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.

I have started building riser blocks for the gantry. After looking around at my materials on hand, I decided to use some very heavy duty 2x2 square steel tubing. Not as easy to deal with as aluminum, but it will be plenty strong.

I took off the gantry and noticed that there is a recess machined into the base plate of the machine where the gantry columns go. Presumably, this eases assembly and ensures alignment during manufacturing. The recess is about .063". On the risers I have machined a mating protuberance.

With the risers, I'll have about 3 1/2" clearance from the table to the Z bracket. I can always make taller risers at some point if need be.

The work envelope is ~24" X,  12" Y and whatever I work out for Z. I'm thinking of going for about 4" travel in my mechanism.

Building my first CNC machine

Well, I am not so much building the machine as re-jiggering it. I am starting with a really nicely made X&Y axis, ball screws and nuts, and steppers.

A few years back I acquired a Dahlgren system 400 engraver system with a non-functional electronics box. It turns out that the power supply and the stepper drivers are functional, and at this point I have been able to drive them from Mach3 via an $11 parallel port breakout board. So far so good, at least for X&Y.

I have taken the thing mostly apart and have cleaned everything really well, and am now reassembling. The biggest issue remaining is what to do about the Z axis.

This machine was a 2.5 axis machine. The Z axis was simply up or down and used an air solenoid to do this. I need a full Z axis. I am planning to mount a Porter Cable trim router (at least initially). I hope to do things like engraving of panels and signs, pc board fabrication, and light machining of plastics and aluminum.

Here is a pic of the machine with the table off. The machine is build very robustly with very high quality machining.

But here is a problem. There is less than 1 1/2" clearance between the table and the Z mounting bracket. This imposes a design limit on my Z axis mechanism. 

Table installed.

How rigid will this 1 1/2" linear bearing be? It is a very high quality Thompson rail and bearing, but it is not very wide, hence inherently limited on resisting twisting force.

The mounting columns are 6" x 1" solid aluminum. I am considering machining riser blocks to raise the entire gantry up an inch or 2 or 3. If I'm designing a new Z axis, I want to have at least a few inches of travel.

The gantry columns are attached from the bottom with 4 1/4-20 allen head cap screws. I'll just need to buy longer ones.  I have a very nice Rockwell mill with DRO and I will be able to machine accurate riser blocks with no problem. It is just a question of how tall to make the risers.

Also, how much Z travel is reasonable to design into a machine like this? If I make 2" risers, that will give me 3 1/2" clearance between the table and the Z bracket. and that will be the max Z travel. Should I take it up another inch?

About the Z axis- I have a number of parts but my current plan is to use 8mm ground rods (from ink jet printers), cheapo recirculating ball linear bearings I got on ebay, stainless lead screw and delrin zero backlash nut from Nicosia, Cypress, bought on ebay. For a Z stepper, I have a few that I got from laser printers, and I also have some old Slo-Syn steppers. I'm trying to keep the mass of the Z axis mechanism to a minimum. 

I also have a 16mm ball screw and ball nut that I could use, but that will involve more work- notably I'd need to machine the ends. The cheaper non-ball screw and delrin nut will require no machining. It will ride in 8mm bearings and I have an 8mm flex coupler to connect it to a stepper.

It seems to me that you can't make use of very much Z travel anyway unless you have a bit protruding quite a ways from the router. The Z axis mechanism and the router body would get in the way- so maybe having more than a couple inches of Z travel is pointless. Thoughts?