Tuesday February 09, 2016
Shaping and Bending
Working with our woods for the flat surfaces of soundboards and backboards is a rather straight-forward task, but not all woods can be shaped and bent with equal ease and results when preparing instrument rims (ribs), especially where acute bends for cutaways and similar tight curves are concerned.
To analyze the bending and shaping attributes of our woods, we enlisted the aid of Roger Siminoff, author of "The Luthier's Handbook" and luthierie consultant in Atascadero, California, who performed tests using several techniques. Complex bends were performed in a fixture that shaped the wood into a tight "S" curve. Each section of the "S" was formed around a 2" diameter post. While the radius of these bends is more extreme than those used for guitars, we wanted to provide you with information about shaping and bending our woods under the most extreme conditions.
For our tests, we considered how difficult it was to bend the wood, how much the wood sheared (checked), and how easily it cracked (if at all).
The wood in the above test cracked and sheared (checked).
The smoothness of the bend was an important consideration in our tests. This sample piece bent beautifully and easily with smooth bends and curves.
The Tests - what was evaluated:
Propensity to shear or check: Each wood was graded on a scale of A-F based on its propensity to shear or check. Aluminum bands were used on both sides of the wood to help form the wood into the bend. The aluminum bands were prepared to be 1/8" narrower than the wood to test how the exposed edge of the wood would react to being bent without support. If the wood had good structural integrity, and no edge tearing (shearing) occurred, it would be graded as "A". If the wood had poor structural integrity, and edge tearing was excessive, it would be graded as "F".
Propensity to crack: Each wood was graded on scale of A-F based on its propensity to crack across grain. If the wood bent with smooth, clean, flowing bends it was graded as an "A". If the wood presented small steps or erratic bends, it was graded as an "F". Wood that cracked immediately was graded as an "X". (Notes: 1-This does not suggest that the wood cannot be bent in curves with a larger radius or that it cannot be coaxed with more heat or a greater concentration of heat. 2- Woods rated "X" for cracking might best be used for flatted areas such as soundboards and backboards and not for ribs. Please contact us if you would like to purchase a small sample to test with your own bending methods.)
Curvature (smoothness) of bend: The shape and evenness of the curvature is very important. Each wood was graded on a scale of A-F based on how satisfactory the bend was. Bends that were very smooth, clean, and well shaped were graded as an "A". Bends that were erratic and not well shaped were graded as an "F".
Grain bias (preparation): Typically, woods bend more easily when the grain is flat (parallel to the wide side of the piece). Grain that is quartered or that runs across the piece imposes greater difficulty in bending. Most dense hardwoods can be bent well in both directions because the density and stiffness of the wood is more similar both across and through the annular rings. Our tests did not take grain direction into consideration and the pieces were prepared primarily in the vertical grain, quarter-sawn method.
Bending method: The wood was wetted for 5 minutes before being bent and we used steam as the heating and wetting agent. Steam at 216° was applied directly to the wood during the bending process from a hose, with an aggressive blast of steam coming from the steam chamber at 40psi. Steam was applied globally for 10 seconds and then directed at the bending location as the fixture was forming the wood into a bend.
Table of Results
A perfect bend on Higuerilla.