CNC Build – Part 1: Mill Design
May 3, 2011 Leave a comment
I’ve spent a few weeks toiling over possible designs for my CNC mill. I’ve fought to keep a balance of effectiveness and simplicity. I want it to be a full-fledged CNC, but I also continue to hear my dad’s voice in the back of my head reminding me of the engineer’s cardinal rule: K.I.S.S. (keep it simple, stupid). The key element was that it needed to work without requiring hard-to-find or expensive parts. This project is meant more as a way to prove to myself that I can produce something this complex and less of producing something that will be immensely helpful in my long-term woodworking. That being the case, I didn’t want to spend a lot of money on this if it turns out to be a massive failure.
With this mindset, I came up with the following criteria:
- The unit will be made of wood. Metal might be stronger and might last longer, but if I find that a part of my design isn’t working, I won’t feel bad about ripping out a 7 inch long chunk of 1×6. That kind of mistake only costs a few cents to fix.
- A CNC that can cut a 4’x8′ sheet of plywood would be really cool, but A) I don’t have space for that in my garage and B) what would I be working on that would need computer-controlled cutting of something in that scale? If I’m going for the detail that only a computer can give me, it is bound to be something small anyways, so the max size can and should be less than 24″x24″. This will give me a mill that can sit on the corner of my workbench when not in use.
- Pre-engineered electronics. Yes, I could probably design my own circuit board for a 3-axis stepper driver, but why? So many already exist. I don’t need to re-invent the wheel on this one. Furthermore, interfacing the drivers with the computer via the parallel port is done with a breakout board. These, again, already exist. I don’t need to design my own. It may cost a few dollars more, but it is just that: a few dollars. In the grand scheme of things, the extra $10 it will cost to buy pre-designed electronics will save me more than that in the value of my electronics R&D time.
So I sat down behind MS Visio (a tool I love for my woodworking projects) and began to mock up mill designs. Using elements of mills I had seen in other people’s projects, I came up with what I call CNC-V1.
In version 1, a fixed gantry is centered over a 2’x4′ unit. The table slides back and forth under the arm for the Y axis. The arm contains a sliding panel to create the X axis. The sliding panel holds the router carrier, which moves up and down for the Z axis.
The Y axis table contained bearings with a grove in the middle, which were designed for sliding patio doors. These ran across the edge of steel angle iron, which was bolted to the bottom of the unit. Overall, this worked. The table top rolled nicely over the angle iron, but the unit was much too big. This type of design requires twice the footprint of the maximum carving size. Back to the drawing board.
After a few days, I found a concept that I liked. Rockler sells a CNC mill that uses a moving arm instead of a moving table. This allows for a smaller footprint. Their CNC is a couple thousand dollars, but I figured I could create the mechanicals of mine for under a hundred.
Here, two under-mount drawer slides are attached to the top of the base. These support a cross arm which then rolls across the depth of the unit (Y axis). A stationary table is then supported above the arm. The arm is then attached to two uprights to support the X axis as it did in the first version.
The drawer slides worked significantly better than the v-groove bearings on the angle iron. Tolerances are much tighter and the fear of the bearings lifting off of the angle iron was removed. The only issue is that when the arm slides back, the upper part of the two drawer slides extend out of the base. This was not an issue since the table support across the back could be shortened to allow them to pass by. I found this to work so well that I’ve changed the X axis to be based on the same slides. For $10 a pair at Menards, this was a fantastic find.
One Friday night and one Saturday morning and the unit took shape.
At this point, the upper slides are attached and a panel connects them. A stub of a 2×4 comes off the back of the panel and passes between the 1×4 upper arms. I still need to design the Z axis, but I assume it will act much like the first two. Stay tuned for updates.