Z-axis Ball Nut Carrier

5 Bears Home Homebrew CNC bench mill

With the CAD drawing complete for the column, I engineered a ball nut carrier which would bridge the gap between the ballnut and the spindle backplate. A ball nut carrier is nothing more than a piece which connects the ball nut, which is permanently attached to the ballscrew, to whatever you want the ball nut to move.

With CAD, it is a simple matter to ensure that your drawing accurately reflects what you have done so far in the real world; then, simply draw a nut carrier which fits around the nut, attaches via the nut's 4-hole bolt pattern, and joins the spindle backplate, with just enough adjustment, via slightly loose mounting holes, to join the two without adding stresses to the system.

Here is the drawing. The carrier is open at the bottom so that it may be removed entirely from the ballscrew. If the bottom had been closed, it would have to be mounted before the ballscrew is affixed to the ballscrew backplate.

The four holes in the end view are tapped to accept the SHCS which affix the ballnut to the carrier.

On the top, two holes are drilled and tapped for 1/4 x 20 SHCS. These wil be inserted through the spindle backplate and into the nut carrier, thus making the spindle backplate, nut carrier, and ball nut, one fixed unit. Spacing of the two big SHCS is 46mm.

A block of FORTAL aluminum was milled to external dimensions. It is especially important that the angle formed by the top and the end of the block, where the ballnut flange mates with the carrier, be exactly 90 degrees. The other angles and dimensions are not as critical.

Work begins on the top, with the two 1/4" SHCS tapped holes being drilled and heavily countersunk.

The countersinking makes for a neat hole, and keeps any burr raised by the tap below the surface. It also allows the tap, and ultimately the SHCS, to start with ease.

Here, I am using a spring-loaded tap plunger to get the tap started straight. Once the thread is started, the remainder of the tapping is finished by hand outside the vise.

The block is rotated ninety degrees, and the two holes we just drilled and tapped are on the far side of the block, against the fixed jaw of the mill vise. The process of drilling and boring the main diameter of the nut carrier commences with a 1/2" through-hole.
  This is followed by a series of passes with a boring head. The diameter is opened up to ballnut diamater + 0.010". This allows for some later positional adjustment. The ballnut carrier does not grip the ballnut on the body; rather, 100% of the connection occurs between the ballnut flange and the nut carrier, where four 8-32 cap screws are used to make the connection at the front of the nut. Given this, and I stress it once more, it is very important that the surfaces defined by the end (mates with ballnut face) and the top (mates with spindle backplate) be truly perpendicular, or once again, when the cap screws are tightened, the system will be stressed.
With the boring complete, the bottom of the nut carrier is opened up with a roughing end mill.

Look at the end view of the drawing... the squared-off modifications to the circular boring serve two functions. At the bottom of the nut carrier, they allow the carrier to be inserted over the ball screw aft of the nut, and the nut carrier may then be brought forward to ride over the ball nut. The upper end of the nut carrier is milled to clear the ballnut's return tubes, which extend beyond the diameter of the nut body.

  The roughing is nearly complete. Once the required roughing is made, a finishing end mill cleans up the ridges left behind by the rougher, for a neat appearance.

Finally, the nut carrier is repositioned to present the face for drilling and tapping of the 4 ea. 8-32 holes. Like the larger holes on the top of the carrier, these are heavily countersunk before tapping.

The finished product, ready to install.

In position on the nut! With the 4 ea 8-32 SHCS loosely attached, the nut carrier can be slightly lifted a few thou, perhaps 0.005", as well as shifted laterally slightly, for final installation.