The actual information that a CNC machine uses to determine where to move is called G-code. There are many ways to not only transfer the G-code to the CNC but also to actually generate the G-code. Today I’m going to show you a simple workflow that can be used with SketchUp, MakerCam, and Universal G-code Sender. At the end of this article I will have download links for the programs and plugins used in this workflow.
There might be some better programs out there that can do it all but the main objective here is to get control of both the design phase and the tooling phase without spending money on expensive programs. I also have been playing around with a SketchUp plugin to directly generate G-code in SketchUp but that will be for another day.
Designing exactly what you want in SketchUp
The benefit of designing in SketchUp is its incredibly easy to work with and it has great accuracy. For this example project I am going to be making a 3” 5 point rounded star knob. It will also have a riser block to elevate it slightly. The material I am going to be cutting is 3/4” cheap CDX pine plywood. I made the model in 3D for illustration purposes only. It’s not necessary to actually create a 3D model for this step as you are really only going to be using one plane to get the faces from. The rise block will be located in a shallow recess on the knob. This will ensure that both interior holes for the bolt remain concentric.
Export Faces To SVG Plugin.
After making sure all of the faces we need are on the same plane (they don’t technically have to be, it just makes things easier) we can select the faces to export and click on our Export Faces To SVG plugin (download link at the end). The first thing to do is click the “…” button to determine where to save our SVG file. Then name the file as you wish and also add the .svg file tag at the end of the file name. Select inches or millimeters depending on what you are using. Then click save.
Defining exact toolpaths in MakerCam.com
Now we have a precise drawing of what we are wanting to cut that can be opened in MakerCam. MakerCam is a flash-based 2.5d CAM and nesting program that outputs standard G-Code for use in CNC fabrication devices and runs directly in your browser by going to MakerCam.com. Once there you can click on File > Open SVG File. Browse to where you saved your SVG and click open. I won’t go into great detail on actually using MakerCam as in the top right corner of the MakerCam window is a link to a great tutorial page to get familiar with the program.
The actual tooling specifications and speeds will depend on your setup. I was using a 1/4” bit in a Bosh Colt router. I’ve had success running some pretty aggressive speeds in it so far but I backed it off slightly for this example. It’s good practice to create the toolpaths that you want to cut in the actual order that you want them cut. For this model I used the following sequence:
- Drill hole in center of riser block
- Cut outside profile for riser block
- Drill hole in center of star knob
- Cut interior shallow pocket for the riser block in the star knob
- Cut outside profile for star knob
With all of your toolpaths complete go to CAM > Calculate All. This will calculate the paths needed based upon your bit diameter to complete your specified cuts. After everything is calculated green paths will be displayed letting you know exactly where the center of the bit will travel. There are also some green triangles that indicate where the bit will plunge down and also the direction it will travel. Because the outside cuts are through cuts we need to add some tabs so the piece doesn’t get damaged when it is freed from the rest of the material.
First, the paths where you want the tabs to be on are selected. Then go to CAM > Add Tabs To Selected. From here you can specify the spacing between tabs (you can move them around once placed), the tab width, and the tap height. It is important to note that the tab width does not take into consideration the diameter of your bit. For example, if you want a 1/4” wide tab and are using a 1/4” wide bit you need to specify a 1/2” tab width in the menu. I found this out the hard way and confirmed that was the case after a quick Google search. Then press OK to view your tabs. Click on the tabs and move them around as needed.
All the tooling should be complete at this stage. To generate actual G-code go to CAM > Export Gcode. If you created the toolpaths in the order you want them to be cut they should already be in the correct order. But if you didn’t you can always select one of the toolpaths and then select + or – to raise or lower it on the list of cuts. Once you have them in the proper order to cut select the all button and then Export Selected Toolpaths. In the next menu you can specify the name of your Gcode file and the location in which you want to save it.
Universal Gcode Sender to communicate with your machine
With the Gcode generated you need to actually send it to the machine. For that I use Universal Gcode Sender. Once the program is launched click Open in the connection box to establish a connection with your machine (the machine must be on by the way). Your machine may be setup a little different than mine but before I cut I always put the cutting tool at the home position with Z=0 directly on the top of my work surface. With my machine in the home position I always go to Maching Control and cick Reset Zero. Then go to File Mode and click Browse to locate your saved Gcode file. Once selected you can select Visualize to see a representation of exactly what your cut will look like. If everything looks as it should press Send to send the cutting information to the machine. Make sure you turn the cutting tool on as you do so. The machine should now be going through the created Gcode.
The scrap plywood I was using had some ebony stain on the top layer which actually looked pretty neat when it was all cutout.
After cutting the piece lose from the tabs You can test the fit of the riser block. If everything went according to plan the riser block should fit securely into the shallow pocket. I ended up making a bunch of these star knobs for my new router table fence. The center holes are 1/4” in diameter which is perfect to manually thread a 5-1/6”x18tpi carriage bolt into to create a threaded knob.
- SketchUp: I have been using the free SketchUp version 8 for years. I already have multiple versions that you can download listed on this page.
- Export Faces To SVG plugin for SketchUp. Google is your friend if you don’t know how to install plugins in SketchUp.
- Universal Gcode Sender
- Right click here and select “save target as” or “save link as” to download my SVG file for these star knobs.