I skipped right to the section on CNC machining because that's what I have some expertise in. A couple pieces of feedback:

- In section 2.1.1 there is a note that states "CAM applications are designed to fail safely; that is, if any of the features of the model cannot be reached without plowing through another essential section of the geometry, the problematic region simply won't be machined at all." This is really, really bad advice for someone learning CNC, because it's the kind of statement that may be true 90% of the time but the remaining 10% where it is false can have serious consequences (ruined workpieces, broken tools, crashed machines, injuries etc). Every one of the 6 CAM programs I have used has both intended behaviors and bugs/edge cases that will violate this assumption. A CNC learner should instead be instructed to have a step-by-step verification checklist to determine the correctness and safety of a new program. This includes steps utilizing both the simulation functions within their CAM program and dry running on the machine. In addition to behaviors within CAM, there is a whole additional class of unintended (unsafe) behavior that can emerge once the program is actually run on the machine and will not be caught in CAM simulation. The exact composition of this verification process will vary depending on what you are doing, but the main idea is to never assume your CAM programming will fail safely like this article suggests.

- In regards to Total Indicated Runout in section 2.1.3. The article has a good discussion here, however I would add that the smaller tool you use, the greater effect TIR has on tool longevity and surface finish. As overall tool diameter get smaller, allowable chip load generally decreases. TIR effectively changes the chip load on each tooth as the tool rotates. If TIR is large enough relative to chip load, this imbalance will destroy the tool in short order. Why is this important to a new CNC user? Lots of new CNC operators assume that since smaller tools reduce cutting forces, they can use very small endmills on their benchtop end mills and not worry about rigidity. However, due to the TIR + chipload issue described above, demands on spindle precision actually increase if you want to use smaller end mills. There is a sweet spot where the end mill is cutting, has decent life and does not exert overly high cutting forces, which will depend on the machine and tool holding setup. But this does not necessarily coincide with using the smallest end mill possible.

- In section 2.1.7, jaw chucks (like a drill chuck) should not even be mentioned, except to caution users away from them. The article describes them as if they are just a non-optimal choice, but they are outright dangerous to use for milling. They are not designed to deal with the lateral forces created by end mills. They also are often mounted on tapers that can't deal with those forces either. Please do not reply and tell me you have had success milling with an X-Y table on your drill press. You may have, but you won't be doing it in my shop. It's not safe.

- Section 2.1.8 is overall good info, but misses mentioning what is one of the most important keywords to understanding how CNC machines interface with CAM software: the postprocessor. All G-code is interpreted and comes in many different dialects, with varying degrees of compatibility across CNC controller manufacturers. Which dialect your CAM system outputs is controlled by the postprocessor, which is a build script that can be interchanged to support different CNC controllers. Of special interest to the HN audience is the fact that these postprocessors can be written and (usually) modified, which may be advantageous to support unusual machines or customize your production process. IIRC fusion360 postprocessors are javascript. Professional machine shops without in-house software dev expertise pay big money for custom postprocessors.

If anyone is interested in getting deeper into this subject, I have been curating a list of resources for learning machining on my website here:

https://www.r-c-y.net/posts/machining/

I began compiling these because I was mostly self-taught when I started machining and at the time found it pretty tough to find good learning resources that weren't primarily focused on hobbyist-scale machining. These should provide a good introduction to industrial scale, professional quality machining rather than small scale benchtop milling like this article. However, the fundamentals apply to both, so even if your ambitions are small it's good to learn from the pros.