Latta Thicknessing Gauge Lie-Nielsen Toolworks
The tool is essentially a scraper. Set one opening for roughing materials to approximate size. Set the other opening for final thickness. To thickness your material, pinch the stringing between your fingers and pull it upward through the opening. Position two fingers in front of the mouth as guides. Pulling the stringing upward prevents it from instinctively going to the bottom of the opening.

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