Step 5: Make the Anti Backlash Screw Blocks
Take the three pieces of 2 1/2" x 1 7/8" plywood (you could also use Delrin or MDF) and cut them according to the drawing. Drill and counterbore the holes for the mounting bolts. Drill and tap the 1/4" Aluminum rod for the 4mm bolts and insert them into the blocks (make sure you can see the threaded holes.) If you use Delrin to make the parts you can skip the Aluminum inserts and just thread the holes in the blocks for the 4mm bolts that hold the metal plate that retains the ACME nut.
My blocks were cut by hand- and it shows! The best way to cut the 1/2" wide slots for the ACME nuts is to make a jig and cut them using the router table. Clamp the screw blocks in the jig and then run the jig over the router bit to cut the slot. Don't try to cut the slot to full depth in one pass- just slowly increase the bit height and make multiple cuts until you get the desired depth. Cut the slot for the non spring side first and then flip the piece over to cut the other slot- that way both of the ACME nuts are guaranteed to be in perfect alignment.
Now make some 3/4" x 1 7/8" retaining plates for the front by drilling holes for the mounting bolts and a 5/16" hole so the ACME rod can slide through.
Check the fit of the parts by assembling it as shown in the drawing. The spring should be 1 1/4" long ( or close to that) before being compressed.