Some comments on straightening saws

Discussion in 'Saw Makers Forum' started by Dusty Shed Dweller, Mar 11, 2019.

  1. Dusty Shed Dweller

    Dusty Shed Dweller Most Valued Member

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    Demonstrated at the Lost Trades Fair on the weekend, lots of "ooohs" and "aaaahs" when I de-kinked and re-tensioned a couple of hand saws with the hammer. Had the pleasure of meeting two elderly gentlemen who had done trade courses back in the late 1960's, and worked maintaining big mill (5-6') circulars and 84' long, 14" wide band saws. We had an interesting chat about hammering saws.

    1. The anvil and saws are critical. The anvil should have a 7 thou crown across the top and a tool steel face for rebound. The rebound ("spring") makes life easier when you hammer all day.

    The next concept is the hard one to grasp; both discussed tension as absolutely critical. Most work is done with the dog head hammer (essentially a round face) which does spot work, each strike adding tension. It is typically light and used gently, not like the hammer of Thor. When striking circulars, the cross head (with rectangular faces) either adds or removes tension depending on whether or not you strike tangential or perpendicular to the circumference.

    They both told me that its all about equalising the tension across the plate. A warp on one side is corrected by working the opposite to "pull" the mis-alignment true.

    Both made their own hammers as part of their trade qualifications, maybe that's why they are so rare.

    2. With the circulars they used a small spot light above the saws to detect issues with the straight edge, typically starting by hammering around the rim to remove the tension before selectively adding it back in. One mentioned using heat (with a double burner set up) to spot ("just enough to colour") around the rim to pull the tension... the spot heat expands then contracts the metal in. I'll experiment with this one and post on my experiences, it might be a method for removing those awful expansion kinks along the toothline in backsaws, but you wouldn't want to get too excited and drawl the temper.

    3. Both worked with "light box set ups"... in which a red light supplied controlled illumination when sharpening. Apparently this makes dull teeth reflect white and sharp teeth reflect pink. Not sure on the physics of this one.