Low Cost CNC Machine Part I

I am currently in the midst of trying to build a low-cost 3-axis CNC milling machine.  I have no basis for making one, I don't even have a project in mind that needs one; I just want one, and specifically one that I've made myself.  This means that I will invariably (a) spend way more time/money building it than needed if I just ordered one, (b) will probably end up with a sub-par machine and (c) will be heartbroken when it has to be taken apart because it is absolutely deafening when running. Oh well.

I've seen a number of DIY machines online, and while I find them interesting, none really do it for me.  I would like to end up with a mill that can handle a variety of materials: wood, plastic, protoboard and ideally, aluminum.  Milling hard materials at sensible feedrates will require a stiff machine, so plywood and MDF framed machines are out, as are (in my opinion) belt and chain drives since they can be backdriven.

Beyond having just a plain CNC, I would also like the ability to add an extruder head to make a 3D extuder.  This means that the feedrates must be reasonably fast; based on the Makerbot at the Vancouver Hack Space, (VHS). I would like feedrates of about 50 mm/s, with a maximum feed of over 100 mm/s so the motors aren't running full-out all the time and so the machine has decent acceleration which will keep it responsive.  This means that I will also need a fairly light machine, which is completely at odds with the stiffness requirement.  Beyond that, I would like to be able to build the machine with a minimum of machining, in order to keep costs down.

Nevertheless I plunge ahead.  Compromising between the stiffness and lightness requirements, I have decided to make the machine from aluminum.  To give maximum flexibility in the design and to maximize component reuse when the design inevitably changes, I have decided to make all frame components from stock sections of aluminum, i.e. bar and plate stock.  These have good tolerances (relatively speaking) to begin with and nice factory edges from which to reference other features.  I've further decided that all components should mount with 1/4-20 UNC bolts on 1" centers.  These bolts are cheap in a wide range of lengths, and using this spacing allows me to prototype on the optical table at the lab.  This will mean a lot of tapping, but it's time I learned how to tap anyway.  The frame components themselves will be drilled with 5/16" holes to allow a sloppy fit, which will allow minor offsets and angular misalignments to be corrected before tightening everything down.

I've also decided to take advantage of my excellent access to the Makerbot at VHS to prototype the various parts before machining them from aluminum.  As conceived, everything can be made with a cutoff saw (which Metal Supermarkets has, at 1$ per cut) and a drill-press (which the hackspace has).  However the machining is a time-consuming and error-prone process for me, so having a design without interference or other gotchas will cut down on frustration.

That's all for now....