I'm not entirely sure this is necessary, do you have a source?

It's necessary. If this is to be CNC milled from a solid block of material then the internal edges must have radii. This is a limitation of milling.

Can you expand on what you mean? I've self taught everything and I don't even know what's right and wrong so far.

Why would I need to do this?

I will do my best. I will try and give an explanation assuming no prior knowledge in order to make it as simple as possible.

First, I used the word 'fillet'. Here is the definition of that word : In mechanical engineering, a fillet /ˈfɪlɨt/ is a rounding of an interior or exterior corner of a part design.

That is, to give an edge a fillet is to give it a

**radius**.

An internal edge with a fillet/An internal edge with a radius :

Show Image

An internal edge without a fillet/An internal edge without a radius :

Show Image

First, understand the basic concept of milling : You have a cutter that spins in order to move material. This cutter (in the case of a 3-axis mill, for example) can move along three axes : x,y,z. In other words : left and right, forward and backward, up and down. CNC mills are exactly the same, but the movements along the axes are precisely controlled with motors and a numerical input, rather than manually controlled.

These cutters come in many shapes and sizes. You have ones for cutting regular holes that look like drillbits, you have

these for cutting chamfers.

However, regardless of the shape and size of the cutter, one thing remains constant : They spin in a circle. Therefore, they

**CANNOT** form

**internal** square edges! All

**internal** edges

**MUST** have a radius.

For example, the below would not be possible :

Show Image

However, the below would be possible, since cutters can form those circular edges. Note that EXTERNAL square edges are absolutely possible, just not INTERNAL ones :

Show Image

It may be helpful to imagine a cutter as a cylinder, which removes any material that any part of that cylinder touches. Realise that this cannot possibly form square internal edges.

I do hope this helps to explain things.

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I recommend giving external edges fillets too for aesthetics, and so that people don't cut themselves on those sharp edges! I recommend giving the top rim chamfers (like

this for the same reasons.