Primitive Archer
Main Discussion Area => Bows => Topic started by: Badger on December 05, 2013, 01:42:42 pm
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I had a few requests for the bow mass program on excel
The Program was written by Dave ( Aussie Yeowman) after he read my chapter.
The program only asks a few basic questions and then there are a few tricks one need to know to generate the proper answers. I just started this thread to go over it with the ones that get the download.
My Email is Badger5149@aol.com I will send it to anyone who e mails me.
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I have that, forgot where I got it. It's great, before, I had to paper pencil it. I can recommend it to anyone.
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After a quick look I'm assuming I need to read TBB4 first?
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I'm not entirely certain that I'm thsmart enuf tu understand all the stuff this little gizmo is telling me, but I'm all ears!
OneBow
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I'm not entirely certain that I'm thsmart enuf tu understand all the stuff this little gizmo is telling me, but I'm all ears!
OneBow
It would help but we can cover the basics right here. going to weight for a few more guys to get it. Then I will start expaling it. Questions would be helpful.
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I need the basic premise first, I'm not sure what we are doing. But I can wait. I'm not antsy or anything, no rush >:D
Really, wait for more people
Don
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haha I'm with Don! just what are we talking about anyway? :laugh: im assuming... that it involves strategically placing the mass of a bow (limb width and thickness) to obtain the desired draw weight using the least mass, which would mean optimum performance? something along those lines?
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got it. Thanks!
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Help those of us out who haven't earned their bowyer IQ yet... ::)
Thats exactly the question that started me on the mass priniciple. 1st of all I am a backyard bow builder and not a geek by any stretch of the imagination. I do everything the hardway by actually building bows and then looking for patterns.
The first thing I had to do was develop some very basic common sense" bow logic" taking into consideration a few givens. The most important is that wood only has 1 correct thickness and it will be different on every bow we build. The challenge is to find and be able to express working techniques that will allow us to ge close to this perfect number on every bow we build. Obviously we are not going to do bend tests and calculations that are way above my head anyway.
The most common denominator I found that best predicts both the demensions and the proper tiller is mass. It is always good to think of thickness as how far a bow can bend and width as controlloing how far it will bend. If you have to reread that and think about it do so, it is an important basic piece of info that is good to drill into our heads.
When we bend something we will always create a radius, the radius may not be the same for the entire length of the limb and certainly the radius wont always be the same for different bows. working with the most elastic woods we have the inside radius needs to be less than 1% smaller than the outside radius, most woods are about 25% less than that. This is just something to keep in mind when you see a narrower section of the bow that is thicker but bending more than a wider thinner part of the bow. You know it just aint right. You don't have to figure anything here just keep it in mind.
Bow logic tells you that if you have paralell limbs it could only be for one reason and that is you want the outer limb to bend more than the inner limb giving you an eliptical tiller shape. Bow logic will alos tell you that if you have a tapered limb the bow should bend at least equally and the thickess should be about the same. Theoreticaly you could build a bow at one specific thickness and then tiller it completely from the sides for a nice round tiller.
Nearly all the woods we work with are going to fall into the specific gravity range of somehwhere between 50 and 100, the great majority of those will be between 60 and 80. The majority of the bow we build are going to be between about 45# and 65#. This is where I feel the mass principle is most accurate.
The mass principle can be very valuable in determining how to execute a particular design for instance. You may want to build a hickory backed ipe english longbow. You want it 6ft long but you only want it to be 50#. You don't want the bow so narrow that it is uncomfortable to shoot just so you can make mass, so what you do is modify the tiller shape until it is the right mass at the width you feel will be comfortable. Lets say for instance you have roughed the bow out and got it bending. it feels like a 100# still but is bending. You weigh the bow and find out it is 6 oz too heavy to hit your target mass and you know only about 2 oz more wood will come off to make your weight. You go the the program and start adding length to the handle and fade number. If you add 4 then you will use just a slightly elyptical tiller, if you add 8 you will use a full elyptical tiller, if you add 12" it will be whip tillered. They should perform pretty well if built like this and not have handshock.
Say you are working on an American longbow with parallel limbs most of the way down. You keep in mind the weight of the wood when you rough it out, for mid 60 bows for intance you might figure 1 3/8 for osage as a starting point 1/1/2 for locust and 2" for a lot of the white woods, + or minus depending on the density of the specimen. You simply rough out the bow, get it bending and then check the mass weight. Figure you have at least a couple more ounces comming off just to make weight so if you are within 2 or 3 ounces you just keep tillering the belly as you get closer you can adjust the width a little at a time or more to adjust for mass weight and fine tuning tiller.
