Spine Measurement

Well we all know about spine measurement... or do we?
I've made a quick and dirty spine tester, having noticed one rogue arrow, and using my digital verniers to measure spine is a right fiddle.

A while back on an archery forum on a post regarding how point weight effects spine I made a comment that infinite point weight would effectively give zero spine! e.g If you take a warbow to full draw, put the arrow point against a brick wall (which approximates to infinite weight) the arrow will just explode when you loose (e.g it will flex infinitely).
This was tongue in cheek, but was to illustrate a point and demonstrate a way of reasoning by extrapolating to extremes.
Well some bloke "corrected" me and said I meant infinite spine.
It became apparent that the target archers way of measuring spine to AMO standard is the opposite to the traditional wooden arrow way of measuring spine!
Now this is what is technically known as bloody stupid, however I'll let you decide the rights and wrongs.
The tradition way is to support the arrow at two points 28" apart. OR any other convenient distance, and this is important for shorter flight arrows. You hang a 2 lb weight on the middle of the arrow and measure the deflection in inches. You divide the distance between the two support points by the deflection in inches. (This automatically compensates for differing support distances... clever eh?)
E.G. If we have the supports 28" apart and we get 0.5" deflection that gives 28/0.5 which is 56
So the spine is 56, this number bears some rough relationship to the bow poundage for an average bow.
Note:- as deflection of the shaft increases the spine reduces, so 1" deflection would give 28/1 = 28 spine.
The spine figures are not linear, this shows on the uneven spacing of the scale. Also note, for convenience I've used 26" between the supports.

As far as I can tell the AMO method simply measures the deflection in thousandths of an inch with an 880gram (1.94 lbs) weight applied using a 29" shaft with supports 28" apart.
Has this annoyed you yet? They are randomly mixing metric and imperial and I have no idea what the 29" shaft length is about!
BUT the most worrying thing is they are just measuring deflection. Why does this matter?
If you plot  distance between supports divided by deflection (x) as a graph y=28/x you get a curve and as x gets bigger y gets smaller.
If you plot simple deflection y=x you get a straight line and as x gets bigger y gets bigger...
So the AMO spine runs the opposite way to the old method, bigger number is weaker rather than stiffer!

Now which is right?
Not for me to judge, but I'll just mention that  bending stiffness of a beam k=p/w Where k= bending stiffness.  p = force.  w = deflection
Oh, btw, my 2 lb weight is just a carefully calibrated bag of pebbles :-)

Trilam all Done

I've got it finished and over 60 arrows through it (I'll take that up to 100 before final waxing). I've just had it on the tiller and it's almost 50# which is odd, as it looked more like 46# the other day, this illustrates the difficulty of getting good data. The camera is mounted about level with the bow so is effectively looking down on the string at full draw giving a parallax error. Whereas if I sit down to pull the rope my eye is about level with string at full draw removing that error, so in reality it's a tad over 47# generally it's prob' sensible to give draw weights rounded down to the nearest 5# as a bow is likely to settle a bit rather than gain weight (unless it starts off not fully seasoned).
It's been interesting shooting the bow, the arrows seemed to wag their tails rather, but a little work around the arrow pass and inlaying the Abalone helped. Then I got suspicious that maybe there was one odd arrow, so I marked the one that flew high and right with an inconspicuous dot. Next five arrows, sure enough, one waggled off right, when I looked, yes it was the same arrow. Maybe that accounts for some of my poor shooting at the last field shoot as it was marked as a number 1 arrow.
I shall try the same test with my field bow and see how that throws 'em, I suspect the effect will be
slightly less marked as that bow is prob' a tad slower.
In terms of feel the tri-lam is lovely, it feels like it would draw further just as smoothly and I've been making sure to shoot it from a good full 28" draw which is extending me slightly. It's good to be shooting more as it seems to be helping the tennis elbow and shoulder niggle. I've been doing push ups night and morning, at least 10 and 15 or so if I'm feeling bouncy.
Couldn't resist adding the pic of the grip again as it is so gorgeous :)

Tri-lam Nearly There

The outers still need to come round a bit more.
I'm mostly adjusting the tiller by rounding off the corners, so the inner third is a sort of rounded rectangular section and as you move up the limb it gets narrower and the belly gets more rounded until at the tip it's circular as it goes into the nock.
Since that pic (shows it at 27" ) I've had it back to 28" from a full brace, I've also blended in the riser block a little more so it looks a smoother transition from riser to limb.
It needs the proper string making and then it's on to the pretty stuff. I've given it the first wipe of Danish Oil this evening so tomorrow I can pore over it taking out any remaining tool marks. Then string, arrow plate, grip. Maybe I'll get a load of arrows through it on Sunday to confirm the arrow plate position before doing that.
I'm not usually one for going bonkers on the finish, yes, I want it good and my standard is probably pretty high, but on this one I'm going for broke.
If I can find the right materials I'm trying for a top notch finish. Mind there are limitations with the wood, a few belly knots and the residual marks from the heat treating of the Yew billets before it was glued up, but hopefully I've dealt with them in an appropriate manner and in any case you want some character in a bow...
Right gotta get out the sub-aqua gear and fly off to South Africa to wrestle some Abalone for the arrow plate .... or maybe E-bay has some?
Need some Unicorn tail hair for the string too ... anyone know a good source?

