The Digital Craft of CNC Flip Milling

We have long been fascinated by CNC flip milling. It is a common technique for overcoming the limits of 3-axis CNC routers that cannot perform undercuts. Just as it sounds, the process involves milling Side A, flipping your stock over, and then milling Side B. By giving the endmill access to multiple sides of the stock, one can achieve a wider range of shapes and geometries than simple profiles, contours, and pockets. The latitude this gives designers and makers is compelling!

While the concept of CNC flip milling is simple, achieving it is fraught with challenges. Reconciling the precision of digital fabrication and the inherently analog process of flipping isn’t easy. Toolpath alignment from Side A to Side B is only as precise as the human maker. After milling Side A, and turning over the stock, that maker must precisely re-align the stock on the router bed. Only then will the Side B toolpaths align with those already cut on Side A. Even the most exacting human maker may fall short of the robot’s perfection.

Up until now, our projects prioritized simplicity as a means of achieving repeatable, digital technique. Repeatability is what enables a global, distributed network of local fabricators to match the work of centralized factory. So, for AtFAB furniture, we stuck to profile cutting and basic fabrication technique. And, while we introduced some complex fabrication concepts in our book Design for CNC, we found them too advanced for the projects.

As we research new designs, we’re exploring ways to achieve complex digital fabrication technique, with the intuitive ease and attainable digital craftsmanship of our earlier projects. Using Fusion 360 and our Shopbot, we are seeking repeatability, with complex methods like flip milling, toolpath surfaces, and parametric design.

We channeled our explorations in flip milling with these interlocking discs and rings that  form a stack. The stack serves as a test, fabrication prototype, and sampler of edges, details, and joinery. We’ll shape future designs around the successful outcomes. Every element in the stack has a pocketed top and bottom, and beveled outer profile. The base and top discs have contoured surfaces. We leave each part unfinished, so we can evaluate the effects of our toolpath settings.

Milling the Stack Parts – Side A
Side A Contouring Detail
After the Flip – Milling Side B
Final Toolpath Spiral Releases Part


To fabricate the stacks, we work with prefabricated wood blanks, which double as fixtures. We cut each blank to a dimension that fits our part. We then, measure and pre-drill four holes at each corner of the stock. Each hole aligns with bolts that we anchored to the T-track of our CNC router bed.

The holes and anchors create a two-sided hold-down system. They secure the part when cutting, and when flipping, they preserve the alignment of Side A and B toolpaths. So, as we mill parts from the stock, we’re confident that they are secured to the router bed, and aligned throughout the flip.


To facilitate repeatability, we designed a beveled edge profile that forgives the most minute misalignments. In Fusion360, we defined a steep spiral toolpath that contours the outside of each part. Starting at the top of Side A, the end mill spirals down to just past the part’s equator.

After the flip, we mill Side B with this same toolpath. As the end mill spirals past the equator, from the B Side, it gently separates the part from the stock. By directing the tool to slowly break through the stock material, we eliminated the need for tabs that would mar the part’s crisp edge.