| Plokkvibu "Primus" | Compound bow "Primus" |
Interesting stuff... | ||
| Longbow | Plokkvibu "Blackman" | "Strange" bows |
This is a story about how to build yourself a compound bow...This is
no high-tech device for top archers...but makers joy is bigger than buyers....
Also this is corroboration that everybody can build himself a whatever
type of bow...
FACT: Better bows shoot over long
distances (hundreds of meters) and arrows with broadheads can penetrate
steel sheets...
with one word... the compoud bow is an extremly powerful weapon...
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General data
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Draw weight (peak): 60 lbs
Draw lenght: 29" Brace height: 148 mm (5,8") Axle to axle: 1070 mm (42") Mas weight: 1,8 kg (4 lbs) (AMO speed): |
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| Riser
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Type: Reflex
Material: 6063-T6, sheet 15 mm (Al-alloy, sheet 0,59") Finish: Marine blue-RAL5002+metallic
Grip: Oak
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| Limbs
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Type: Solid, straight
Lenght: 430 mm Material: laminated glassfiber-wood and glass-, carbonfiber composite Finish: paint coated (black)+varnish)
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| Cams
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Bearing: 625 half-closed+teflon
coated bearing
Material: Al-alloy, maybe T6 Finish: anodized-black Let-off: 60 %
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| String &
cables
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String: Fast Flight
Cable: dia.=1,5 mm (steel) thermal shrinking tube
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| Cable guard
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Materials: 6063-T6 Al-alloy
dia.= 8 mm Stainless steel-calibrated slider: 100 % teflon  |
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| Rest/
overdraw
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Overdraw: 50 mm (2")
Materials: 6063-T6 Al-alloy
Fire zone: Teflon tips
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| Sight
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Adjust X-Y: +/- 10 mm
Materials: 6063-T6 Al-alloy,
dia.= 5 mm
Finish: Powder paint
coated (yellow)
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| Laser
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Laser: 680 nm / 1mW (Laserpointer
off the shelf)
Adjust X-Y: +/- 2 deg Material: 6063-T6, Al-alloy, sheet 0,59" Finish: Powder paint
coated (yellow)
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Of course you must figure out your riser design. Maybe next points help you:
1) Risers geometry - REFLEX, DEFLEX, STRIGHT etc...(Angle of the limb
pockets and the brace height)
2) Riser must be very rigid but very light at the sme time - material
must be choosen as good as possible (most common is:
aluminium alloys, also magnesium or aluminium moulding...very
good is carbon fiber)
3) Manufacturing methods - mill it from one massive piece (very
expensive), weld method - it is 10 times cheaper than milling),
moulding (you have to check quality of riser - x-ray
snapshots. Also you can do it with welding joints
(no 'bubbles' inside the riser are allowed). NOTE: If
the riser breaks, you have bad injuries like a free-air skull.
4) Vibration is very important factor. Think about, how to decrease
it...main problem is angle and the building of the limb pockets.
5) With one word - you have to think about everything. How to fasten
rest, quiver, stabilizer(s), cable guard, grip. Type of finish (anodizing,
painting, coating with the other metal)
6) etc.
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It is not bad to check the strength of riser. You can use some final
elements analyze program (FEA). Red color is biggest force and violet is
smallest force .
The forces inside the riser are no danger at all (AL-alloy 6063-T6). Sorry, I have no 3D FEA program.
Compoundbow "Primus" with accessorys
(click to zoom)
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Fig. 1. Riser details before welding...(you have to reserve some material
for the welding process)
I have no aluminium weld apparatus-it is too expensive to buy one....you
better go to specialists...
If you have good tools, then cut out all risers
details (I did it with hacksaw...it was a real nightmare because
this alloy is extremly hard to cut. The use of
laser-, water-, or erosion cutting systems is much quicker.
--> Look for a friend working with one of these
machines ;-)
Fig. 2. The first version of riser (after weld and moderate machining)
You MUST be sure that the riser is 100 % through-welded! Next step is
to machine risers firing zone.
After that you can weld the finger protector. If riser weld operations
are finished, then you can start with limb pockets machining.
