The Craftmans Shop > Backyard Ballistics
Fibreglass laminated longbow - how to build
Ray:
As stated before, that is a recurve...not a longbow. Do you make longbows also? Ray
jcs0001:
This particular mold is one that I bought from a fellow who decided to sell off ones he had made. I have made my own molds also including longbows and molds for take down recurve limbs. I don't know of any one selling molds although there may be some around.
A bow mold isn't that hard to make - one way is to use a thin template (eg. 1/4 in. particle board) cut to the design you want and then use it as a router template to do the final shaping on the mold itself. The riser template is made from the bow mold (the bottom must match the mold exactly and the top (belly side) is designed for the final riser shape.
I started with fairly straight longbows and then began to build them with more curves in the risers and limbs. If you are designing your own a review of what's available and different designs on the net will give you ideas for a design.
Recurve / longbow - see previous explanation - feel free to call it what you want. Note that the limbs on a recurve are thin and wide - the limbs on this bow are thick and narrow.
John.
MrFluffy:
Watching with interest, I have a recursive wooden bow with detachable wooden limbs and I did think this was a recursive bow on first glance but looking and checking the local regs I can see where the distinction appears. On my bow the string actually lies touching the limbs of the bow when strung but not under drawn position as the final end of the tips curve away from the archer, whereas this has some pre loading nearer the riser end of the limbs. Exactly as you state.
I doubt I'm going to go out and start laminating my own bows, but I'm still interested to watch it come together, whatever it ends up being classified as.
jcs0001:
MrFluffy - it doesn't really matter what the classification is as the process of making the bow is pretty much the same - in the end it is just laminating wood and fibreglass on a mold. Glad to see your interest.
Grinding laminations
Laminations for bow limbs can be made from a variety of woods including yew, rock maple, ash, elm, zebrano, black walnut and exotics. My preference is for light in weight strong woods with good flex. If clear fibreglass is used usually the wood underneath will be chosen for pattern and beauty. A plain piece of maple may give good results as far as speed and durability but does not look that good under clear glass - of course stains and the like can be used if needed.
As mentioned previously we will use a layer of 50 thou glass on each side of the bow and 4 pairs of wood laminations between the glass. In this case we are aiming for a bow of about 60 to 65 lb. - this is determined by glass thickness and wood thickness (all other things being equal). With this bow we want a wood lamination thickness of about .336 in. measured at the thick end of the tapers. Being that we will use 4 pairs of laminations we are aiming for two tapered pairs (one will also work well with this design) of about 0.100 at the thick end and two (.336-.100-.100)/2 = 0.068 in. thick parallels. This total thickness can be a bit less or more (say 5 thou or a bit more either way and be compensated for when finishing the bow - more on that later).
Note that the total wood thickness is the key - the figures above are just estimates as I don't like to go less than 0.100 on tapers - less and the thin end is extremely thin. We could use 3 pairs of laminations or 5 however thicker laminations become difficult to bend and 5 pairs become somewhat difficult to get into the mold before the epoxy starts to thicken.
I rip laminations from 2 in. thick x 36 in. long stock with a well tuned bandsaw aiming for a finished thickness of about 0.135 in. or so. Once cut they are run through a thickness sander to get the final thickness.
Running a pair of laminations through the thickness sander.
A poor pair of book matched osage orange laminations shown as they would be placed in the bow limb (overlapped just for the photo).
Adjoining laminations will be book matched in the bow limbs so that they are in the same position in the limb but a mirror image of each other - for example the first lamination cut from a given block will be on the lower limb against the fibreglass at the back of the bow and the adjoining piece will end up against the fibreglass on the back of the bow but on the upper limb.
72 in. laminations can be cut and used however they will not be book matched and they are difficult to handle so I seldom use them. 36 in. laminations will be scarf joined before glue up if necessary.
