Primitive Archer
Main Discussion Area => Bows => Topic started by: KS51 on June 09, 2014, 08:38:39 pm
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First, Hi! This is my first post. I'm not new to bow building, but I have been out of the loop for a bit.
Anyway, I am going to jump right in and ask for some tech advice with a build I've been working on for a bit. I started a take-down a while ago and I think I may have designed myself into a corner. To start with some pics.
here's the handle
(http://i221.photobucket.com/albums/dd44/kmsnake/20140609_164003.jpg)
(http://i221.photobucket.com/albums/dd44/kmsnake/20140522_174758.jpg)
(http://i221.photobucket.com/albums/dd44/kmsnake/20140609_164011.jpg)
(http://i221.photobucket.com/albums/dd44/kmsnake/20140609_164022.jpg)
the bow is 65" - 12" riser boo-cherry-oak. Riser is cocabolla.
my problem is with the mass equation, my target is 40# @ 29", but with 26-1/2" working limbs, the equation tells me the limbs should be around 16 oz when done (I ignore handle weight because I can take it off). Currently, they are right at 14 oz and only bending this much @ 40# (with long string)
(http://i221.photobucket.com/albums/dd44/kmsnake/20140609_164410.jpg)
Now, I still have all the reflex I started with so all is not necessarily lost. I'm thinking of adding 1-1/2" to each limb, thus "reducing" the weight from the mass equation and "adding" mass. Does anyone have any other suggestions?
Oh, the limbs are 1-1/4" wide for the first 12" then taper down to 7/16" with 4" of straight tip before the nock. Thickness runs from .605 down to .530 from fade to end of taper.
Ken
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That weight don't mean anything on the long string, you need it on the short string at low brace to get a true reading
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You have to get those outer limbs bending and coming around more.....I can see it's bending a lot right at the fades and not much anywhere else....Also the right limb appears (from the pic) to be much thicker than the left.
I like the simple hardware you are using. What are those and where did you get them? need to do that sort of handle attachment for a couple I am making from some perfectly quarter saw white oak wine barrel staves (which are actually "billet" size but in the barrel world they are called staves).
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Yeah, it's gonna break at the fades if ya don't get the outer limbs working.
EDIT: Well really all the limb should be working for that matter, unless you want stiff tips, etc.
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Thanks for the input. I managed to get it to a 4" brace and saw right away that the left limb is really stiff through the middle 60% of the limb. Unstrung it and will work it down until I get a good even bend when strung and go from there.
I have a feeling this bow is going to lower poundage than I intended. But, it is an experiment in a take-down building technique I've been working on (3rd try so far). I should note that the idea is that a 3 piece take-down can be made with only a drill press (hobby size) and without any special fixturing.
I'll get a picture of the hardware I used tomorrow and post it up for anyone interested.
Ken
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You can do it , I've built quite a few with just a band saw and a cordless drill
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Well, it's been a few days and I thought I'd check back in. I had to fix up my tillering tree and put in a chalkboard back. Now to get a beam compass and chalk so I can draw in target curves as needed.
But, I got the bow braced at 6-1/2" (I probably stick with that).
(http://i221.photobucket.com/albums/dd44/kmsnake/20140613_201804.jpg)
and here it is drawing to 17" @ 40#
(http://i221.photobucket.com/albums/dd44/kmsnake/20140613_202132.jpg)
I notice a little bit of limb torque when I first took it to full brace, but I'm working the right side as I tiller and it is coming back toward center.
(http://i221.photobucket.com/albums/dd44/kmsnake/20140613_202230.jpg)
I have lost about 3/4" of reflex at this point, but Boo usually gives up more than a lot of other woods.
(http://i221.photobucket.com/albums/dd44/kmsnake/20140613_202910.jpg)
for those interested, here is the hardware. The SS threaded insert has the same OD as the brass bushing (3/8") and the small brass tube is 7/32 with an .014 wall, so an 1/8" brass rod slides in snug.
(http://i221.photobucket.com/albums/dd44/kmsnake/20140613_202621.jpg)
by glueing the limbs to the square riser, I was able to drill straight through with 2 seperate diameter drill bits and guarantee that I had perfect alignment without trying to make precision drill fixtures. The trick is to use weak white glue (kids glue) and glue a piece of 30# cheap construction paper between the limb and riser. After drilling, a quick twist of the limb shears the paper without damaging either wood surface (old wood bowl turning trick I picked up). All you have to do is make sure everything is in line when you glue it up. One caveat - each limb is dedicated to one side of the riser, they aren't swappable.
Ken
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coming along nicely thanks for the build along I've been looking at my lams going....uhh now I just might have to try it!
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"Oh, the limbs are 1-1/4" wide for the first 12" Ya thats a little narrow, I have been playing around with some laminates at 1 1/2" and ended up with twist on both bows. I going to up the ante by cutting the limbs at 1 5/8" by the time I clean them up they should end up at 1 9/16" . Seems like the narrow limbs do have a tendency to twist easier for flat limb bows anyway. I have one survivor thats 1 1/2" wide. If there is one thing I hate thats Twisted bows. >:D
Anyway just keep tillering the bow by narrowing the tips.....
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Had to stop this bow, it is going to have tension failure.
(http://i221.photobucket.com/albums/dd44/kmsnake/20140622_104758.jpg)
the alignment pin has 2 little lift offs starting on either side. When stringing it, I did an inspection and noticed the limbhole at the pin had begun to elongate and the boo had begun to seperate in line with the outer edges of the pin. You can just see the lines in the photo.
