CNC machining sharp internal corners can be challenging due to the inherent limitations of machining processes and the tool geometry. Here are several methods that can be employed to tackle this issue:

1. Adding reliefs: This is a commonly used method where certain shapes are added to the corners to accommodate the roundness of the milling tool. There are various types of reliefs such as:

• A hole centered in each corner: This is the cheapest solution and involves placing a hole at the corner where two internal edges meet
• Half-Moon relief: A slightly more expensive option than the hole, shaped like a half-circle at the corner
• Internal Radii: This is another form that is equivalent in cost to the half-moon and involves creating a rounded internal corner

2. Avoidance of sharp corners: Where possible, the design can be altered to avoid sharp corners altogether. Instead, radii are included on all internal edges, using an end mill with a radius slightly smaller than the added edge radius

3. Dog-bone or T-bone fillets: These are specific types of undercuts that can be used to extend the corner shape beyond the cut area. This allows the milling tool to either complete a full rotation or to back up slightly to create a sharp angle following the tool path

4. Electrical Discharge Machining (EDM): For cases where sharp internal corners are essential and cannot be avoided, electrical discharge machining can be used. This process involves creating sharp corners by eroding the material with electrical discharges. It is noted, however, that EDM tends to be an expensive method

It is generally suggested to design parts in a way that avoids the need for sharp internal corners due to the complexity and additional cost involved in manufacturing them. The best approach often involves collaboration between designers and machinists to find a manufacturable compromise that meets the design requirements while being feasible to machine.