Tillering Limb Twist

[hillbilly61]

 My limbs are starting to twist as I'm tillering. They both twist in the same direction. I think this is do to weakening the same side on both limbs. Is this correct or do I need to look at another problem. This is an experamental bow for me, just to see if it will work. It's 60" end to end 1" wide tapering to 1/2" at the tips. The wood make up is. Hickory backed by 1/8" IPE backed by Boo. I know all these are powerful tnbsion woods. But like I said, it's just an experament.... Thanks for the help in addvance.

[Lombard]

I have had that happen to me, and I assumed that it was due to taking to much material from one side. However using calipers revealed this not to be the case. So my next assumption was it must be internal stresses inside the wood. Like so much of my bowyering activities I proceed with lots of theorems and very few proofs. Ended up using dry heat to correct the twist, and the bow is still being used today.

[half eye]

Hillbilly,
      Seems if they measure equal side to side, might be that one of the glue joints or wood layers may be slightly more dense, stornger, something like that. With two joints and three layers of wood it seems pretty logical that there is some room for differing strengths within the mix.....not necessarily your tillering.
      It is my experience that if I get tillering twist it's usually on opposite sides, limb to limb. and not twisting on the same sides.JMO
rich

[hillbilly61]

 Why I didn't notice this when I put the tiller string on is beyond me, but my center line is off to the same side it's twisting to. With it being 1/4 sawn I'm assuming that it's trying to compress the weaker side. Does that make sense? If that be the case then even though I take some off, the off center side, It may still try to compress the weaker side. Would that be the case too?

[half eye]

I'm no expert on laminated bows, but have a made a few. With this problem showing up after you started the tillering (ie: cutting material from the belly) I would first suspect that maybe one or all of the individual lam's may be thicker on one side than the other, perhaps a very tiny amount. As they got glued up the "strong side bias" was not visible because the actual wood could overcome it, but as the belly became weaker during tillering the bias could become progressively more noticeable. If that is the case (I dont know for a fact) you may be able to correct some of it by shaving the strong side EDGE of the bow.
     After a few strokes of the scraper/file and measuring to see if ya get correction in the string tracking that should let you know without removing too much wood.

Brought this up because lately I have run my backer wood through a thickness planer and have some that come out slightly thicker on one side. I ignore that during glue up....but when cured I run the resulting stick through the surface planer again and measure the entire stick for equal thickness. May not be your problem....just thought I'd pass it along for you look at as a possible answer.
rich

[hillbilly61]

 Thanks Rich...I'll take a few scrapes off the stronger side and see what happens. I'm almost betting the IPE is over powering the rest. I'll report back when I know something.