The pictures don't really do it justice as it's hard to photograph something long thin and shiny! This is just the first stage of finishing too, anyhow  a load of perfect lams machined and glued up with robotic precision would be soulless.

Just to give an idea of scale, where the top limb enters that nock it is 10.65mm diameter, that's why I've left the pics small.
Also note the top nock is shaped to take a stringer above the string groove and the lower nock has one big deep groove which will accommodate both string and stringer.
Should get the final full draw vid/pic tomorrow, unless I get called up for sliding wardrobe door fitting duty!
Update here's the grip and arrow plate:-

Tri-Lam Glued Up

I did a dry run of the glue up to make sure everything would go smoothly. I spent a few quid on G clamps (you can never have too many).
Immediately before gluing I ran each surface on the belt sander to ensure it was clean. Having clamped it I proceeded to add rubber strapping between the clamps, this was a good thing, as I could see it squeezing out more glue. After doing the strapping I nipped the clamps up again.
Note, on the glue up I've added an extra clamp at each tip, I need the full length of the bow as the Purpleheart is only just long enough, so I want to make sure it's a good glue up right to the tip.

24 hours later, I've got it off the form, cleaned up the edges with a farriers rasp and I'm now running it through the bandsaw to an approximate shape. 26mm wide at the centre section (for about 2') then tapering to 15mm which will allow plenty of slimming down in the later stages.
I've trimmed off the ends so there are no odd laminations sticking out to get snagged. I'm very pleased with the glue line, e.g, you can't see it :-)
One pic shows a thin sliver taken off with the bandsaw where you can really see the glue line. It's worth all the time and preparation, 'cos if there are gaps, then it all needs re-doing.

I've just this minute glued on a 12" riser blcok made from an off-cut left over from the 3rpi Yew. I like to have bits of wood with some history behind them rather some bit of random hardwood from an old window frame. That's me done for the day, still a bit tired from my cold, but I've been doing 10 push ups night and morning which seems to have firmed up the tennis elbow and left shoulder... been shooting half a dozen or so every day and the decent grouping has returned.

Boo, Purpleheart, Yew Progress

I've got the purple heart tapered and joined with a short scarf joint. The Yew belly billets have been tapered, a short Z splice cut, they have then been heat treated and glued.
I've got to plane up the boo and the glue it all up. I've asked on Primitive Archer whether I'm better off gluing all 3 at once or gluing the Yew to the purpleheart first. There's a hint of deflex in one half of the yew still and maybe gluing it up to the Purple heart would pull that out. Mind I'm thinking of gluing the whole thing up with a hint of reflex / backset, maybe just an inch or so, although it probably won't take much set during tillering.

More Thicknesser Development

It's almost finished now, here's a pic with the power plane missing, but it's sole plate is screwed to the top to give alignment as it is all built up.

I've put feet across the bottom spaced so that can be clamped on to the workmate. The adjust wheel is more central now, slightly less convenient, but better mechanically, the screw that pushes upwards bears against a bit of steel plate. I've made lots of improvements to take out slop. The biggest factor is probably the thickness of the table which rides up and down smoothly with no real room to twist.
The only problem is I don't know where I'll have room to store it, maybe I need another major sort out.
Most of the wood is off-cuts from shelving that I've been doing and bits that were hanging around.
Just added a pic with it all finished and set up with a length of Purpleheart in there for tapering.

Update:- It works a treat, no perceptible ripple, bit noisy of course and you have to take the cut in stages, con't just rip a great big taper off in one go. An advantage over a sander style thicknesser is it produces chippings rather than dust. The chips are much easier to collect/sweep up. It does throw 'em out of the front with some force so you can feel 'em pinging onto the back of your hand.

Improved Taper Sled and Thicknesser

The thicknesser was made a while back from an old power plane. One fault was the adjustable plate was hinged at the back and didn't come up parallel to the sole of the plane. I've improved this by hinging both ends so it moves parallelogram fashion, being pushed up by an adjusting screw under the front edge.
The other problem was the taper sled which was wooden and not very stiff or flat. I bought some aluminium U channel and made a better sled, it is still somewhat flexible over a 3' length... but then, so is almost anything unless you go to box section steel.
I gave it a quick try out on a strip of Ash, I set it to quite steep taper and a deep cut. It did the job but with some chattering.
I learned a few things.
1. Stick the whole lamination down with double sided tape.
2. Only take a fine cut, and/or maybe gradually increase the taper.
3. don't try to pull it back out with the plane running. It will take other cuts and spoil the work

I'll have another try on a bit of scrap and report further.
Some of these taper/thickness machines that people make use a drum with sandpaper on, I reckon they must take an age to run through if you are taking off any amount of material, though they doubtless give a better finish. I imagine using this for rough taper, then running the lamination over the belt sander.
We'll see how much wood I ruin before resorting to working staves ;-)

Update:- I've given it a go with finer cuts and it works well, there are still some improvements to make as the parallel motion doesn't work at low angles and there isn't enough height for thicker billets. The wooden adjustable plate wasn't flat, but a few strokes of the plane have improved that.
I'll work at the design to try and make it good and rugged and versatile. It looks promising.