Blackbutt/How not to make a Static Recurve/First BBB
Posted: Mon Dec 01, 2014 3:04 pm
I recently decided to start on a bow based off of a profile I read of in an old English book on archery (The Book of the Bow, Gordon Grimley, 1958). Despite being overtly racist in place, it's not a bad little read.
The bow was also to be my first real attempt at a Bamboo Backed Bow (hereafter referred to as BBB, because I'm lazy), having previously only backed with Fibreglass or cloth (or not backed, as per selfbows). To make sure I really broke with the scientific method and changed as many things as I could at one time, it was also to be my first time using Blackbutt as a compression timber. I had read elsewhere on these forums that it was a usable timber and it marked a change from the usual Ironbark or Spotted Gum I tend to use.
Being a BBB, I'd read that that these tend to take a lot of set & it was advisable to make recurved tips static to minimise this. The final stack ended up,
Pole bamboo (back)
Reconstituted bamboo (core), 0.002 taper
Red Gum tip wedges/3" powerlam (wedges to ensure static recurved tips, powerlam to shift point of bending away from the glued on riser)
Blackbutt belly, 0.001 taper (belly)
I glued up with Titebond III, first time I've used this since I lived in Canada. I had great success with it there and it makes for a great alternative to the more expensive epoxies. I used an adjustable-style form, using the natural bend of the grass/timber stack to dictate the precise Reflex/Deflex profile. The gluelines came out great and after trimming the profile I cleaned up the edges.
After gluing on a beauty of a red gum/spotted gum riser (I really do like this combination),
I thought I'd give it a quick floor tiller, just to see how it might bend. I thought only a couple of inches wouldn't do it too much harm.
Cue the compression crack at the wedge tip (ie, where the tip wedge ends).
I had had my concerns about these wedges after trimming the profile and seeing the gap in the glueline.
It was here that the belly cracked, which is no surprise really. Lesson learned - those wedges have to be really smooth in transition. Or Blackbutt isn't all that great a timber in compression??
Some time wasted, but a few lesson learned. I started on a second BBB today with the lams currently gluing up end to end, ready for the full glue up when I next have a few hours to myself. I'll be using a powerlam with this one too (I've had success in the past with these) but will be avoiding tip wedges, for now. Or maybe I will give them a shot, I have some 1/16th thickness red gum that might be better suited for that.
Reflections
- Don't try too many new things at once, the scientific method exists for a reason.
- Don't be so sure about Blackbutt. Maybe.
- Pole Bamboo is great to work with and if you fail with it, it's far less stress than a failure with expensive fibreglass.
- Similarly, a failure using Titebond III is much easier to cope with than a failure with an expensive epoxy. Far, far fewer hypothetical tears.
- I should've put as much attention on my wedges as I did my powerlam, which came out great.
- Drum/Thickness Sanders make life very, very much easier than an improvised system. First time using it and I love it already.
The bow was also to be my first real attempt at a Bamboo Backed Bow (hereafter referred to as BBB, because I'm lazy), having previously only backed with Fibreglass or cloth (or not backed, as per selfbows). To make sure I really broke with the scientific method and changed as many things as I could at one time, it was also to be my first time using Blackbutt as a compression timber. I had read elsewhere on these forums that it was a usable timber and it marked a change from the usual Ironbark or Spotted Gum I tend to use.
Being a BBB, I'd read that that these tend to take a lot of set & it was advisable to make recurved tips static to minimise this. The final stack ended up,
Pole bamboo (back)
Reconstituted bamboo (core), 0.002 taper
Red Gum tip wedges/3" powerlam (wedges to ensure static recurved tips, powerlam to shift point of bending away from the glued on riser)
Blackbutt belly, 0.001 taper (belly)
I glued up with Titebond III, first time I've used this since I lived in Canada. I had great success with it there and it makes for a great alternative to the more expensive epoxies. I used an adjustable-style form, using the natural bend of the grass/timber stack to dictate the precise Reflex/Deflex profile. The gluelines came out great and after trimming the profile I cleaned up the edges.
After gluing on a beauty of a red gum/spotted gum riser (I really do like this combination),
I thought I'd give it a quick floor tiller, just to see how it might bend. I thought only a couple of inches wouldn't do it too much harm.
Cue the compression crack at the wedge tip (ie, where the tip wedge ends).
I had had my concerns about these wedges after trimming the profile and seeing the gap in the glueline.
It was here that the belly cracked, which is no surprise really. Lesson learned - those wedges have to be really smooth in transition. Or Blackbutt isn't all that great a timber in compression??
Some time wasted, but a few lesson learned. I started on a second BBB today with the lams currently gluing up end to end, ready for the full glue up when I next have a few hours to myself. I'll be using a powerlam with this one too (I've had success in the past with these) but will be avoiding tip wedges, for now. Or maybe I will give them a shot, I have some 1/16th thickness red gum that might be better suited for that.
Reflections
- Don't try too many new things at once, the scientific method exists for a reason.
- Don't be so sure about Blackbutt. Maybe.
- Pole Bamboo is great to work with and if you fail with it, it's far less stress than a failure with expensive fibreglass.
- Similarly, a failure using Titebond III is much easier to cope with than a failure with an expensive epoxy. Far, far fewer hypothetical tears.
- I should've put as much attention on my wedges as I did my powerlam, which came out great.
- Drum/Thickness Sanders make life very, very much easier than an improvised system. First time using it and I love it already.