Complex Geometry on a 3-Axis CNC Mill

The 3-axis CNC machine is most commonly used for one sided milling (profile or contour cuts). In order to create more complex geometry or surfaces with two finished faces, jigs will have to be used.

This example will look at how jigs were used to facilitate two sided milling for fabrication of the table legs of a QuaDror table. 

Setting up the part:

This tutorial begins with you having the object fully modeled in Rhino and oriented flat on the xy plane. We're going to duplicate the object and rotate it so there are two objects with each side facing up. It is helpful to know the distance that these two objects are displaced from each other.

Milling notes - consider tabbing the part if it wont be resting on the table after the final cut

Setting up the jig:

The process of designing the jig is relatively straightforward. The primary concern is alignment. Making sure you can place the jig on the spoil board in the same place with respect to the 0,0 point in the rhino model is extremely important as well as ensuring that you can align the stock to the jig.

The first step is to use the reference points defined in the previous step. An easy way to do this by doing a 'Save As' on the file you created the geometry in and removing the extraneous geometry. The jig in this example was created by rough cutting and stacking mdf on top of each other. The rough geometry was then brought into rhino, and the CNC was used to cut the part to the precise dimensions we needed (tip - you can create an irregularly shaped box stock by using the 'Stock from Selection' command.

The tool-paths we ran on the jig were; once along the raised face so we had a straight face to press the board to, a surfacing path along the bottom of the jig so we knew the elevation of the piece precisely, and guide holes for the jig to spoil board and the jig to the part.

Milling the object:

Because of all the prep work we've done, the actual milling of the piece is pretty straightforward. We first run the jig locator tool path and mark on the table where we should place the jig. We place the jig on the table and secure the stock to it. We mill the first side and make sure that included in this tool path is a way to locate the part on the jig when we flip the stock. Depending upon how the part is set up, you might need to include tabs in the part so it stays attached during the mill process. We flip the part, and locate it using the guide holes and run the tool path for the second side. If you've tabbed the part, you'll need to take the piece into the wood shop to cut the piece out.

And the final result is...

This tutorial was originally written for the University of MN's Digital Design website.

Design and Tool Making

The role of tool making in architecture and product design is often an ignored part of the process. For this seven week design exercise, I partnered up with fellow classmate Hank Butitta to develop a series of analog and digital tools to assist in the fabrication of the QuaDror geometry as designed by Dror Benshetrit of Studio Dror.

We isolated the 3-axis CNC routing machine as the ideal CAD/CAM machinery to design our manufacturing process around because it represents the most accessible type of CNC equipment. In theory, this process could be replicated at any of the thousands of cabinet shops in the country that have access to a 3-axis CNC machine.

We developed two different types of processes for two different types of products; a high-end hardwood version and low-end plywood version. The hardwood system was optimized for speed and efficiency. We built off of a Grasshopper definition that was provided by Dror that is used by his office to define the articulating QuaDror geometry by inputting the desired width and height. Our modifications would automatically create and bake the Rhino curves into layers that would create a toolpath that could be used for the three different routers that we were using (dovetail, chamfer and 1/2" endmill). The hardwood version couldn't be simplified to a degree that we could just lay our stock material on the bed of the CNC machine and mill the desired shape. Instead we had to create a jig that would place the part in the correct location and allow us to flip the stock after the first pass and place the piece in the correct location for milling on the reverse side.