B Andersen

You can avoid the issue of flattening the spar by making sure the grooved in the bending rollers 1) exactly as wide as the spar you are bending and 2) >50% of the spar's diameter deep. This gets the widest part of the spar down into the groove and prevents it from spreading or flattening as you bend the spar. The distance between the rollers shorter determines how close to the end of the spar you can achieve bending.