Little CNC Z-axis

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I am trying to rebuild a better Z-Axis for the smaller CNC machine. My requirements are:

  1. Increased travel. The current axis has 3/4" of travel, which limits both the depth of cuts, the size of the material I can cut and even the length of bits I can use.
  2. Better adjustments for squareness. The current axis has effectively none, and the bit is not square to the milling surface. Not a big deal for some engravings, but not very good when trying to cut out parts.
  3. Easier tool changing capability. The current assembly makes it challenging to change tools. There's too much structure around the collet.
  4. Reduced backlash. I want the bearings / slides / whatever to be tight. No play or slop.
  5. Light weight. The gantry style machine is flinging a lot of weight and adding more only reduces overall performance.
My first serious overhaul attempt was to have a pair of bearings squeeze a steel rod. The idea was that by squeezing the bearings, I could adjust for tension and even angle. The problem was the bearing were not only rolling, but also sliding as the rod went past (think of your car tire rubbing against the curb when you park too close). The friction was too much.

My current attempt is the common angle aluminum linear bearing assembly seen all over the net.

CNC z axis

This design gives a lot of travel, is rock solid (so far) and enables easier tool changing.

CNC z axis

It is 2" wider than the current stand.

Little CNC Z axis.

And it does weigh a fair bit, so I'm not sure how much I've improved there.

CNC z axis

The threaded rod across the back and the slots in the cross bars allow for tension adjustment and some squaring of the tool to the surface.

CNC z axis

These ribs are how the angle aluminum is mounted to the cart.

Left to do:

Adding a stepper motor with threaded rod and nut for motion control. I will need to cut/drill into the ribs to make room for the rod and the nut will be mounted on the bottom rib. Some form of structure to mount the stepper motor to. For now I will probably use a basic 1/4" 20TPI threaded rod. This will limit the speed of the vertical travel, but is simple.

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