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A SIMPLE, ACCURATE SPINE TESTER developed by John C. Johnson ( 2jays ) Most of us have, at some point, thought "Sure would like to have an accurate spine tester, but that's a weeks groceries
for the four of us."
Well, if you can scrounge up a few simple things shown here, chances are you can build yourself a spine tester accurate
enough to measure shaft deflection within about .020" deflection. That is 0.00077 lbs of spine. Pretty dern tight tolerances.
I was inspired to develop this spine tester after seeing and building one developed by Rob Goebel (ArcheryRob).
All I have done is to combine the best parts from a number of sources. There are no new ideas in the world,
just new ways of putting them together.
There are only two hard-to-get parts involved. We're going to give you one and teach you how to make the other.
NOTE: if at all possible, do all drilling with a drill-press, as alignment is important to accuracy. Wear Safety goggles and avoid loose
clothing when working with machinery. Remember, the best safety device is between your ears. You and you alone are responsible
for your safety.
STANDARD DISCLAIMER: I am not responsible for anything you screw up, including but not limited to your body, your marriage,
or your budget.
PARTS LIST (2) pieces 1" x 6" x 26" lumber (1) 1/8" x 3" machine screw (3) 1/8" self-locking nuts (6) 1/8" fender washers (2) end-caps 1/4" x 5-1/2" x 6-1/4" plywood ( 1) shim - 3/4" x 1/2" thick square of hard foam-rubber or something similar. I snipped a piece from my GI surplus sleeping-pad. THE FIRST HARD-TO-GET PART After lots of research on the dial-design, I finally figured out how that bad boy was designed and why equal
increments on the dial didn't work. Thanks to Bruce Woodard, my Machining Technology instructor, for his encouragement and mentoring.
MAKING THE DIAL 1. Photo-copy the dial illustration. Most commercial photo-copy machines have an enlargement feature. Enlarging/reducing does
not normally affect the accuracy of this dial in any appreciable way if you keep the short spoke equal to "Line A". However, I find letting
"Line A" equal 2" to work best for me.
2. Glue it to a stiff backing (sheet aluminum, thin Plexiglas, scrap wall-paneling, etc.).
3. Drill a 5/32" hole at "X marks the spot". If you use Plexiglas, use a 16d finish nail to drill a pilot hole, then run the drill backwards
to avoid breaking the backing.
4. Drill a second hole accurately centered on one end of "Line A".
MAKING THE NEEDLE AND ASSEMBLING THE SCALE TO IT
You need something round and flat for a hub and some stiff wire for spokes. I used a 3/4" thick slice from a hardwood closet
rod and two bicycle spokes.
1. Drill a 5/32" hole through the center
of the hub for the axle.
2. Drill two holes opposite one another around the rim. Be sure they are the right size to epoxy your spokes into. Install the
spokes into these holes. Don't trim them yet. Be sure not to plug up the axle-hole.
3. Bolt the needle-hub through the hole in "Line A". Make a 90?bend outwards in the short spoke at the other end of "Line A".
Be as accurate as possible. THIS LENGTH IS MOST IMPORTANT MEASUREMENT ON THE WHOLE DERN THING!!
3. Remove the hub and put a washer on the machine screw, install the hub to it, another washer, and a self locking nut. Tighten
the nut so that the hub spins freely but has a minimum of side-play.
4, Put the axle through the hole in the dial and mark where you want to cut the indicator end of the needle. Cut and shape it to
a rounded point.
5. Now locate and make the bend on the other end od the needle assembly. Measure "Line A" on the dial. Whether you enlarged the
dial or not, this line establishes the correct ratio. It is important that the length of this line "A" is the distance from the
center of the axle-hole to the bend.
6. Then install another washer, the dial, another washer and another self locking nut. Here again, tighten the nut so that
the dial spins freely but has a minimum of side-play.
1. Glue and nail the 1" X6" boards together edge to edge. 2. Glue and nail the end-caps on. The 4-3/4" x 26" inside surface is the floor. 3. Locate and mark the center of the top edge of the back wall. 4. Measure and mark the length of line A to the left of this mark. 5. At this point measure down 1/2" and drill a 1/8" hole front to back. 6. Put another fender washer on the axle, install through this hole, add a washer and lock nut. This one is to be tight. 7. Glue the shim on the back wall about where the degree numbers will pass over it. This is so you can zero-adjust the scale and
it will stay where you put it. 8. Place an arrow shaft across the two end-caps to locate where you want your grooves. This may vary according to what you used
for a hub. 9. Use a 1/2" round rasp to cut the grooves 1/2" deep and sand smooth. 10. Cut the arrow-rest portion of the short spoke to the length you want and balance the needle. I used split-shot fishing
weights crimped on the short spoke. 11. Remove the needle assembly, stain and finish the frame to your liking. and re-assemble.
THE "NO-BOX" METHOD
One young man, Andrew Bochnovic, "DamnYankee" on the Leather Wall, had the idea of mounting two screws in his garage wall
26" apart and placing the pivot bolt at the appropriate place between them . I might suggest using nails or cup-hooks
rather than screws so the shaft doesn't get scarred. Thanks, Andrew, for sharing this.
THE OTHER HARD PART
The second hard part is getting an accurate 2-pound weight. After all, if we're going to talk about thousandths of an inch,
1/2 ounce is not close enough. Here's how I did it.
(1) Empty PVC pipe cleaning fluid can Be real sure it's clean and aired out. (1) 1/8" machine-screw hook (2) nuts & washers (6) 4-oz fishing weights (3) 2-oz fishing weight (1) 4-oz tube BB shot or lead shot (2) baggies 1. Remove the brush from the lid and dispose of it. 2. Punch a hole in the lid and install the hook with a nut &
washer on each side. Put as much thread as possible through the lid and cut off the excess.
Gather all this stuff up and head on down to your local Feed 'n Seed store. Buy a sack o' feed for Fido and talk nice.
If you don't have a Fido, buy it anyway and give it to a neighbor. Notice that they probably have a government-inspected
pharmaceutical-type balance for weighing seed, (Man, that stuff is costly, so they want it to be right on the money). Ask
if they'll help you get a good weight. You brung the baggies so your lead wouldn't contaminate their scale.
This may be a 2-lb balance but chances are it is only a 1-lb'er, so you can do this in 2 stages:
3. Use 1 baggie for the can and enough lead to get a pound. 4. Use the other baggie for the lid/hook and enough lead to get the other pound. Shot only weighs a few grains each, so you can trim this real close. Take it home, put it together, and start making arrows. GOOD HUNTIN', NEIGHBOR!! Revised 10-25-2000 Copyright © 2000 John C. Johnson ( 2jays )
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