I'll need to re-figure a way to drill an alignment pin hole without violating the back. Though I wonder if it will move the problem up to the limb bolt? My goal is to figure a way to do this without using fiberglass reinforcement.
Ken
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I don't know how you would figure the mass on a takedown but for a 65" bow drawing 40#@29" with a stiff handle and fade area of 12" and 2" of reflex I would figure about 19 to 20 oz including the handle. 14 oz sounds about right for the limbs. I wouldn't worry too much about the mass anyway, thats just a guide to help decide how wide to make a bow. If you bow is taking set and the tiller is spot on then either the belly wood is wet or it is over strained. In your case I think the boo may be over powering the red oak a bit.
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"I'll need to re-figure a way to drill an alignment pin hole without violating the back."
I would think you can do it, as long as you keep the limb thick enough at that section to not actually work, and make sure to get the rest of the limb working. Maybe you can try a wedge glued into your limbs on the ends to make sure it does not work. Your tiller still bends way too much off the fades. I would want as little as possible working limb near the violation of your inserts.
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I'm debating how far back I can move the alignment pin and not have too much side pressure on it. On this bow, it was about 1/2" back from the end of the handle (1" from the beginning of the bend). The tension in the backing needs to spread out over the full unbending surface to help prevent the affect I experienced.
I may be able to drill from the handle side and stop before I go all the way through, but that presents problems since the boo isn't flat and would force me to find a way to fixture the handle. My goal is to come up with a method of doing this without the need for any special fixtures.
also, I suspect the geometry of this bow was off (my fault). For it to reach the draw length I was after, the working limbs would have to bend into an arc of around 21" radius. From the years I've been building bows and all the full draw pics I've seen here and on other forums, I think there is minimum radius that wood can take safely and efficiently for bow purposes. (This is a theory I've been thinking about for a while and anecdotal evidence looks like the minimum radius is between 23" and 25" depending on the species)
Ken
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Ken, the radius doesn't mean as much as the difference between the radius of your outer limb and your inner limb. If they are at 1% difference you have allready gone too far. You really don't need to worry about any of that anyway as watching how much set the bow is taking will tip you off as to wether or not your design is working.
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I'm debating how far back I can move the alignment pin and not have too much side pressure on it. On this bow, it was about 1/2" back from the end of the handle (1" from the beginning of the bend). The tension in the backing needs to spread out over the full unbending surface to help prevent the affect I experienced.
I may be able to drill from the handle side and stop before I go all the way through, but that presents problems since the boo isn't flat and would force me to find a way to fixture the handle. My goal is to come up with a method of doing this without the need for any special fixtures.
The problem with this bow does not lie in the positioning of the alignment hole, or the alignment hole itself. It has to do with the tiller shape and the lack of extra thickness of the ends of the limbs, where you drilled in holes (and thus make the limb effectively narrower and thinner there). There are a few issues you should address with this bow, and especially with any future attempts:
- Your limb width profile dictates an elliptical tiller. You need to make the midlimbs bend more, and have the fade area bend less. You can still correct this tiller on this bow, and will loose maybe 5-8 pounds draw weight in doing so.
- The first 4" of the limb (the part that touches the riser) need to be make thicker. This is best done with a powerlam kind of principle. Create a thin lamination of wood that tapers from 1/8" to zero (a wedge) and glue this in between the backing and belly (/core) wood. It should extend past the riser by an inch or two/three. This additional lam beefs up the beginning of the limb, creating extra thickness and thus stiffness around those drilled holed.
- Red oak is pretty poor compression wood. It is not the best candidate for a bamboo backing. It can work (you've just proven that), but there are better belly woods if you want to back it with bamboo.
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Darksoul,
I think you are right that the ends need to be thicker in my next attempt. I will also be making the limb a bit longer (and the handle a bit shorter) so that the overall geometry lends itself to the drawlength I'm after.
I did score some Ipe for the next try, and now that I've gotten as far as I did on this one, I have a lot more confidence in the strength of the bolts than I did going into it. (Engineering calcs are all fine and good, but until you test it, it doesn't matter)
That piece of Oak was an exceptional piece I had left over from a few years ago. It had an SG of around .68 and some of the biggest late growth in the rings I've ever seen. The next one is with Ipe, so that should remove that potential issue.
Steve, I agree that the radius vs thickness is just a matter of the %. But I come at this as an engineer and generally try to do some calcs at the front end. I come at it from the radius because this is easier for me to do the math. It all works out to the strain-stress relationship.
Ken
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Ken, if you come at it as an engineer then you will need to figure the strain on the wood based on the radius it is bending and the thickness. Most wood has less than 1% elasticity.
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Here's how I drilled the alignment pin in my T.D. I got all my hardware from bingham. First, I marked out the center line on my riser pads and drilled and tapped it for the threaded insert and drilled a 1/4" hole half the depth of my alignment pin(I kept it about 3/8" away from the end of the riser).
Next, after i have epoxied in the threaded insert and put the alignment pin in the riser, I mark out center on the limb pads and drill for the limb bolts (but not the alignment pins).
Screw in the limbs till the alignment pin in the riser is just touching the the limb pad. Put it in a vise so you can look down the limbs and verify everything is lined up.
after its lined up, take a clamp and squeeze the limb pad just enough that the alignment pin leaves an indentation on the limb. then drill just enough that the top half of the pin bottoms out. Doing it this way will mean that you wont have the alignment hole showing on the top of your limb.