Now that I have raised the gantry up by 2", I have started to make the Z axis mechanism. After looking at several approaches and materials on hand and those that are available for low cost, I have settled on this design.

I salvaged a nice 8mm ground rod from an ink jet. I purchased linear bearings to fit this rod on ebay for stupid cheap- I think $2 each (more on these in a minute).

For the lead screw, I have gone with a stainless screw and delrin anti lash nut from ebay for $23 shipped (from Nicosia Cypress, of all places!). This lead screw is nearly 8mm o.d. and I obtained cheap flex couplers and ball bearings to accommodate it.

Here, I'm indicating the top surface of the aluminum block for the moving block for the Z axis. I tap the block with a nylon hammer until it is trued in the vise.

Here, I'm drilling the hole for the lead screw. First with 1/4" bit, then 3/8". Note that I'm using a 1/4 collet to hold the bit rather than a jacobs chuck. This is for rigidity and accuracy.

I don't seem to have taken pics of squaring and surfacing the block from a rough chunk of aluminum block. Also, no pics of the boring process. On the extreme ends of the block, I had to bore 15mm dia holes 24mm deep for the linear bearings. I used my new (old) Bridgeport boring head for the first time. It worked great.

Here is the nearly final result, after having machined the pocket for the delrin nut.

I'll be drilling and tapping holes for set screws to lock in the linear bearings, as well as mounting holes for the router.

About those linear bearings...I'm quite disappointed how stiff they are on the shaft. I was hoping for a silky smooth, low drag feel. These have a noticeable roughness to them, and they require a fair amount of force to move. I think the stepper will not have a problem moving them, especially given the fine pitch of the lead screw. This means that it won't take that much torque to move the block, as compared to a ball screw. I have some pretty high torque, old, Slo-Syn steppers, but they are pretty heavy. I also have scavenged a nice looking stepper from an old laser printer. This is considerably lighter and may serve well. I'll know when I hook it up and see how easily it moves the block along the rails.

While taking the thing apart just now, one of the linear bearings leaked some teensy ball bearings. At least 5, probably more. These are low quality bearings. I may use them, or I may try another approach.

One idea I have is to machine some delrin of PTFE bushings (I don't recall which I have). I have some 3/4" plastic rod stock, that is a white, low friction plastic of some sort. I also have an 8mm drill bit. So, I may turn some of this down to 15mm od, center drill with the 8mm bit and cut off to 24mm. I think this stuff would make better bearings than these cheapo linear bearings.