Main Discussion Area > Bows
Bows by the numbers - modelling a bow before cutting timber
Hamish:
Do you have any pics of the bow unbraced, that show where the set occurred? 40mm is a good result in my books too, from an oak board stave.
I find oak can take a lot of set, unless it's very dry or heat tempered.
willie:
--- Quote from: Hamish on September 08, 2024, 07:46:41 pm ---Do you have any pics of the bow unbraced, that show where the set occurred? 40mm is a good result in my books too, from an oak board stave.
I find oak can take a lot of set, unless it's very dry or heat tempered.
--- End quote ---
Hamish, I think he could have designed for any amout of set he preferred.
Not to sidtrack the design part of this thread, but should one prefer more or less set with any particular kind of wood? ie do white oak bows work out the best with a moderate ampunt of set (40mm)?
Aussie Yeoman:
--- Quote from: sleek on September 08, 2024, 02:15:10 am ---Thanks that's perfect. If you had made them 11 to 15 % less in draw weight what does your calculations say you should predict for set? Per mine, 11 to 15% less the ideal range for no set for white oak.
--- End quote ---
Sorry Sleek I don't quite follow - is your objective to get less set or to have a lower draw weight? The thickness of the 40 lb and 50 lb bows is pretty much identical - the difference between them is the width. 40 lb is 80% of 50 lb, so the lower draw weight bow is 80% of the width.
--- Quote ---deliberately designing to 1-1/2" of set and coming in at 40mm is great. I wished some of mine came in that close. Guess it is all in the test data. Have you considered or tried a simple bend test to obtain a working strain?
--- End quote ---
Yeah I've done heaps of bend tests. And I got a copy of al the bend test data that Tim Baker did decades ago for TBB1. The working strain is what I used to calculate the dimensions of these bows. In this case, the working strain is about 0.75%.
I've done a few different bend test styles - simple cantilever, simple supported beam, three point cantilever... But the one I want to try is a four-point bend test. This spreads the load evenly between the two inner supports, and so better simulates a bow's limb in that part of the test sample.
--- Quote ---Do you have any pics of the bow unbraced, that show where the set occurred? 40mm is a good result in my books too, from an oak board stave.
I find oak can take a lot of set, unless it's very dry or heat tempered.
--- End quote ---
Stand by and I'll take some pictures.
I have never tried heat treating, but I may for this 50 lb job to see what happens. I'll have to brush up on my HT knowledge.
--- Quote ---Not to sidtrack the design part of this thread, but should one prefer more or less set with any particular kind of wood? ie do white oak bows work out the best with a moderate ampunt of set (40mm)?
--- End quote ---
I'm not sure on that one. I haven't done the testing. Designing these bows to have this level of set is what allowed me to get 1 lb per mm or width at this length without an outrageous amount of set.
Aussie Yeoman:
I tried to take photos of the set, which turned out to be a challenge. Most of the set comes from the inner third of the limb.
Easier to show is where the stress is distributed throughout the limb - more stress = more set at that location. I could fiddle with the design more to distribute the stress throughout the limb more evenly, but the layout of this bow is super simple, and the tillering almost takes care of itself. So it has its trade-offs.
Woodbear:
Looks like you have a couple of nice bows there. And they appear to match the modeling to within a good tolerance.
I have a few questions:
You state that the thickness is within 1 mm of the design dimensions. Does this mean you tillered to get the desired weight at the desired draw and then measured the thickness for comparison to the model?
Are the computed cross sections and actual bows rectangular at all points on the bow, or does the modeling accommodate non-rectangular cross sections?
Have you taken draw curves to check against the model?
Why did you make 2 sets of string grooves?
How do the bows shoot?
Does your Virtualbow compute the set, or did you calculate that separately? (my copy of Virtualbow does not do set)
Do you have any setup for, or intention to measure the dynamic properties of the bows for comparison to the model?
By the way, I also think a 4 point bend test setup should be best for characterizing wood. The other test methods mentioned preferentially test a single location, which is bound to add uncertainty to the results. A 4 point test should average over more wood. It should also fail at the weakest spot in the tested section. If you have a test setup please post a picture some time.
Dave
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