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Intermediate level build/help-along - Laburnum Pyramid - UPDATED WITH PART 3!

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bownarra:

--- Quote from: NicAzana on August 19, 2022, 03:46:40 am ---Surely that depends on other variables than thickness and width!

A long bow will have thicker and narrower limbs for the same strain, and, conversely, a short bow need wider and thinner limbs to experience the same strain as a longer bow.

Same goes for reflex/recurving - if the stave has a lot of reflex (like this one), it must bend more to be braced, therefore requiring thinner, wider limbs to be equally strained, compared to a straight or deflexed bow.

I guess for laburnum (or yew), with its low bend resistance and high elasticity, the real answer is that it should be thicker and narrower than an otherwise identical bow of a stiffer wood? In this case, I have four inches of reflex, a pyramid taper, and will have very slim/light and stiff tips. It might be a little long, but I can shorten it a bit if it turned out to be understrained later.
Does anyone else think this is way overbuilt.

Of course bow design is a combination of many factors.
Really bow making is about judging maximum thickness for the wood you are working and marring width taper and tiller profile.
you aren't telling me anything I don't already know re. reflexing/recurving etc :) been there done that many times.
I'm simply trying to help you by telling you (all other things being equal)that labernum bows should be thicker /narrower than virtually all other woods - due entirely to the properties of the wood. I've probably made around 10 labernum bows. I've had 185fps @ 10gpp with the later ones.
Anyway I hope it works out for you.


--- End quote ---

NicAzana:
Shortish update:

Tillering is progressing along - When I first got it braced, the stave/bow was very twisted:



I'd monitored the limb sides, and they were equally thick, so the twist was a little bit of a mystery to me.
I then realized that the crown is a little uneven - it slopes a little more gradually on one side of the limbs, making it effectively thicker on one side.

Bows with this much reflex is a challenge to me - My ability to judge the bend is limited untill I get in on the tiller, but to get it there, it has to bend much more than a straight stave.
I think I've finally learned my lessen this time - next time I'm going to wait with flipping the tips untill I'm further along, and have the limbs already bending evenly.

Feeling whith my fingers and looking at the tiller, comparing with photos on the computer, and watching for set, I slowly try to get it bending more, without taking too much set.


Here it is, marked for another round of scraping.

It still has developed about 1-1 1/2" of set, subtracted from the original 4 1/2. But I've now have even thickness taper, and the twist is much reduced.
Here's my latest set of image analyses:



First I superimpose the braced and drawn profiles on top of the unbraced image. This way it is much easier for me to judge the bend on a stave with some character.
You can judge the set based on the chalk line behind the stave. The set is happening in the inner third of the left limb, and in the right limb, at the two kinks.



I can also compare drawn circles that touch each end of the bending part of the limb.
As you can see, the right limb is stiffer (circle is larger), and how the limb follows the cicle can help identifying stiff and weak sections (but remember to account for reflex/deflex and wiggles).



Finally, I'll "walk" the cut out bent stave along the unbraced bow in my program, help to see the details of where the bow is bending.

All, in all, these images, along with set, and what I can feel with my fingers, tells me that:

1) right limb is currently stiff
2) right limb needs to bend more on both sides of the wiggle, and is a little weak in the outer part of the bending limb.
3) left limb is stiff in the outer 1/3-1/2

I think this bow should ideally have almost circular/slightly elliptical tiller, so I've got some scraping to do.

I don't dare drawing it to full weight, until the bend is better, but I'm a little worried that it is approaching target weight fast.
But I think it is long enough at 68" NTN, that I'll be able to plug an inch off each end, as long as the tiller is good.

We'll see!

Stay tuned!

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