Long Acme threaded rod

Try not to laugh...
 
I'm working on designing and building a CNC mill/engraver. I'd like it to have a cut volume of approximately 1000x1500x100mm (about 3ft by 5ft by 4 inches) that can cut aluminum plate and hardwood, at most. I expect to take multiple passes to cut through an aluminum plate.
 
I'm basing the design on the ShapeOko machine sized up, using 8020 in the 25mm size with Openrail hardcoat and grooved steel ball or delrin bearings rather than ground rods and linear bearings. The Y travel will have two steppers driving it.
 
Something like this, except the Y rails will be fully supported by another piece of 8020 extrusion bolted alongside and the 8020 along the front and back will be raised so I can slide a longer sheet in:
http://powercordlabs.com/1200mm-x-1200mm-shapeoko-build/
 
Openrail:
http://openbuildspartstore.com/openrail-hard-coat/
 
The ShapeOko uses timing belts to move things, but for the size I want, I am worried about stretching. So I'm planning on using Acme rod. However, I'm worried about a 1500mm long Acme rod whipping. I already have some 6ft 1/2-10 and there are anti-backlash Delrin nuts available, but that seems thin for such a long rod.
 
I have considered ballscrews, but the price is fairly high and it is cheaper to get a more powerful stepper to make up the power loss. Although if there will be a significant performance difference, I'll save a bit longer. I am not trying for 0.5 thou cut accuracy. I'll be cutting aluminum panels for equipment builds (slots for slide pots, holes for controls and buttons, holes for displays), and engraving/2-1/2D machining of plywood, hardwoods, machineable wax, plastics, etc. Probably the parts requiring the most accuracy will be wooden gears for clocks, driven by a stepper motor.
 
Am I completely off track? There are also steel wire reinforced timing belts available.

Steve Greenfield AE7HD
http://www.linkedin.com/in/stevenjgreenfield

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