I use a drum style thickness sander get good accuracy on the laminations as long as I run them through the sander several times once they reach a point near the final thickness. It is best to use fairly aggressive sandpaper (60 grit) as that roughs the wood up for the epoxy. It is a bit of a slow process to grind a few pairs of laminations.
All laminations are run through the sander using a board underneath - the board has sandpaper glued on it so that the laminations stay onboard - otherwise they may shoot out of the sander like a small projectile. The photo shows two laminations beside the sandpaper covered boards.
Tapers are created by running laminations through the sander with a tapered board underneath. I commonly use tapers that are tapered 0.002 in per 1 in of length - others may use 0.001 per inch. My bows have the tapers arranged to be thick end in the centre of the bow and thin end at the tips. One company I know of tapers laminations from thick at both ends (middle of bow to each tip) to thin in the middle. That particular model shoots very well.
Bow Glass:
Fibreglass is available in different widths, thicknesses and colours. I normally use clear and black and have 0.040 and 0.050 thick for bows. Since my molds are 2 in. wide that is the width I use. For longbows (thick limbs) I use 0.040 or 0.050 depending upon the desired bow weight, for flatbows and recurves (bows with thin wide limbs) I use 0.040 glass.
John.
jcs0001:
Continuing on:
I had a few things come up that prevented continuing on this thread. Will continue now with the hope that I can finish it soon.
Referring back to the photo of the bow mold:
The back of the bow will be at the bottom of the pile of laminations while the top of the pile will be the belly. When we put the laminations together in the mold the stack will be as follows (top to bottom):
- one layer fibreglass (in this case clear glass).
- two layers of wood laminations (usually includes the tapered pair)
- riser
- two pairs of wood laminations (parallels)
- final layer of fibreglass (since we will be putting snakeskin on the back of the bow we will use black glass here).
Before laminating the bow the two pair of matched laminations that go on the back of the bow (under the riser on our form) must be joined. If they are not then usually they will float apart when placed in the form under pressure, leaving a gap in the bow. I do a scarf joint on them and use some quick set epoxy to join that, making sure that they are book matched before gluing them together. Once the glue has set it is easy to use a sanding block to smooth both sides of the joint.
The scarf joint can be done on a belt sander or by hand with a sanding block. The photo below shows a scarf joint between two yew laminations on a built bow. Look just to the right of centre of the photo, near the back (bottom) of the bow 3rd lamination up.
Glue up:
Once all the lamination pairs are ready they are laid out on a table and the epoxy, application sticks etc. are prepared. It is important to have everything ready to go because there is a limited amount of time to mix and apply the glue. With epoxy the warmer the room is the less time there will be available. However if the room/epoxy is too cool it will not mix or spread well. I like to have the room at about 70 F or so - it is a good compromise.
Latex/plastic gloves are used to minimize skin contact with the epoxy and the table is covered with plastic or newspaper to reduce cleanup.
Before applying glue the mold is set up with bolts and air hose ready to go. Since I leave my compressor attached to the air hose while the glue is hardening, all of this hardware must be ready to go. This includes turning the regulator on the compressor down to 0 (zero) and bleeding the air hose. It is helpful to have two people to do the glue up and put the laminations in the mold however I have done this alone in the past.
Glue is applied to all bare surfaces (excluding the outside of the fibreglass laminations). Thus each joint will have two applications of glue. This insures that there is sufficient glue in the joint and also that no areas are missed. A good light is a help both to assist in seeing that glue is applied evenly and to see any dirt, hair or other objects that may have been introduced into the joint. (One bow I saw a few years ago had a piece of grass embedded between the clear glass and the adjoining lamination).
Extra amounts of epoxy are put in some areas - the fadeout areas and on the bottom and top of the riser.
Once the laminations have sufficient glue applied, they are stacked together with the riser in the middle and fibreglass tape is used to hold things together as the bundle is placed in the mold. It is also possible to use a couple of elastic bands near the tips of the bow the help hold things together.
To be continued.....
John.
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