Z-axis Spindle Backplate

5 Bears Home Homebrew CNC bench mill

At this stage of the game, the vertical (God's eye) view of the column to the left is complete except for the uppermost piece outlined in red... the spindle backplate. The spindle backplate is a heavy piece which joins the two NSK linear guides into a solid unit; it also connects to the Z-axis ballscrew via the ballnut carrier. The ballnut carrier is shown on the drawing, but for clarity purposes, none of the ballscrew assembly except the ballscrew backplate is shown.

This drawing is a simple but powerful example of why CAD is so nice. The Z-axis column was pieced together, purely in CAD, starting with the Bosch 90x90mm column. I knew a ballscrew would be required. The outline of the ballscrew was simply added, floating next to the column, with clearance around it.

I had the NSK linear guides on hand. The drawings for these are available on the net as .DXF files, which were imported with ease, as was the 90x90 column. The NSK guides were added to the drawing, showing me that the sideplates would be needed to both separate the guides, and elevate them somewhat from the column.

With the NSK guides and the ballscrew roughly in place, a little dimensional refinement set the drawing up for the addition of the spindle backplate in red. When adding stock to the drawing, it is best to keep in mind the starting dimensions of your stock. For example, rather than using sideplates of CAD derived thickness 1.133", I altered the drawing to use stock of a more common dimension like 1.250". The same applies to the spindle backplate. Mic6 stock comes in imperial dimensions such as 1", 1.25", etc. Start your drawing with stock available, rather than being forced to mill large sections of plate to some obscure dimension.

In place on the drawing, then, I had everything except the ballnut carrier. It was a simple job to draw with a few lines the outline you see here, respecting things such as the major OD of the ballnut, the need for some "meat" in the ballnut carrier to attach it with 1/4" hardware, and finally, creating an output to my printer of a dimensioned drawing. When milled to print, these parts come together amazingly well, much better than measure, cut, and paste in the shop with calipers, a pencil, and paper.


On to the backplate!

The spindle backplate starts as a slab of 1.5" thick mic6 jig plate, which is a cast aluminum product whose surfaces are finished via some form of grinding or accurate milling. Note the plastic sheet protecting the faces of the plate. It is good practice to keep the sheet in place as much as possible to protect the plate's accurate surfaces. Mic6 plate is relatively inexpensive, cheaper than I thought it would be, and worth every penny for the accuracy it delivers with minimal fuss.

While the faces of the plate were perfect, the edges were band-sawed and required a cleanup operation. All four edges were face milled, being sure to maintain 90 degrees between edges.
There is a lot of aluminum (volume-wise, at least) to remove from the bottom of the spindle backplate. We start with the middle section.
This particular face mill removes aluminum fairly quickly due to the insert geometry. A roughing end mill would probably do better for hogging, with a final pass or two done with a face mill or flycutter.

Personally I detest flycutting. I'm always amazed at the sheer number of beginner machinist's books which advocate the joys of flycutting. I'd rather mow a few lawns and make the $100 or so a quality face mill will cost, than deal with the horrible geometry requirements of a HSS bit for a flycutter, which bangs like a dowel on a coffee tin with any cut other than uninterrupted.

  With the central channel cut, I can begin cutting the seat areas, where the backplate will contact the NSK blocks.

The blocks chosen for the Z-axis are 25mm series (rail width) "long" blocks. The extra length gives the pair of blocks more than enough rigidity, and using two blocks instead of four is a good idea, as it is both cheaper, and it increases the available Z-axis travel. My goal from the outset for the "Z" is to have an almost ridiculous amount of travel... when you have tall work in a vise, a drill chuck + bit, the Z can be eaten up in short order.

Final cleanup of the bottom of the backplate. The concave cut on the near side of the photo is the seat for the ballnut carrier.

Accuracy of these cuts is important both for depth, and to make appropriate surfaces coplanar. For example, both NSK block sets must be in the same plane as each other. Further, these in turn must form a plane perfectly parallel with the opposite face of the backplate, which is the surface to which we will ultimately mount the spindle.

When possible, make cuts like these without disturbing the mill's quill depth.

  Turned over and set on parallels, all that remains is to drill and countersink the 8 NSK block holes, and do likewise for the two ballnut carrier holes.

The nearly finished backplate in place on the Z-axis column. When cinched down, the backplate is first moved full travel by hand to check for binding, before attempting movement via ballscrew. Finally, the smoothness of the ballscrew installation is tested with a handwheel, which allows the operator to physically feel for tightness.

In this case, all was well.

The last remaining operation on the backplate will be the mounting attachment for the spindle.