Crossbows

[Lucasade]

My soon to be 13 year old nephew has a Technic Lego set I want to trade him for. It's worth £75 so I asked his parents what he might like me to make for him. He came back with a crossbow. 

I've already explained to his parents that I doubt I have the engineering skills for a crossbow, I don't have any metal fabrication equipment and va hand crafted crossbow is probably worth many times more than £75, and while I don't mind giving him a good deal because he's my nephew I'm not sure that trading a £75 Lego set for a several hundred pound piece of hand crafted engineering is going to teach him anything about how life works. Anyway, today he came back to me and said he still wants a crossbow, and now he's specified a stock and a metal prod too.

To get to the point, I need to let him down gently. So that I can give him a reasoned answer, please can you guys help me with a few questions...

1. What sort of draw weight would a metal prod produce, and would it be remotely safe for a 13 year old to use?
2. What is the simplest possible trigger mechanism - I really don't have a clue so I don't know if I would be even capable of making the thing.
3. What sort of labour time would be involved in making a supremely basic crossbow that is actually safe to give to someone else assuming a moderate engineering and craft knowledge?

I've had a look around t'internet and seen a few things. They all scare me witless. 

[GlisGlis]

I don't have any metal fabrication equipment and va hand crafted crossbow is probably worth many times more than £75, and while I don't mind giving him a good deal because he's my nephew I'm not sure that trading a £75 Lego set for a several hundred pound piece of hand crafted engineering is going to teach him anything about how life works

there is not an absolute value for an object.
You can set it but as long as no one wants to buy it  it's irrelevant
this is especially true for handcraft objects. you can account hundreds of pounds for materials and many more for your time but as long as the product is not appealing it will have no value.
And if it is you proposing the deal you should be more than sure that your nephew will like the trade
So first question is. Will you be able to make the object he is looking for?

If you google around you will find many cheap options for crossbows at your price but here arise the safety issue.
You better investigate with the child parents if a 100+ lbs toy would be accepted 

[Lucasade]

My Mum's sold things she makes for a living her whole life so I know all too well about the value of an object is in the eye of the beholder. She also taught me and I've learnt from my own experience that most people catastrophically undervalue hand made objects because they compare it to something out of a factory.

What I said to his parents the first time was that a crossbow would be something I might make as a gift due to the time and education costs involved in producing something I would be prepared to let out of my hands, whereas he needs to understand that if you are negotiating a trade the two objects should have a similar value. Obviously that message hasn't been passed along as the example pictures he sent me show three different, beautiful, full size crossbows that appear to have draw weights in the 300lb range.

[GlisGlis]

You may propose him to build the crossbow together
This will give him a notion of the time and effort involved

[Zuma]

You may propose him to build the crossbow together
This will give him a notion of the time and effort involved

Good point Glis
This may help.
Zuma

[Zuma]

trigger

[Del the cat]

You can make a simple effective all wood crossbow  .
Posts in my blog from start to finish here...
https://bowyersdiary.blogspot.co.uk/2011/10/crossbow-progress.html
https://bowyersdiary.blogspot.co.uk/2011/10/trigger-mechanism-try-out.html
https://bowyersdiary.blogspot.co.uk/2011/10/detail-pics.html
A wooden or steel prod can be bound on with sinew or rawhide. 40# draw weight should be plenty for a reasonable performance but will need light bolts (2 flights angled for spin). It won't be too dangerous due to the short power stroke.
this design is easy enough to build as a quick try out, and the trigger mechanism is in use today by tribesmen in the rain forest.
Del

[Lucasade]

Thanks Del, that's what I wasn't able to find!

[Lucasade]

Well that was easy - nephew doesn't want a wooden prod and parents don't want 13 year old boy running around Portsmouth with a massive weapon in his hands!