The Craftmans Shop > Backyard Ballistics
Fibreglass laminated longbow - how to build
jcs0001:
Jonny - Not sure what you mean by making V springs. The way I am describing for laminating a longbow is one of many different methods.
DavidA - you have touched on a controversial subject. As I understand it, according the the rules of the North American Longbow Safari, a longbow is such as long as the riser shelf is not centre cut (needs to be at least 1/8 in. off centre) and as long as the string does not touch the belly side of the limbs (except at the tip). I think this is the general application in other areas although I have no idea what the "rules" are in your area.
Having said that, longbows of this type perform very similarly to recurves. Bigger archery meets in our area will sometimes differentiate between "primitive" bows and laminated bows - primitive bows usually are longbows without fibreglass laminations and tend to be fairly straight although I have seen wooden recurves occasionally.
Certainly will not argue that this design is really pushing the envelope as far as longbows/recurves go.
John.
jcs0001:
Continuing:
We have to get by the tedious but necessary parts before stringing the bow up. I will start with forming the riser. The riser can be made of wood and even plastic (micarta) - I prefer strong heavy woods as the weight will help in shooting and the strength may be necessary particularly in a heavy bow. There are lots of forces involved. In the case of this bow we are using actionwood - dyed fine maple laminated into blocks - fancy expensive plywood. ::) However other options are available including purpleheart, bubinga, kingwood - your imagination and budget are the limit. I believe that Jonny's post earlier shows something similar to actionwood.
You will see in the photo below a plexiglass template showing the shape of the riser. In this case the riser will be about 18.5 in. long and will be 2 in. wide to match our mold and the fibreglass and wood layers. Before marking out the riser block with the template, it is important to have the top and bottom surfaces (sides of the riser) parallel. Planing them or using a thickness sander is a good idea.
Once the riser is marked out in pencil it is cut out using a bandsaw. I take a lot of care to square the blade to the table and install a good blade as the better the cut the less work sanding it to shape after. The bottom of the riser, will eventually be against the back of the bow and it is critical that it fits the mold. The top is less critical as long as the curves are very even and clean. It is also critical that the bottom and top of the riser be at a right angle to one of the sides. If they slope in relation to each other the glue up will become almost impossible. :hammer: Epoxy in it's unhardened form has a very close relationship to grease.
This is one type of sander that works well to shape the riser after it has been cut to shape. I tend to use a horizontal belt sander (6 x 48 in) but it takes a bit more care as the table cannot easily be used. Notice the two risers on the sander table - both have the bottoms to the left.
The fadeouts must be less than paper thin once finished. Using a piece of wood to help hold the fadeout area against the sander helps to get them even.
Getting the riser even and ready can take some time - it must not be rushed or the final bow will be a mess.
All sorts of decorative ideas can be pursued with the riser. You will notice the riser of the bow in the first photo on this thread. That riser was cut in a very even curve with the bandsaw and three contrasting veneer layers were glued between the two riser pieces. It is best to do this before cutting the riser shape.
John.
Powder Keg:
Looking great so far!!! Do you have to bake the glue to get it to set up? I need to finish putting my crossbow togather. If there were 50 hours in a day I might be able to get caught up? Maybe not :(
jcs0001:
Wes:
Yes, any bow building epoxies that I am familiar with need heat. The "smooth-on" epoxy I use will harden at room temperature but it can take quite a few hours. The heat strips shown in an entry above (the photo of the mold lying on my table saw) are hooked up to an electric controller plugged into the mains. I regulate the heat to between 130 to 140 F for about 3.5 hours - this isn't very critical.
Each heat strip is surrounded by a strip of galvanised sheet metal (2 in. wide) - one on each side and then a piece of wide teflon tape. The tape prevents the epoxy from sticking to everything and the fibreglass from being scratched up. The heat strips lie directly against the bottom and top fibreglass laminations and I use an oven thermometer to monitor the temperature.
In the past I've used a large heat box made with plywood enclosing 4 - 100 watt electric bulbs. A salvaged stove oven thermometer can be hooked up to this to keep the temperature fairly even. The box is fairly big as it has to fit the mold with a decent amount of space around it. In use one would leave the mold in the box under heat for up to 6 hours as it takes a while for the mold and laminations/glue to get warmed up sufficiently.
It is my understanding that heating the epoxy also cures it so that it won't release until the outside temperature exceeds the curing temperature. Bows sometimes end up in hot places - the inside of a car in the sun for example. Being that epoxy can soften with heat, one does not want to have a bow de laminate in hot weather.
John.
J. Tranter:
Is the riser template and the bow mold your own design or is there some place you can get them?
John
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