The root of the external thread or the crest of the inner thread .
大径指过外螺纹牙顶或内螺纹牙底的圆柱面直径。
Through metallographic analysis of the material, microstructure change and deformation mechanism of crest, flank and root of the thread along the layer depth direction were obtained.
结果表明，牙顶处金属材料受到丝锥棱齿的压力相对较小，并且金属流向牙顶两侧上方的区域时受到的阻力最小；
Through metallographic analysis of the material, microstructure change and deformation mechanism of crest, flank and root of the thread along the layer depth direction were obtained.
结果表明，牙顶处金属材料受到丝锥棱齿的压力相对较小，并且金属流向牙顶两侧上方的区域时受到的阻力最小；
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