I am really working on two machines simultaneously. One will be a small scale machine with a bed size of maybe 1' x 2'. I am leaving some flexibility as I have no real firm size in mind or real use for it other than as a test platform. Just small and easy to deal with. I plan on maybe using it for PCB cutting and small scale stuff. I might also put a plastic filament extruder on it for fun. The second will be a 4' x 8' machine bed that will accept full size sheet goods.
The control of each will be drastically different. The small one is going to be controlled by an Arduino Mega with EasyDriver 4.2 stepper controllers and some small steppers I picked up at Sparkfun ( http://www.sparkfun.com ) Why? Because I already had the parts. If I didn't, I would have grabbed some of the pre-made Sanguino stuff that RepRap and Makerbot use. It would be less hassle to use stuff that was already integrated.
The bigger one will be computer driven from a parallel port using parts I have ordered on Ebay from Hong Kong. It should be a month or so until they actually get here. Since it was only $260 including shipping for a 4-axis set including 4 steppers, controller board and power supply, I am happy with it in theory. What's not to like? The steppers are 1 NM (about 140 oz/inch) stepper motors which should be more than powerful enough for what I plan to do with them. Why so small? That goes to the whole idea of my building ideas. If things are designed well and you are making reasonable cuts, it shouldn't take a lot of power. This is going to be a hobby machine and not some production monster that will be going nonstop for 2 shifts a day.
The linear bearings will be cheap. I really can't justify going beyond stuff based on standard 608 skate bearings. They are high quality bearings that are dirt cheap and readily available. I really can't see spending lots of money for the fancy recirculating bearings and such. Life is full of compromises. In this case, the price is going to be the #1 priority in the decision making process.
The last design choice that has already been made is for the actuator mechanism. I thought about this for a long time. I considered plain threaded rod lead screws, acme threaded lead screws, ball threaded lead screws, rack and pinions, timing belts, and timing chains. The choice really was super easy in this case. I went to SurplusCenter.com and found the #25 (1/4 in pitch) chain for 60 cents a foot in a 100' spool. The sprockets came in at $1.75 a piece. Pretty hard to beat that short of getting it for free. Is it ideal? Well there are a lot of factors to consider but the price was so good on this that it outweighs everything else. It should also have very little backlash issues to deal with.
Software. Well, I really don't know yet. The only CNC experience I have had so far is to play with a little nearly antique Emco Unimat-PC lathe and a Roland CAMM3 that I brought back from the dead. I have never had a real CNC machine and I will probably start with EMC2. I am also following the development over on Phlatboyz.com for a plugin for SketchUp! that generates G-Code. Looks like it is coming along really well and will be working long before I have a machine to use it on. We shall see. If I don't like it, I am also looking at the USB controller and software combination that they have over on http://www.planet-cnc.com/. It looks like it could be a really nice solution as well. I will try the free stuff first.
Most of the parts will be off the shelf. I know I will probably have to fabricate some things. For that I have a little Harbor Freight 7x10 lathe with a milling attachment. Nothing fancy but it should be good enough for what I need. Beyond that I will just be using basic hand tools and a few portable power tools. If I get in over my head, I do have access to a fully outfitted machine shop and a pretty cool array of rapid prototyping equipment at work so I am not worried about painting myself into a corner. I also have access to a whole building full of engineers to pester if I have anything I can't figure out for myself. Standing on the shoulders of giants and all that....
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