The secret is knowing how to do the input, modify the length of handle and fade input to accomadate your tiller shape. If you want circular limbs use the exact measurement of your handle and fades if you want elyptical then add 2 or more inches depending on how extreme you are going.
If your tips are stiff say for six inches you may want to use a draw lenght figure 1 or 2 inches longer than you are actually going to draw it. If the tips are stiff for 10" you may want to use a figure 2 or 4" longer than your actual draw.
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got it. Thanks!
Not really the least mass, you need ample mass but in the right places. My bows are heavier than they ever have been but are performing better and are more durable than they were when I was just making them as skinny as I could.
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Bow logic tells you that if you have paralell limbs it could only be for one reason and that is you want the outer limb to bend more than the inner limb giving you an eliptical tiller shape. Bow logic will alos tell you that if you have a tapered limb the bow should bend at least equally and the thickess should be about the same. Theoreticaly you could build a bow at one specific thickness and then tiller it completely from the sides for a nice round tiller.
Ok, finally. I've been asking this question on other threads and not getting answers. Badger, would you deal with my question?:
In vol.4 theres a hand drawing of limb profiles and proper tiller of each. Maybe I'm confused about eliptical vs. circular tiller. Eliptical to me is like the curve of an egg. Circular is like, well a circle. So the outer limb on a circle is bending more than with the eliptical? And if its bending more than it should be wider--wider as in parallel? So fully parallel should be fully circular and pyramid would be more egg shaped, with the wide part of the egg on the handle end? That's actually what the hand drawing in the book shows but the actual description says the opposite. Hence my confusion. thanks!
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I guess it depends which way you lay the egg, parallel should bend more as it nears the tip.
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Im confused... When you say parallel you mean constant width right? Wouldnt that be bending more out of the fades and stiff at the tips?
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I will tell you why that is not logical, your goal is equal stress, you only have one correct limb thckness at any given point in the limb and you want equal strain. If the bow bends the same it will be thinner at the outer edge of the parallel but not quite as strained, in order to be equally strained for it's thinner thickness it has to bend a tad more.
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Ahhhh... So you are saying that if you had a parallel limb... You would NEED it to bend more near the tips in order for there to be equal stress. And a perfectly circular tiller would only be correct in a pyramid design.... So you use this mass principal to determine the correct tiller shape for a given limb profile? Am i on the right track of what you are saying? Cant wait to read your chapter. This is intriguing stuff!
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Read the above long paragraph, it explains a lot of this.
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Just read it all for about the tenth time. here is what is throwing me for a loop. Say i have two bows... The limbs on each are 3/8 thick from fade to tip. One is a pyramid... One is an American flatbow. If i throw them both on the tillering tree the pyramid will be closer to circular and the flatbow would be bending more at the fades.... Cause it doesnt have the width tapered to give it closer to even bend. That logic is sound right?
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Yes, thats right so you need to thin the limbs on the wide limb bow. The width allows the wood to bend. the thickess just determines how far it can safely bend.
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so what your method does, is attempt to make every working inch of the limbs under the same amount of stress, even though that may mean something other than circular tiller depending on profile? for instance a wide/thin portion of the limb may be bending more than a narrow/thick portion, but still be under less stress? so our flatbow would be elliptically tillered, and our pyramid would be circular.
so on every bow i have built, where i just try to make it circular, no matter the profile.... (in other words the tillering gizmo or straight edge shows the same bend throughout) is less than ideal? could this be a significant contributor to the amount of set that my bows have taken?
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My question to "adb" on the other thread, after discussing this issue, was, "why ever have parallel limbs then?". He basically said, "you never need to".
But it makes sense to me that you may NEED to have the outer limbs bending on some bow in order to get the DRAW LENGTH that you want out of, say, a shorter bow. So if that whole limb is bending alot, vs. the pyramid that is narrowing and therefore should NOT be bending as much there (the outer), it may need to be wide in the outer in order to be bending if, for example, it was an inferior species or inferior piece of wood. Capish?
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Thanks Badger!!!
For the great conversations. Current/hands-on discussion is so much better than just reading past posts.
Thanks! again for keeping these discussions going. You know, you are a good teacher.
DBar
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"why ever have parallel limbs then?"
Cause the stave is not wide enough to go full pyramid in the inner limbs. Or if you would, you'd get a real skinny bow. You need quite a wide stave with quite a flat back to make a good pyramid.
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Steve B:
I would say it's more important to get the inner limbs bending more on a short bow to reach draw length, because this contributes less to a shortening of tip-to-tip length as draw progresses. That's why holmgaar/molly designs do well at short lengths.
