Using a CNC machine you will inevitably run into the issue of rounded inside corners. The radius of the cutting bit makes it it impossible to machine these interior right angles using a CNC router (or any router for that matter). This can be a problem as it can have an effect on assembly or simply affect your desired aesthetic. 

However, there are a few different way to work around this issue:

1. Dogbones

One way to work around this issue is to include a dogbone. This is when extra material is removed around the inside corners to allow for parts to assemble together without an issue. The dogbone will only have to be as big as the cutting bit that is being used. For example a part machined with a .125″ bit will only have a dogbone that size (not too noticeable). One big advantage of dogbones is that they can be easily added into the CNC program and don’t add any extra post-processing time to parts. 

2. Radius the exterior corners

For instances where your parts assemble inside each other (inside corner to outside corner) you can simply add a matching radius to both corners. This way, the parts will sit inside one another perfectly without having to do dogbones (see image). 



3. Chisel the radius (post processing)

Ultimately, you can achieve a right angle on inside corners in post processing. To achieve the desired angle, corners can be chiseled by hand. This is the least efficient method but there is no alternative to achieve the same result. 


4. Consider another method of fabrication

If none of these options work for you, consider another method of fabrication. For example laser or water jet cutting both have no problem cutting inside right angles. You can compare these services here