CNC Routing

This page is intended to help anyone who wishes to build / operate a 3 axis CNC router. It discusses software compatible with commercially available driver board, microcontroller development board, and a raspberry pi computer. It specifically discusses using the bCNC program on the raspberry pi to communicate with the microcontroller running grblHAL. Pease note that CNC milling is not my area of training and that my experience here is very limited. If you see glaring errors, feel free to point them out to me (j6summers@gmail.com).

Hardware

We have a 3 axis CNC mill that we assembled from a kit we bought from OpenBuilds. While this supplier is no longer in business, here is a link to a hardware kit by another manufacturer that is based on the same design. Here is a video showing how it is assembled. The machine is made from aluminum V-slot rails, which are available from a number of online suppliers. MakerStore.cc is one such supplier. MakerTechStore.com is another. In addition to the aluminum rails, the build requires the following: 1. gantries that ride on wheels along the rails, 2. plates at the ends of the gantries that accept the drive screws and motor hardware, 3. drive screws, nuts, bearings and standoffs for the motors, 4. motors (NEMA 23 steppers) and couplers (that connect the motors to the drive screws, 4. a small router and mounting hardware, 5. a hefty power supply (we use 24V DC) and 6. a controller board (we use the T41U5XBB with a Teensy 4.1 microcontroller which was one of the earlier available for grblHAL), and stepper motor drivers. Of course, you need a computer to drive it. We use a Raspberry Pi model 4b.

We also built a plywood box to keep the noise and sawdust down. Sawdust made out of aluminum chips is especially hard on electronics. Don't ask how I know.

Controller board

As mentioned above, we use the T41U5XBB driver board with the Teensy 4.1 microcontroller. Since we built our current configuration, a number of other boards have become available. If I were to do it over, I would go with the PicoCNC board. This choice has two advantages, first is the low cost of the pico ($5 USD for the Pico versus $30 for the Teensy (at the time I wrote this). The second benefit is the ability to program the Pico via drag-and-drop.

As an add-on, we built a jog-controller (described in the next section) that allows us to jog the tool around without sitting in front of the computer. The jog controller connects to the I2C interface pins, which are broken out as shown in the upper right of the figure on the left for the PicoCNC.

Keypad jog controller

Since our machine is in a plywood box, it is not easy to see the position of the cutter while sitting at the control computer. The jog controller (photo on the left) allows me to jog the tool around while physically in front of the router. The controller incorporates a RPi Pico microcontroller on the back of the pcb and a bunch of buttons on the front. The Pico communicates with the router controller using the I2C protocol. The grblHAL firmware is already set up to interface with a keypad, but requires that you include the "keypad" plugin when you compile. This is easily done using the web builder (described below in the Firmware section). The jog controller has buttons to jog in X, Y, and Z (plus and minus) and also 45 degrees between the x and y axes (+X+Y, +X-Y, -X+Y and -X-Y). It also has a button to toggle between three jog speeds (rapid, slow, and step). The oled display shows the current jog speed setting.

Software

Computer control software.

The first thing you need is a program that talks to the microcontroller on the driver board. A program called bCNC works well on the Raspberry Pi. You can download bCNC for free. While the instructions for installing the code are basically correct, downloading packages onto the current Pi operating systems requires you do so in a virtual environment. Here is a description of how that is done.

Like other CNC control software, bCNC has a terminal that allows you to look at the code passed between the computer and the machine. In this terminal (accessed on the upper right tab) you can view and edit a number of machine settings. To view the settings, type $$ in the command line. Here is a page to help understand what the settings are.

Firmware.

The firmware you need will depend on the driver board you use. We use the Teensy 4.1 microcontroller with grblHAL firmware. One benefit of grblHAL is that you can download compiled firmware from a web builder. The web builder has a number of tabs that allow you to customize your software. One of the tabs is "plugins". On the plugin tab you can chose to enable the keypad functionality. Even if you don't have a keypad, it doesn't hurt to have this enabled in case you decide you want one later.

Machine Settings

Setting up your CNC router requires getting the machine settings correct. The settings include things like the number of steps the microcontroller sends to make the machine move a certain distance. To find the machine settings, type $$ into the terminal. grblHAL will respond by sending a list of machine settings and their values. Unfortunately, grblHAL does not give a description of what each setting is in this list (I think that Universal GCode Sender, UGS, does). Here is a link to a page with a listing of settings.

Common Errors: Once you load up the firmware, grblHAL is expecting normally closed limit and probe switches. If you use normally open limit switches, you will get error message from bCNC that say: "XYZHSEP". These are seven individual error messages that are associated with not having the expected hardware configuration. You can fix these by opening the terminal and sending the following commands: $5 = 7 (this inverts the x, y, and z end stops, fixing the X, Y, and Z errors). Sending $14 = 70 inverts the emergency stop (E), hold (H) and start (S) switches. Sending $6 = 1 inverts the probe switch, fixing the P error.

Design Software: CAD and CAM

The fast and easy way to get from idea to product is to throw money at it. That is not what we do and I am not going to pretend to know the ins and outs of what is involved. Here is a review of some options. Autodesk Fusion offers a version with limited functionality for personal use is free for 3 years. After that, I assume that you need to pay for it.

CAD software.

The first step in CNC milling is drawing what you want planned. This is done in Computer Aided Drafting (CAD) programs. While there are a number of good CAD programs for generating .stl files for 3D printing, this doesn't work for CNC milling. Inkscape is a commonly used program for creating 2 dimensional drawings. Once you have a two dimensional drawing, you need to translate that into GCode using CAM software.

CAM software.

CAM is an acronym for Computer Assisted Manufacturing. Here is a reddit page that discusses some of the options.

While 3D printing usually uses .stl files, milling requires files written in GCode. Some programs require files in DXF format. We have used a program built into our controller software (bCNC). The CAM functions of bCNC are not intuitive and it takes a bit of time to figure out how to use them. There is no documentation or video available.

PyCAM, a cautionary tale: One CAM program that looked promising is PyCAM, an open source python based program that works with Linux. Pycam has a problem with one of the modules () being deprecated. This can be gotten around using code in a pull-request from the GitHub page.

GCode can be generated from drawing files (usually in DXF format) by a number of programs. Here is a link to a web page where you can download a program for converting DXF files to GCode. Here is a link to an online program available from the folks at OpenBuilds. Here is another online program.

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