Where more aggressively elliptical tiller (that is, bends more near the tips) is beneficial is in longer bows. Say I have an order for a bow that for whatever reason, HAD to be 90 inches long but only 28" draw. What I would do is make the tiller so aggressively elliptical that it would almost appear I put two 28" normal limbs on a barely bending handle 34 inches long.
Others,
I'm not sure if the question was answered sufficiently, so here's some more fat to chew:
For a beam of uniform thickness to bend with equal stress along its length, it must taper in width in something very close to straight lines.
For a beam of uniform width to bend with equal stress along its length, the thickness must taper in a convex fashion, becoming more steeply tapered about 1/4 out from the tip.
We want a bow to bend with even stress as much as possible. Why is this? If a bow is bending to experience 10 units of stress everywhere except a 2" length of limb around midlimb, which experiences 7 units of stress, then that bit of limb likely has excess mass which slows the bow down. It also means the wood inside of that patch likely took more set, which projects to bigger set out at the tips.
Where you can afford to have less than equal stress is in the riser/handle/fade/flare area, as the amount of bend here (for flatbows most usually) is very low anyway, and usually takes no set.
Here's a design idea I just thought of: Think of a tiller shape you want to tiller. Some sort of ellipse. Start with a bow profile cut to pyramid design and tiller it to that desired tiller shape. What you will find is that the further out towards the tip you go, the more set it will have taken.
Now make another bow. But this time, using your first bow as a reference, make the profile a little wider wherever you see the set being too much. Just a little wider. You likely will still have a bow that has too much set in the outer half. So rinse and repeat.
Eventually, after a few tries, you will have empirically come to learn the perfect thickness taper and profile for that exact tiller shape you want.
Labour intensive, but an interesting exercise.
A bit more:
Strain is the percentage a beam's surface expands or contracts as a result of tension/compression. It is directly related to the radius of curvature the beam is bent to and the thickness of the beam. It can also be calculated by knowing the stiffness of the beam material, and the bending stress. A thick beam that bends a small amount might feel a great deal more strain than a thinner beam bent quite a bit further.
Theoretically, a properly tillered pyramid bow will not taper in straight lines, but will bulge slightly near the fades and taper to a zero-width at the tips. This is of course not possible in the real world and so there must be some width here. Adding width to a part of the beam increases bend resistance locally, meaning it becomes more stiff in proportion to the rest of the limb. As such, to make this section of limb less stiff, thickness must be reduced.
I've found lately that for a pyramid profile bow, the outer 1/3 to 1/2 of the limb need to be thinned slightly to achieve proper circular tiller.
A pyramid bow can be so narrow in the outer half because it is so thin in the inner half. Being thinner in the inner half, it bends further than the same part of a limb that has parallel width. The deflection here on the pyramid bow projects to a large displacement out at the tips, which means the outer half of the limbs can be of a cross section that bends less compared to the same section of limb in a bow with a parallel profile. It's kinda like a race between two cars. One has high acceleration and decides to coast along, while the other has lower rate of acceleration, but maintains it for longer.
Man it feels late. I'm just blabbering along now with no real point. Not even sure if, when I read this in the morning, it will make any sense.
Badger, I hope I'm not detracting all too much from your original intent...but I suspect I am.
Dave
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Good post Dave, sometimes my explanations are not that clear, I thought yours was great.
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My question to "adb" on the other thread, after discussing this issue, was, "why ever have parallel limbs then?". He basically said, "you never need to".
But it makes sense to me that you may NEED to have the outer limbs bending on some bow in order to get the DRAW LENGTH that you want out of, say, a shorter bow. So if that whole limb is bending alot, vs. the pyramid that is narrowing and therefore should NOT be bending as much there (the outer), it may need to be wide in the outer in order to be bending if, for example, it was an inferior species or inferior piece of wood. Capish?
No... I didn't say "you never need to"... I said I haven't made one in years. Big difference.
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Maybe I missed this but is circular tiller more preferred than elliptical or does it depend on design? I'm thinking that if the inner limbs are stiffer(elliptical) that they should be thinned and the bow shortened.
Don
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Don, it just depends on what kind of bow you are building. The tiller shape should correspond with the front view of the bow.
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Don, it just depends on what kind of bow you are building. The tiller shape should correspond with the front view of the bow.
