Author Topic: Heat-Bending a Backed Bow: Lesson Learned  (Read 1261 times)

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Offline kentowl

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Heat-Bending a Backed Bow: Lesson Learned
« on: January 23, 2011, 05:23:56 pm »
I was working on a hickory-backed oak stave, glued up with EA-40 mixed 2A:1B for max heat tolerance. Steaming had very little effect on the oak (it would bend, then go right back to nature afterwards). So I tried adding more reflex to the tips using dry heat, and one end delaminated. I threw some more EA-40 in the gap and clamped it for 24 hours - which worked through bench testing to my draw length. But it let go as soon as I drew full length. After that I worked harder on the joint, cleaning with sandpaper and acetone, glued, wrapped, and clamped. That has worked through test shooting with no signs of letting go.

But now I'm hesitant to use dry heat on a glued assembly. I used steam on my next (hickory-backed black cherry) and in future will either steam, or steam for awhile, then use the heat gun on the steamed limb. Anyone have experience/suggestions with a hybrid bending process?

Offline Grunt

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Re: Heat-Bending a Backed Bow: Lesson Learned
« Reply #1 on: January 23, 2011, 08:32:42 pm »
Glue in your bends by doing the backing to bow glue up in a pre-made form. For this to work the bow limbs have to be almost finished thickness. I don't think trying to heat bend a glued up bow is a good practice.
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Offline Del the cat

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Re: Heat-Bending a Backed Bow: Lesson Learned
« Reply #2 on: January 24, 2011, 08:08:23 am »
Like Grunt says, glueing in the shape is the way, you are then building in the recurve tension you want, so it won't pull out so much.
Del
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Offline kentowl

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Re: Heat-Bending a Backed Bow: Lesson Learned
« Reply #3 on: January 24, 2011, 01:37:27 pm »
Thanks, grunt and del the cat - that's the lesson learned.

I was gambling, based on TBBs heat-treating chapter, suggesting repetitive heat treating would not damage the belly wood. I had also mixed the EA-40 for maximum heat performance - just didn't know how much it could take. Now I do - less than I used javascript:void(0); There's really no excuse; in my day job I build high-temp (700C+) nitrogen work stations made to break down military-grade epoxies and reflow eutectic bonding.

The good news is that regluing let me clamp it fully on the form and the tip appears strong with no change in draw weight. But I went to steam on the next one (hickory over black cherry) and I'm likely to use steam followed by no more than light dry heating in the future...coming up, sycamore-backed black cherry.

Amazing how many "note to self - don't DO that" messages just a couple of bows can generate.

Offline Jesse

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Re: Heat-Bending a Backed Bow: Lesson Learned
« Reply #4 on: January 24, 2011, 03:43:59 pm »
.  If you use urac I believe you can recurve the tips with heat after glue up. I never tried but I believe RyanO does it with good results. I wouldnt use this method for the whole limb profile but it might be ok for just flipping the tips. I wouldnt try it with EA-40/smooth on
"If you can find a path with no obstacles, it probably doesn't lead anywhere."
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Offline kentowl

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Re: Heat-Bending a Backed Bow: Lesson Learned
« Reply #5 on: January 25, 2011, 11:42:18 am »
appreciate that, Jesse:

I'd be interested to see tech data on Urac 185 to see if it is supposed to handle high temperature much better than EA-40. I believe the internal temperature where wood starts to plasticize is above the 180C-210C glass transition temperature of even the Mil-grade (150C cure) epoxies I face at work.

Based on my limited experience, my light heat treating (not as deep heating as Marc recommends) pushed EA-40 to the edge; one limb separated under stress, one didn't. For what it's worth, heat treating the glued-up bow didn't cause any apparent problems (I shot it successfully several times after that) only heat treating and clamping when I wanted to add more reflex. But I'd rather keep a larger cushion than that - will depend more on steam, and perhaps heating afterwards to help dry. Something more to learn about..

ken