Crossbow Design (help needed)

Designing a crossbow around making a prod out of handsaws. The draw length from what I read is about 18 cm (7 inches) with a draw weight of 25-30 kg (55-65 lbs)

 
http://weaponcollector.blogspot.no/2011/05/how-to-make-crossbow-prod-limbs-from.html

(link is not working for me currently, I blame slow internet).

In short:
You cut 5 pieces that are 45 cm (18 inches) long.
Attach them to each other by making a end cap/clamp out of aluminium and screwing it on.


This is what I came up with, plan is to use a AR-15 pistol grip and stock (maybe folding).




I am intending to use this as a training aid (i shoot rifle) and for fun (due to being cheap, and arrows reusable).

So my questions are:

1# is this a good idea/design?
2# how to attach prod?
3# Is a safety really necessary? ( I have good trigger dicipline and dont intend to hunt with it)
4# what to make string out of? (or should I just buy a premade string)?

I my opinion the trigger and stock feasible. Saw blades for a prod? I am sure it will work but a lot of work might want to consider using fiberglass batten strips to make a no-glue laminate prod. Hope Kenh sees this and gives his experience.


Different ways to attach the prod, What is your stock going to made of?


A safety is  a good idea but not absolutely necessary, and if you can incorporate one into the design why not it would be worth the effort.


The best strings are made with regular bowstring material available from an archery supply house like Three Rivers Archery. Although if the poundage is not to high paracord can work.


Todd

The stock I plan to make out of regular planks I can get at my local hardware store(s). I am thinking 3 planks each 1 cm thick sandwiched together. That way I could cut out pieces of the middle plank to attach the trigger without having to inlet.

 The trigger I would cut out of steel plate (sheet metal) around 1 cm thick or thinner. I have a friend who has machine class who has access to materials and tools. Specifically a machine that cuts out patterns out of sheet metal (you input a shape and it cuts it out). I plan to use this to cut out the parts for the trigger then file them to smooth them out.

I built one 12" prod out of loose laminate 1/32" thick spring steel that was 1/2" wide.  Cost me a whopping $6.  But with 12", 10", 8", 6", 4" laminates it still only draws around 20# at 6".  Loose laminations are not all the same length, they are stepped to "tiller" the prod by tapering --somewhat the same way a flat bow is tapered, to make most efficient use of the material in energy transfer.  The laminates are not clamped at the ends, merely help in place with cord ties or I used heat shrink tubing.   The end cap clamped in place would net work very well IMHO as the laminates need to be able to slide against one another.  Check out my Loose Laminate Pinlock build.

I would think that the cutting out process would destroy the spring temper of saw blades.... maybe not.  

It seems you're European (it's to metric measures that give you away).  So finding what we call a fiberglass chainlink fence tension bar may not be possible.  But if you can find a fiberglass bar about 6mm x 18mm x 700mm that should work just fine.  Another possibility for the draw weight you want is one of the fiberglass "replacement prods" for the commercial crossbows.

I have considered buying one of those replacement prods and using that. But I am worried that it might be stopped at the border. Would it be considered a weapon part (crossbows are controlled like firearms here, unlike firearms they dont currently have any lawfull purpose)?

Depends on your country.  But archery things are not on prohibited lists in most (if any) EU countries.

Read the lawbook. In my country the only thing illegal is a stock with mounted trigger. The parts are not illegal by themselves.