Limb pockets are milled with a vertical mill (look fastening
limbs). Also a correct hole (M8x1 metric thread) for a stabilizer was
milled.
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We have no company regarding photo anodize in Estonia. You can paint
it with strong and
abrasion proof powder-paint (the riser is not so cold any more).
If the riser is finished and painted, then you can plan a grip. Good
materials are wood or rubber.
The walnut is the wood for advanced grip makers (in one or two piece(s)).
But the oak is good enough..it is strong. Some notices before building
the grip:
a) Ergonomical angles
for grip are 20 deg (straight) and 15 deg (side)...other sources talk about
different numbers,
b) care for minimal vibration from riser
to grip,
c) comfort.
Find some suitable wood, draw your lines on it and go!
If you are interested in handcraft then use different colored wood
(mahogany or birch)
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Use notching cutter...knife and a lipstick to make it more accurate...
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There are two different types of limbs:
1) Solid limb
2) Split limb
Which are already in straight and bended versions.
Split limbs are more rigid (in lateral direction) than solid (if the mass
weight is the same).
The most common material is wood, glass-fiber
and carbon-fiber. Extremely good limbs are made from 100 % carbon-fiber,
because they have very small moment of inertia
(small mass). If your bow limbs are longer (approx. ATA=40" and more) then
you have more comfort
with finger release, but arrowspeed is lower
(physical pendulum). The bows with shortest limbs (...28,5"...38" ATA)
have more speed, but
you can't shoot with fingers, you have to use
a release. To make limbs yourself is not so complicated, but you need the
right material.
This is very expensive (often you have to buy
many meters, but you need only 2 meters). You need a specially designed
oven to heat glued parts
which are under heavy pressure. Cheapest way
is to design limbs yourself and to order it from local manufacturer. We
have no pro shops for
compounds in Estonia, but we have a company which
makes limbs for sport bows and crossbows - Falco.
If you start to design limbs, you should remember
that the cross-section of the limbs fork has to be a couple of times bigger
than the cross-section of
flexible part. In static case (drawn bow) the
forces near to cam are not so big, but if you release, the dynamical forces
are extremely high.
In release case, if the cam arrives in zero position,
the strength of the fork must be huge. How to prevent your limbs from damage:
a) enlarge the cross-section of the fork (disadvantage:
more limb top mass decreases the bowspeed),
b) use cam breaks and -smoothers (not so wide-spread...cam
have bigger mass, but cam arriving in zero position is very smooth),
c) limb savers - specially designed stuff (look
SIMS VIBRATION LABOROTORY),
e) NEVER DO DRY FIRE! (shooting without an arrow
or arrow with lowest mass weight). It will cause limb or risers damage.
Arrow weight rule for compound
bow: 5-6 grain (1 grain = 64,8 mg = 0,0648 g) per 1 pound bows max draw
weight (If you have
70 pound compound bow then
arrow weight must be at least 350 grain (22,7 g)).
Fig. 4. The area of limb cross-section near the
cam
nuki poolne ots=cams fastening side
sisselõike lõpp= the fork ends here
paindele töötav ala=flexible area
kaare ristlõike pindala=area of limb cross-section)
ATTENTION! Cam fastening to the limb has to be
very accurate. If the axle of cam is tilt then the string or cable may
drop off from cams groove
(once it was happend to me...two hours my hands
have quailed...there is 350 kg between axle-to axle (ATA)).
Taking off string and cables without a special
bow press may be dangerous for your LIFE!
I use tighten belt for cars (tested with 650
kg), but its is not safe. Be careful!
Limb is 17,7" or 450 mm tall... it is enough for finger shooter. Limbs are coated with special paints... |
The fork. The end of fork must be very-very smooth... |
This picture is bad quality but it shows limb profile... |
Main source of vibration are moving limbs and
the rotation of the cams ...think about how to hamper the vibration circulation
from limbs to riser...
There are lots of methods...decreasing connection
area between limb and limb pocket, stabilizer has direct connection
to limb pocket through the limb....last word
is using sand traps from BowTech...
I tried to use a rubber sheet for decreasing
vibration, but the rubber was gone between the details...
