The Craftmans Shop > Backyard Ballistics
Fibreglass laminated longbow - how to build
jcs0001:
Here are a few photos to go with the discussion about scarf joints and preparation of the mold.
Shows the two laminations being placed together - use a straight edge along side to ensure they are glued straight. In this case the 72 in. piece of clear fibreglass next to them was used as a straight edge. This is being done in a heated room as my shop is too cold in the winter.
Using a couple of small weights to hold down the joint while it cures. Plastic around the joint keeps the epoxy from curing to the hold down.
The mold ready to go. Note the white fire hose in the background, heat strips laid out. The bolts are also ready to go along with their spacer blocks. It is worth doing a dry run before applying epoxy to the laminations.
The laminations and riser are now in the mold and it is bolted together. Note the two clamped "fingers" used to help hold the riser in place. There are matching fingers on the other side. The thermometer can be seen between the two fingers.
Fibreglass tape can be used to hold the riser and laminations down to the bow mold as the top is being put in place. It won't however hold things in place once pressure is applied with the air hose.
This is a critical time in the layup as once the fire hose starts to inflate the laminations and riser will try to move around. Wet epoxy has very good lubricating properties. At this stage the air hose is connected to the firehose and the pressure is very slowly increased from 0 to about 60 psi. I do this in stages, checking carefully to ensure that the laminations have not moved in relation to each other. It is always nerve racking as it is difficult to remedy any problems at this stage.
Note - use floor wax on any parts of the mold or fingers that may come in contact with epoxy. It will prevent it sticking.
Once the pressure reaches about 60 to 65 psi and everything is being pressed down nicely (may take 5 min. or so), the controller for the heat strips is turned on. I use an instant read thermometer between the air hose and the top heat strip to monitor the temperature. Over about 1/2 hour I will let the temperature rise to about 135 F or so. This isn't critical but don't be too far out on either side of that figure.
The mold and bow are then left at temperature for about 3 hours (again not critical) and then the controller is turned off, the pressure left on and it is left to cool. After an hour or more (depends upon how impatient you are), the pressure is relieved and the top of the mold can be removed. This is the result:
The belly of the bow is facing up - this turned out flawless with no air bubbles under the clear glass and all laminations stacked nicely.
John.
spuddevans:
--- Quote from: jcs0001 on May 18, 2011, 11:32:58 AM ---Note - use floor wax on any parts of the mold or fingers that may come in contact with epoxy. It will prevent it sticking.
--- End quote ---
You can also use plain old brown parcel tape, there is virtually no glue that can stick to the shiny side of parcel tape.
Tim
Brass_Machine:
This is very cool to see!
jcs0001:
Gentlemen:
Thanks for the tip on the parcel tape.
Yes the white "bulging" part is the firehose. Just conventional fire hose with a fitted aluminium plug at the far end and another fitted plug with a 1/4 in. tapered pipe thread in the other to accept an air hose fitting (or a schreader valve if using a foot pump to inflate it). It pays to be cautious with these plugs as they can become a real projectile if they "release". Quite a while ago I tested an airhose on the back deck, pumped it up to 60 psi and left it while I was doing other things. The sun got at it (it was dark blue) and when I came back one plug was gone and I never did find it. None of the neighbours was missing or injured so I guess it "missed".
Continuing on:
Most of the photos from here on in were taken by my friend as he did the finishing on the bow.
Once the bow is lifted off the form, it is very rough - lots of very sharp epoxy on the edges and the ends aren't even. At this point the edges are cleaned up on a belt sander and each limb cut to about 1.5 in. over the final length. The length of this bow will be 66 in. (nock to nock) so it is initially cut to about 69in. I put masking tape in the back of the bow to allow for layout lines.
It is helpful to have a long piece of plate glass to use to lay out the centre line but I have used a chalk line also.
A centre line is drawn the length of the bow and measurements are taken to each side to lay out the limbs. The limbs can be cut with a wood bandsaw blade or ground down with a disk grinder. If a bandsaw blade is used the teeth will be ruined in about the first inch leaving it to cut/burn it's way through the remainder. I have a couple of old blades I use but because they are so dull they do wander some.
I cut well off the lines and then sand down close to them. The final shaping will be done by eye using the centre line as a guide. Initially one does not wish to take too much off the limbs or round them excessively as this can be done later to bring the bow down to the desired weight.
The riser can be cut to rough shape with the bandsaw or with a hand hacksaw or it can be sanded to shape with various machines or a sanding drum in a drill press (I don't like to do this without a bearing support under the drum as it's hard on the drill press).
For some reason photobucket won't work right now so I will leave for now and include photos relating to this part of the narrative in the next day or two.
John.
jcs0001:
A few photos to go with the last part of the narrative:
The bow in rough form, before cleaning up the edges. The glass table is my friend's layout table. (Not sure if SWMBO is aware of this.)
Bow with centre line marked out. This is fairly advanced as the limbs are close to being finished and it is now strung. I get the limbs close to their final shape before laying out and shaping the riser since the unfinished riser makes it easier to "eyeball" the finished limbs.
Once the shape of the limbs and riser are roughed out overlays are glued on. These may be one or more layers and are usually applied to the tips (for reinforcement) and to the back and belly of the riser. Epoxy is used and once the pieces are clamped in place, heat is used to cure it. Short heat strips or a light bulb can be used to provide heat.
In this case my friend had a sheep horn (garage sale find) so used strips for overlays on the back of the riser.
The horn with one piece cut out.
Pieces cut and ready to apply. The tape will be removed and the fibreglass will be sanded to get a good surface for the epoxy. Note small "form" behind the bow. This will be used to apply consistent pressure to the overlay as it is being glued.
Sheep horn now applied and the riser mainly shaped.
John.
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