Would not the side view more or less dictate the front view first Steve? Making it a requirement instead of a preference? For example, you wouldn't want to use a pyramid design on a highly reflexed (out of the handle) stave. Nor use a rectangular limb design for a deflexed stave...........Art
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Art, I waqsn't very clear, the front view should determine the thickess of the limbs at various points and how much bending they should do. The front view of the bow is subject to the type of side profile you are looking for.
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Badger
been followin the discussion with interest , and it seems that mass program is the most useful after the bow is bending and ready to be put on the tiller tree. Most of my difficulties as a newbie have been from trying to brace and pull the bow to soon I think. How does a guy that has not built too many bows know how much to reduce the stave before pulling it to hard? Is there an amount of deflection with a long string that should be reached before trying to brace? can yhe program help during the floor tiller stage?
willie
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Willie, thats a good question. When working with a long string you can't use a set amount of deflection as your base because it all depends on how long your string is, if your string is haging down 6" it will register a lot different with the same amount of deflection than it will if it is hanging down 1". I have seen where guys have figured that out and posted it but I never have done that I just go by feel. The longer your string is the higher it will read. I think Jawge might talk about that on his web site. If not I am sure someone else will come along and give you a rule of thumb you can work with. Almost everyone new has the same issue. I just checked a finished 50# bow, the string was hanging down to the 8" mark with my finger just taking out the slack, I pulled it to 23" to get enough bend to brace the bow and it was reading 40#, so for a straight bow I would say about 50# is where I would have first braced this one. The draw weight your bow is reading has a lot more to do with the string angle than it does how much the limbs are bending.
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Badger
I checked a lighter bow the same way and the results were similar. So if I follow the guideline to never pull a bow beyond it's intended final draw while tillering, then I quess the bow should be 90% done before I try to brace it? I have read where many boyers have said that they are pretty much done once they have a nice brace. If this is so, then most of the art of tillering is floor tillering?
willie
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I haven't done floor tillering in over ten years.
My approach to long string tillering is a little different.
I start with a stave that's bandsawed out to dimensions that will bend a few inches. I use a long string that is just barely long enough to reach over both nocks and pull no further than intended draw weight. As the amount of deflection increases, I shorten the string. Because a long string will make a stave bend differently to a short string, I focus mostly on the inner 1/2 to 2/3 at this stage.
Eventually the string gets so short that the bow starts to have a low, then increasing brace height. When the bow finally reaches full brace, I use the tiller string as a guide as to how long to make the proper string. Also, by this stage, the bow's pretty close to full draw too.
Actually, when I say that 'I' do those things I wrote above, I actually mean that it's what I have participants on my courses do. What I actually do, personally, is pretty much go from bandsaw to short string (with a reasonably low brace) straight to half draw or pretty close. Going from bandsaw to finished bow lately I've only been taking about an ounce off. But for beginners I wouldn't recommend that.
A bit of ephemera:
The tiller shape the bowyer is seeking should determine the thickness taper (or lack thereof). The width should be appropriate to the thickness to allow sufficient stiffness, and minimise set.
I've made pyramid bows that were quite highly reflexed right out of the handle, and they were rippers. And as for rectangular section, I don't think having a deflexed stave would preclude it. It'd just likely have a thicker section than another bow of the same specs without deflex.
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aussie
Eventually the string gets so short that the bow starts to have a low, then increasing brace height.
using your method for a say 50# bow, I could hang a 50# weight (temporarily, of course) on the short string and tiller the limb up to full draw, and at the same time be shortening the string so that it arrives at fullbrace about the same time the limbs arrive at full draw with the 50# weight?
willie
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Not quite. Usually, once we get to full brace height, the draw weight is reached at about 10" draw measured from the back. I suppose I was exaggerating when I said the full draw was nearly done by the time full brace is reached. Mea culpa.
The bows we make are man-tall longbows.
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thanks aussie,
no Mea culpa's necessary, just trying to understand better how to tiller before it's time to brace
willie
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My advice would be:
* use the shortest long string you can
* never apply more force than desired draw weight
* get a short string on when tips deflect about 8"
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For the program: How does one input nock position when gluing reflex into a backed bow?
For instance, if one were to attempt to glue in 2" of reflex using a form, but the bow only shows 1.5" of reflex out of the form prior to tillering, does one use the 2" number for the calculation or the 1.5" number?
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if you only got an 1-1/2" to start with <that's what ya got bro! >:D!
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So how would I use it for a static recurve and how do I account for sinew?
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Sinew bows come in a bit lighter, just yes for backed bow and it will take off 10%, for the recurves just add how much the tips were behind the back of the bow when you started the bow, say 5" in the tip position column. I never really figured it with sinew but they tend to come out about liek a wood backed bow.