In experiment was found better material for floor
covering (plastic, linoleum).
The best bolt for fastening is a high-strength
M8 (12,9). Weaker bolts are not recommended.
Also you must watch that bolt and limb has no
direct connection.
The nut is the yellow cylinder...the other yellow thing is a specially
designed aluminium spacer ring...there is no tension fit... |
The limb pocket... there are 6 support areas... it gives more accuracy... |
Damping materials fit
between the limb-limb pocet and spacer ring-limb... |
ATTENTION! The connection limbs to riser must
be very accurate!
4. A LITTLE BIT THEORY OF THE CAMS
There are lots of work to do with a cam...the
first idea was: if I do the compound bow, then I want to have best
from and so I choosed agressive cams - this types
of cams gives more energy than others...look fig. 5.
Most common types of cams are round (excentric,
soft), medium (between soft and hard (also one cam types))
and agressives (one cam, too). Round types are
for target shooters and hard cams for bowhunters (not legal in Estonia
????)
and advanced target-3D shooters. The other story
is through shooting system (dubble grooved cams).
The last word is one cam bows...there is no adjustment
needed if the sting (cable) will stretch. But still 2 cam bows
are the most fastest in the world (cams mass
weight is many times smaller as with cam bows).
Fig. 5. Compound bows draw weight characteristcs
(blue curve-soft cams, magenta curve-hard cams)
(black line is regression line for hard cam characteristic).
So...if the area below the draw characteristc
is bigger then the bow saves more potential energy (Ep=F*h,
F- draw force [N] , h-draw [m]). If you have
regression equation (like fig. 5. y=-1E-13x.....) then you can calculate
Ep
by integrating (equation was found by MS Excel).
But the equation has to be at least 6th or 7th gradation of polynoma.
Also you can find it by adding the small areas.
Now if you are shooting then potential energy
will transform to cinetic energy 
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(m- arrows mass weight,
[kg], v- arrows base speed [m/s]). Of course,
part of this energy transforms to noise, vibration, friction and air friction.
This is about 5...30 % (bows operate efficiency).
Arrow rotation will add more cinetic energy....you must summarize both
(better look some theory of mechanics).
After shooting agressive types of cams will increase the vibration and ahead moving.
The second very imortant factor is the let off
(anchored-holding weight). This is percent value of maximum draw weight.
Often bowhunters use 70-80 % let off (because
you must sit in tree and watch the animal, so you wait with full draw about
2 minutes).
If you have a 70 pound bow, then 21-14 pound
is for let off. This percentage is depends by the cam(s) geometry.
Best material for cams is aluminium alloy 6061-T6.
As I can't get it, I found old russian aircraft material D16.
Its strength is between 6063 and 6061. With one
word -cams material must be extremely hard, solid and light.
Shafts are 5 mm calibrated stainless steel (use
temper hardening and normalize it).
If you figured out what geometry to use then cut
them of from Al- sheet (i used 5 mm sheet).
Ther are two separate part for string and cable.
Bearing holes are broached.
Sheets are glued together and fixed by M3 bolts.
Also bearings and bolts are glued. If all are assembled, you must check
the cams for rotation (movement) without resistance.
Cams are anodized because the paint gives too much weight.
After that you should have something like the following...
Upper cam (from right) |
upper cam (from left) |
Cams bearins... (shaft, ball-bearing, E-clips, plain bering) |
...assembled... |
...just see... |
...look the grooves... |
...with shaft (from left)... |
...with shaft (from right). |
Fig.6. Fastened cam (lower). Use bronze- and
steel spacer rings for axial bearing.
I used 1,5 mm steel rope for cables (steel cable
is good only for soft cam type bow). Steel rope is good-there is no strech
at all, but synthetic material strech will cause
cam timing problem. You must add more strands (min. 16...better 20) to
cable
to decrease it. Steel ropes are coated with thermo
shringing tube, it will decrease the friction and abrasion between cam
and cable.
Cables must be exactly at the same size. There
are about 5 turn in cable...
Centre serving was made from special FF centre
servin material...naylon is nood good enough..
