Supercharged Sherline Spindle - Part III, Motor Mount

5 Bears Home Homebrew CNC bench mill

With the sheaves completed, we need some way to mount the much larger motor, and do so in a manner that wil allow for precise and positive tensioning. Spindles near 10,000 RPM, when belt driven, can be very finicky with tension.

A hefty spindle and motor standoff (I call it a foreplate) was accurately made using a 3/4" sheet of mic6 tool and jig plate, and a pair of 1" X 1.250" extruded aluminum rectangles. Before these are attached to the foreplate, they are milled so that they present a truly flat surface to the plate. Once attached with a series of 1/4" SHCS, the foreplate/standoff assembly was cleaned up all around. The very last task was to clamp the foreplate inverted on my mill table, and use a face mill cutter with a locked quill to generate a foreplate that is true and parallel. Once mounted, the plate will not be canted or otherwise out of truth with the spindle backplate.
The newly-made foreplate is then mounted to the backplate with 6 ea. 5/16" SHCS, as shown. The holes for the SHCS are not yet drilled and tapped in the backplate. The backplate, of course, is what actually rides on the NSK trucks and is motivated by the Z-axis ballscrew.
I sat at my bench and pondered motor-mounting for quite some time. If I wanted the Sherline spindle block to be centered on the foreplate at the bottom (I did), the motor would have to "hover" somewhere to the upper right of the foreplate.

In the end, I decided to make it fancy. To begin, I mounted a stub length of THK RSR12 rail and two trucks onto the upper-right surface of the foreplate. The marker on the foreplate are dimensions for positioning the motor sheaves relative to the spindle sheaves. "Sharpie" marker is great for aluminum, as it washes clean in a moment with any alcohol.

Another aluminum extrusion was milled as shown, and mounted to another flat (which rides on the RSR12's) so as to create roughly a "T" shape. The four holes on the right side of the plate accept 4 ea. 10-32 SHCS to secure the motor to the plate.

The motor mount plate is now free to travel left and right relative to the foreplate.

From behind, you can see the heavy extrusion which forms the stem of the "T" and is drilled for mounting to the RSR12 trucks.

Needing some way to move the plate, I mounted an endcap piece (red lines) to accept a small 1/4" leadscrew.

At the correct location, a spot-face was milled with my automatic boring and facing head. This ensures that the bottom of the bore is smooth, and not stepped as would be the case with a normal, manual boring tool.

The thickness of the end cap is being "miked" in this picture in preparation for the leadscrew and handle.

A neat little handle was fabricated, along with a 6" 1/4x20 leadscrew. This in turn is fixed to a brass nut which is mounted to the foreplate. It will all come together in the next installment.