Greetings, fellow maker-person! Today I make your dreams come true! This is, of course, assuming that your dreams involve making wooden tops with a 3-axis CNC. This top would traditionally be carved with a lathe, which would work beautifully, except that I do not have access to one. However, I do own a CNC router, which (as you will soon see) falls nicely into the category of Good Enough. This set of instructions will take you through the entire process, from the virtual design to the finishing of our wooden top.
Step 1: The Design
The main limitation (or primary frustration) of a 3-axis CNC is the fact that its cutting tool hovers over the piece--unlike a lathe, a 3-axis CNC can only cut from the top, and thus, can only carve one side at a time. To make this project possible, the top I designed is composed of two parts joined together. To spin properly, the top must have A) a sharp point to balance on, and B) equally-distributed weight, so that it doesn't immediately fall over. Therefore, as long as the two pieces are centered on each other, distributing their weight equally over the sharp point, the top design will work.
Thankfully for me, because I am lazy, most CAD programs have a tool that lets you rotate a 2-D drawing to create a solid model, instead of actually doing the 3-D work to create one. Using this glorious function, I created the top and bottom sections of the top. For your viewing pleasure, I was wonderful enough to include my sketches above. All units are in mm, because my CNC prefers its files in mm, and is highly emotional when I try to give it anything else.
To save you from potential future agony, I recommend making a small hold in the center of your top piece at this stage in the process. This is to help you when drilling the dowel hole (or just nicely ask your CNC do the drilling, if its feels up to it). When you are all done designing (HOORAY FOR YOU!!!), export your design as a Step or Stl file.
Step 2: Creating the G-code
To make your CNC do what you desire, you must learn to speak its language. The CNC's native tongue is called G-code, and you will learn to love this language, because, hey, you have no choice. To translate my instructions into this language (for my 3-D object), I used a program called FreeMill. As the name implies, it is a free, as in, "cha-ching," which naturally implies that it is the best software available on the current market. (HAR!)
When you start up FreeMill, you first will need to open the file containing your design. FreeMill will then take you through a number of steps to create your g-code.
Set Cutting Direction: First, you will need to tell the program which direction is up, to make sure that you and your CNC are on the same page. You can use the rotate buttons to do this (make the Z direction point up).
Create Part Bounds Stock: This step allows you to define how large the chunk o' wood you'll be cutting from will be. You'll do this by adjusting x, y, and z values to create a "box" around your object that you're going to be cutting. This box represents the chunk o' wood it will be cut out of. Just make sure that your ENTIRE OBJECT is within this "box!" Sometimes, it's a good idea to raise your "Z+ Off" number by a small amount; this raises your entire system, in the eyes of the CNC, and makes sure it cuts all the way through.
Set Work Zero: Next, you need to decide where the "0,0,0" place--or the "Start Cutting Here" point--will be. Since this is a top, I recommend using the highest Z at the center of the part.
Create Cutting tool: Because your program is smarter than you, it will automatically take the size of your cutting tool into account. The reason that you own it and it does not own you, however, stems from the fact that only YOU know how big that cutting tool actually IS. Enter the size information in this step, wise CNC master.
Set Cutting Feeds and Speeds: In this step, you will tell the program how fast to run your CNC. These values depend on the material you are using and how much your CNC likes that material. Figuring out these numbers will take some testing.
Create Machining Operation: In this tab, you will instruct the program how to cut your part, and how much of an overlap you want each of your cutting passes to have. FreeMill works by making your CNC follow the surface of the part back and forth. If your CNC (like mine) does not like to cut an entire part at once, then you can use parting planes to cajole it by telling the program to only cut the piece down to a certain point. To do this, select "parting plane" from the surface menu, select your part and chose where to place the plane. Then, create a different file for each cutting depth.
Post Process Operations: For this final step, chose your CNC from the list (or one close to it) and press post. A text file will come up containing your g code, and you will then rejoice.
Step 3: Create Your Stock
If you already have wood thick enough to use then you can skip this step. Because I am poor, I did not, so I took the corpses of some failed previous projects COUGH some oak plywood scraps and glued them together to create a thicker piece large enough for my parts. Once this was dry, I screwed this piece underneath my CNC for cutting.
Step 4: Start Your CNC!
It is now time to bask in your sheer maker brilliance as you watch the CNC obey your will. In the mean time, this will take a while, so we recommend going and making dinner or something (while staying within earshot, of course).
Step 5: Join the Pieces Together
Once the CNC is done ("Good CNC. Goooooood CNC,"), take the pieces and glue them together. Make sure the keep the 2 parts lined up! An easy way to make sure that you didn't mess up is to place a small nail into the center of the top and (after allowing the glue to set for a minute) spin the top gently. Do this with confidence, because these types of projects can smell fear. If the parts are lined up, the top should balance relatively (this can be a generous word) well when spinning. Sanding the balancing tip into a sharp point will improve the balance more, because it will demonstrate to the top that you are its master.
Step 6: Drill the Dowel Hole and Finish
Remember the hole I suggested you make in advance, all the way back in step Long Time Ago? Now is the time to use it! Using this hole to guide your drill, drill increasingly large hole until your dowel can be snugly and lovingly crammed into it. Glue and clamp this in (like I have shown) (you're welcome), and then go make yourself another meal while you wait. We recommend fried chicken.
After this is dried, take some sandpaper and sand down the milling marks. Once you have a smooth surface, continue sanding and spinning the top until you are happy with its balance. Finally, apply whatever stain, wood burning, paint, etc. that you want until it looks as beautiful as you.
Yeah, yeah, we know. Impossible. But pat yourself on the back, because you just got DARNED close. (WINKY FACE EMOTICON!)