CNC stands for Computerized Numerical Control. It is a process in which pre-programmed software and code controls the movement of production equipment. CNC machining controls a range of complex machinery, such as grinders, lathes, and turning mills, all of which are used to cut, shape, and create different parts and prototypes.

Inner Edges

Avoid sharp inside corners. Vertical corner radius larger than 1/3 x cavity depth is recommended. For internal vertical edges, the larger the fillet the better. Increasing the corner radii by as little as 0.005” (0.127mm) can give the tool enough room to move around and follow a more circular path.

If a corner radius is the same diameter as the cutter being used to form it, it can cause chatter and premature tool wear.

Walls Thickness

Recommended: 0.8 mm (metals), 1.5 mm (plastics)

Feasible: 0.5 mm (metals), 1.0 mm (plastics)

You may be able to achieve thinner sections without significant risk, but this needs to be assessed on a case by case basis. 

With metal, thin walls increase chatter, which can compromise the accuracy of the machining process and the surface finish of the part. With plastics, thin walls can cause warping and softening.

Another important DFM considerations that designer should know while designing parts for CNC machining is around ‘Holes’ and ‘Outside Fillets’. 

Holes

Avoid creating blind holes with a flat bottom, especially small holes  and instead use a standard twist drill to create holes with one-shaped bottoms. Cone angles are commonly 118° or 135°.

Keep the drilling depth low. Flat-bottomed holes often cause problems down the line for subsequent operations like reaming.

Outside Fillets

Outside fillets, or fillets on the top edges of pockets, bosses, and slots, require an exceptionally sharp cutter and a precise setup. 

Add the large fillets (at least ⅓ times the cavity depth) to all internal vertical corners.

If fillets are not given there would be stress concentration near sharp edges which can lead to part failure during machining