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 Press details for cable loops ... |
There are two different types arrow rests:
1) for shooting with fingers,
2) for shooting with realese.
Arrow rest main factor is adjustability. Also
minimum friction is welcomed.
If you are using an overdraw, then it is possible
to shoot with shorter (lighter) arrows (flat arrow excursion).
As a rule of thumb: all well adjustable arrowrest
is very expensive...you better make it yourself.
Adjustable is:
a) arrow position in X axle,
b) arrow position in Y axle,
c) pole stage for arrow has 2 scenery and 2 rotation
movement.
Rest was made from different materials. Example
pole stage was made from 2 mm knitting pole.
Bigger problem is, where to find pure teflon...Teflon
barrel made by using electric drilling machine
and regular file.
Arrow rest details... |
...assembled... |
...mounted to bow... |
Arrow rest with attachment of laser sight.
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The best (cheapest) guiding bar is calibrated
rounded steel (stainless) and a teflon slider.
You have to polish the guide bar very strictly.
For finishing use water-sand paper no. 1000.
You need sharp drills, knife and broaching tool
(same diameter as guide bar...example dia.=8 mm).
As a precaution a taller slider is better (no
cleaving). Also smaller mass is good. Slider moving must be free.
Teflon slider... |
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A good sight is simply to adjust...there are four
sight pins (hunting sight) for different distances.
The sight pins are made from safety pins and
painted red with nail polish...
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You need a sling to perform a relaxed shot with
no finger squeeze. Also the sling is best against
bow jumping after release. You can buy it for
20$.....or do it yourself. It takes just an hour.
You need bag belt, velcro band and a piece of
leather, metal or plastic link, filament, sewing machine.
GO !
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9.2. LASERSIGHT
The Lasersight is a nice detail for shooting in
the dark. Also it is a good training aid (shows any vibration).
The best laser for a bow is a simple green
laserpointer but red is good enough. You can buy it for 5$.
To fasten the laser you have to build a special
detail which gives you vertical and horizontal adjustment.
There are several methods to turn laser on and
off. You can do it with pull action or a simple toggleswitch.
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 take apart for trigger rework...
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Testing... laser ja märk=laser and light spot märk esimesest lasmisseeriast=mark from first testing (After 5 th shot the string fastening was broken (look 11.2) and string hited to me....also both limbs was damaged (look 11.1)...I'm alive... |
The most important compound bow parameter is saved
energy (potential energy).
This is equal with drawing characteristics area
(fig. 7). Graph was taken up by scale (dynamometer) and meter
and the data are converted to"Excel" (red line).
Excel will make regression line (trendline) and equation.
Fig. 7. Compound bow's "Primus" draw-force chart.
Integrate the equation and the result is potential
energy:
You can calculate arrow's base speed. You have
to know arrows mass weight-m. Also you have to multiply by
bow's operating efficiency coefficient 'n'. Best
bows have n=0,95:
(291 fps (n=0,75), 327 fps (n=0,95))
If you know an arrows base speed then you can
calculate maximum flying distance (without air resistance):
Ready for 1st shot... |
Feel the power... |
Elapse 11 month to design and produce it...
11. OBSERVATIONS AND SUPPLEMENTS
11.1
Problem: Limb has a long crack between
its fork
Reason: Dry fire or disbalanced tension
Correction: Use
spacer rings or make limbs top with cross-fiber tehnology
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11.2
Problem: String fastening to cam was broken
(extremely
hazardous)
Reason: Wrong nut geometry, was poor material
and low dimensioned bolt
Correction: Use
bigger bolt (M4, 12.9), choose nut geometry and use stronger alloy
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11.3
Problem: Positioning
vane was "hurt"
Reason: Finger release is not good for
this type of rest
Correction: Use
release..
11.4
Problem: Thermo
shrinking tube on the steel cable was broken
Reason: Geometry of agressive cams causes
too big tension to the plastic
Correction: Use
only polymer materials...like a Fast Flight
Also, you can see the movie how to shoot with this bow...
Also special thanks to Rolf for translateing...
ARCHERY FAN, WRITE TO ME!
leintz@hot.ee
