Before diving in, we need to note that the answer previously posted to Q&A 607 incorrectly listed "angle" as a legal material type. This is in direct conflict with the last sentence of VUR3, and has been edited to reflect this correction. We apologize for this error and the confusion that it caused.
With that in mind, hopefully your question becomes more straightforward to understand. The full text of VUR3 reads as follows:
<VUR3> Teams are allowed to fabricate their own unique components for each of their Robots from the following additional raw materials. These parts may be fabricated using techniques that may otherwise e prohibited in VRC, such as welding, brazing, casting, forging, hot/cold rolling, tempering, or gluing.
a. An unlimited amount of non-shattering plastic from the following list: polycarbonate, acetal monopolymer (Delrin), acetal copolymer (Acetron GP), POM (acetal), ABS, PEEK, PET, HDPE, LDPE, Nylon (all grades), Polypropylene, FEP.
b. An unlimited amount of silicone, polyurethane, or other rubber.
c. An unlimited amount of composite materials, such as G10 (Garolite), FR-4, or carbon fiber.
d. An unlimited number of plastic 3D printed parts.
e. An unlimited amount of steel, aluminum, brass & bronze.
The intent of <VUR3> is to encourage Teams to explore fabrication techniques like milling, 3D printing, injection molding, sheet metal punching, etc., to develop their own new robotic components in addition to the “standard” set of VEX components permitted by <VUR2> . To utilize these techniques, raw materials from the list provided in <VUR3> may be used.
However, the intent of <VUR3> is not to legalize all commercially available items made from these materials. The only commercial components (other than pneumatic components) that may be used are those purchased from VEX Robotics, as specified in <VUR2>.
For example, aluminum billet may be used to machine a custom bracket. However, purchasing a custom aluminum bracket is not within the spirit of this rule. Similarly, pre-drilled or extruded metal, such as angle aluminum, is not permitted, unless it can be found on www.vexrobotics.com.
The bottom three paragraphs, i.e. the "red box" in the manual, comprehensively explain the intent and spirit of VUR3. It would be impossible to provide a blanket answer that concisely defines all hypothetical materials that can or cannot be used.
If you have a specific item you are concerned about, we are happy to clarify via the Q&A. However, this is an inefficient way to determine component legality in a way that can be consistently applied across all teams, events, and regions; we would prefer to utilize the intent and spirit conveyed in VUR3's red box.
When determining if a given design option is legal, try to ask the following "thought experiment" questions:
"Am I making a brand-new custom part, or am I taking advantage of a feature on a component that I have purchased?"
- Milling slots into a piece of aluminum bar stock would be legal, but buying pre-slotted aluminum would not. The goal of VUR3 is to encourage Teams to explore fabrication techniques of their own.
"Would another team with access to our same fabrication resources (3D printer / CNC mill / manual lathe / metalworking forge / etc) be able to replicate the same custom part with a different commercially available raw material?"
- A tube that is lathed into a custom spacer would likely be functionally similar to another, regardless of whether it was aluminum or steel, or whether you purchased it from McMaster or from a hardware store. A pre-made honeycomb lattice will likely rely on a specific manufacturer in order to be functionally equivalent.
"Can I explain how this part was made in our Engineering Notebook or in a judged interview, or is the answer 'we bought it that way'?"
- We are not going to request that teams submit a piece-by-piece bill of materials to "prove" that items were purchased as raw materials; we are much more interested in the engineering design process and fabrication techniques that you learned and applied to make the part.