Hi to all, beginner here with a few questions

Hi all, I'm just a beginner starting to learn about this so pardon me for my noob questions lol. I'm quite curious about the DIY of modern crossbows and in general how they work especially those reverse draw/limb types from an engineering perspective.

What tools and materials are actually needed to build one of those?
E.g. Do I need a CNC milling machine? I've noticed those commercial crossbows that they sell have laminated fibreglass prods and maybe duralumin(Not sure about this though) stock? Are there different grades of fibreglass or duralumin that they use that can be produced by the hobbyist?

Are there dimensions to the prod or stock that must be followed strictly for optimal performance? How are those calculated?

There are folks here who have built "reverse action" crossbows literally from garden sticks and string (see the paleo-legal compound bow discussion), from DIY store wooden blocks and lumber, and from the highest of high-tech materials like carbon fiber, cast and machined exotic metals, plastics and a pair of junked snow skis.

That's the fun of making your own as compared to buying OTC.  The bow in my avatar picture has its prod made from two pieces of fiberglass chainlink fence tension bar, while the Skane or pinlock action is probably a thousand year old design.

Here are a couple of my favorite discussions to check out:

https://thearbalistguild.forumotion.com/t689-paleo-legal-compound

https://thearbalistguild.forumotion.com/t774-compound-project

Do you have links or resources that can teach me how to make laminated fibreglass or work with carbon fibre/dural?

What about the dimensions of the crossbow? I would expect that there must be some calculations involved in getting the right size of each part.

You don't "need" a computer controlled mill, any normal milling task can be done on a normal mill and most parts can be made using some combination of milling, forging, casting, welding, sawing, grinding, stamping, filing etc. Usually there is a "best" way to do something but there is almost never an "only" way so it pays to keep an open mind and design your project around the tools and materials that you actually have available.

If you mean "laminated fiberglass" in the sense of a laminated fiberglass recurve bow, there are dozens of resources on the web that can teach you that.  The  'loose laminate' fiberglass that comprises the prod on my avatar crossbow is not glued together, just lightly held together like old fashioned leaf springs. 

I've never worked with carbon fiber, personally.  Much too expensive for my blood. 

Dimensions of prods, tillers, bolts, actions depend upon the desired draw weight of the prod you want to build, the materials you want to use, etc.  Give us some specifics and we can probably help.   If you're just starting out, a good resource are the "replacement fiberglass prods" available on Ebay in 50#, 80# and 120-150# draw weights.

We're not really a "tell me everything I need to know" kind of place.  But we will help you once you've done some research and decided upon the rough dimensions and specifications of your first build.  Read into the discussions here.  Read the Building Wiki referenced on the right side of this page.  Then ask us some pointed questions, not vague generalities, please.