This instructable is a quick guide to desktop CNC milling with an Othermill, a common small-scale machine that's mainly used for making PCB circuit boards. This machine can do everything a larger machine can, it just has a smaller build platform.
I have another instructable up on how to make the CAM setup for this particular model: 3D-Milling-CAM-Setup-Fusion-360
Step 1: Tool Change + Homing
Like every other CNC mill, the Othermill uses collets and collet nuts to attach the end mill to the spindle. It's important to insert shaft to the proper depth as described in the graphic below.
To insert a tool, insert the collet into the nut, then insert the shaft so that the flutes are exposed while keeping the shaft in the collet at the maximum possible depth. Use a pair of wrenches to tighten the collet. Don't tighten with too much strength though- this can strip the threads.
With the tool in place, you will need to home the X, Y, and Z axes. Homing tells the machine where the zero points are on each axis. The Othermill has proximity (prox) switches for the X and Y (horizontal) axes, so this can be done automatically. The Locate button in the Configure menu will allow you to automatically home the X and Y axes.
The Z axis cannot be homed with a prox switch because the depth of this axis depends on the length of the tool inserted into the spindle. The Othermill has a conductive platform that will automatically set the proper Z zero height when the end mill touches the plate.
To set this height, click on Change under the Configure menu, select the type of tool from the list (1/8" ball nose end mill in our case), then move the X and Y axes as necessary to locate the tool over an empty spot on the bed. Click Locate Tool, and the machine will move the spindle down until the end mill touches the plate. This will automatically locate the proper Z axis.
If your CNC mill doesn't have a conductive Z function, just put a piece of paper between the bed and the end mill, then nudge it down until you can move the paper with a little bit of resistance.
This process will depend on the CNC mill you're working with, but the concepts are more or less universal.
Step 2: Fixturing Stock
In order for a CNC mill to cut out a tool path, the stock must be properly fixtured to the bed. There are lots of different ways to do this which include vices, clamps, hot glue, and many other methods. Since our part is so small (3" X 3" X 3/4"), double-sided tape is sufficient for a firm hold on the bed.
Be sure to properly align the stock with the bed on of the corners. I'm choosing the bottom left corner of the bed which is X=0 / Y=0. Also, be sure to account for the thickness of the tape and add it to the thickness of your stock when you're setting up your cut.
Step 3: Running Machine / Cleanup
With the stock fixtured and the machine homed, you can run the gCode you created in Fusion 360. To run a file, just click Open Files in the Othermill dialog box and select the G-Code you exported from Fusion 360 in post processing.
Note that the Othermill control software lets you specify which corner the material is in ("at left of spoilboard origin"). My piece of wood is .76" thick, but I've changed it to .77" to account for the double-sided tape on the bottom of the wood.
For the Othermill and most other desktop CNC machines, a computer must remain plugged in to run the tool path via USB. You can pause the job, meaning it will resume where it left off if you click the Play button, or you can stop the job by clicking Stop. This will reset the spindle position and won't let you restart from where you left off. This toolpath takes about 1.5 hours as indicated in the progress bar.
Step 4: Finished Product
With the toolpath settings we created, the piece comes out kind of like a topographical map. With a smaller stepdown value, the layers would be less pronounced.