Basics of making custom robot brackets and skeletal parts

Basics of making custom robot brackets and skeletal parts
—Cutting Sheet Metal with a Sherline Mill
Check out how Matt Bauer cuts and shapes metal pieces to make robot brackets. Click here for an ERB.zip file that contains .nc g-code files provided by Matt for the Sherline mill and a “how-to” .pdf.—the editors

I am the proud owner of several robots that range from simple toys to advanced humanoids, and one of them is seldom given the credit it deserves, given its abilities—my Sherline Products 2010 CNC mill. This precision machining tool is responsible for all of my latest custom and prototype work, and it's the workhorse behind some of the lighter manufacturing we do at Bauer Independents Ltd. The mill dramatically cuts down on the expense of having small quantities of parts machined or laser-cut elsewhere. And it is much more convenient to be able to trek on out to the shop, mill the part there and test the finished product not long after first drawing it on the computer.

For many, the greatest benefit of owning a CNC mill lies in its ability to cut three-dimensional parts out of stock materials that range from semi-rigid plastics to titanium. The stock is usually relatively thick in comparison to the part being milled, and it's held down with step-block clamps or placed in a vise. Rook's Pawn III, our competition-level humanoid, has a number of parts made of solid blocks of Delrin stock. His aluminum brackets, though, were all cut out of 0.040-inch-thick sheet and then bent on a box-pan break to form 3D shapes.
It doesn't take much effort to flex metal sheet that thin, and it can be a challenge to prevent the end mill from lifting a sheet that thin. This article provides hints on how to hold down sheet stock while you cut parts out..

I have received several emails inquiring about the cutting method that works best for me. A good solution was to make a tooling plate specific to my mill that could hold very thin sheet metal flat and secure at multiple points.
I recently discovered that Sherline Products does indeed list a mill tooling plate in its accessory catalog (item no. 3560). A tooling plate is typically used as an easy-to-modify work table for mounting job-specific tools or clamps. They are also an inexpensive alternative to exposing your mill's tabletop to damage resulting from machining mishaps. If the thin sheet you're cutting falls within a 4x10-inch footprint, this accessory is definitely worth buying, or you can make one from scratch.

Making a Custom Tooling Plate

I start by finding the mill's maximum travel—its range limits—in both the X and Y axes. The 2000 series mills allocate 7 inches of Y and 9 inches of X travel. These dimensions may vary according to your model and the way in which your mill is set up. I had a slab of 5/8-inch-thick, 6-inch wide 6061 aluminum lying around—plenty big for most of the work I do.
In this example, the maximum size of part being cut from one single sheet will reside within an area of 5.5 inch. by 9.0 inch. I measured 2 inches down from the top of the aluminum plate and 1/2 inch over from the left. This marks the center position of one of the capscrews that will hold the tooling plate on the mill's table. From there, I measured 1 1/2 inches down and marked the second position. Then, I repeated these measurements, this time 1/2 inch in from the right side (see photos with diagrams).

Tooling plate in extreme negative-Y position.

Tooling plate in extreme positive-Y direction.