I've made a couple dozen bows but have yet to make a serious, working crossbow. The precision work required for them seems to be the challenge.
It's more of a prototype toy than a serious weapon. The bow draws 4lbs at 12" and shoots bamboo skewers. I wanted to see if a simple roller nut mechanism could be done with crude materials (worked fine with a 25lbs draw) and if I could wrap my little head around how wheelie bows work (still not too sure). I was inspired by this crossbow's mechanism: http://thearbalistguild.forumotion.com/t774-compound-project
There is no let off on these round cams, but they do allow a much longer draw length for a short prod. The cams are made of apricot wood; slices I took from a branch to make buttons... The roller nut is a section of an oak branch. Trigger pull is very light and there is no safety either