CNC Mill Design

[MattP]

I just graduated from High School and need a project. I took every metal course I could at school, even had the teacher make a new one for me. I always loved machining, never got the opportunity to work with a CNC though.

So, I'd like to make a CNC Mill. My original design idea was like a typical CNC Router, with the bit moving, stationary table. I started thinking about that, and it would seriously lack the stiffness needed for any reasonable quality of machining. I'm hoping to be able to (slowly) machine steel with this.
I'm working on my second design now. It's all 2x2x.125" square tubing, with the X Axis being the table and Y Axis being the bit. Both have a 2" square tubed rail running their whole length, with skateboard bearings or similar pushing into the rail, stiffening it up, so it'll freely slide along the axis. I'll weld 5/16" nuts running along a pair of 5/16" threaded rods moving it along the axis.

On the electronics side, I don't know much. For the bit, I'm thinking of using an electric mower motor, using gears to adjust the speed accordingly. I probably won't have that computer controlled, just on/off.
X, Y, Z axis, would the 305oz/in 3 axis kit here be a good fit? http://hobbycnc.com/hcncpropkg.php I'll probably change the ratio with gears to increase the steps and torque. Is this what most people are doing? It says it needs an input of ~35V DC, does that just mean a DC adapter that steps down to 35V?

Did any of that make sense? My terminology is.. lacking..
Am I on the right track here? What kind of tolerances can I expect?

[btopley]

I would like to offer help, although I am afraid I am not wise in the way of CNC. For a first simple project to introduce you to the world of cnc, try the instructable design
http://www.instructables.com/id/Easy...hine/?ALLSTEPS

or

http://www.instructables.com/id/How-...and-/?ALLSTEPS

[JoeDawg]

Hey Matt,

Once you've been bitten by the bug, its hard to turn back... I recently ordered a cnc ready mill and cnc'd it, and the only thing stopping me from building another from scratch is, of course, $$$. I would highly recommend buying you first mill and going through the process of converting it and upgrading it to get familiar with the concepts of cnc. EVEN IF YOU DON'T INCLUDE THE VALUE OF THE HOURS OF WORK YOU PUT INTO IN (there will be many), BUYING THE MILL PRE-MADE WILL STILL BE CHEAPER!!!!!!! Sorry to shout but you can't go into this thinking your going to save money by building your own mill. You can get an X1 for $340 bucks and can reuse the spindle and motor when you build your own (or at least design around it planning to buy another) http://www.harborfreight.com/cpi/cta...emnumber=47158. Going this route you can get an operating cnc mill for less than $1000 bucks (I got $2000 in my taig). Dont count on this figure for building your own unless you score some serious deals on used components. If you decide on building you own however, plan on hundreds of hours of research before beginning to purchase parts. Heck, i did that just for my conversion...

Concerning your frame.....
I've considered many different designs, including a similar box type design, and in the end I always end up back at the c pillar design. Pretty much all "mills" are c pillar, and "gantry"s are the stationary table, moving tool (the cutting tools are called mills also, and for simplicity, most just refer to is as a "tool", bit, not so much), even if they are used for metal. The reason being that driving a tool through metal generates a moment force that deforms (elastically, hopefully) the structure of your machine. The magnitude of this force is proportional the the length of a moment arm (think using a cheater pipe on a ratchet). With that in mind, you want as much of these forces as possible absorbed by your static structure (i.e. your frame) and as little as possible by your dynamic components (you ways), and the c pillar design does this very well and still manages not to limit one axis of motion (the x). So basically, 200 lbs of c-pillar will be stronger than 200 lbs of what ever other design you might go with. Also, the c-pillar design minimizes the amount to precision required in construction, (Only 1 or 2 angles) verses the many that would have to be made in your design even inaccuracies of .1 degrees will compound and could have detrimental effects on the accuracy of your machine (get ready to buy a really good machinists square).That being said your design could be made accurate and it could be use to cut steel (maybe not with 2x2x.125), if you have the right combination of time, motivation, and, of course, money.

Of course there are several other concepts what ever you decode, just let me know which route you decide on and i'll try to point you in the right direction.