the bed of the machine. Below is an example of the simulated output from VCarve Pro. Also included are pictures of me milling out my clock parts on a Shopbot CNC machine. Enjoy!

MLT-13_CNC

MLT-13_CNC_Template

MLT-13_toolpath_simulation

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