Moving a part: which method

[originator]

Hello all, I have an application where this 2020 tube will be used in a crazy contraption. There is a vertitcal main section, I call it Y since from the front it looks like Y travel. This Y has a so-called "bearing" with a plate on it as shown in the pic. On the Y it will have two of the plates/bearings stuck together, and some 45 degree supports that hold another section of perpendicular 2020 as well. This new horizontal piece will be called X, and will range from 4 to 8 feet. If it is 8 feet, another Y will be added to its end for support, as X would be too long. Then on the X boom, there is a Z boom mounted, this part is maybe 16 inches long.

Here is the current plan but I am open to better ideas:

Route the center of one side of the extruded 2020 leaving that side split down the middle( on all axis'). Drill a hole through their plate down through the center, and mill a part that fits slides in the center of the 2020 extrusion with a nut on it (or just tap it). High precision is not required, just repeatable to within .05. I am using a 23 bipolar 3.5 amp for each axis, and mounting the motor directly at the top of the square tube, connected to a threaded rod that will travel through the center of the extrusion, connected to the milled insert that will slide when the motor turns. As the stepper turns on the Y, the X boom lifts into the air. As the motor turns on the X, the Z boom slides along the X boom on the bearing/plate as shown. There is a third plate/bearing on the Z moving an object as well. This thing should be able to position any object anywhere along a wall 8' x 8' for example, and it has a GUI to control it already done to move it with presetalble locations for it to move to.

Now finally the question:

What to use to move the bearings/slides? I settled on the insert/threaded rod method as it was cheap, fast, standard rod could be bought at any hardware store, using any standard nut. The problem is the X length with produce some sag unless it is large rod, but I don't want the weight of 1/2 stock. 1/4 would be great but it will sag and bang around inside, unless some crazy support was done to hold the rod up when the nut wasn't around. A timing belt will not work for Y since it would mean the motors would have to stay braked to hold postiion, or add more braking hardware.

Thanks for reading this short book, any suggestions for cheap motion/threaded acme etc would be greatly apprecited.

If using long threaded rod, any good sources for 4' - 8'? Also what about acme threads tapping? How to tap it either with machine or hand? I have 1/4 28 and 20 threadmills, but never saw an acme version.